Transformer voltage rating

Figure 15.6(a) Capacitor voltage transformer (CVT) rated voltage 36-420 kV and above (Courtesy ABB)... 

Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 V, provided all of the following conditions are met ... 

Power systems for offshore platforms and onshore plants, which have their own generators seldom have feeder transformers with ratings greater than about 10 MVA. These transformers usually feed radially to the consumer switchboards. Several levels of operating voltages are used in these situations, for example ... 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

A low voliage of 5Vc. at which the transformer is required to maintain its accuracy limit, is of great significance. A protection iran.sformer is required to operate under a fault condition, during which the primary voltage may dip to a value as low as 5% of the rated voltage. 

This is the error in the transformed secondary voltage as generally caused by the excitation cutrent l, and as shown in Figure 15.1. It is the variation in the actual transformation ratio from the rated and is expressed by... 

K = rated transformation ratio V = actual primary voltage (r.m.s.)... 

Rated primary voltage The normal practice for an HT system is to provide a separate LT feeder for the auxiliary supplies. The primary voltage will be the normal system voltage, V, when the transformer is connected line to line or V,/ l3 when connected line to neutral. 

The rated frequency, insulation systems and the requirement of creepage distances will generally remain the same as for a voltage transformer (Section 15.4,1). For the remaining parameters, the following may be noted. 

These are protection CTs lor special applications such as biased differential protection, restricted ground fault protection and distance protection schemes, where it is not possible to easily identify the elass of accuracy, the accuracy limit factor and the rated burden of the CTs and where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application tind the relay being used determine the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design parameters for such CTs. They are classified as class PS CTs and can be identified by the following characteristics ... 

To determine the grounding parameters, consider a generator rated for 200 MW, 15 kV and the ground fault current limited to 15 A. Considering GFF as -J3, the voltage ratio of the grounding transformer with a 220 V secondary will be... 

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