document classification: title: network planning...

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GUIDELINE

Document Classification: Controlled Disclosure

Reference: 34-1408

Document Type: DGL

Revision: 0

Published date: APRIL 2008

Total pages: 34

Title: NETWORK PLANNING GUIDELINE FOR ELECTRICAL MOTORS

Review date: APRIL 2011

TESCOD APPROVED COMPILED BY APPROVED BY FUNCTIONAL RESP AUTHORISED BY Signed _ _ _ _ _ _ _ _ D DUKE CG CARTER-BROWN

Signed _ _ _ _ _ _ _ _ CG CARTER-BROWN

Signed _ _ _ _ _ _ _ _ V SINGH

Signed _ _ _ _ _ _ _ _ MN BAILEY

Authors Planning SC Chairman for TESCOD CMDT for MD (Dx)

Content

Page Foreword..............................................................................................................................................2 Keywords .............................................................................................................................................3 Bibliography .........................................................................................................................................3 1 Scope..........................................................................................................................................4 2 Normative references..................................................................................................................4 3 Definitions and abbreviations......................................................................................................4 4 Basic theory of induction machines ............................................................................................5 5 Motor starting ..............................................................................................................................8 6 Power Quality............................................................................................................................12 7 Application guideline .................................................................................................................17 8 Modelling Motors in PSA Software ...........................................................................................19 9 Worked examples .....................................................................................................................24 Annex A: Typical motor parameters ..................................................................................................28 Annex B: Typical motor load characteristics .....................................................................................30 Annex C: Impact assessment............................................................................................................31

DOCUMENT CLASSIFICATION: CONTROLLED DISCLOSURE NETWORK PLANNING GUIDELINE FOR Reference: 34-1408 ELECTRICAL MOTORS Type: DGL Revision: 0 Page: 2 of 34

When downloaded from the EDS database, this document is uncontrolled and the responsibility rests with the user to ensure it is in line with the authorised version on the database.

Foreword

The electrical motor is one of the most common types of loads. Electrical motors can cause power quality problems (rapid voltage change during motor starting and voltage flicker with certain mechanical loads). The ability of electrical motors to fulfil their required function is influenced by the network supplying the motor. If fault levels are too low in relation to the size of motor and method of starting then the motor may not be able to be started. Power quality characteristics such as voltage unbalance and harmonics affect the efficiency and performance of motors.

It is important that Eskom Distribution network planners have an understanding of the basic theory of electrical motors, how these motors are affected by various power quality characteristics, and how motor starting characteristics need to be considered when planning network extensions or performing network strengthening.

Revision history

This edition replaces edition no A of guideline no. DISAGABK6.

Date Rev. Compiler Remarks

Apr’05 A D Duke Original issue for comment. Document was never formally published via TESCOD.

Apr’08 0 C Carter-Brown

Re-formatted and reference number changed from DISAGABK6 to 34-1408. Process for the assessment of motor loads simplified.PowerFactory modelling included. Additional worked examples.

Authorisation

This document has been seen and accepted by:

Name Designation Rob Stephen General Manager – Distribution Capital Program Kurt Dedikend Network Services Manager – Eastern Region (NSM Planning custodian) Riaan Smit Network Planning Manager – Western Region Simphiwe Hashe Network Planning Manager – Southern Region Mike Pallett Network Planning Manager – Eastern Region Kobus Barnard Network Planning Manager – North West Region Chris du Toit Network Planning Manager – Central Region Monde Bala Network Planning Manager – Northern Region

This guideline shall apply throughout Eskom Holdings Limited, its divisions, subsidiaries and entities wherein Eskom has a controlling interest.

Development team

This guideline is based on the efforts and contributions of the following individuals representing their respective regions on the Reticulation Planning Electric Motor Starting Working Group:

D. Duke Western Region A. Sprunt Eastern Region S. Hashe Southern Region B. van Niekerk Northern Region



The guideline was originally developed in 2005, but was not formally published. It has been updated for formal publication. Keywords

Network planning, network design, motor, power quality. Bibliography

“Electric Machinery”, 4th edition by AE Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans McGraw-Hill Book Co., Singapore, 1985

“Alternating Current Machines”, 5th edition by MG Say, Longman Scientific & Technical, England, 1983

EASA Electrical Engineering Handbook

Eskom Power Quality Reference Guide, Revision 2: April 1996

Flicker Prediction Techniques for Sawmills, I. Jefferies, G Atkinson – Hope, Department of Electrical Engineering, Cape Technicon

“An Investigation of Flicker Emission Prediction Techniques for Crushers in Power Systems” M.Y. Martin, G Atkinson – Hope, Department of Electrical Engineering, Cape Technicon

“The Performance and Design of Alternating Current Machines”, M G Say, 1958

DigSilent Technical Documentation: Induction Machine, March 2007



1 Scope

This guideline covers the theory of electrical motors, affect of power quality characteristics on the efficiency and performance of motors and considerations when connecting electrical motors to the Eskom network. It specifies what studies need to be performed by the Network Planner, how results should be interpreted and what actions are required to mitigate possible power quality and/or motor starting problems.

This guideline is applicable to HV sub-transmission and MV&LV distribution network planners. For a summary of the key information jump to the Application Guideline on page 17. 2 Normative references

Parties using this guideline shall apply the most recent edition of the documents listed below:

NRS 048-2: 2004 Electricity supply – Quality of Supply Part 2: Voltage characteristics, compatibility levels, limits and assessment methods

NRS 048-4: 2006 Electricity supply – Quality of Supply Part 4: Application guidelines for utilities

34-542 Distribution voltage regulation and apportionment limits

34-618 Network planning guideline for voltage, technology and phasing

34-539 Network planning guideline for MV step-voltage regulators

34-335 Distribution network planning standard

3 Definitions and abbreviations

3.1.1 Constant power load: A load that draws a constant power irrespective of an increase or decrease in voltage. An increase in voltage will cause a decrease in current to maintain constant power.

3.1.2 Constant impedance load: A load where the impedance remains constant irrespective of an increase or decrease in voltage. An increase in voltage will cause an increase in current as the impedance is constant.

3.1.3 Constant current load: A load where the current drawn remains constant irrespective of an increase or decrease in voltage. An increase in voltage will result in increased power consumption as the current is constant.

3.1.4 Resonance: The amplification of currents and voltages at certain frequencies.

3.1.5 DOL: Direct On Line

3.1.6 YD: Star Delta

3.1.7 PCC: Point of Common Coupling

3.1.8 HV: High Voltage

3.1.9 MV: Medium Voltage

3.1.10 LV: Low Voltage

3.1.11 PF: Power Factor

3.1.12 SC PF: Short Circuit Power Factor

3.1.13 QOS: Quality Of Supply

3.1.14 ARC: Auto recloser

3.1.15 PCC: Point of Common Coupling

3.1.16 POS: Point Of Supply

3.1.17 VSD: Variable Speed Drive



3.1.18 LPU: Large Power User

3.1.19 SPU: Small Power User

4 Basic theory of induction machines

4.1 Introduction

Induction machines are supplied with alternating current directly to the stator and to the rotor by induction action from the stator. When excited from a balanced polyphase source, induction motors produce a magnetic field in the air gap rotating at synchronous speed as determined by the number of poles and the applied frequency, which is 50Hz in case of South Africa. Induction motors convert electrical energy drawn from the power supply into a mechanical power, usually as a shaft rotating at a speed determined by the frequency of the supply. The power available from the shaft is equal to the torque (moment) multiplied by the shaft speed (rpm). From an initial value at standstill, the torque varies up and down as the machine accelerates, reaching a peak at about two-thirds synchronous speed and ultimately becoming zero at synchronous speed. This characteristic means that induction motors always run at a slightly less than synchronous speed in order to develop power, slip speed, and, hence the term asynchronous. Figure 1 below shows an induction motor torque/speed curve that illustrates most of this characteristic.

Figure 1: Torque/speed characteristics of an induction motor

It is to be noted that for each type of motor, there is a different torque/speed characteristic that is defined or determined by the load type. Figure 2 below shows the characteristic coupled with load.

