Grounding of Hydrogenerators

RESISTANCE GROUNDING:

THE NEUTRAL POINTS OF TRANSFORMER OR GENERATOR ARE GROUNDED THROUGH IMPEDANCE, THE PRINCIPAL ELEMENT OF WHICH IS RESISTANCE. THIS METHOD IS USED WHEN THE EARTH FAULT CURRENT WOULD BE TOO LARGE IF NOT RESTRICTED.

Here, a resistor is connected intentionally between the neutral and earth. This is to limit the earth fault current. The reasons to limit the earth fault current are: In rotating electrical machines like motors and generators, if the earth fault current is high, as in the case of solid earthing, the core damage would be high. To limit the damage to the core, machine manufacturers allow only a limited ground fault current. This is given in the form of a core damage curve.

A typical value would be 25A for 1 second. This value is used as a guide in selecting NGR and setting stator earth fault relays in generator protection. Winding damage in rotating electrical machines is

not of serious concern. The repairs to winding damages can be done by the local re-winder. But,

in case of core damage, repairs cannot be carried out at site. The machine has to be sent back to

the manufacturer's works for repairs thus resulting in prolonged periods of loss of production.

Since rotating electrical machines are not present in voltage levels from 22kV onwards, these

systems are usually solidly grounded. At EHV level too, solid grounding is universally adopted for two reasons:

- Cost of insulation at EHV level is significant.

- Primary protection in such systems would clear the fault within 5 cycles.

If rotating machines are present in 3.3kV, 6.6kV & 11kV levels, the systems are grounded through

resistor or reactor to limit the earth fault current. If rotating machines are not present at these

voltage levels, then these systems can be solidly grounded.

In case of LV Systems, though rotating electrical machines are present, the system is solidly grounded to confirm to IE Rules. [Rule 61 (1) e]. Since LV System is also handled by general public,

for safety reasons, solid grounding is mandated. Sufficient ground fault current is allowed to flow so that protective devices can operate and clear the faults at the earliest. Of course, the core damage at the point of fault in rotating machines will be high.

Since a large number of rotating machines are present at the LV level, it may be worth considering resistance grounded system, even at this level to limit the earth fault current. The LV Bus can be segregated into those supplying rotating machines (this should be a resistance grounded system) and those supplying static loads like lighting and heaters (with solid grounding).

Other reasons for going in for resistance grounding systems are:

• Reduce burning and melting in electrical equipment (which are caused by the high fault currents in a solidly earthed system).

• Reduce mechanical stresses (F ∞ i²) compared to solidly earthed systems.

• Reduce re-striking/arcing faults when compared to unearthed systems.

Depending upon the value of the limiting current, this is further classified as:

Ø High Resistance Grounding and

Ø Low Resistance Grounding

In high resistance grounded systems, the ground fault current is limited to about 10 to 15A. The value of resistor is selected such that for a ground fault, current through the resistor 'I_NR' is equal to the total system capacitive current 'I_C^'. Consider a 11kV system. Let the ground fault current be limited to 10A. The value of the NGR is approximately given by: R_G = (11000/√3)/10 = 635Ω.

One method for achieving the above is to connect a 635Ω resistor directly in the neutral circuit. But a more economical solution is to connect the resistor across an NGT (Neutral Grounding Transformer). This uses the elementary fact that an impedance 'Z' connected to the secondary side of the transformer gets reflected as T²_RZ on the primary side, where 'T_R' is the turns ratio of the transformer.

The voltage ratio of NGT is: (11000/√3)V/240V Turns ratio of NGT: 26.5.

Value of Resistor R^'_G on the LV side: 635/(26.5)² = 0.90Ω

The use of a low voltage, low-value resistance is economical.

A voltage relay (64ND) - also known as Neutral Displacement Relay - is connected across the resistor to detect ground faults.

But, the problem is that the earth fault relays on feeders cannot be connected in a residual circuit as the fault current magnitude is very less compared to the rated current of the feeder.

Assuming that the feeder rating is 800A, the CT ratio 800/1A and the minimum earth fault relay setting as 10%, then the pick-up amperes is 80A (10% of 800A), which is much higher than the fault current of 10A. So, this relay cannot sense this fault. In such a case, a separate CBCT is required for earth fault detection and a very sensitive earth fault relay is connected to the CBCT. Primary earth fault currents as low as 2A can be detected with this scheme.

In low resistance grounded systems, the earth fault current is limited to about 400A. A widely used criteria here, is to limit the fault current to the full load rated current of the generator or the transformer. On a 11kV System, with the earth fault current limited to, say, 400A, the value of NGR would be:

R_G = (11000/√3)/400 = 16Ω.

The resistor is directly connected between the neutral and earth.

Now, a current relay in the neutral circuit is possible, as the earth fault current is not very low. But, compared to a high resistance grounded system, the core damage to rotating machines will be more.

Earth fault relay can also be connected in a residual circuit. An earth fault relay with a setting range of, say, 10 to 40% is adequate.