power system protection

Unit 1Power System Protection

Generation-typically at 4-20kV

Transmission-typically at 230-765kV

Subtransmission-typically at 69-161kV

Receives power from transmission system and transforms into subtransmission level

Receives power from subtransmission system and transforms into primary feeder voltage

Distribution network-typically 2.4-69kV

Low voltage (service)-typically 120-600V

TypicalBulkPower System

2GE Consumer & Industrial

Multilin

Objective of Power System Protection

Why power system protection?

Protection Zones

4GE Consumer & IndustrialMultilin

1. Generator or Generator-Transformer Units2. Transformers3. Buses4. Lines (transmission and distribution)5. Utilization equipment (motors, static loads, etc.)6. Capacitor or reactor (when separately protected)

Unit Generator-Tx zoneBus zone

Line zoneBus zone

Transformer zone Transformer zone

Bus zone

Generator

~XFMR Bus Line Bus XFMR Bus Motor

Motor zone

Application Principles

5

Protection zones

Generator

Xfmr Bus

Line

Bus


6

Protection zones – determined by CT location

Generator

Xfmr Bus

Line

Bus


7


Generator

Xfmr Bus

Line

Bus

Transformer Zone


8


Generator

Xfmr Bus

Line

Bus

Transformer zoneUnit generator-transformer

zone


9


Generator

Xfmr Bus

Line

Bus


zone

Bus zone


10


Generator

Xfmr Bus

Line

Bus


zone

Bus zone

Line zone


11


Generator

Xfmr Bus

Line

Bus


zone

Bus zone

Line zone

Bus zone


12


Generator

Xfmr Bus

Line

Bus


zone

Bus zone

Line zone

Bus zone

Line zone


13


Generator

Xfmr Bus

Line

Bus


zone

Bus zone

Line zone

Bus zone

Line zone

Note the overlap, so that there is no location where a fault would go undetected


14

Protection zones

MotorXfmr Bus

Line

Bus


15


MotorXfmr Bus

Line

Bus


16


MotorXfmr Bus

Line

Bus

Line zone


17


MotorXfmr Bus

Line

Bus

Line zone

Bus zone


18


MotorXfmr Bus

Line

Bus

Line zone

Bus zone

Xfmr zone


19


MotorXfmr Bus

Line

Bus

Line zone

Bus zone

Xfmr zone

Bus zone


20


MotorXfmr Bus

Line

Bus

Line zone

Bus zone

Xfmr zone

Bus zone

Motor zone


21


MotorXfmr Bus

Line

Bus

Line zone

Bus zone

Xfmr zone

Bus zone

Motor zone

Again, note the overlapAlso note: A fault on a major element is covered by how many zones? A fault within a circuit breaker is covered by how many zones?

Zone Overlap


1. Overlap is accomplished by the locations of CTs, the key source for protective relays.

2. In some cases a fault might involve a CT or a circuit breaker itself, which means it can not be cleared until adjacent breakers (local or remote) are opened.Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at both sides of CB-fault between CTs is cleared from both remote sides

Zone A Zone B

Relay Zone A

Relay Zone B

CTs are located at one side of CB-fault between CTs is sensed by both relays, remote right side operate only.

Factors affecting power system protection

Selectivity= isolate only the faulty network and maintain the normal supply

Reliability = operate properly during the period of service

Speed= quick disconnectionDiscrimination = between fault and loading

conditionsSimplicity= simple and straight forwardSenstivity= operate correctly within its

zoneEconomics= max protection+ min cost

Art & Science of Protection


Selection of protective relays requires compromises:• Maximum and Reliable protection at minimum

equipment cost• High Sensitivity to faults and insensitivity to

maximum load currents• High-speed fault clearance with correct selectivity• Selectivity in isolating small faulty area• Ability to operate correctly under all predictable

power system conditions

Art & Science of Protection


• Cost of protective relays should be balanced against risks involved if protection is not sufficient and not enough redundancy.

• Primary objectives is to have faulted zone’s primary protection operate first, but if there are protective relays failures, some form of backup protection is provided.

• Backup protection is local (if local primary protection fails to clear fault) and remote (if remote protection fails to operate to clear fault)

Primary Equipment & Components


• Transformers - to step up or step down voltage level • Breakers - to energize equipment and interrupt fault

current to isolate faulted equipment• Insulators - to insulate equipment from ground and

other phases• Isolators (switches) - to create a visible and

permanent isolation of primary equipment for maintenance purposes and route power flow over certain buses.

• Bus - to allow multiple connections (feeders) to the same source of power (transformer).

Per unit and percent valuesRatio of actual to base values is per unit

values.Convert from per cent to per unit values by

dividing 100.

But why per unit values?

Advantages of per unit representation1. Ordinary parameters vary considerably with

variation of physical size, terminal voltage and power rating etc. while per unit parameters are independent of these quantities over a wide range of the same type of apparatus

2. It provide more meaningful information.3. The chance of confusion between line and

phase values in a three-phase balanced system is reduced.

