Protection voltage transformers

X6/1987 Measuring voltage transformers Protective voltage transformers Capacitor voltage transformers. "1156-2/1992. 1156-3/1992 3156-4/1992 BS 7729/1995 BS 7625/1993 ... 

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

Note For CTs this multiplying factor has been specified as 2.5 for all voltage systems, as in lEC 60044-1. See also Section 15.7 for metering and protection current transformers. 

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. 

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... 

Figures 16.4 and 16.5 show typical protection schemes for two circuits of a 60 MW generator, a generator transformer with tripping logic and a 2 MVA transformer. The figures illustrate the protection devices provided the current and voltage transformers supplying them and the tripping scheme associated with each.

Iligh-voltage controllers which regulate prirnaiv input voltage to the rectifier and wiper transformer and house primary current-limiting protection, meters, and instrumentation are designed for local or remote operation. 

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 difference in phase between the primary and the secondary voltage phasors (S). The direction of the phasors are so ehosen. that the angle is zero for i perfect transformation. Refer to Ihe phasor diagram. Figure 15.2. and Table 15.5 for measuring and Table 15.6 for protection VTs. 

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 ... 

The front of the transferred surge will, however, be less steep and dampened than on the primary side due to capacitive dampening. But sometimes this may also exceed the BIL, particularly of the tertiary (if provided) and also the secondary windings of the transformer, as well as the cable and the terminal equipment connected on the lower voltage side. This is especially the case when the primary side voltage is very high compared to the secondary. Protection of the secondary windings, in all probability, will be sufficient for all the cables and terminal equipment connected on the secondary side. 

Surge travels from higher voltage side of the transformer to the lower voltage side. Consider a surge protection on the primary, with details as follows ... 

In single-phase bridge circuits for ac connections and for very low ac output voltages below 5 V, single-phase center tap circuits are used as rectifier circuits for CP transformer-rectifiers. They have an efficiency of 60 to 15% and a residual ripple of 48% with a frequency of 100 Hz. A three-phase bridge circuit for three-phase alternating current is more economical for outputs of about 2 kW. It has an efficiency of about 80 to 90% and a residual ripple of 4% with a frequency of 300 Hz. The residual ripple is not significant in the electrochemical effect of the protection current so that both circuits are equally valid. 

Fig. 8-4 Circuit diagram of a transformer-rectifier with overvoltage protection (protected against high voltage).

---