Reasonable selection of current transformer transformation ratio -

Magnetic material: Reasonable selection of the transformation ratio of the current transformer In the design process of 10kV power distribution, the selection of the transformation ratio of the 10kV current transformer is very important. If the selection is improper, it is very likely that the relay protection function cannot be realized. Problems such as the failure of dynamic stability verification and other problems should be paid enough attention to by designers. 10kV current transformers can be divided into two types according to the purpose of use, one is for relay protection, and the other is for measurement; they are respectively installed in the incoming line, metering, outgoing line, and contact cabinets of the power distribution station. In the design practice, the author found that in the design of the distribution substation, the selection of the transformation ratio of the current transformer is not uncommon. For example, the author once found that in the power supply circuit of a 630kVA substation with a transformer (the rated primary current of the 10kV side is 36.4A), the transformation ratio of the current transformer in the outlet cabinet of the power distribution station is only 50/5 (using GL type overcurrent Relay, DC operation), which will cause a series of problems such as the current relay cannot be set. For the selection of the transformation ratio of the 10kV current transformer for relay protection, at least the following conditions should be selected: one is to calculate the ratio of the current on the primary side to the rated current of the primary side of the current transformer; the other is to follow the requirements of relay protection; the third is to The calculated primary current multiple mjs of the current transformer is less than the saturation multiple mb1 of the current transformer; four is heat stability; five is pressure stability. As for the selection of 10KV current transformer for measurement, because it is used for the measurement of normal working conditions, there are no requirements for the second and third above; The effect of the selection of transformers, and find out the main factors affecting the selection of current transformer transformation ratio. 1. Calculate the ratio of the current to the rated current of the primary side of the current transformer according to the primary side<<Code for design of electrical measuring instrumentation devices for electrical installations>>(GBJ63-90), under the operating conditions of the rated value, the indication of the instrument is at 70%~100% of the range, and the maximum transformation ratio of the current transformer should be: N=l1RT/(0.7*5); 11RT---Rated current of primary side of transformer, A; N---Transformation ratio of current transformer; Obviously, when selecting the transformation ratio of current transformer according to this principle, the transformation ratio will be very small, and the 400~1600kVA transformer is listed below. Click here When selecting the principle, the maximum transformation ratio of the current transformer: 400KVA11RT=23A N=6.6 take 40/5=8500KVAI1RT=29A N=8.3 take 50/5=10630kVAI1RT=36.4AN=10.4 take 75/5=15800KVAI1RT=46.2AN= 13.2 take 75/5=151000KVA11RT=57.7AN=16.5 take 100/5=201250KVA11RT=72.2A N=20.6 take 150/5=301600KVAI1RT=92.4AN=26.4 take 150/5=30 It can be seen from the above table that for 630kVA transformer , the maximum transformation ratio of the current transformer is 15, when 50/5=10, the rated current only accounts for 3.64/5=72.8% of the current range. This may be one of the reasons why some designers take the transformation ratio of the current transformer at the power outlet circuit breaker of the 630kVA transformer as 50/5. In addition, in many cases, the power supply department often cannot provide the short-circuit capacity or system of the power supply leading to the user during design. Impedance, which makes it difficult to verify several other conditions, which may be another reason for improper selection of transformation ratio. From the following analysis, we will find that when selecting according to this principle, the transformation ratio is obviously too small and cannot be used. 2. According to the requirements of relay protection, in order to simplify the calculation and facilitate the discussion, it is assumed that: (1) the short-circuit capacity at the outlet of the circuit breaker remains unchanged in the maximum and minimum operating modes; (2) the current transformer is a two-phase incomplete Star connection; (3) GL-11 overcurrent relay is used for overload and quick break protection: (4) The operating power supply is DC 220V, and the opening form of the circuit breaker is shunt tripping. Figure 1 is the main wiring diagram of the distribution transformer. The short-circuit capacity of B and C is considered in three levels of 200, 100 and 50MVA. Analysis of the impact of distribution transformer overload protection and current quick-break protection on the transformation ratio is as follows: 1. Overload protection The overload protection should meet the following requirements: IDZJ=Kk*Kjx*Kgh*11RT/(Kh*N)IDZJ-- --The operating current of the overload protection device; Kk----reliable coefficient, take 1.3; Kjx.----connection coefficient, take 1; Kgh----overload coefficient; Kh----relay return Coefficient, take 0.85; a. For the power distribution station for civil buildings, the overcurrent multiple caused by the self-starting of the motor is generally not considered. At this time, Kgh is taken as 2. In order to meet the requirements of the relay setting range, the minimum transformation ratio of the current transformer should be: N =Kk*Kjx*Kgh*11RT/(Kh*IDZJ) (GL-11/10 type relay setting range 4~10A). When Kgh=2, the minimum transformation ratio of the current transformer for each capacity transformer meeting the requirements of the above formula is as follows: 400KVA11RT=23A N=7.0 take 40/5=8IDZJ take 9A500kVA11RT=29A N=8.9 take 50/5=10IDZJ take 9A630kVA11RT = 36.4A N=11.1 take 75/5=15IDZJ take 8A800KVAI1RT =46.2AN=14.1 take 75/5=15IDZJ take 10A1000KVA11RT=57.7A N=17.6 take 100/5=20IDZJ take 9A1250kVA11RT=72.2A N=22.1 take 150Z/5J=30ID 8A1600kVAI1RT=92.4A N=28.3 take 150/5=30IDZJ take 10A Note: The transformation ratio selected according to the above table generally cannot meet the instantaneous current multiple NS requirements of the current relay (see the following analysis for details). b. For factory power distribution stations, it is generally necessary to consider the overcurrent multiple caused by the self-starting of the motor. For the sake of safety, Kgh is taken as 3 at this time. Obviously, the transformation ratio of the current transformer meets the requirements, and its minimum value is N==Kk *Kjx*Kgh*l1RT/(Kh*IDZJ). When Kgh=3, the minimum transformation ratio of the current transformer for each capacity transformer meeting the requirements of the above formula is as follows: 400kVA11RT=23A N=10.6 take 75/5=15IDZJ take 8A500kVA11RT=29A N=13.3 take 75/5=15IDZJ take 9A630kVA11RT= 36.4A N=16.7 take 100/5=20 IDZJ take 9A800kVA11RT =46.2A N=21.2 take 150/5=30IDZJ take 8A1000KVA11RT=57.7AN=26.5 take 150/5=30IDZJ take 9A1250KVA11RT=72.2A N=33.1 take 200/J5=4 Taking 9A1600KVA11RT=92.4AN=42.3 taking 250/5=50IDZJ taking 9A to compare the above data, the following conclusions can be drawn: the transformation ratio selected according to the ratio of the primary side to the primary side rated current of the current transformer is generally smaller than the actual required ratio. the