Reasonable Choice of Transformer Ratio of Current Transformer

In the process of 10kV power distribution design, the selection of the transformation ratio of 10kV current transformer is very important. enough attention from designers. 10kV current transformers can be divided into two types according to their use, one is for relay protection and the other is for measurement; they are respectively located in the incoming line, metering, outgoing line, contact and other cabinets of the power distribution station. In the design practice, the author found that in the design of the distribution and substation, the selection of the current transformer transformation ratio is not rare. For example, the author once found that in the power supply circuit of a 630kVA station attached 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 the 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 must be selected: one is the ratio of the primary side calculated current to the rated current of the primary side of the current transformer; the second is according to the requirements of relay protection; the third is The calculated primary current multiple mjs of the current transformer is less than the saturation multiple mb1 of the current transformer; the fourth is the thermal stability; the fifth is the pressing stability. As for the selection of the 10KV current transformer for measurement, because it is used for the measurement of normal working conditions, there is no second and third requirements above; The influence of the selection of the transformer, and find out the main factors that affect the selection of the transformation ratio of the current transformer. 1. Calculate the ratio of the primary side current to the rated current of the primary side of the current transformer according to<<Specification for design of electrical measuring instrument devices for electrical installations>>(GBJ63-90) stipulates that under the operating conditions of the rated value, the indication of the instrument is at 70%~100% of the range. At this time, the maximum transformation ratio of the current transformer should be: N=l1RT/(0.7*5); 11RT--- rated current of the primary side of the transformer, A; N---the transformation ratio of the current transformer; obviously when the transformation ratio of the current transformer is selected according to this principle, the transformation ratio will be very small, and the 400~1600kVA transformer is listed below according to this When selecting in 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 taking 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 current transformer ratio of 50/5 at the power supply outgoing circuit breaker of the 630kVA transformer. 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. impedance, which makes the verification of several other conditions difficult, which may be another reason for the improper selection of the 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 under the maximum and minimum operating modes; (2) the current transformer is a two-phase incomplete Star connection; (3) GL-11 type overcurrent relay is used for overload and quick-break protection: (4) The operating power supply is DC 220V, and the circuit breaker opening form is shunt tripping. Figure 1 is the primary main wiring diagram of the distribution transformer. The short-circuit capacity of the two places B and C should be considered according to the third gear of 200, 100, and 50MVA. The analysis of the influence of the overload protection and current quick-break protection of the distribution transformer 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----can be *coefficient, take 1.3; Kjx.----connection coefficient, take 1; Kgh----overload coefficient; Kh----relay return The coefficient is taken as 0.85; a. For the power distribution station for civil buildings, the overcurrent multiple caused by the self-starting of the motor can generally not be considered. At this time, Kgh is taken as 2. In order to meet the requirements of the setting range of the relay, 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 variation ratio of the current transformer for each capacity transformer to meet 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 150/5=30IDZJ take 8A1600kVAI1RT=92.4A N=28.3 take 150/5=30IDZJ take 10A Note: Selecting the transformation ratio 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 the power distribution station used in the factory, 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 the minimum value is N==Kk *Kjx*Kgh*l1RT/(Kh*IDZJ). When Kgh=3, the minimum variation ratio of the current transformer for each capacity transformer to meet 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 200/5=4 Taking 9A1600KVA11RT=92.4AN=42.3 and 250/5=50IDZJ and comparing the above data with 9A, the following conclusions can be drawn: The transformation ratio selected according to the ratio of the primary side to the rated current of the primary side of the current transformer is generally smaller than the actual required ratio.​​