machine protection ppt.pdf

Ass. Prof. Abdelsalam M. ElhaffarD. Sc. Tech., Senior Member IEEE

Electrical Engineering Dept. College of Engineering, SQU

Block C, Office 211, [email protected]

Power System ProtectionECCE 5302

Introduction to MachineProtection

What are Machines and what is Machine Protection?

Generators (asynchronous and synchronous)Motors (asynchronous and synchronous)Machine protection includes all numerical

devices which are able todetect faults and abnormal operating

conditions on machines andreact according the situation. It protects the

objects againstinadmissible stress and is indirectly also a

human protection.

What are Machines and what is Machine Protection?

The protection devices must fulfil the following 3S criteria: Selectivity (switch off of the faulty system) Speed (fast as necessary) Security (high security and reliability)

MachineFault Categories

1. Winding and terminal faults

2. Sustained or uncleared external faults

3. Abnormal operating conditions such as overload, overvoltage and overfluxing

4. Core fault

5. Rotor Faults

Faults in Synchronous Generators (1)

Stator:Earth FaultsShort Circuits(two and three phase)Interturn Faults(at separate stator windings)Overload

Rotor, Excitation Circuit:Earth Faults(single, double)Failure in the Excitation(partly or loss of excitation)Voltage RiseOverexcitation (U/f)

Internal Faults:

Faults in Synchronous Generators (2)

Grid/Transformer:Earth FaultsShort CircuitsOverloadUnbalanced LoadAsynchronous Condition(cap.Load, long short circuit duration)Torsion StressUnderfrequency (PG < PL)

In Turbine, Regulation: Loss of Prime Mover(Reverse power) Over- and

Undervoltage Over- and

Underfrequency Overexcitation (U/f)

External Faults:

Generator - Differential Protection 87G Transformer - Differential Protection 87T Overcurrent Time Protection 50, 51V Impedance Protection 21

Stator Earth Fault Protection for: Generators directly connected 67N Generators connected via unit transformer Protection Range 90 % 64 Protection Range 100 % 64 100%

Rotor Earth Fault Protection 64R Unbalanced Load Protection 46 Reverse Power Protection 32R

Main Components of Machine Protection (Part 1)

Overvoltage Protection (Undervoltage) 59, (27) Over and Underfrequency Protection 81 Underexcitation (Loss of field) Protection 40 Stator Overload Protection 49 Overexcitation (U/f) Protection 24 Out of Step Protection 78 Breaker Failure Protection 50BF Rotor Overload Protection 49R (Overcurrent at excitation transformer) Interturn Fault Protection 87G / 87T (separate Stator windings - hydro generators) Tripping matrix (Software matrix) 94

Main Components of Machine Protection (Part 2) Proposal for Selection of Protection Functions

Y: Pump storage stations (motor /phase shift operation) O: option

In unit schemes, protection is required to trip more than one breaker.

It depends on the fault type, the plant design and plant operation, generator operation and general operation philosophy

Generator Breaker Excitation Breaker Turbine Rapid Shut Down HV Network Breaker Auxiliary Supply Breaker Aux. Supply ChangeoverUnit-transformer , Aux.-transformer

Tripping Concept Redundancy (Large Units) 2 devices per Object

Requirements for Communication Eaxmple 1 Requirements for Communication Example 2

Introducing IEC61850 Ethernet based Communication

IEC61850 Configuration Principles

Protection functionsRotor Faults:Resistance measurement with a square wave voltage of 1 to 3 Hz

A higher sensitivity is required for larger generators. On the one hand, the disturbing influence of the

rotor earth capacitance must be eliminated more effectively

On the other hand, the noise ratio with respect to the harmonics (e.g. 6th harmonic) of the excitation equipment must be increased.

Injecting a low-frequency square wave voltage into the rotor circuit has proven itself excellent.

Protection functionsRotor Faults:

The square wave voltage injected through the controlling unit leads to permanent recharging of the rotor earth capacitance.

By way of a shunt in the controlling unit, the flowing earth current is measured and is injected into the protection unit (measurement input).

In the absence of a fault (RE=), the rotor earth current after charging of the earth capacitance is close to zero. In the event of an earth fault, the fault resistance including a coupling resistance, and also the injecting voltage, defines the stationary current.

