CT in the high-voltage cabinet is a current transformer, for the protection of the current transformer, such as high-voltage cabinets, transformers with high-voltage cabinets and bus high-voltage cabinets, the ratio is often two times the rated current or higher to choose, so that the protection can guarantee the protection of the fixed value of the appropriate protection accuracy is relatively high, in the vicinity of the fixed value, there will not be a current is too small relays or institutions do not act, and will not be too much current will be too big. The current is too small for the relay or mechanism to operate, nor is the current too large for the transformer to be saturated and not reach the fixed value and not operate.
Current transformers for metering, such as metering cabinets, the ratio is often rated current or slightly higher to consider, but generally not greater than 1.2 times the rated current; this ensures the accuracy of the measurement of small loads, but also saturated in the large current, to prevent the secondary current and burned metering instruments.
Of course, in the current transformer selection, but also to calculate the short-circuit current at the use of the transformer, according to which the calibration of the current transformer dynamic, thermal stability, etc., to prevent the short-circuit current is too large and burned the transformer.
Expanded information
Precautions for use:
1) The wiring of the current transformer should comply with the principle of series: that is, the primary winding resistance should be connected in series with the circuit to be measured, while the secondary winding resistance is loaded with all the instruments.
2) According to the size of the measured current, select the appropriate ratio, otherwise the error will increase. At the same time, the secondary side must be grounded, in order to prevent the insulation once damaged, the primary side of the high-pressure into the secondary low voltage side, resulting in personal and equipment accidents
3) the secondary side is absolutely not allowed to open, because once open, the primary side current I1 all become magnetizing current, causing φm and E2 sudden increase, resulting in over-saturation of the core magnetization, serious heating and even burned coils; at the same time, the magnetic circuit over-saturation magnetization At the same time, the magnetic circuit over-saturation magnetization, so that the error increases.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering and other devices, in the generator, transformer, outgoing, bus sectional circuit breaker, bus circuit breaker, bypass circuit breaker and other circuits are equipped with two to eight secondary winding resistance of the current transformer.
5) for the protection of current transformers should be installed in accordance with the location of the main protective device to eliminate as far as possible to set the unprotected area. For example: if there are two groups of current transformers, and the location allows, should be located on both sides of the circuit breaker, so that the circuit breaker is in the cross-protection range.