19 GENERATOR NEUTRAL EARTHING TRANSFORMERS

 GENERATOR NEUTRAL EARTHING TRANSFORMERS

The practice of earthing the neutral of large generators via a high resistance

was developed in the USA in the 1950s with the object of restricting the stator

earth fault current to a low value and thereby limiting the damage caused in

the event of a fault. The aim, in selecting the value of resistance to be used is to arrange that its kW 

dissipation in the event of an earth fault on a generator

line terminal is equal to the normal three-phase capacitative charging kVA of

the combined generator windings and its connections. The equality between

kW dissipation and charging kVA can be shown to give critical damping to the

restriking transients generated by arcing ground faults. This value of resistance

results in earth fault currents for a full phase to earth fault on the generator terminals of the order of 2–3 A but in the UK the CEGB adopted the continental

European practice of using a slightly lower value of resistance to limit the current for an earth fault on the generator terminals to between 10 and 15 A. This

value makes little difference to the critical damping at the point of the fault but

simplified es the setting of the protection to avoid spurious operation due to third harmonic currents in the neutral. 

On a 23.5 kV generator this requires a resistance of about 1400 Ω. If connected directly into the generator neutral a resistor

of this value for such a low rated current would tend to be rather flimsy as well as expensive. 

The solution is to use a resistor of low ohmic value to load the secondary of a single-phase transformer whose 

primary is connected in series with the generator neutral earth connection.

When this system was fi rst devised the practice was to use a low-cost standard

single-phase oil-filled distribution transformer. Since that time generator ratings

have increased considerably and the need for high security means it is no longer

considered acceptable to use an oil-filled transformer located near to the generator

neutral because of the perceived fire hazard and, although some utilities have

used both synthetic liquid-filled and class H distribution units, once the principle

of using other than an off-the-shelf oil-filled item is placed in question, it becomes

logical to design a transformer which is purpose made for the duty.

The following section describes the special characteristics of generator neutral

earthing transformers which have been developed at the present time. 

Modern Power Station Practice

The generator neutral connection to the primary of an earthing transformer,

or any other high-resistance neutral earthing device, must be kept as short as

possible since this connection is unprotected. An earth fault on this connection

would go undetected until a second fault occurred on the system and then a

very large fault current would fl ow. 

Hence the desirability of locating the neutral earthing transformer immediately adjacent the generator neutral star point.

In the mid-1970s, the CEGB decided that this was an ideal application for a

cast resin transformer and therefore drew up a specification for such a device.

When the system operating at generator voltage is healthy, the neutral is at  

earth potential, so a transformer connected between this neutral and earth is

effectively de-energised. It only becomes energised at the instant of a fault and

then its ability to function correctly must be beyond question. 

For the neutral of a 23.5 kV, 660 MW generator a voltage ratio of 33/0.5 kV

was selected. The primary voltage insulation level of 33 kV corresponds to that

used for the generator busbars, thus maintaining the high security against earth

faults. However the main reason for selecting an HV voltage considerably

higher than the rated voltage of the generator is to exclude any possibility of

ferroresonance, that is resonance between the inductive reactance of the transformer and the capacitative reactance of the generator windings, occurring

under fault conditions. This could give rise to large overvoltages in the event

of a fault. Such a condition could be brought about on a non-resonating system

by a change in the effective reactance of the transformer as a result of saturation in the core when the generator phase voltage is applied at the instant of

a fault. Occurrence of a severe fault is likely to cause the generator AVR to

drive to the fi eld forcing condition thus boosting the phase voltage to up to 1.4

times its rated value. To avoid the risk of saturation under this condition the

transformer flux density at its ‘normal operating voltage’ must be well below

the knee point for the core material. Normal operating voltage in the case of

a 23.5 kV machine is 23.5/Ï3 kV 5 13.6 kV and if increased by a factor 1.4,

this would become about 19 kV. A transformer having a nominal flux density

of, say, 1.7 Tesla at 33 kV would have a flux density of around 1 Tesla at 19 kV

and so a good margin exists below saturation.

In aiming at a minimum 10 A earth fault current under ‘normal’ phase voltage conditions a current of 14 A would result for the fi eld forcing situation,

hence the maximum transformer rating must be 14 3 33 000 5 462 kVA, single phase. However, since an earth fault of this magnitude would lead to rapid

operation of the generator protection, this only need be a short-time rating.

CEGB specified that this duty should apply for 5 minutes although the use by

some utilities of ratings as short as 15 seconds has been suggested.

The transformer must also have a continuous rating and the required continuously rated current is that which is just too low to operate the protection, plus

an allowance for third-harmonic currents which may fl ow continuously in the

generator neutral. The aim is to protect as much of the generator windings as

possible and so the minimum current for operation is made as low as possible.

This is taken to be 5 per cent of the nominal setting of 10 A, that is 0.5 A. Tests

on 660 MW turbine generators suggest that the level of third-harmonic current

in the neutral is about 1 A. The transformer continuous rating is thus (0.5 1 1) 3

33 000 5 49.5 kVA. In practice a typical cast resin transformer able to meet

the specified 5 minute duty has a continuous rating of 20–25 per cent of its

5 minute rating, hence the continuous rating is accommodated naturally