In the design process of 10kV distribution substation, the selection of transformation ratio of 10kV current transformer is very important. If it is not selected properly, it is likely to cause problems such as failure of relay protection function and failure of dynamic stability verification, which should be paid enough attention by designers. 10kV current transformer can be divided into two types according to its use, one for relay protection and the other for measurement; They are respectively set in the incoming line, metering, outgoing line, liaison and other cabinets of the distribution substation. In the design practice, the author found that the selection of current transformer transformation ratio is too small in the design of distribution substation. For example, the author has found that in the power supply circuit of a 630kVA station auxiliary transformer (the rated primary current at 10kV side is 36.4a), the transformation ratio of the current transformer in the outgoing cabinet of the distribution substation is only 50 / 5 (GL type overcurrent relay and DC operation are adopted), which will cause a series of problems such as unable to set the current relay.

The transformation ratio of 10kV current transformer for relay protection shall be selected at least according to the following conditions: first, the ratio of calculated current at the primary side to the rated current at the primary side of current transformer; Second, according to the requirements of relay protection; Third, the calculated primary current multiple MJS of current transformer is less than the saturation multiple MB1 of current transformer; Fourth, heat stability; Fifth, press to stabilize. For the selection of 10kV current transformer for measurement, because it is used for measurement under normal working conditions, there are no requirements in Article 2 and 3 above; The following takes the common distribution transformer as an example to illustrate the influence of the above conditions on the selection of 10kV current transformer, and find out the main factors affecting the selection of transformation ratio of current transformer.

I The proportion of current in the rated current of the primary side of the current transformer is calculated according to the primary side. According to the provisions of < < code for design of electrical measuring instrument devices of Electrical Devices > > (gbj63-90), under the rated operating conditions, the indication of the instrument is at 70% ~ 100% of the range. At this time, the maximum transformation ratio of the current transformer shall be: n = L1RT / (0.7 * 5); 11rt --- rated current at primary side of transformer, a; N --- transformation ratio of current transformer; Obviously, when the transformation ratio of current transformer is selected according to this principle, the transformation ratio will be very small. The maximum transformation ratio of current transformer when 400 ~ 1600KVA transformer is selected according to this principle is listed below:

400kva11rt = 23a n = 6.6, take 40 / 5 = 8

500kvai1rt = 29a n = 8.3, 50 / 5 = 10

630kvai1rt = 36.4an = 10.4, 75 / 5 = 15

800kvai1rt = 46.2an = 13.2, 75 / 5 = 15

1000kva11rt = 57.7an = 16.5, take 100 / 5 = 20

1250kva11rt = 72.2a n = 20.6, take 150 / 5 = 30

1600kvai1rt = 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 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 reason why some designers take the transformation ratio of the current transformer at the circuit breaker of the power supply outlet of 630kVA transformer as 50 / 5. In addition, in many cases, the power supply department often cannot provide the short-circuit capacity or system impedance of the power supply introduced to the user in the design, which makes it difficult to verify other conditions, which may be another reason for the improper selection of transformation ratio.

From the following analysis, we will find that when selected according to this principle, the transformation ratio is obviously too small to be adopted.

II 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 outgoing line of the circuit breaker remains unchanged under the maximum and minimum operation modes; (2) The current transformer is two-phase incomplete star wiring; (3) Gl-11 overcurrent relay is adopted for overload and quick break protection: (4) the operating power supply is DC 220V, and the opening form of circuit breaker is shunt tripping. Figure 1 shows the primary main wiring diagram of distribution transformer. The short-circuit capacity of two places B and C is considered as 200100 and 50MVA.

The influence of overload protection and current quick break protection of distribution transformer on transformation ratio is analyzed as follows:

1. Overload protection

Overload protection shall meet the following requirements:

IDZJ=Kk*Kjx*Kgh*11RT/（Kh*N）

Idzj --- action current of overload protection device; KK refers to the * coefficient, taken as 1.3; Kjx.---- Wiring coefficient, take 1; KGH - overload coefficient; KH - return coefficient of relay, taken as 0.85; a. For the distribution substation for civil buildings, the overcurrent multiple caused by motor self starting can generally not be considered. At this time, KGH is taken as 2. In order to meet the requirements of relay setting range, the minimum transformation ratio of current transformer shall be:

N = KK * kjx * KGH * 11rt / (KH * idzj) (setting range of gl-11 / 10 relay is 4 ~ 10a).

When KGH = 2, the minimum transformation ratio of current transformer with each capacity meeting the requirements of the above formula is as follows:

400kva11rt = 23a n = 7.0 = 40 / 5 = 8idzj = 9A

500kva11rt = 29a n = 8.9 takes 50 / 5 = 10idzj takes 9A

630kva11rt = 36.4a n = 11.1 = 75 / 5 = 15idzj = 8A

800kvai1rt = 46.2an = 14.1, 75 / 5 = 15idzj, 10A

1000kva11rt = 57.7a n = 17.6 takes 100 / 5 = 20idzj takes 9A

1250kva11rt = 72.2a n = 22.1 takes 150 / 5 = 30idzj takes 8A

1600kvai1rt = 92.4a n = 28.3 takes 150 / 5 = 30idzj takes 10A

Note: the transformation ratio selected according to the above table generally can not meet the requirements of instantaneous current multiple ns of current relay (see the following analysis for details).

b. For the factory power distribution station, it is generally necessary to consider the overcurrent multiple caused by the self starting of the motor. For the sake of reliability, at this time, KGH is taken as 3, obviously the transformation ratio of current transformer

To meet the requirements, the minimum value is n = = KK * kjx * KGH * L1RT / (KH * idzj).

When KGH = 3, the minimum transformation ratio of current transformer with each capacity meeting the requirements of the above formula is as follows:

400kva11rt = 23a n = 10.6 takes 75 / 5 = 15idzj takes 8A

500kva11rt = 29a n = 13.3 takes 75 / 5 = 15idzj takes 9A

630kva11rt = 36.4a n = 16.7 = 100 / 5 = 20 idzj = 9A

800kva11rt = 46.2a n = 21.2 take 150 / 5 = 30idzj take 8A

1000kva11rt = 57.7an = 26.5 takes 150 / 5 = 30idzj takes 9A

1250kva11rt = 72.2a n = 33.1 takes 200 / 5 = 40idzj takes 9A

1600kva11rt = 92.4an = 42.3 takes 250 / 5 = 50idzj takes 9A

By comparing the above data, it can be concluded that the transformation ratio selected according to the proportion of the calculated current at the primary side to the rated current at the primary side of the current transformer is generally less than the actual transformation ratio required

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