Protection functions

Rotor Faults:

The current square wave voltage and the frequency are measured via the second input (control input).

Fault resistance values up to 80 k can be measured by this measurement principle.

The rotor earth circuit is monitored for discontinuities by evaluation of the current during the polarity reversals.

Protection functions100% stator earth-fault protection with 20 Hz injection

Injecting a 20 Hz voltage to detect earth faults even at the neutral point of generators has proven to be a safe and reliable method.

Contrary to the third harmonic criterion, it is independent of the generators characteristics and the mode of operation.

it is independent of the generators characteristics and the mode of operation. Measurement is also possible during system standstill This protection function is designed so as to detect both earth faults in the entire generator (100 %) and all electrically connected system components.

Protection functions100% stator earth-fault protection with 20 Hz injection

The protection unit measures the injected 20 Hz voltage and the flowing 20 Hz current.

The disturbing variables, for example stator earth capacitance, are eliminated by way of a mathematical model, and the ohmic fault resistance is determined.

Out-of-step protection


This protection function serves to detect powerswings in the system.

If generators feed too long onto a system short-circuit, a compensation process (active power swings) may take place between the system and the generator after fault disconnection.

If the center of power swings is in the area of the unit, the active power surges lead to impermissible mechanical stressing of the generator and the whole generator mounting including the turbine.

23 > Transformer Protection



Since these are symmetrical processes, the positive-sequence impedance is calculated from the voltage and current positive-sequence components and the impedance curve is evaluated.

The symmetry is monitored by evaluating the negative-phase sequence system current. Two characteristics in the R/X diagram describe the range of effect (generator, unit transformer or system) of the out-of-step protection.



The extending of the characteristic in R direction determines the detectable power swing angle. 120 are practicable.

The characteristic can be tilted at an adjustable angle to adapt to the conditions when the system is feeding off several parallel generators.The impedances of the protected zone seen fromthe protection relay are decisive for determiningthe setting values.

Out-of-step protectionProtection functions

In the direction of the generator, the power swing reactance of thegenerator must be taken into account;

It can be set approximately equal to the transient reactance xd'.

This means the transient reactance related to thesecondary side is calculated and set for Zb= xd'

Out-of-step protectionProtection functions

Parameter overview for out-of-step protection


LOSS OF FIELD PROTECTIONProtection functions

DISTANCE SCHEME 1: UNQUALIFIED TRIP SCHEME

This scheme employs a single mho element connected at the generator terminals oriented to measure impedance looking into the generator and trip with a time delay.

An impedance element cannot detect a failure in the excitation system directly. Instead, it is set to detect the generators post failure operation as an induction generator. The impedance of a generator operating asynchronously at final slip with no field excitation is unique and can be used to differentiate a LOF event from other power system transients.




The reactance measured by the LOF relay as the generator slips poles each slip cycle will then vary between X'd and X'q if the generator was initially at full load and between Xdand Xq if initial operation was at light load. Given that Xd is greater than Xq and that X'd is less than X'q , LOF protection must be set to encompass all reactance values between X'dand Xd if it is to operate for all initial values of generator loading. It has become standard practice to set the impedance element for this type of scheme with a diameter of Xd and an offset of 1/2 X'd






DISTANCE SCHEME 1: UNQUALIFIED TRIP SCHEMEDISTANCE SCHEME 1: UNQUALIFIED TRIP SCHEME

The conventional settings for Scheme 1 are:Outer elementDiameter = Xd =1.48 puOffset Time delay = 1.0 secInner elementDiameter =10 puOffset Time delay = 0.4 sec

Impedances on generator base 104 MVA 13.8 kVXd = 1.48 pu Xd ' = 0.196 puXtr = 0.07 pu, Zsyst = 0.078 pu @ 85min (normal)

= 0.225 pu @ 75max (line out)

Traditional Generator Protection Package

Overcurrent Protection

Requirements

Fast operation for primary short circuits

Discrimination with downstream protections

Operation within generator withstand

Non-operation for short or long term overloads

Transformer Overcurrent Protection

Concerns relay response to offset waveforms (DC transient)

Definition

I1 - I2I2

x 100

I1 = Steady state rmspick up current

I2 = Fully offset rms pickup current

I1

I2D.C

.

