Design current transformers

General specifications and design considerations for current transformers 15/471... 

Type tests These are conducted on a finished voltage or current transformer, one of each design and type, to verify their compliance with the design data and relevant Standards. 

The instmments discussed above (and many of the others which follow) act in response to the system voltage and/or current. In most cases, the values of these two parameters are very high, which presents problems in the design of the insulation and current-carrying capabilities of the instmment. In these instances, the instmment is supplied with a known fraction of the measured quantity using a voltage transformer or a current transformer, as appropriate. 

The power requirements for heating the crucible are adequately met by a 20-A variable transformer (120-V input, 0-120 V output) feeding a 1.5-kW step-down transformer designed for a 15-V output with a 120-V input. Cables to the reactor electrodes must be of adequate size to handle the large currents, which may range up to 300 A. A rough idea of the voltage and current requirements for specific metals may be obtained from Table I. 

Electrical isolators include fiber-optic and photo-electric couplers, transformer-modulated isolators, current transformers, amplifiers, circuit breakers, and relays. Isolators used in NPPs are designed to prevent the maximum credible fault in the transverse mode on the non-Class IE side of the isolator from degrading the performance of the Class IE side of the isolator below an acceptable level. 

Unlike the partitions chosen by hand in the previous section for designs B and C, the partitions chosen by APARTY and shown in Figure 8-2 were not chosen with high level concurrency in mind. However, the low number of data dependencies between the two partitions make the creation of processes from the partitions an interesting prospect. Although the current transformations cannot handle the multiple entry points that would be needed to create these processes, such a transformation could be defined. 

The half-wave rectification circuits (Equations 47.1, 47.2 and 47.3) are generally used only for low-current applications, of the order of 0.5 A or less. At higher currents, transformer efficiency is low and special core design is required because of the large direct current polarization effect. 

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. 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

Through the tap-offs of the bus, the unit auxiliary transformers (UATs) are connected to feed the station auxiliary services. For more clarity we have taken out the portion of the tap-offs from Figure 13.21 and redrawn it in Figure 13.18 to illustrate the above system and its interconnections. The tap-offs are now subject to the cumulative inOuence of the two supply sources. In the event of a fault on this section, both the sources would feed the same and the fault current through the tap-offs would add up. The tap-offs should thus be designed for the cumulative effect of both fault levels. For the sake of an easy reference, Table 13.8 suggests a few typical values of fault currents, worked out on the basis of data considered for the G and GT. One such example is also worked out in Example 13.3. 

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 ... 

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