In the case of the same rated current and the same rated output capacity, the secondary current of the current transformer adopts 1A or 5A, and its structure and characteristics are greatly different. The use of 1A is 5A, and its structure and characteristics are quite different. The 1A is 5 times larger than the 5A current transformer, the secondary winding is 5 times larger, the open circuit voltage is high, the internal resistance is large, the excitation current is small, the manufacturing is difficult, and the price is slightly higher. However, using 1A can greatly reduce the active loss in the cable (reduced to 1/25 of 5A), and under the same conditions, the allowable length of the current loop cable can be increased.
The secondary current rating of the current transformer is 1A or 5A, which needs to be determined by technical and economic comparison. When 1A is used, the investment of the current transformer itself increases, while the investment of the current loop control cable is less; on the contrary, when 5A is used, the investment of the current transformer itself is reduced, and the investment of the secondary cable is increased.
When selecting the secondary rated current of the current protector, it should also consider whether the rated current of the protection device and the measuring instrument can be matched with it; in the unified substation, the secondary current of each side current transformer is as uniform as possible.
Generally speaking, in the 220kV and below voltage level substations, there are not many 220kV loops, and the number of 10-60kV loops is large, and the cable length is short. It is economical to use 5A for the secondary current rating of the current transformer. In the 330kV and above voltage level substation, the number of circuits of 220kV and above is more, the current loop cable is longer, and the current rated current of the current transformer is 1A.
After the primary and secondary rated currents of the current transformer are selected, the rated ratio of the current transformer is also determined. In the actual project, the initial conformity of the project is often lighter, and the design load of the loop is much different, and the secondary current of the current transformer is small. The pointer ammeter reading has difficulty or does not guarantee the minimum precise operating current of the electromechanical protection device. This requires changing the current ratio without changing the current transformer. To change the ratio of the current transformer, the following methods are usually used:
1 uses a double-variable current transformer. This current transformer has two primary windings that are changed by a primary winding connected in series or in parallel. For example, a current transformer with two primary windings rated at 600A each, when the two windings are connected in parallel, the ratio is 1200/5A; when the two windings are connected in series, the ratio is 600/5A.
2 Set the tap on the secondary winding of the power transformer.