Protection voltage unbalance

Protection against voltage unbalance or negative phase sequence 12/291... 

Thus, besides voltage unbalance it can also detect an inter-turn fault, which leads to a current unbalance. A small amount of unbalance is already detected by the thermal element of the overcurrent protection but a severe unbalance, such as during a single phasing, would require quicker protection and hence, this protection. The relay may be set for an of around 3% or so. 

System unbalance protection. As discussed earlier, an asymmetry in the supply voltage causes... 

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 ... 

Nevertheless, unbalance is a major occurrence in either case and an unbalanced protection, in addition to short-circuit protection is imperative in all types of capacitor units. Hence it is also possible to design series protected capacitor units in larger sizes and for higher voltages, which are more economical, compared to smaller units. 

This method is applicable to single-star or delta-connected capacitor banks. Unbalance can be detected through the use of an RVT (residual voltage transformer) (Section 15.4.3). See Figure 26.4. The theory of operation is that any unbalance, of the system or the capacitor bank, will shift the neutral and reflect as the residual voltage across the open delta and can be used for the protective scheme. The unbalance voltage across the open delta in the event of failure of a unit in any series group can be expressed by... 

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