Figure 2: Torque/speed characteristic coupled with load

The difference between the developed torque (motor) and the absorbed torque (load) causes the acceleration of the motor-load. The larger the difference, the greater the acceleration, however this



increased acceleration also places greater stress on both the supply and drive systems during starting. Figure 3 below shows the acceleration torque as mentioned above.

Figure 3: Torque/speed characteristics with accelerating torque

Assume that the rotor is turning at n r/min in the same direction as the rotating stator field. Let the synchronous speed of the stator field be n1 r/min. The rotor is then travelling at a speed n1-n r/min backward with respect to the stator field. The speed n1-n r/min is referred to as slip of the rotor. Slip is usually expressed as a fraction of synchronous speed; i.e., the per unit slip S is

1

1

nnnS −

=

The relative motion of flux and rotor conductors induces voltages of frequency Sf, called slip frequency, in the rotor. Thus, the electrical behaviour of an induction machine is similar to that of a transformer but with additional feature of frequency transformation.

When used as induction motors, the rotor terminals of the machine are short-circuited such that the rotor currents are then determined by the magnitudes of the induced voltages and the rotor impedance at slip frequency.

At starting, the rotor is stationery, the slip S = 1, and the rotor frequency equals the stator frequency f. The field produced by the rotor currents therefore revolves at the same speed as the stator field, and a starting torque is developed, tending to turn the rotor in the direction of rotation of the stator-inducing field. If this torque is enough to overcome the opposition to rotation created by the shaft load, the motor will come to its operating speed. The operating speed can never be equal to the synchronous speed n1 as the rotor conductors would then be stationery with respect to the stator field and no voltage would be induced in them.

The motor speed/torque characteristic is controlled by the rotor resistance. A motor with high rotor resistance can generate its peak torque at standstill thus giving the high break-away torque characteristic as shown in figure 4 below. It reduces steadily as the speed increases, becoming zero at synchronous speed. A motor with low rotor resistance will produce a low starting torque but will generate its peak torque closer to the synchronous speed. This type of motor runs at full power with higher operating efficiency and low slip speed.



Figure 4: High break-away torque characteristic

4.2 Starting torque

At the instant of motor starting, the rotor current/phase at standstill is defined as follows:

20

22

00

XR

EI

+=

Where:

0I = rotor current per phase at standstill

0E = rotor emf generated per phase at standstill

2R = rotor resistance per phase

0X = rotor reactance per phase at standstill

Thus the starting Torque,

20

22

20

XRREk

Ts +Φ

=

φcos0IkTs Φ=

Where

k = proportionality constant

( )( ) 0

220

22

2200

20

22

2

=+

Φ−Φ+=

XRREkEkXR

dRdTs

02

=dRdTs

( ) 02 2200

20

22 =Φ−Φ+∴ REkEkXR



20

22 XR =∴

02 XR =∴

The starting torque is therefore at maximum when the rotor resistance is equal to rotor reactance at standstill.

Since Φ∝0E and 1V∝Φ

22

2

o2

21 s

XRRV T

+∝

If the rotor resistance and rotor reactance are constant at standstill, the starting torque varies with the square of the applied voltage.

4.3 Relationship between torque and slip

As the rotor accelerates during the period of starting, the slip, the rotor emf, rotor frequency and rotor reactance decrease continually. Therefore the rotor current decreases as well, but not in proportion to the slip. For a considerable portion of the starting period the increasing power factor causes the torque to increase, but later, when the reactance becomes very small due to low slip, (X2 = sX0), the decrease in rotor current becomes the dominating factor and the torque drops suddenly to zero at synchronous speed. The maximum torque is developed when the slip is such that the rotor resistance is equal to rotor reactance. The change in rotor resistance does not influence the value of the maximum torque but only the slip at which maximum torque is developed. Therefore, the maximum torque is inversely proportional to the standstill rotor reactance.

A change in the applied voltage affects the torque at any given slip in a similar way that it affects the starting torque, so that, if the applied voltage decreases, the torque decreases and hence the slip to develop a given torque, increases, therefore the speed decreases.

5 Motor starting

5.1 Introduction

Induction motors are frequently started, by connecting them directly to the supply. This is referred to as Direct On Line (DOL) starting Due to the amount of current that gets generated during DOL starting (in the range of 500% to 800% of rated current at full load), different techniques are used with induction motors to reduce the starting current. These techniques provide reduced starting currents and in many cases also result in reduced starting torque. Thus, their applicability to any particular situation must be carefully considered along with other issues related to their implementation (annex B provides an indication of the starting torque requirements of common motor loads). There are many and varied types of loads that have different torque characteristics which in turn affect the manner in which these motors are started. The usage of the different techniques requires a basic understanding of the power system and the phenomena associated with the induction motor contribution on the network.

When starting a de-magnetized induction motor from standstill all the energy necessary to magnetize the motor, to provide the acceleration force and to supply the kinetic energy of the rotor and load, must be present together with the energy to overcome the mechanical and electrical losses. To do so at full supply voltage places considerable stresses on the supply, the motor windings, and the iron cores of the stator and rotor.

Excessive acceleration of a rotor when the mechanical load is small can produce torque oscillations in the shaft, causing severe wear to transmissions, gears and drives. Excessive acceleration when the load inertia is high, such as in centrifugal fans, causes belts to slip in the pulleys, producing rapid wear and early failure.



5.2 Direct On Line (DOL) starting

The simplest means of starting an induction motor is to connect the power supply by a single, solenoid operated, 3-phase switch, known as a contactor. Very widely applied, the method is known as direct on line, and is the usual form of control where low cost is the first, and most important consideration. As a result, it is most often used on smaller motor sizes (7.5 up to 22kW), or where the supply is strong enough to withstand the inrush and starting current surges without causing unacceptable voltage drops. Normally, the starting current of induction motors is six to seven times the rated current. It should be noted that the high starting current can cause supply disturbances to other consumers on the network due to the voltage drop in the main power supply.

The simplicity and apparent low initial capital cost of DOL starting must be balanced against risks and costs associated with increased maintenance, reduced equipment life and higher risk of motor failure, particularly when frequent stopping and starting is needed. In larger sized motors special strengthening is necessary, at higher cost, before they can be safely used with direct-on-line starting. However, the shortcomings of the direct-on-line starter have been recognized ever since motors have been used and alternative systems have been developed over the years to reduce the damaging effects of this form of starting.

5.3 Star-Delta starting

Inrush current (and motor torque) is proportional to the square of the terminal voltage. Reduced starting current can be achieved by reducing the applied voltage during motor starting. The most familiar type of reduced-voltage starter is the star-delta starter.

Consisting of three contactors and a time switch (which can be mechanical, pneumatic, electrical or electronic), the star-delta starter changes the motor winding configuration from an initial star connection to a delta as the motor accelerates. The change-over or transition point is controlled by the time switch and is usually arranged to be at 80% of full speed. The effect of starting in star is a reduction in the voltage across each stator winding to 58% of the normal running voltage. This reduces the starting torque to a third of locked rotor torque (LRT) with a consequent reduction in starting currents and acceleration forces.

Although star-delta starting is an apparent improvement over DOL starting, it does have significant disadvantages. The transfer from star to delta momentarily removes the motor from the supply. During this time the motor is under the mechanical influence of the rotating load and, at the instant of disconnection, current will still flow in the rotor bars due to the time delay necessary for the magnetic flux to die away. As a result of this there is a residual flux "frozen" on the surface of the rotating rotor which cuts the stator windings and generates a voltage whose frequency is dependant on the rotor speed. If the load inertia is small, such as in a pump, or if the friction is high, there could be a significant loss of speed during the time the supply is disconnected.

In this case, when the reconnection to delta is made, a large phase differential can exist between the supply and the rotor fluxes. This can give rise to very large current surges (as much or more than full-voltage locked rotor current), together with massive transient torque oscillations. (These oscillations can be as much as five times full-load torque.) Although the effects described are only present for a very short period of time (about one fifth of a second), they are sources of great stress and damage to the whole drive system, and where frequent starting is necessary, invoke high maintenance costs.

The star-delta starter also has the disadvantage of restricted starting torque available (if 35% or more of the LRT is required to ‘break-away’, one will need to either oversize the motor or consider an alternative method of starting). Combined with the severe effects of the re-switching surges, and the additional costs of bringing six conductors from the motor to the starter as opposed to only three, star-delta stating only offers an imperfect solution to the problem of starting the induction motor.

