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Generator Protection - Class-A Protection

Class A Protection:
It initiates a main CB trip as fast as can be arranged. Electrical failure damage propagates so quickly that class A tripping is essential. Taking the trip relay and CB operating time into account, this means about 120 ms (6 cycles) after initiation. With most modern CBs this time may be reduced to about 80 ms. The following situations are commonly protected by class A trip through unit lock out relays 86G-1 and 86G-2(Instantaneous Tripping of unit).


A.
Tripping conditions for 86G-1 will energize on the operation of the following protections :

1.                 
Differential protection of Generator Transformer and Unit Auxiliary transformers-A/B.

2.                  
Generator and Transformer overall differential protection.

3.                  
Generator transformer and UAT fire protection.

4.                 
Generator inter winding protection.

5.                  
Generator Transformer over flux protection.

6.                  
Bus bar protection of switchyard and breaker back up protection.

7.                  
Generator pole slipping.

8.                 
Generator main earth fault (64G-1).

9.                  
Generator second rotor earth fault.

10.              
Low forward power relay when ESV and IV are closed.

B.                
Tripping of 86G-1 will result in following operations:

1.                 
Tripping of UAT-A & B breakers of 6.6 KV side.

2.                  
Opening of Generator main CB.

3.                 
Opening of transfer bus breaker(if unit is running on transfer bus)

4.                  
Opening of sequential isolators.

5.                  
Tripping of turbine lock out relay.

6.                  
Tripping of master fuel trip relay.

C.                 
Tripping of unit lock out relays 86G-2 will energise on the operation of following protection :

1.                 
Generator stands by earth fault protection.

2.                  
Generator transformer (HV Side) – restricted earth fault protection.

3.                  
Buchholz relay operation of generator transformer or UAT-A/B.

4.                 
Generator transformer pressure relief valve operation.

5.                  
Excitation system trouble-over current, loss of thyristors and over voltage.

6.                  
Operation of 86G-1.

7.                  
Any of HP heaters level high (4250 mm)

8.                 
Turbine in trouble-high axial shift (-1.7 mm, +1.2 mm), very low vacuum (540mmHg), or very low lubricating oil pressure (0.3 kg/cm2).

D.                
Tripping of 86G-2 will result in following operations :-

1 to 6 operations are same as those of B.

7.                  
Tripping of field breaker.

8.                  
Tripping of unit lock out relay 86G-4

E.                 
Tripping of 86G-1 and 86G-2 energise 86G-4 which in turn will affect the  following operations :

1. 
Tripping of following auxiliaries:-
 .  CW pumps A and C.
 .  Condensate pumps A and B.
 .  Boiler feed pumps A,B and C.
 .  PA Fan A and B.
 .  FD Fan A.

2. 
Auto closing of 6.6KV tie breaker to unit buses from the 6.6KV station buses.

3. 
AVR mode changes over from Auto to Manual.

4. 
Stops the generator transformer cooler pumps and fans.
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Comments

  1. Miracle Electronics28 November 2016 at 01:17

    The maximum arresting force probably won’t play into your planning very much. When worn with a body harness, both of these classes meet the OSHA requirement of limiting the arresting force on the human body to 1800 lbs.
    Transformer Manufacturer in India

    ReplyDelete

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