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TRANSMISSION TRANSFORMERS AND AUTOTRANSFORMERS
Transmission transformers are used to provide bulk supplies and to interconnect the separate EHV transmission systems. In the UK they will have nominal HV voltages of 400, 275 or 132 kV. Both double wound and auto-connected types are used and these are usually of three-phase construction having three limb or five-limb cores and dual ONAN/ODAF cooling. The ONAN rating is usually 50 percent of the ODAF rating. Tappings are provided on the HV winding of double-wound transformers. On autotransformers tappings, if pro vided, will generally be at the line end of the lower-voltage winding. However, because of the high cost of line-end tapchangers, some transmission autotransformers do not have on-load tapchangers.
To ensure security of supply, transmission transformers are installed in two, three or four transformer substations, such that, in the event of one transformer being unavailable for whatever reason, the load can be carried by the remaining transformers. This might, on occasions involve some modest degree of over loading within the limits permitted in EN 60354. For larger more important transformers, the overload capability will generally be made a requirement of the specification so that this can be accurately determined at the time of the transformer design.
Autotransformers and the HV windings of double-wound transformers are, in the UK, almost exclusively star connected, with the HV neutral solidly grounded and thus employing non-uniform insulation. All other windings have uniform insulation. In the UK the 66 kV system is in phase with the 400, 275 and 132 kV systems, so that double-wound transformers stepping down to 66 kV from any of these voltages will be star/star connected. All autotransformers and the majority of star/star-connected double-wound transformers have delta-connected tertiary windings. Autotransformer tertiary windings are usually rated 13 kV and are brought out to external bushing terminals to enable these to be connected to up to 60 MVAr reactive compensation equipment. Grounding of this 13 kV system is provided by means of an interconnected star grounding transformer (see Section 7.7). The ratings of the main windings are not increased to allow for the loading of the tertiary winding. Should these transformers not be required to supply reactive compensation equipment, then two connections from the phases, forming one corner of the delta, are brought out for linking externally to close the delta, and for connection to ground via protection current transformers. Any decision to omit the tertiary winding from a star/star-connected transmission transformer would only be taken following careful consideration of the anticipated third-harmonic current in the neutral, the third-harmonic voltage at the secondary terminals and the resultant zero-sequence impedance to ensure that all of these were within the prescribed values for the particular installation.
The maximum permitted value of nominal core flux density varies according to the rated HV voltage. Because the upper voltage limit on the 400 kV system under normal operating conditions is restricted to +5 percent, autotransformers and double-wound transformers with this primary voltage are allowed to operate at up to 1.7 Tesla nominal flux density. The 275 kV system voltage can rise to +10 percent above nominal so for 275 kV transformers the flux density is limited to a nominal 1.6 Tesla. At 132 kV the transformer tapchanger can be used to boost the voltage of the lower-voltage system as explained in Section 4.6, so the nominal flux density needs to be low enough to ensure that saturation will not be reached at the highest system volts applied to the lowest likely tap position. For 132 kV transformers flux density is thus restricted to a nominal value of 1.55 Tesla.