Figure 5 shows the voltage recording at a 55kW (D250S, RPM 2970, 380V) motor started using a Star-Delta starter. The motor is supplied from a pole mounted 200kVA 11000/400V transformer, on mid tap, with 30metres of 70mm² 4c CU PVCA LV cable. The 11kV, 3 phase fault level at the transformer is 938A. The LV voltage dropped from 102.6% to 97% (-5.6% drop). The motor sped up for almost 2 seconds, changed over to “Delta” causing a second voltage drop to a voltage slightly lower than the first dip and then ramped



up to full speed. This is typical of star-delta starting i.e. two rapid voltage changes, which if excessive can annoy other customers in the vicinity.

Figure 5: Example 55kW motor voltages during starting using a Star-Delta starter

5.4 Primary resistance/reactance starter

It has been recognized that the transition step in the star-delta system was a source of problems such as welded contactors, sheared drive shafts etc., and for many years a method of stepless control has been available in the form of the primary resistance/reactance starter. This type of controller inserts a resistance/reactor in series with each of the phase connections to the stator at start-up, after which it is progressively reduced and shorted out at the end of the acceleration process. Frequently, the resistances are movable blades that are gradually immersed in an electrolyte liquid. The mechanism is usually large and expensive, both to purchase and to maintain, and considerable heat is created by the passage of current through the electrolyte resistor. This limits the starting frequency (because the electrolyte has to condense back to liquid before a new start can proceed), and these restrictions prevent this starter from being a popular option when selecting a control system. However, it has the distinction of being the smoothest and least stressful method of accelerating an induction motor and its load.

The reactor is an electromagnetic choke winding that has been designed to allow fixed current to pass through it. The current is a function of the tapping that has been selected where the required current limitation is set by design. The starting voltage is reduced to the set tapping percent at zero speed by 40% on the 60% tap and the current limited to an acceptable level. The inductance of the reactor is in opposition to that of the motor and is controlled by means of the motor itself. The voltage drop is caused by the high magnetic field that is created by excess reactive power on the motor. As the motor speed increases, the



motor reactive power decreases. The magnetic field sustained by this reactive power decreases as the motor speeds up. The decreasing magnetic field reduces the back EMF in the reactor windings. Thus the motor speed and the reactive power are proportional to the magnetic field of the reactor and the voltage drop. The motor torque varies proportionally to the square of the voltage.

5.5 Auto-transformer starter

This method of starting requires that a step-down autotransformer be installed to reduce the voltage during starting. As the motor approaches full speed, the autotransformer is switched out of the circuit and rated voltage is applied to the motor terminals. The transformer used requires only a short time current rating however its leakage impedance should be small to avoid undue limitation of the starting current. It needs to be noted again that the reduction in voltage not only results in the reduction of the starting current but also in a reduction of the starting torque which is proportional to the square of the terminal voltage. This is one of the better methods of starting as it reduces the starting current quite considerably. The starting current is normally 2.1 to 2.3 times the rated normal running current of the motor.

5.6 Soft starter (solid state)

During the 1950's, much effort was put into the development of a four-layer transistor device that had the power to switch large currents at high voltages when triggered by a very small pulse of current. This device became known as the silicon controlled rectifier (SCR) commonly known as the Thyristor. It is the basis on which all soft starting systems are built. The characteristic of most interest is the ability of the Thyristor to switch rapidly (in about 5 millionths of a second) from "OFF" to "ON" when pulsed, and to remain "ON" until the current through the device falls to zero, - which conveniently, happens at the end of each half-cycle in alternating current supplies.

By controlling the switch-on point of a Thyristor relative to the voltage zero crossing in each half wave of an alternating current, it is possible to regulate the energy passing through the device. The closer the turn-on point is to the voltage zero crossing point, the longer the energy is allowed to flow during the half-cycle. Conversely, delaying the turn-on point reduces the time for the energy to flow. Putting two thyristors back-to-back (or anti-parallel) in each of the phase connections to a motor, and by precisely controlling their turn-on points, an electronic soft starter continuously adjusts the passage of energy from the supply so that it is just sufficient for the motor to perform satisfactorily.

By starting with a large delay to the turn on point in each half cycle, and progressively reducing it over a selected time period, the voltage applied to the motor starts from a relatively low value and increases to full voltage. Due to the motor torque being proportional to the square of the applied voltage, the starting torque follows the same pattern providing a smooth, stepless start. This starting method can reduce the starting current considerably to some 1.9 to 2 times the rated normal running current of the motor.

In recent years further development of SSRV starters have produced improvements such as the controlling the current to a preset limit while ramping the voltage up. Some versions also include a voltage ramp with a “kickstart” or boost pulse to overcome the inertia of hard to start loads. Most also incorporate a soft stop feature which enables the user to achieve controlled deceleration instead or uncontrolled coasting to stop. Another feature incorporated in some models is an energy saver feature that reduces power input to the motor at light loads.

SSRV starters have taken over from the older electro mechanical starters due to several factors such as being less bulky, more reliable - no moving parts, more controllable – smooth, stepless acceleration and generally cost less, especially with larger motor applications.

5.7 Variable Speed Drives (VSD)

Also known as adjustable-frequency (A-F) drives, are the ultimate in AC motor control technology, offering refined control by adjusting the frequency of power supplied to the motor. These starters permit smooth, universal control of motor speed from zero to rated speed and beyond at any value of loading (with high torque across the entire speed range). Their use increased dramatically in the 1980’s due to their energy saving advantages. Variable speed drives inherently provide soft starting.



The main problem with variable speed drives is the initial capital costs, which in some cases can approach the cost of the motor to be controlled. However with the ability of controlling the speed makes this type of control suitable for almost all applications.

5.8 Capacitor start capacitor run starting

Starting of single phase induction motors requires some arrangement (so that the motor can produce starting torque) as the stator windings of these motors inherently do not produce any starting torque. It is only in the running condition that the single phase motor is able to produce torque with the stator winding. The simplest method of starting single-phase motors is to provide an auxiliary winding on the stator in addition to the main winding, and start the motor as a two-phase machine. The two windings are placed in the stator with their axis displaced 90º electrical. The impedances of the two circuits are such that the currents in the main and auxiliary windings are phase-shifted from each other. The motor is equivalent to an unbalanced two-phase motor. The result is a rotating stator field that can produce the starting torque. In the running condition, the motor can develop a torque with only the main winding. Therefore, as the motor speeds-up, the auxiliary winding can be taken out by making use of a centrifugal switch.

Single-phase motors are classified in terms of the methods of starting employed to produce the phase difference between the currents in the main and auxiliary windings. One of the methods used is the Capacitor start capacitor run. In this method, two capacitors, one for starting and one for running are connected to the motor. The starting capacitor is normally higher in value compared to the running one and is of the electrolytic type. The running capacitor is connected in series with the starting winding. The starting capacitor is connected across the running capacitor and in series with the switch as shown in the diagram below. Capacitor run capacitor start induction motors have a starting torque of 100-300% of their rated torque and breakdown torque of up to 250% of rated torque.

Figure 6: Capacitor Start Capacitor Run

6 Power Quality

The network planner needs to be aware of the following power quality aspects with electrical motors:

Note: Only a basic description of the power quality problem and a “rule of thumb” measure to identify a potential problem will be covered. Further information can be found in the “Eskom Power Quality Reference Guide” on the different issues.

6.1 Voltage regulation

The effect of low or high steady state voltages on a motor are as follows:

Low Voltage: Supplying voltage below a motor’s rated voltage will cause the motor to operate at higher temperatures under full-load conditions. This is caused by the motor drawing more current to compensate for the lower voltage level in order to achieve its rated power. The increased temperatures may result in premature motor failure.



High Voltage: Voltages above the motor’s rated voltage will result in increased starting currents, increased torque and decreased power factor. Core magnetising currents will increase due to core magnetic saturation, which may result in increased operating temperatures.

34-542 (Distribution voltage regulation and apportionment limits) provides steady state voltage regulation limits for HV, MV and LV networks such that motor voltages will be within acceptable limits.