Transient Overreach

Earth Fault Protection


EARTH FAULT PROTECTION FOR THE COMPLETE STATOR WINDING

The earth fault protection schemes (percentage bias differential protection or neutral overcurrent relay or voltage relay) protect a certain portion of the winding leaving a part of winding at the neutral end unprotected.


Two different schemes are available forcomplete protection of the stator winding:

1. Low frequency injection scheme.

2. Third harmonic voltage scheme

STATOR PROTECTION


Sub-harmonic injection methodSTATOR PROTECTION

1. Injection transformer and a grounding resistor

2.Injection transformer and distribution transformer as a grounding

3. Voltage injection through the grounding distribution transformer

4. Neutral reactor grounded generators


STATOR PROTECTIONThird harmonic voltage method1. Third harmonic under-voltage

scheme2. Third harmonic overvoltage scheme3. Third harmonic ratio voltage scheme Differential Protection

Basic principles: measuring circuit for a 3-phase system

Rest. current

Diff.

ConventionalDifferential Protection

L1

L2

L3

Basic circuit for a 3- phase system:Generator / Motor / Reactor


Motoring protectionMechanical protection: steam turbinesReverse-power relay (Device 32)

Steam turbine 3.0%

Water wheel turbine 0.2%

Gas turbine 50.0%

Diesel engine 25.0%

BACKUP FAULT PROTECTION


Over excitation protection

Protection1. FIELD MONITORING RELAYS

2. V/HZ LIMITER

EXCITATION PROTECTION


Loss of field protection

ProtectionDevice 40

EXCITATION PROTECTION

1. DISTANCE SCHEME 1: UNQUALIFIED TRIP SCHEME

2.. Modified Scheme: Two Impedance Elements

3. Frequency-sensitive excitation systems


Interturn faults in a generator with a single winding can be detected by observing the zero-sequence voltage across the machine terminals.

INTER-TURN PROTECTION BY ZERO SEQUENCE VOLTAGE MEASUREMENT

Overfluxing

Generator transformersUsually only a problem during run-up or shut down, but can be caused by loss of load / load shedding etc.

Flux Vf

Effects of overfluxing : Increase in magnetising current Increase in winding temperature Increase in noise and vibration Overheating of laminations and metal parts (caused by

stray flux) Protective relay responds to V/f ratio

Stage 1 - lower A.V.R. Stage 2 - Trip field

m2

Ie

mV = kf

Overfluxing Basic Theory

CAUSESLow frequencyHigh voltageGeomagnetic disturbances

EFFECTSTripping of differential element (Transient overfluxing)Damage to transformers (Prolonged overfluxing)

V f K

Trip and alarm outputs for clearing prolonged overfluxing

Alarm : Definite time characteristic to initiate corrective action

Trip : IDMT or DT characteristic to clear overfluxing condition

Settings

Pick-up 1.5 to 3.0 i.e. 110V x 1.05 = 2.3150Hz

DT setting range 0.1 to 60 seconds

V/Hz Overfluxing Protection

1000

100

10

11 1.1 1.2 1.3 1.4 1.5 1.6

M =

Operatingtime (s)

V Hz

Setting

K = 63K = 40K = 20K = 5K = 1

t = 0.8 + 0.18 x K(M - 1)2

V/Hz Characteristics

Enables co-ordination with plant withstand characteristics

RL

AVR

Ex

VT

G

Overfluxing Relay

Thermal Overload Protection

90 100 110 120 130 140Hot spot temp C

0.1

1.0

10

100

Relativerateof usinglife

With ambient of 20 C.Hot spot rise of 78 C isdesign normal.A further rise of 6 Cdoubles rate ofusing life.

98

80

Effect of Overload on TransformerInsulation Life

10000

1000

100

101 2 3 4 5 6

Trip time (s)

Current (multiple of thermal setting)

Single characteristic: = 120 mins

Dual characteristic

Single characteristic: = 5 mins

Thermal OverloadOil Filled Transformers

machine protection ppt.pdf

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