4. Impedances of machines are specified by the manufacturer in terms of per unit values.

5. The per unit impedance referred to either side of a single-phase transformer is the same.

6. The per unit impedance referred to either side of a three -phase transformer is the same regardless of the connection whether they are ∆-∆, Y-Y or ∆-Y.

7. The computation effort in power system is very much reduced with the use of per unit quantities.

8. Usually, the per unit quantities being of the order of unity or less can easily be handled with a digital computer. Manual calculation are also simplified.

Advantages of per unit representation

Per Unit CalculationsA key problem in analyzing power systems is

the large number of transformers. – It would be very difficult to continually have to

refer impedances to the different sides of the transformers

This problem is avoided by a normalization of all variables.

This normalization is known as per unit analysis.

actual quantityquantity in per unit base value of quantity

30

Per Unit Conversion Procedure, 1f1. Pick a 1f VA base for the entire system, SB

2. Pick a voltage base for each different voltage level, VB. Voltage bases are related by transformer turns ratios. Voltages are line to neutral.

3. Calculate the impedance base, ZB= (VB)2/SB

4. Calculate the current base, IB = VB/ZB 5. Convert actual values to per unit

Note, per unit conversion affects magnitudes, not the angles. Also, per unit quantities no longer have units (i.e., a voltage is 1.0 p.u., not 1 p.u. volts)

31

Per Unit Solution Procedure1. Convert to per unit (p.u.) (many problems

are already in per unit)2. Solve3. Convert back to actual as necessary

32

Three Phase Per Unit

1. Pick a 3f VA base for the entire system, 2. Pick a voltage base for each different

voltage level, VB,LL. Voltages are line to line. 3. Calculate the impedance base

Procedure is very similar to 1f except we use a 3f VA base, and use line to line voltage bases

3BSf

2 2 2, , ,3 1 1

( 3 )3

B LL B LN B LNB

B B B

V V VZ

S S Sf f f

Exactly the same impedance bases as with single phase using the corresponding single phase VA base and voltage base!33

Three Phase Per Unit, cont'd4. Calculate the current base, IB

5. Convert actual values to per unit

3 1 13 1B B

, , ,

33 3 3B B B

B LL B LN B LN

S S SI IV V V

f f ff f

Exactly the same current bases as with single phase!

34

Example 1A 5 KVA 400/200 V, 50Hz, single phase transformer

has the primary and secondary leakage reactance each of 2.5 ohm. Determine the total reactance in per unit.

Sb= 5000 VAPrimary Base Voltage Vb1 = 400 VSecondary Base Voltage Vb2 = 200 V

X1e=X1+a2X2

a=N1/N2=400/200=2X1e=X1+a2X2=2.5 + 2.5 * 22 =12.5 ohm

Xpu= (Xactual/Zbase)Zb1= (V2

b1/Sb)=4002/5000=32 ohm Xpu= (Xactual/Zbase)= 12.5/32 = 0.390625

pu………………..1

X1pu = X1/Zb1X2pu = X2/Zb2Zb2 = (V2

b2/Sb)= 2002/5000=8 ohmX2pu = X2/Zb2 = 2.5/8 = 0.3125X1pu = X1/Zb1 = 3.5/32 = 0.078125Xpu= X1pu + X2pu = 0.390625……………………………..2

Both 1 and 2 matches which confirm that pu impedances are same on both sides of a transformer

Symmetrical Components

Symmetrical components

In a three-phase Y-connected system, the neutral current is the sum of the line currents:

Three phase fault

Single line to ground fault

V=Vf – IZ

Line to line fault

Double line to ground fault

sudden testA 7 KVA 1000/250 V, 50Hz, single phase transformer has the primary and secondary leakage reactance each of 5 ohm. Determine the total reactance in per unit.

50

A phasor is a representation of a sinusoidal voltage or current as a vector rotating about the origin of the complex plane.

AC Power and Phasors

phasors

Advantages of Phasors

Less Cumbersome (short hand notation)

Simpler Calculations (complex arithmetic, calculators can do), generally less need for integration and differentiation

Additional insights may be obtained about relations between currents, voltages, and power

Limitations

Applies only to sinusoidal steady-state systems

Power Calculated using phasors is only the time average


• Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc.

• CT ratios are expressed as primary to secondary; 2000:5, 1200:5, 600:5, 300:5

• A 2000:5 CT has a “CTR” of 400

Current Transformers

Current into the Dot, Out of the DotCurrent out of the dot, in to the dot


Forward Power

I P

I S

I R

Relayor Meter

Forward Power

I P

I S

I R

Relayor Meter


VP

VS

Relay

• Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relay

• VT ratios are typically expressed as primary to secondary; 14400:120, 7200:120

• A 4160:120 VT has a “VTR” of 34.66

Voltage Transformers

Typical CT/VT Circuits


Courtesy of Blackburn, Protective Relay: Principles and Applications

CT/VT Circuit vs. Casing Ground

Case ground made at IT locationSecondary circuit ground made at first

point of use 56GE Consumer & Industrial

Multilin

Case

Secondary Circuit

power system protection

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