6.2 Voltage unbalance

Voltage unbalance results in a negative sequence voltage. The negative sequence impedance of an induction motor is much lower than the positive sequence impedance. Thus for a given negative sequence voltage, a disproportionately high negative sequence current will flow resulting in increased heating and a reduction in efficiency. Negative sequence currents also cause a degree of reverse torque, which will have a retarding effect on the motor. The motor’s negative sequence impedance is approximately the same as the locked rotor impedance, which is usually 10 – 20% of the normal full load impedance. Hence motor currents are sensitive to relatively small levels of voltage unbalance. A voltage unbalance of 10 to 20% could cause a current unbalance of 100%, which will usually result in overheating and destruction of the motor.

Figure 7: Motor de-rating factor due to voltage unbalance

In order to prevent overheating problems associated with voltage unbalance, motors need to be de-rated. Figure 7 provides typical motor de-rating factors for a range of voltage unbalance levels.

34-618 (Network planning guideline for voltage, technology and phasing) provides voltage unbalance limits for HV, MV and LV networks such that voltage unbalance will be within acceptable limits.

6.3 Harmonics

6.3.1 Effect of harmonics on motors

As a general rule of thumb, harmonics cause increased losses (I²R) and therefore heating, in electrical equipment due to the skin effect, which reduces the area of effective current flow at increased frequencies. Due to resonance in the system and harmonic amplification, insulation of equipment is stressed above normal operating conditions. The specific effects of harmonics on motors include:

a) Rotating machines experience increased heating due to additional copper and iron losses. b) In severe cases harmonics can cause cogging (refusal to start) or crawling (sub-synchronous speeds) in

induction machines. c) The 5th and 7th harmonics in particular could excite mechanical resonance’s in motor/load systems close

to 300Hz if mechanical system parameters warrants it.



6.3.2 Source of harmonics via motor drives

Power electronic soft starters and variable speed drives generate harmonics. Motors (via the power electronic drives) can hence be sources of harmonics. 6.4 Rapid voltage changes

6.4.1 Cause of rapid voltage change during motor starting

During motor starting the step change in current flowing through the network impedance results in a rapid voltage change as illustrated in figure 8.

Figure 8: Rapid voltage change during motor starting

The magnitude of the voltage change is related to the magnitudes of the starting current and fault level. To lessen the voltage drop during motor starting, various methods of starting are employed as already explained in section 5.

A basic rule of thumb calculation to determine the magnitude of the rapid voltage change when starting a motor is:

100% ××

=SCMVA

multipliermethodStartingMWMotorVdrop

Where: MotorMW Motor rated active power in MW Starting method multiplier Ratio of starting to full load current (typically 5 to 8 for DOL) MVASC Fault level at the point of supply in MVA 6.4.2 Effects of rapid voltage change

Excessively large rapid voltage changes can result in the following problems:

a) Motor causing rapid voltage change may not start, possible damage to motor windings b) Damage to equipment, such as load static converters malfunctioning c) Production processes disrupted causing financial losses d) Certain lamp types switch off, until cool, resulting in production problems e) Creates customer anxieties over the reliability of supply



During motor starting high starting currents and poor starting PF can cause excessively large voltage drops, which may stall the motor. The permissible minimum voltage level at start on the motor terminals is normally taken as 85% of nominal, however in circumstances this may be too low if the torque requirements are rated to the motors rated output. At 85% of nominal voltage the motors torque output will only be ±72% of its rating. Installing a larger motor is also likely to increase the rapid voltage change, due to the heavier rotor. So if a high starting torque is required then a corresponding higher voltage level may be necessary.

6.4.3 Rapid voltage change limits

The maximum voltage change during motor starting must be limited to:

a) Ensure that the voltage at the motor windings does not drop too low such that the motor has insufficient starting torque.

b) Comply with rapid voltage change limits at the Point of Common Coupling (PCC) such that other loads are not adversely affected.

The voltage at the terminals of a motor during starting is influenced by the method of motor starting, utility network impedance and the impedance within the customers own network i.e. between the utility point of supply and the actual motor. The utility is hence not directly responsible for the rapid voltage change at the motor terminals. The customer needs to ensure that the customer network and method of motor starting are such that starting voltages will be acceptable for the fault level at the utility point of supply. The motor starting current needs to be limited by the customer such that rapid voltage change limits at the PCC are met as per NRS 048-4 and table 1.

Table 1: Rapid voltage change emission limits [NRS 048-4]

1 2 3 Number of changes per

hour Rapid voltage change ΔUdyn/Un (%)

r MV/LV HV r ≤ 1 per day 6 3-5

1 < r ≤ 4 per day 5 3-4 r ≤ 1 per hour 4 3

1 < r ≤ 10 per hour 3 2.5 The PCC (Point of Common Coupling) with other customers is taken as the secondary of the transformer if other customers are also supplied off the same transformer. If only one customer is supplied from a transformer, the PCC is taken as the primary of the transformer.

Referring to table 1, the allowable voltage change is dependent on the voltage level MV/LV vs HV and number of occurrences per hour (more starts per hour = less voltage change permitted). In most rural water pumping applications, motors are not started more than once a day, so the applicable limit would be 6%. Note that the limit does not refer to the voltage change at the motor, but rather to the PCC. For most of our rural applications, the motor supplies are at LV, however the PCC is taken as the MV side of the transformer.

Example: Can a 100kW motor, proposed starting method DOL, be installed at a location with a fault level of 9 MVA at the PCC (assume DOL multiplier to be 6 x Ir)?

%7.610096.0100

961.0% =×=×

×=

MWMW

MVAMWVdrop

The 6.7% rapid voltage change exceeds the 6% limit, so an alternative means of motor starting would have to be used. Assuming a YD multiplier of 2.2 x Ir a voltage change of ± 2.4% can be expected, which is acceptable for applications with up to 10 starts per hour. Note that this method is only a rough indication of



the voltage drop that will be experienced at the PCC. Motor starting simulations would be required in borderline cases.

6.5 Voltage flicker

The term flicker refers to the variation in light intensity as perceived by the human eye, which is very sensitive to the variations in luminance in the region of 8 – 10 Hz. Although flicker does not affect equipment, it is very irritating and can lead to customer complaints.

Fluctuating loads cause voltage drops across the supply impedance, thereby varying the voltage and resulting in voltage flicker e.g. as a log enters a saw mill and the saw starts to cut the log, there will be a momentary surge in current (and hence dip in voltage).

Voltage flicker is evaluated in terms of a Pst level, or short-term flicker severity level. A Pst of 1 is the threshold at which flicker is perceptible to the human eye, with incandescent lighting being the worst affected. The maximum flicker level dictated by the NRS 048 for MV power systems over a 10 minute period is Pst = 1 and over two hours Plt = 0.8.

There are two main types of voltage flicker:

Cyclic Flicker: Periodic voltage fluctuations from loads like arc furnaces, can be predicted by the application of deterministic curves depicted in IEC 1000-3-7.

Non-cyclic Flicker: Occasional voltage fluctuations from loads like motor starts and motor driven loads, such as crushers and sawmills. The effects have to be calculated using empirical formula.

On MV rural distribution networks the main flicker causing loads are crushers and saw mills. Additional information on voltage flicker, its calculation and apportionment between customers can be obtained from the Plant QOS specialists. The Plant QOS specialists need to be consulted when assessing networks supplying customers with known flicker producing loads e.g. crushers and saw mills. Annex B provides information on likely flicker producing motor loads. 6.6 Motor sensitivity to ARC’s

Motor controllers are usually such that a momentary interruption (due to e.g. an auto-reclose operation of an upstream breaker) will trip the motor supply. When power is restored the motor is no longer connected to the supply. The motor controller usually needs to be reset (either manually or automatically).

On networks supplying a large number of motor loads a momentary interruption can result in a significant amount of load being disconnected (until the motor controllers are reset) which may in turn result in increased voltages. This can be problematic on MV distribution feeders with voltage regulators, as is described in more detail in 34-539 (Network planning guideline for MV step-voltage regulators).

6.7 Impact of existing load on motor starting

The magnitude of the voltage drop during motor starting is influenced by the voltage dependency of the other loads supplied by the network. Constant power loads draw more current as the voltage drops. Hence motor starting in the presence of constant power loads will cause the voltage to drop further due to the constant power loads drawing more current as the voltage drop during starting. Conversely constant impedance loads will draw less current as the voltage drops, and the voltage drop during motor starting in the presence of constant impedance loads will be less than as compared to constant power loads.



