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INTERCONNECTED STAR GROUNDING TRANSFORMERS
On a number of occasions in the preceding sections, for example in Section 6.2, mention has been made of the use of interconnected star grounding transformers to provide a neutral point for connection to ground on a system which would otherwise not have one. This requirement commonly arises at a grid bulk supply point where transformers stepping down to 66 or 33 kV from the 132 kV or higher-voltage system will need to have one winding connected in delta and, since the HV must be the star-connected winding to enable non uniform insulation to be used, the delta winding must be the LV. Transformers for stepping down from the 66 or 33 to 11 kV are normally required to be connected in delta on their higher voltage, that is 66 or 33 kV, windings in order to provide a neutral for grounding purposes on the 11 kV side, as well ensuring that the 11 kV system has the required zero phase shift relative to the 400 kV system reference. Hence the 66 or 33 kV system is without a neutral. The situation also arises where a 13 kV delta-connected tertiary is provided on a star/ star or star auto-connected transmission transformer for connection to a shunt reactor for VAr absorption.
This section examines a little more closely the technical requirements and constructional features of interconnected star grounding transformers which are used in these situations.
An interconnected star grounding transformer is a conventional three-phase oil-filled transformer except that it requires only a primary winding in order to provide the required neutral point. FIG. 22 shows a phasor diagram and connection diagram of such a transformer. Each 'half-phase' is effectively a complete winding for construction purposes so that the transformer is built as if it were a double-wound transformer. Core frame size, flux density and number of turns necessary will probably mean that 66 and 33 kV grounding transformers at least will need to have disc windings throughout, although it is possible that at 13 kV helical windings may be used. Particularly at 66 or 33 kV there will be a need to consider lightning impulse strength. Each winding end will constitute a discontinuity from the point of view of surge impedance and will probably require some form of stress control. In the case of disc windings at 66 or 33 kV a shield between end sections or a dummy strand as described in Section 4.4 will probably be used.
Under normal conditions the steady-state voltage applied to grounding transformers is the LV voltage of the step-down transformer with which they are associated. This voltage is likely to have a maximum value of 10 percent above the system nominal voltage so that a flux density of 1.7 Tesla may be permitted for the grounding transformer without the risk of saturation. It is not usual to provide the transformer with tappings. However it is common practice to provide an auxiliary winding, usually a star-connected 415 V winding, to provide a three phase and neutral supply for substation services. Generally the rating of this auxiliary winding is up to about 200 kVA. The rating of the grounding transformer is however determined by the current it is required to carry in the neutral for 30 seconds (the short-time rated current) in the event of an HV line-to-ground fault, and not the rating of any auxiliary winding.
As explained in Section 6.2 it is normal practice to select the impedance of the system ground connection to such a value as will result in the flow of rated full load current for the supply transformer in the grounding transformer HV neutral in the event of a line-to-ground fault on the HV system, which itself has negligible impedance. It is usual to place a minimum value on this fault current which varies according to the HV system voltage. Values of minimum rated short time currents are listed in Table 1. At the end of 30 seconds the maximum temperature of the copper must not exceed 250ºC. The starting temperature is taken as maximum ambient, 40ºC, plus any temperature rise resulting from operation at the continuous maximum rating of the auxiliary winding. The calculation is performed in the same manner as when determining the temperature rise of a transformer on short circuit described in Section 4.7. Expression (4.3) is used except that the time must be increased to 30 seconds. The same assumption is made that, for the time for which the fault current flows, all the heat is stored in the copper. Although this will be slightly less true in the case of a 30 seconds fault compared with one for 2 seconds, it is nevertheless introducing a small degree of pessimism which is no bad thing. The transformer will also be required to withstand the mechanical forces resulting from carrying the short time fault current and these two requirements usually result in a transformer which is considerably more generously proportioned than would be determined by any requirement to supply the auxiliary loading alone.
Table 1 -- Minimum rated short-time current through the neutral of interconnected star grounding transformers in relation to voltage of delta-connected winding of main transformer
An important factor in determining the HV system single phase to ground fault current is the zero-sequence impedance of the grounding transformer. This is calculated in the same way as the positive-sequence value between half-phases treating these as if they were separate windings and using the expression (2.1). It is usual to quote a minimum value for this, that is with no negative tolerance and a 20 percent plus tolerance and it is also necessary to convert this into a value in ohms per phase rather than in percentage terms, the reason being that the grounding transformer does not have a true continuous rating against which to relate a percentage impedance and it is the ohmic value of impedance which dictates the system ground fault current. If the grounding transformer is provided with a secondary or auxiliary winding, the impedance between the interconnected star winding and the auxiliary winding is normally between 4 and 6 percent based on the auxiliary winding rating and is calculated in the normal manner.
Grounding transformers for 66 and 33 kV generally have HV bushings for line and neutral terminations of the 66 or 33 kV windings. Air connections of cop per bar or tube can then be brought across from the LV terminals of the main transformer and the neutral bushing is usually connected in a similar manner, via any protective current transformers, to a liquid or metal element neutral grounding resistor. The 415 V auxiliary winding will probably be brought out via a weatherproof fuse-switch unit incorporating a bolted neutral link arranged for glanding and terminating a four-core cable to take the auxiliary supply to its associated distribution board. FIG. 23 shows a 33 kV grounding transformer with a 415 V auxiliary output and the associated 132/33 kV bulk sup plies transformer is discernible in the background.