Design Practice Generator Prot.

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DESIGN PRACTICE

GENERATOR PROTECTION SCHEME

0 REV.

07/ 06 / 04 DATE

ISSUED FOR INFORMATION REVISIONS

Tarek

Hussein

BY

CH'D

Hussein

APPR.

DESIGN PRACTICE

Pages 1 page 8

DESIGN PRACTICE

GENERATOR PROTECTION SCHEME

5-DP-001

DESIGN PRACTICE

1.0

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INTRODUCTION:-

This document is concerned with generator protection aspects. It describes various types of generator protection functions provide a basis for applying proper generator protection schemes 2.0

TYPES OF FAULTS :-

The following problems required addressing & consideration from the  point of view of applying protection:• • • • • • • •

Short circuit protection Stator electrical faults Loss of excitation Over voltage Overload protection Unbalanced load protection Under /over frequency protection Mechanical Faults

2.1 SHORT CIRCUIT PROTECTION 2.1.1

PROTECTION AGAINST 3 PHASE , SHORT CIRCUIT (51 V)

Voltage restrained over current relay is one of the most commonly used protection against generator  external short circuit protection. The reason of introducing a voltage signal into over current device is to provide a relay that can ride through momentary overload conditions such as motor starting and still provide proper short circuit protection. The voltage is said to restrain the current element. Typical voltage setting of this relay is (80-90%) of nominal.

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2.1.2

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EARTH FAULT PROTECTION :-( 51 G )

A relay connected to a current transformer mounted on the NeutralEarth conductor can provide back up protection for all earth relays at the generator voltage level, It provides protection against generator  internal earth faults, however not all the stator winding can be  protected against earth fault using this protection function. The relay settings should be higher than the expected harmonic current following the neutral during normal load condition and not more than 33% of the maximum earth faults generator currents.

2.2

STATOR WINDING ELECTRICAL FAULTS:-

Failure of the stator winding or connection insulation can result in a severe damage to the winding and stator core. The extent of the damage depends on the magnitude and duration of the fault current. 2.2.1 DIFFERENTIAL PROTECTION (87)

To respond quickly to a phase fault with damaging heavy currents, high speed differential protection is normally applied to generator equal or  higher than 500 KVA. Differential relays operates only for faults within their protected zone they are inherently Selective and can operate much faster. Accordingly they will provide better protection and in large generating systems maintaining stability by fast clearing faulted generators. The zone of protection of differential protection can be extended to cover the connecting cabling. Typical setting of the differential relay is 5- 10% difference in CT’s current.

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2.2.2 STATOR OVERHEATING PROTECTION (49Q):

Accidental over loading might occur through the combination of full active load current governed by the primemover (KW) output and an abnormally high reactive current component KVAR output, governed  by the level of rotor excitation. excitation. Thermal relay (49 Q ) is normally connected to RTD’S resistance temperature detectors imbedded in the stator winding of generators .It Provides protection for the machine in the over load area but not fault conditions Each phase of the winding shall be connected to 2 RTD’S at the hot spot defined by supplier  The trip & alarm setting of this relay depends on the insulation class & temperature rise of the generator. The generator manufacture manufacture should be consulted during the setting of this relay.

2.3

PROTECTION AGAINST LOSS OF EXCITATION:EXCITATION: -

Loss of excitation can occur when the generator is operating at only 2030% of rated power. In this case the machine runs as induction generator  executed from the others machines on the system. The generator quickly over heats due to the system slip frequency currents induced in it , generator terminal voltage decreases and the stator currents starts to increase. Rapid automatic disconnection is then required in order not only to   protect the generator but also to protect other machines from which the faulted machine withdraw its excitation as an induction generator . The last might cause instability of the power system & over heating of the other machines. Relay (40) loss of excitation is an impedance viewed from the generator  terminals. This relay must be used with a proper time delay to stabilize the protection against maloperation in response r esponse to transient conditions.

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 N.B. This relay should be used only with several generators connected  to isolated bus.

2.4 PROTECTION AGAINST OVER VOLTAGES

Over voltage may occur due to the transient surges on the network and can also be caused from other reasons such as : a) Defective operation of AVR when the machine is running in isolated operation.  b) Sudden loss of load (Due to the tripping of outgoing feeders leaving the machine isolated or feeding over small load). c) Operation under manual central of the voltage regulator. A sudden change in load particularly reactive powered component will give rise a substantial change in voltage. A typical setting for over voltage relay (59) is 107 % of rated stator  voltage with a time delay of 10 sec. to allow for transients due to load switch off / rejection , over voltages resulting from recovery from fault or motor starting 2.5

UNDER VOLTAGE PROTECTION :-( 27) (OVER LOAD PROTECTION )

Under voltage protection is used for generator feeding an isolated system. It protects generators against prolonged over load or failure of  AVR. Setting must be chosen to avoid maloperation during voltage dips during power system fault clearance or associated with motor starting

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PROTECTION AGAINST UNBALANCED CONDITIONS CONDITIONS RELAY FUNCTION (46)

Phase balanced load produces a reaction field that is constant and rotates synchronously with the rotor field system. Any phase unbalance can be resolved into positive and negative sequence component. The positive sequence component is similar to the normal balanced load. The zero sequence produces no main armature reaction. The negative sequence component field reaction rotates in the opposite direction to the DC field system, thereby producing double frequency currents in the field system and in rotor body. The resulting eddy currents are very large and cause severe heating of the rotor. This protection is not required when the generator has large negative  phase sequence capacity. N.B Enppi specification calls for 8 % negative phase sequence capacity 2.7 UNDER / OVER FREQUENCY FREQUENCY PROTECTION PROTECTION 81% SEQUENCE CAPACITY :-

The governor fitted to the prime mover normally provides protection against over frequency, however over frequency may occur in case sudden removal loads. Under frequency as a result of over load of  generators operating in isolated systems. Prime mover & generator load must be protected against excessively low Frequency by tripping of the generators concerned. Typical settings of under frequency is 49 HZ for 20 seconds & 48 HZ for 0.5 seconds. Over frequency setting 51 HZ for 10 seconds and 52 HZ for 1 second

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2.8 MECHANICAL FAULTS:2.8.1 REVERSE POWER PROTECTION :(32)

Reverse power protection in its simple form means motoring of the  prim mover , if the prime mover has lost power for any reason and start to be motored by the system. Diesel engine could catch fire or  suffer from damages to its gear box or shafts. Gas turbine could suffers from gear box damage. The manufacture value for motoring percentage of rated power  should be consulted. The reverse power protection should be provided with time delay, on operation to prevent operation of the relay with transient power  swings that may arise following synchronization Typical protection settings is 50% of motoring power .This setting must be checked during commissioning. 2.8.2 GENERATORS BEARING PROTECTION: -(38)

Failure of the generator lubrication system may cause over heating of generator bearing. Consequently causing mechanical damage. Resistance temperature detectors (RTD’s) embedded near the driving & non driving end bearing to trip the generator breaker in case of over heating of the bearing. The generator manufacture should be consulted to provide this setting.

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REFERENCES: •

ALSTOM NETWORK PROTECTION & ALSTOM

•

BROWN & ROOT DESIGN PRACTICE

•

ENPPI SPECIFICATION 0000-000-510-06

5-DP-001