7 Application guideline

7.1 Supply side versus demand side solutions for motor starting problems

Motor starting problems are generally due to inadequate fault level for the size of motor and method of starting. There are two general solutions (or a combination thereof):

Supply side strengthening: The utility increases the fault level of the supply to the customer. This is usually achieved by reducing the network impedance via the addition of circuits and/or transformers in the utility network.

Demand side modifications: The customer reduces the size of the motors and/or changes the method of starting such that the motor(s) can be started with the available fault level. The customer could in cases also modify and/or strengthen the customers network (between the utility POS and motor) to increase the fault level of the supply to the motor.

The most economical solution will depend on the relative costs of supply vs demand side options and will vary for different networks.

Eskom Distribution is required to provide adequate supply side capacity such that steady state voltages at the customer POS are within regulatory limits (as specified by NRS048). The customer’s motor starting currents must be limited such that a) the voltage dip at the motor is not too large and b) the voltage dip at the PCC is within regulatory limits. It is the customers responsibility to ensure that motor starting currents do not cause rapid voltage change problems at the motor or PCC. Eskom Distribution must provide the customer with sufficient technical information (such as the fault level at the PCC and POS) so that the customer can size motors and select starting methods accordingly.

In some cases it may be more cost effective for Eskom to increase fault levels as compared to customer demand side modifications. In these cases the customer will request Eskom to provide a premium supply (with the higher fault level), and the customer will pay the additional cost associated with the premium supply.

7.2 Planning process when dealing with motor loads

The process to be followed by network planners (when assessing supplies with motor loads) is illustrated in figure 9. This process is to be followed for all new Large Power User (LPU) supplies and upgrades i.e. supplies >100kVA. Small Power User SPU (≤100kVA) supplies are exempt.



Figure 9: Process for assessment of motor loads

Note 1: The network planner evaluates the supply side (network) options in the normal manner. The

preferred alternative is selected as per the requirements of the South African Distribution Network Code and 34-335 (Distribution network planning standard). No special allowances are made for customer motor starting.

Note 2: The fault level at the PCC can be calculated with Power System Analysis software such as ReticMaster or PowerFactory.

New or increased LPU (>100kVA) supply

Evaluate supply alternatives. Select preferred alternative (note 1)

Calculate fault level at the PCC (note 2)

Customer load > 1% of the PCC fault level, OR possible flicker producing load (annex

B).

No further action required

Refer to Plant QOS specialist. Include contractual clauses for the

apportionment of QOS contributions

Customer requests and is prepared to pay for increased fault level at the PCC and/or

POS? (note 4)

No further action required

Investigate premium supply options with Network Planning, Plant QOS and the

customer

Select optimum solution that minimises net cost. Customer to pay additional costs

for premium supply (note 5)

No

No

Yes

Yes

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Note 3: If the customer load is less than 1% of the fault level at the PCC then power quality problems due to motors (rapid voltage change during starting and voltage flicker with flicker producing loads) are unlikely and no further action is required by the network planner. Supplies containing likely flicker producing loads (see annex B) must be referred to Plant QOS.

Note 4: In some cases the customer cost to comply with emission limits (based on the fault level at the PCC) may be significant. The customer may also require additional fault level at the POS so that motors can be started without excessive voltage changes at the motor(s). In these cases the customer may request Eskom to provide alternatives (and associated costs) that result in increased fault level at the PCC and/or POS.

Note 5: The optimum (premium power) solution that minimises the total cost (Eskom and customer) will typically be arrived at iteratively via interaction with the customer. The customer will be required to pay the cost difference between the base alternative (see note 1) and the premium power solution.

Note that regardless of whether a standard or premium supply is provided, it is the customer’s responsibility to comply with the power quality emission limits specified in the customer contract such that customer motors do not cause power quality problems. The customer must ensure that the motors and method of starting do not result in emission limit violations. Eskom must ensure that suitable emission limits are included in customer contracts. Network Planners are not required to perform or review motor starting studies. Where a premium supply for increased fault level is required, the network planner should evaluate options which may for example include: • Network reconfiguration. • Additional lines/cables, including re-conductoring of existing circuits. • Increased transformation capacity at existing substations. • New substations. • Series compensation. The strengthening of shared networks (as is usually performed as part of Network Development Planning) should consider possible motor power quality implications. Network planners should select preferred network alternatives taking into consideration likely fault level requirements (problems are unlikely if individual loads are less than 1% of the fault level). Excessively high fault levels are also problematic as equipment fault level ratings can not be exceeded. 8 Modelling Motors in PSA Software

8.1 Introduction

Power System Analysis (PSA) software can be utilised to simulate motor starting. A range of parameters can be simulated such as motor power, speed, currents and busbar voltages.

Where the simulation of motor starting is required, DigSilent PowerFactory should be used. ReticMaster does offer motor starting simulation, however the PowerFactory models and functionality far exceed that of ReticMaster. Note that as per section 7, the Network Planner is not usually required to perform motor starting simulations. This information on PSA is provided for completeness only.

8.2 Data required for PSA

A motor load consists of three basic components:

Motor: The physical electrical motor as would be purchased from a supplier such as ABB or Siemens e.g. 55kW single pole 400V motor.

Motor driven load: The mechanical load that the motor drives e.g. a pump or fan.



Motor controller/drive: The equipment used to start and run the motor. This equipment could vary from a simple DOL starter, to a Star Delta starter, power electronic soft starter, variable resistor, or variable frequency drive.

In order to accurately simulate motor starting, all three of the above components need to be modelled. Note that motor starting simulation requires modelling of motors, controllers and mechanical loads, and is hence a specialised skill.

Eskom Distribution has established a Master Type Library (MTL). The MTL contains standard motor parameters for common motors utilised in South Africa. The MTL provides a standardised set of motor parameters for PowerFactory. Annex A provides typical parameters for common motors. For simple motor starting simulations basic models for starting methods and mechanical loads can be used, and basic motor data is required as per table 2.

Table 2: Basic motor data requirements 1 2

Motor parameter Description Frequency Rated system frequency e.g. 50Hz Rated mechanical power Shaft output power at rated speed (usually in kW) Voltage Rated voltage Power Factor Power Factor at rated mechanical power Efficiency Efficiency at rated output power, speed and rated voltage Poles Number of pole pairs. Can be calculated from rated speed, poles = 50*60/rpm Winding configuration Star or Delta Locked rotor current Expressed as a ratio of full load current Locked rotor torque Expressed as a ratio of rated torque (at rated speed) Torque at stalling point Maximum pull out torque. Expressed as a ratio of rated torque (at rated speed)

8.3 PowerFactory

8.3.1 Introduction

It is not the purpose of this guideline to cover the detailed modelling of motors, mechanical loads and starting methods using DigSilent. The DigSilent software manual, technical reference on Induction Machines and FAQ section on the DigSilent website should be referenced for details. Motor starting is a transient simulation and is usually covered as part of an advanced PowerFactory course on dynamic studies. The information in this section is confined to basic induction motor simulation for simple DOL type starting with a simple model for the mechanical load.

This section describes PowerFactory functionality and contains software screenshots. It is possible that functionality and interfaces may change in future software versions. The latest software version and user guide should be consulted.



