Industrial Power Transformers -- Special features of transformers for particular purposes [part 13]

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The practice of grounding the neutral of large generators via a high resistance was developed in the USA in the 1950s with the object of restricting the stator ground 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 a ground 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 ground fault currents for a full phase to ground 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 cur rent for a ground 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 simplifies 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 Ohm. 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 ground connection.

When this system was first 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 grounding transformers which have been developed at the present time. For a detailed description of the protection aspects the reader is referred to a specialist work dealing with generator protection, for example The Switchgear Book or Modern Power Station Practice [2].

The generator neutral connection to the primary of a grounding transformer, or any other high-resistance neutral grounding device, must be kept as short as possible since this connection is unprotected. A ground fault on this connection would go undetected until a second fault occurred on the system and then a very large fault current would flow. Hence the desirability of locating the neutral grounding 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 ground potential, so a transformer connected between this neutral and ground is effectively de-energized. It only becomes energized at the instant of a fault and then its ability to function correctly must be beyond question. Such a duty is very demanding of any dielectric but as explained here, this is a duty for which cast resin is well suited. After extensive testing of a prototype cast resin transformer, the system was adopted for the grounding of the Dinorwig generators and this became the standard arrangement for subsequent stations.

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 ground 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 field 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_/3 kV = 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 ground fault current under 'normal' phase volt age conditions a current of 14 A would result for the field forcing situation, hence the maximum transformer rating must be 14 x 33 000 = 462 kVA, single phase. However, since a ground 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 flow 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 percent 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) x 33 000 = 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 percent of its 5 minute rating, hence the continuous rating is accommodated naturally.

Practical arrangement

As stated above, the generator neutral grounding transformer needs to be located as close as possible to the generator neutral. For most large modern machines the neutral star point is formed in aluminum or copper busbar located underneath the neutral end of the generator winding, usually at the turbine hall basement level. It is housed in a sheet-aluminum enclosure which provides protection for personnel from the operating voltage as well as electromagnetic protection to the surrounding plant from the large flux generated by the high machine phase currents. The neutral grounding transformer in its enclosure, which usually also houses the resistor, is arranged to abut the neutral enclosure in such a way as to enable a short 'jumper connection' to be made from a palm on the generator neutral bar to one on the transformer line-end terminal via suitably located openings in the neutral enclosure and transformer enclosure. On generators 4, 5 and 6 at Drax power station, the transformer was made with long flexible connections to the secondary loading resistor and arranged so that it could be 'racked forward' towards the generator neutral bar once the transformer and resistor had been placed adjacent the neutral enclosure, thus enabling a very short connection indeed to be made between the neutral bar and the transformer.

A generator neutral bar and its grounding connection is shown in FIG. 48.

Loading resistor

The value of apparent resistance required in the generator neutral is

V / I_f √3

where V is the generator line voltage and If the specified stator fault current.

When referred to the LV side of the transformer, this becomes

[1 / n^2 ] [V / I_f √3]

where n is the turns ratio of the transformer.

Inserting the values already given for a 660 MW, 23.5 kV generator gives a resistance value of about 0.3 Ohm. Strictly speaking this is the total secondary resistance including that of the transformer, but since a transformer of the rating quoted above has an equivalent resistance of less than 0.01 Ohm, this can be neglected within the accuracy required.

It is also necessary that the X/R ratio for a transformer/resistor combination does not exceed a value of about 2 in order to ensure that the power factor of arcing ground faults is as high as possible and that restriking transients are kept as low as possible.

FIG. 48 Arrangement of Dinorwig generator neutral star-bar with cast resin-insulated neutral grounding transformer at lower right-hand side. Neutral current transformers can be seen, one set on each vertical phase conductor. Generator ground fault CT is just discernible mounted over lower left-hand grounding transformer terminal.

In fact, a practical transformer meeting the other parameters specified above can be designed fairly easily to have a reactance of about 4 percent based on the rating of 462 kVA for the transformer of a 23.5 kV, 660 MW unit, which equates to 0.0025 O. This would give an X/R value of about 0.08, assuming the resistor to be non-inductive, and would allow the resistor to have considerable inductance before causing any embarrassment. The CEGB practice was to specify that the resistor should be non-inductive but this is simply erring on the side of caution and ensuring that there is no likelihood of the maximum X/R value being exceeded inadvertently. Generally, a non-inductive resistor would be flimsier than one which has some inductance because of the construction needed to give this characteristic. Economics might therefore dictate that the resistor is allowed to have some inductance: if so, it is important to know its magnitude and to ensure that the permissible X/R ratio is not exceeded. A typical form of construction of a low-inductive resistor element is shown in FIG. 49.

The resistor rating can be calculated on the basis of I^2 R, where I equals 924 A, equivalent to a primary current of 14 A, and R equal to 0.3 O. This works out to about 260 kVA, which is the required 5 minute rating. This is not equal to the 5 minute rating of the transformer, since the latter has been based on a notional voltage of 33 kV rather than the actual applied voltage of 19 kV.

FIG. 49 Typical low-inductive metallic resistor element used to form a bank of the appropriate resistance and current rating for neutral/ground connection (GEC Alsthom).

The resistor must also have a continuous rating. For the example quoted, this is a 1.5 A in the transformer primary or 99 A in the secondary, giving about 3 kVA for a 0.3 O resistor. As with the transformer, a resistor which can meet the 5 minute rating easily satisfies the continuous rating.

Other parameters of the loading resistor are conventional for metal resistors of this type. It should be housed in a ventilated sheet-steel enclosure which provides protection against personnel access and accidental contact.

This can be a common enclosure with the transformer, as indicated in FIG. 48. However, if a common enclosure is used, there should be a metal barrier between resistor and transformer so that the transformer is protected from any directly radiated heat from the resistor. The external temperature of the resistor enclosure after operation must not exceed about 80ÂșC to avoid possible injury to anyone coming into contact with it.

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