Types of Generator Protection

GENERATOR PROTECTION ABU SAMAH ABU HASAN INSTITUT INSTI TUT LATIHAN LATIHAN SUL SULT TAN AHMAD SHAH SH AH TENAGA NASIONAL BERHAD MALAYSIA

GENERATOR PROTECTION

Overspeed 

Rotor E/F 

Generator  Transformer 

NER

Generator 

IDMT  Earth Fault 

V T 

Reverse Overvoltage Buchholz  Power  Generator  Restricted E/F  Diffirential  Generator 

Negative Phase Sequence Overcurrent  Instantaneous Loss of Field  Earth Fault 

Transformer  Diffirential 

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DIFFERENTIAL PROTECTION Differential protection using high-impedance relays is usual  for stator protection and is applied on a phase-by-phase basis. As the leads between the two sets of current  transformers may be long the resistance will be fairly high but as the maximum through-fault current will be less than 10  times full load current a reasonably low voltage setting can be applied. This means that the CT magnetising current will  be low and therefore a low overall current wetting can be expected. The overall setting has a direct bearing on the amount of the generator winding which is protected.

STATOR DIFFERENTIAL PROTECTION

CT 

Stator 

CT  Red  Yellow  Blue

RELAY  Biasing Coil  Operating Coil 

STATOR EARTH EARTH--FAULT PROTECTION Stator earth-fault protection comprises an instantaneous relay and the Inverse Definite Minimum Time (IDMT) relay. Both relay will be connected to current transformer having a  primary current rating equal to that of the earthing resistor. Earth faults will be detected in 90% to 95% of the generator  winding even though the maximum earth-fault current may  as low as 5% of the generator rating.

STATOR EARTH FAULT PROTECTION

Generator  Red  Yellow  Blue

Relay 

CT 

Neutral Earth Resistance

ROTOR EARTH EARTH--FAULT PROTECTION For detecting earth-faults in the rotor circuit, a highresistance potentiometer is connected across the rotor circuit  the centre point of which is connected to earth through a sensitive relay. The relay will respond to earth faults occurring over most of the rotor circuit.

OVERCURRENT PROTECTION  An Inverse Definite Minimum Time (IDMT) relay is generally  used as back-up protection but the operation of this relay is complicated because of the current decrement in the generator during fault conditions. In some cases a setting is chosen, such that the relay will not operate for a system fault  but will only respond when fault current is fed into the generator, in this way it only acts as a back-up to the main generator protection.

LOSS OF FIELD PROTECTION Failure of the field system results in acceleration of the rotor  to above synchronous speed where it continuous to generate  power as an induction generator the flux being provided by a large magnetising components drawn from the system. This condition can tolerated for a short time but clearly there will  be increased heating of the rotor because of the slipfrequency currents which flow. Loss of field can be detected by undercurrent relay  connected to a shunt in the field circuit.

OVER--VOLTAGE PROTECTION OVER Voltage is generally controlled by a high-speed voltage regulator and therefore over-voltages should not occur and  over-voltage protection is not generally provided for  continuously supervised machines. On unattended  machines a instantaneous relay set at, say, 150% is use to cater for defective operation of the voltage regulator.

NEGATIVE PHASE SEQUENCE PROTECTION (Unbalanced Loading) CT  Red  Yellow  Blue

 X  Positive Sequence

 XL

R

R

V ZB

V ZR

Y  Negative Sequence

R

Ir 

Ir 

VZZR 

VZR 

Y

VZB 60o

60o

I b

VX

VZB

V ZR + V ZB = 0 

Iy

Iy

I b

V  XY  = V ZR + V ZB

NEGETIVE PHASE SEQUENCE (Unblanaced Loading) U nbalanced loading of the

generator phase results in the  production of negative phase sequence (NPS) currents. These current, which have a phase rotation in the opposite direction to the normal phase rotation, produce a magnetic  field which induces currents in the rotor at twice the system frequency. This causes considerable heating in the rotor and  would cause damage. The actual NPS currents is difficult to determine. Relays to detect the condition usually have an IDMT characteristic  matched to I 2 t value.

OVERSPEED The speed is very closely controlled by the governor and is held  constant as the generators in parallel with others in an interconnected system. If the circuit breaker is tripped the set will  begin to accelerate and although the governor is designed to  prevent over-speed a further centrifugal switch is arranged to close the steam valve. There is still a risk, however, that the steam valve not close completely and even a small gap can cause over-speed and so where urgent tripping is not required. It is usual to lower the electrical output to about 1% before tripping the CB. A sensitive under-power relay is used to detect when this value is reached.