8.3.2 Motor data type library

Figure 10: Asynchronous machine type data (Basic Data)

Table 3: Asynchronous machine type fields (Basic Data)

1 2 3 Fields Units Notes Name None Name of the motor Nominal Voltage kV Motor rated voltage Power Factor ratio Power Factor at rated mechanical power Rated mechanical power kW Shaft output power (at rated speed) Efficiency at nominal Operation

% Efficiency at rated output power, speed and rated voltage

Nominal Frequency Hz Rated system frequency e.g. 50Hz No of Pole Pairs None Determines the rated speed based on the nominal frequency Connection None Star or Delta connection of stator windings



Figure 11: Asynchronous machine type data (RMS-Simulation)

Table 4: Asynchronous machine type fields (RMS-Simulation)

1 2 3 Fields Units Notes Nominal Speed rpm Rated motor speed (at rated power) Locked Rotor Current pu Expressed as a ratio of full load current Locked Rotor Torque pu Expressed as a ratio of rated torque (at rated speed) Torque at Stalling Point pu Maximum pull out torque. Expressed as a ratio of rated torque (at rated

speed) Acc Time Constant Sec Acceleration time constant of the motor rotor and shaft



8.3.3 Motor element data

Figure 12: Asynchronous machine element data (Basic Data)

Table 5: Asynchronous machine element fields (Basic Data)

1 2 3 Fields Units Notes Name None Name of the motor Type None The relevant motor type must be specified Out of Service None Only select if motor is to be excluded from study Number of parallel machines None If a bank of identical motors are modelled as a single element then the

number of motors in the bank must be specified. Default 1 Generator/Motor None Specifies if the machine is used as a generator or motor. Default Motor

Figure 13: Asynchronous machine element data (Load Flow)

Table 6: Asynchronous machine element fields (Load Flow)

1 2 3 Fields Units Notes Active Power kW This is the active power consumed by the motor for normal load flow studies.

The reactive power consumption is determined by the motor parameters as specified in the type



Figure 14: Asynchronous machine element data (RMS-Simulation)

Table 7: Asynchronous machine element fields (RMS-Simulation)

1 2 3 Fields Units Notes Proportional Factor pu Per unit torque of the mechanical load at rated motor speed (in pu of the

motor rated torque at rated speed). Default 1 Exponent None Exponent for the torque speed curve. For pump and fan type loads default to

2

8.3.4 Performing a simple motor stating simulation

After performing a load flow study, right click on the motor to be started, select calculate/Motor startup. PowerFactory automatically performs a dynamic RMS simulation and generates the plots for motor active power, current, reactive power, speed and voltage. Additional attributes can be plotted via the standard dynamic simulation functionality provided by PowerFactory.

9 Worked examples

The following worked examples illustrate some of the key issues discussed in this guideline. The associated PowerFactory file (PowerFactory MS case files v13.dz) is published as an attachment to this guideline. Note that the worked examples include dynamic studies. In order to perform these studies the user must

“calculate initial conditions” and then “Start simulation” .



9.1 Rural distribution

Table 8: Rural distribution network example 1 2

Case file Description RD step 1 Base network

Perform a load flow calculation. The 11kV distribution network supplies 1.5MVA of load. The 11kV voltage drops from 103% to 98.2%. Voltages and thermal loadings are acceptable.

RD step 2 New 200kVA supply

A customer requests a new 200kVA supply 2km from the existing 11kV network (11kV BB12). The obvious option is to extend the 11kV network to the new supply point and provide a 200kVA supply at LV. The 11kV network is extended and the load is modelled as a lumped 200kVA load directly connected to the MV network. Perform a load flow calculation. The MV voltage drops to 96% (which is a minimum at the new supply), which is acceptable. Perform a fault level calculation. The three phase 11kV fault level at the new supply is 9.77MVA. The load is 100*0.2/9.77=2.1% of the fault level at the PCC. As this exceeds 1%, problems with motor starting may be experienced. The application should be referred to Plant QOS for inclusion of required clauses in the quote/contract.

RD step 3 Motor starting DOL

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. It is established that the customer intends to run a 110kW 400V 4 pole motor started DOL driving a water pump. There will be no mechanical loading during starting. Once at rated speed the mechanical loading will be gradually applied. It will be started more than four times per day. In order to perform a motor starting simulation the MV/LV transformer and customer network are modelled. Look at the motor starting results. The motor runs up to near synchronous speed in under 0.5seconds. The voltage at the motor drops to 80.4%. The 11kV voltage changes from 97.6% to 92.7% i.e. a 4.9% change. Referring to table 1, the rapid voltage change limit of 4% will be violated. The customer needs to perform another method of starting in order to meet rapid voltage change limits at the PCC. Note that the MV PCC voltage change using the rule of thumb calc is 0.11*6*100/9.77=6.75% which is significantly greater than the simulated voltage change.

RD step 4 Motor starting Star Delta

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. The motor has been modelled with Star Delta starting, with transition from Star to Delta 1second after starting. Look at the motor starting results. The motor runs up to near synchronous speed in a second (twice as long as compared to DOL). The voltage at the motor drops to 94.1% during initial starting, and to 93.5% during the transition to Delta. This double voltage dip is characteristic of a Star Delta starter (see figure 5). The two voltage changes are of similar magnitude. The maximum 11kV voltage change is 97.6% to 96.1% i.e. a 1.5% change. Referring to table 1, this voltage change is within required limits.

RD step 5 Motor starting Star Delta Mlod

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. The motor has been modelled with the mechanical load applied during starting. Look at the motor starting results. The maximum voltage change occurs during transition from Star to Delta. Even with Star Delta starting the 11kV voltage dip increases to 97.6% - 92.8%=4.8%, which exceeds the limits of table 1. Star Delta starting is not acceptable (excessive voltage change at the PCC) with the mechanical load applied during starting. Another form of starting would need to be investigated. This illustrates the impact that the mechanical load also has on motor starting studies!



9.2 Urban distribution

Table 9: Urban distribution network example 1 2

Case file Description UD step 1 Base network

Perform a load flow calculation. Each of the 11kV distribution feeders supplies 1.8MVA of load. The 11kV voltage drops from 103% to 102%. Voltages and thermal loadings are acceptable.

UD step 2 New 200kVA supply

A customer requests a new 200kVA supply near an existing 500kVA minisub supplied at busbar 11KV F1 T5. This minisub presently supplies 288kVA. The obvious option is to supply the new 200kVA load from the existing 500kVA minisub. The load is modelled as a lumped 200kVA load on the terminals of the minisub. Perform a load flow calculation. All thermal and voltage requirements are met. Perform a fault level calculation. As the new load is supplied off a minisub that also supplies other customers, the PCC is at the LV terminals of the minisub. The minisub LV three phase fault level is 9.73MVA. The load is 100*0.2/9.73=2.1% of the fault level at the PCC. As this exceeds 1%, problems with motor starting may be experienced. The application should be referred to Plant QOS for inclusion of required clauses in the quote/contract.

UD step 3 Motor starting DOL

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. It is established that the customer intends to run a 110kW 400V 4 pole motor started DOL driving a fan. The mechanical load will be connected during starting. It will be started once per hour. In order to perform a motor starting simulation the customer LV network is modelled. Look at the motor starting results. The motor runs up to near synchronous speed in under 1second. The voltage at the motor drops to 90%. The LV voltage at the PCC changes from 104% to 96.9% i.e. a 7.1% change. Referring to table 1, even if the motor is started infrequently the rapid voltage change limits will be violated. The customer needs to perform another method of starting in order to meet rapid voltage change limits at the PCC. Note that the LV PCC voltage change using the rule of thumb calc is 0.11*6*100/9.73=6.78% which is close to the simulated voltage change.

UD step 4 Motor starting Star Delta

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. The motor has been modelled with Star Delta starting, with transition from Star to Delta 1second after starting. Look at the motor starting results. The motor runs up to near synchronous speed in a second (similar to DOL). The voltage at the motor drops to 100% during initial starting, and to 90.4% during the transition to Delta. The voltage change during transition to Delta is the larger of the two and results in a 101.9% to 96.9% i.e. a 5% change at the LV PCC. Referring to table 1, this voltage change violates the required limits. Note that with the mechanical load applied during starting, Star Delta starting does reduce the voltage change, but the voltage change is still not within acceptable limits. Alternative methods of starting, or a higher fault level are required.

UD step 5 Dedicated LV supply

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. To demonstrate the application of a premium supply, the 11kV network has been extended to provide the new 200kVA customer with a dedicated LV supply via a dedicated 500kVA transformer (the customer may in reality take a MV supply and perform their own transformation). The PCC is now at the 11kV point of supply, and not the LV side of the transformer. The motor is started DOL with the mechanical load applied during starting. Look at the motor starting results. The motor runs up to near synchronous speed in under 1second. The voltage at the motor drops to 97%. The MV voltage at the PCC changes from 102.4% to 101.9% i.e. a 0.5% change. Referring to table 1, even if the motor is started frequently the rapid voltage change limits will NOT be violated.



9.3 Sub-transmission

Table 10: Sub-transmission network example 1 2

Case file Description Stx step 1 Base network

Perform a load flow calculation. The 132/11kV 20MVA transformer is loaded to 10.6MVA. The 11kV busbar voltage is regulated to 103%. Voltages and thermal loadings are acceptable.

Stx step 2 New 5MVA supply

A customer requests a new 5MVA supply near the existing 132/11kV substation. The substation has a single 20MVA transformer and presently supplies 10.6MVA. The preferred option is to supply the new 5MVA load from the existing substation via two 11kV cable circuits. The load is modelled as a lumped 5MVA load at a new 11kV busbar. Perform a load flow calculation. All thermal and voltage requirements are met. Perform a fault level calculation. The PCC is at the MV busbar of the 132/11kV substation. The PCC three phase fault level is 180MVA. The load is 100*5/180=2.8% of the fault level at the PCC. As this exceeds 1%, problems with motor starting may be experienced. The application should be referred to Plant QOS for inclusion of required clauses in the quote/contract.

Stx step 3 Motor starting DOL

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. It is established that the customer intends to run two 1.6MW 6.6kV 2 pole motors started DOL driving water pumps. There will be no mechanical loading during starting. The motors will be started sequentially. Once at rated speed the mechanical loading will be gradually applied. They will be started once per day. In order to perform a motor starting simulation the customer network including 11/6.6kV transformation is modelled. Look at the motor starting results. The motor voltages drop to 92.7% and 92.6% during the starting of the 1st and 2nd motors respectively. The 11kV voltage at the PCC changes from 102.8% to 98% i.e. a 4.8% change during the 1st motor start. The voltage change during the starting of the 2nd motor is similar. Referring to table 1, the rapid voltage change limits will NOT be violated. Note that the MV PCC voltage change using the rule of thumb calc is 1.6*6*100/180=5.33% which is similar to the simulated voltage change.

Stx step 4 Additional trfr

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. To demonstrate the application of a premium supply, a second 132/11kV 20MVA transformer has been installed to increase the fault level at the 11kV PCC. The motors are started DOL as with step 3. Look at the motor starting results. The motor voltages drop to 94.9% and 94.8% during the starting of the 1st and 2nd motors respectively. The 11kV voltage at the PCC changes from 103.2% to 100.6% i.e. a 2.6% change during the 1st motor start. The voltage change during the starting of the 2nd motor is similar. Referring to table 1, the rapid voltage change limits will NOT be violated.

Stx step 5 Motor starting Star Delta

THIS STEP DOES NOT NEED TO BE PERFORMED BY THE PLANNER. IT IS INCLUDED FOR INTEREST ONLY. The motors have been modelled with Star Delta starting, with transition from Star to Delta 9.4second after starting. Look at the motor starting results. The motor voltages drop to 99% and 98.2% during the starting of the 1st and 2nd motors respectively. The 11kV voltage at the PCC changes from 102.6% to 101.1% i.e. a 1.5% change during the 1st motor start. The voltage change during the starting of the 2nd motor is similar. Referring to table 1, the rapid voltage change limits will NOT be violated.



Annex A: Typical motor parameters

Table A1: C frame motors

Efficiency Power Factor Starting kW r/min Frame F.L. F.L. Star Delta Direct-on-line SC

Torque Current Torque Current PF 0.75 955 C160M 84.0 0.77 0.35 2.0 1.6 5.0 0.453 11 2900 C160M 84.0 0.79 0.35 1.5 1.7 5.0 0.391

1435 C160M 84.0 0.82 0.30 1.7 1.2 5.0 0.504 15 2890 C160M 84.0 0.83 0.30 2.0 1.3 5.0 0.390

1435 C160L 88.0 0.84 0.40 2.0 1.9 5.0 0.491 18.5 2920 C160L 86.5 0.81 0.40 2.0 1.8 5.2 0.394

1435 C160L 86.0 0.80 0.45 2.0 2.0 6.0 0.474 22 2880 C160L 86.5 0.82 0.35 2.0 1.5 5.0 0.396

1450 C180M 90.0 0.80 0.35 2.0 1.4 5.0 0.424 30 2895 C180M 87.0 0.84 0.35 2.0 1.4 5.0 0.406

1445 C180L 90.0 0.78 0.45 2.0 2.0 5.0 0.448 37 2910 C180L 88.5 0.82 0.35 2.0 1.5 5.0 0.431

1440 C200M 88.0 0.77 0.40 2.0 1.7 5.0 0.403 45 2920 C200M 88.5 0.88 0.35 2.0 1.6 5.5 0.454

1465 C200L 89.0 0.77 0.35 2.0 1.7 5.0 0.416 55 2935 C200L 90.0 0.86 0.35 2.0 1.4 5.5 0.470

1475 C225M 93.0 0.87 0.35 2.0 1.8 6.0 0.433 75 2930 C225M 92.0 0.92 0.35 2.0 1.5 6.0 0.433

1470 C250S 93.5 0.87 0.30 2.0 1.0 6.0 0.353 90 2960 C250S 92.0 0.91 0.35 1.9 1.5 6.0 0.442

1475 C250M 94.0 0.87 0.40 2.0 1.7 6.0 0.340 110 2950 C250M 92.0 0.92 0.30 1.9 1.1 6.0 0.439

1475 C280S 93.0 0.89 0.30 2.0 1.2 6.0 0.360 150 2955 C280S 93.8 0.90 0.35 2.0 1.0 6.5 0.312

1470 C280M 94.0 0.89 0.35 2.0 1.1 6.0 0.308 185 2950 C280M 94.0 0.89 0.35 2.0 1.1 6.5 0.335

1480 C315S 95.0 0.88 0.30 2.0 1.1 6.0 0.280 220 2960 C315S 93.5 0.88 0.45 2.0 1.0 6.5 0.305

1470 C315M 94.0 0.89 0.45 2.0 1.0 6.0 0.261 260 2968 C315M 94.5 0.88 0.50 2.0 1.6 6.5 0.310

1485 C315MX 95.0 0.88 0.30 2.0 1.0 6.0 0.361 300 2970 C315MX 94.5 0.88 0.40 2.0 1.75 6.5 0.320 320 1480 C315MXB 95.0 0.88 0.40 2.0 1.75 6.25 0.330 350 2973 C315MXB 94.5 0.88 0.40 2.0 1.75 6.50 0.310



Table A2: D frame motors

Efficiency Power Factor Starting kW r/min Frame F.L. F.L. Star Delta Direct-on-line SC

Torque Current Torque Current PF 0.75 2800 DY80 75.0 0.86 -- -- 3.4 6.5 0.72

1385 DY80 74.0 0.79 -- -- 2.6 4.4 0.66 1.1 2825 DY80 77.5 0.87 -- -- 2.8 6.7 0.68

1400 DY90S 78.0 0.79 -- -- 3.5 6.2 0.64 1.5 2885 DY90S 78.0 0.82 -- -- 3.5 7.5 0.66

1410 D90L 79.5 0.81 -- -- 2.6 6.0 0.72 2.2 2830 D90L 81.0 0.88 -- -- 3.0 5.7 0.64

1410 D100L 78.0 0.84 -- -- 2.0 6.0 0.67 3 2850 D100L 84.0 0.88 0.80 2.0 2.6 6.5 0.48 1400 D100L 79.0 0.79 0.80 1.8 2.5 5.7 0.61

4 2880 D112M 82.0 0.88 0.80 2.4 2.6 7.1 0.62 1400 D112M 81.0 0.81 0.70 1.6 2.0 6.0 0.59

5.5 2870 D132S 83.5 0.89 0.70 1.9 2.0 6.0 0.52 1440 D132S 85.5 0.82 0.70 1.7 2.2 6.0 0.50

7.5 2885 D132S 86.0 0.88 0.70 1.9 2.2 6.3 0.50 1440 D132M 86.5 0.84 0.70 1.7 2.5 6.0 0.49

11 2880 D160M 85.0 0.86 0.50 1.7 2.0 6.0 0.42 1450 D160M 88.0 0.86 0.80 2.1 2.6 6.3 0.48

15 2900 D160M 87.0 0.85 0.50 1.8 1.6 6.0 0.41 1450 D160L 89.5 0.86 0.70 1.9 2.1 6.0 0.47

18.5 2895 D160L 88.0 0.87 1.00 2.1 3.0 6.2 0.43 1460 D180M 89.5 0.84 0.60 1.7 1.9 6.0 0.38

22 2930 D180M 89.0 0.88 0.60 2.0 2.3 6.5 0.37 1450 D180L 90.0 0.83 0.70 1.5 2.2 6.0 0.49

30 2965 D200L 91.0 0.80 0.90 2.1 2.9 6.3 0.47 1465 D200L 90.5 0.85 0.50 1.5 1.6 6.0 0.39

37 2940 D200L 88.5 0.79 0.90 1.6 2.6 6.0 0.45 1475 D200S 93.0 0.89 0.60 1.9 1.8 6.0 0.38

45 2950 D225M 92.5 0.93 0.40 2.9 1.9 7.5 0.32 1475 D225M 93.5 0.88 0.40 2.1 1.9 6.2 0.39

55 2970 D250S 93.0 0.91 0.50 2.2 1.4 6.0 0.34 1485 D250S 94.5 0.88 0.70 2.2 2.0 6.8 0.37

75 2965 D250M 83.5 0.91 0.40 2.2 1.8 6.5 0.32 1485 D250M 94.5 0.89 0.70 2.0 2.1 6.8 0.38

90 2955 D280S 94.0 0.91 0.40 1.9 1.3 6.0 0.34 1475 D280S 94.0 0.82 0.60 1.9 1.7 6.0 0.32

110 2970 D280M 94.5 0.91 0.40 1.9 1.2 6.0 0.34 1475 D280M 94.0 0.90 0.50 1.9 1.4 6.0 0.30

132 2975 D315S 93.0 0.87 0.40 2.0 1.2 6.2 0.29 1480 D315S 94.0 0.88 0.40 1.6 1.2 6.3 0.32

150 2975 D315M 95.0 0.87 0.40 2.0 1.0 6.1 0.25 1480 D315M 95.0 0.87 0.40 1.6 1.4 6.0 0.30

200 2985 D315L 95.0 0.89 0.40 2.0 1.2 6.5 -- 1485 D315MX 95.0 0.91 0.40 1.9 1.3 6.0 0.29

300 2970 D315L 95.0 0.88 0.50 2.0 1.3 6.0 -- 1485 D315L 95.0 0.88 0.50 2.0 1.3 6.0 --

335 1490 D315L 95.0 0.91 0.50 1.9 2.0 7.0 --



Annex B: Typical motor load characteristics

Table B1: Load types and torque requirements

1 2 3 4 5

Load Type Coupling Starting torque Likely flicker issues

Direct Low No Centrifugal Fluid coupling Low No

Direct Low No Screw Fluid coupling Low No

Direct Low No

Water pump

Piston Fluid coupling Low No

Direct High Yes Centrifugal Fluid coupling Medium Yes

Direct High Yes Compressor

Piston Fluid coupling Medium Yes

Direct High Yes Jaw Fluid coupling High Yes

Direct High Yes Roller Fluid coupling High Yes

Direct High Yes

Crusher

Drum Fluid coupling High Yes

Direct High Yes Chain Fluid coupling Medium Yes

Direct High Yes Frame Fluid coupling Medium Yes

Direct High Yes Circular Fluid coupling Medium Yes

Direct High Yes

Saw

Multi-blade circularFluid coupling Medium Yes



Annex C: Impact assessment

Impact assessment form to be completed for all documents.

1 Guidelines

o All comments must be completed. o Motivate why items are N/A (not applicable) o Indicate actions to be taken, persons or organisations responsible for actions and deadline for

action. o Change control committees to discuss the impact assessment, and if necessary give feedback to

the compiler of any omissions or errors.

2 Critical points

2.1 Importance of this document. E.g. is implementation required due to safety deficiencies, statutory requirements, technology changes, document revisions, improved service quality, improved service performance, optimised costs.

Comment: The guideline provides Distribution network planners with the theory and practical process to adequately plan and manage the connection of loads containing electrical motors such that power quality compatibility levels can be met.

2.2 If the document to be released impacts on statutory or legal compliance - this need to be very clearly stated and so highlighted.

Comment: No statutory or legal impacts. The guideline simply assists planners in ensuring compliance with power quality limits as dictated by national standards.

2.3 Impact on stock holding and depletion of existing stock prior to switch over.

Comment: None.

2.4 When will new stock be available?

Comment: Not applicable.

2.5 Has the interchangeability of the product or item been verified - i.e. when it fails is a straight swop possible with a competitor's product?


2.6 Identify and provide details of other critical (items required for the successful implementation of this document) points to be considered in the implementation of this document.

Comment: None.

2.7 Provide details of any comments made by the Regions regarding the implementation of this document.

Comment: (N/A during commenting phase)



Annex A (continued)

3 Implementation timeframe

3.1 Time period for implementation of requirements.

Comment: The guideline is suitable for self study. It should be fully complied with within a 1 year period.

3.2 Deadline for changeover to new item and personnel to be informed of DX wide change-over.


4 Buyers Guide and Power Office

4.1 Does the Buyers Guide or Buyers List need updating?


4.2 What Buyer’s Guides or items have been created?


4.3 List all assembly drawing changes that have been revised in conjunction with this document.


4.4 If the implementation of this document requires assessment by CAP, provide details under 5

4.5 Which Power Office packages have been created, modified or removed?


5 CAP / LAP Pre-Qualification Process related impacts

5.1 Is an ad-hoc re-evaluation of all currently accepted suppliers required as a result of implementation of this document?


5.2 If NO, provide motivation for issuing this specification before Acceptance Cycle Expiry date.


5.3 Are ALL suppliers (currently accepted per LAP), aware of the nature of changes contained in this document?




Annex A (continued)

5.4 Is implementation of the provisions of this document required during the current supplier qualification period?


5.5 If Yes to 5.4, what date has been set for all currently accepted suppliers to comply fully?


5.6 If Yes to 5.4, have all currently accepted suppliers been sent a prior formal notification informing them of Eskom’s expectations, including the implementation date deadline?


5.7 Can the changes made, potentially impact upon the purchase price of the material/equipment?


5.8 Material group(s) affected by specification: (Refer to Pre-Qualification invitation schedule for list of material groups)


6 Training or communication

6.1 State the level of training or communication required to implement this document. (E.g. none, communiqués, awareness training, practical / on job, module, etc.)

Comment: This guideline is suitable for self study and can be applied without any further training. However in order to ensure that is is disseminated and applied the material contained in the guideline will be developed into formal training material via a separate project.

6.2 State designations of personnel that will require training.

Comment: Is applicable to all distribution (MV&LV) and sub-transmission (HV) planners in Distribution Network Services Network Planning.

6.3 Is the training material available? Identify person responsible for the development of training material.

Comment: No. Training material will need to be developed via a new Research project that has been initiated within R&S for the development of Dx Network Planning training material.

6.4 If applicable, provide details of training that will take place. (E.G. sponsor, costs, trainer, schedule of training, course material availability, training in erection / use of new equipment, maintenance training, etc).

Comment: To be decided.

6.5 Was Training & Development Section consulted w.r.t training requirements?

Comment: Yes, this is being done as part of the broader Dx Network Planning training framework.



Annex A (continued)

7 Special tools, equipment, software

7.1 What special tools, equipment, software, etc will need to be purchased by the Region to effectively implement?

Comment: None. The guideline utilises existing tools and simply enhances there application and the interpretation of results.

7.2 Are there stock numbers available for the new equipment?


7.3 What will be the costs of these special tools, equipment, software?

Comment: None.

8 Finances

8.1 What total costs would the Regions be required to incur in implementing this document? Identify all cost activities associated with implementation, e.g. labour, training, tooling, stock, obsolescence

Comment:

As this guideline has been developed for self study, no direct costs are associated with implementation. Costs for the development of formal training material are being managed under a separate project for the development of formal training material for network planners.

Impact assessment completed by:

Name: ___Dr CG Carter-Brown________________________________________________________

Designation: __Chief Engineer, IARC, Power Plant Technology__________________________

document classification: title: network planning...

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