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It is protected for the main protection of the voltage transformer that the voltage transformer is differential, it determines the exactness of CT wiring is very important to protect operation principle, do not lack and can’t protect the situation of the normal work to cause in the real work by differential return circuit wiring mistake. So necessary analysis is differential protects receiving the law correctly of CT wiring, find out the law, offer basis on which the wiring after constructing and constructing live is adjusted. The following discussions are on the basis of the following prerequisite:

The voltage transformer polarity is correct;

CT own polarity is correct, L1 end of the same name is K1;

The intersection of CT and two wiring looks correct return circuit, namely have A, situation that C meet by mistake at son, end of switching.

The voltage transformer is differential to protect a kind of correct wiring diagram which receives the law and is shown as in Fig. 1: Use I AY shows the star-shaped side A looks of the voltage transformer once electric current, I Ay expresses electric current two times of A looks, I Ay’ shows that flows into the differential electric current which protects the relay.

Use I a Show the triangular side a looks of the voltage transformer once electric current, I a ‘ express electric current two times of a looks. According to the intersection of Fig. and 1 illustrated the intersection of electric current and direction, as follows,

I ay’ = I by – I ay

I by’ = I cy – I by

I cy’ = I ay – I cy

The looks scalogram is shown as in Fig. 2, find out from Fig. 2, the three-phase both sides electric current, against the looks, have I CD =I ay’ I a ‘ =0. Certainly if the electric current direction that the star-shaped side of voltage transformer flows into the relay fetches the reverse direction, the three-phase both sides electric current is the same as the looks, and still have at this moment I CD = I a ‘ – I ay’ =0.

First kind of situation. Receive the law of the star-shaped side CT of voltage transformer to publish instead, such as Fig. 3a Shown. Have I ay’ = I ay – I cy

I by’ = I by – I ay

I cy’ = I cy – I by

Looks scalogram such as Fig. 3b Show, it is obvious, link with b the intersection of A and looks and the other side the intersection of c and looks, the intersection of B and looks and the intersection of a and looks, C, there is ICD =0.

The second kind of situation, the law transposition draw the end and reverse too of connecting the triangle above ,Such as Fig. 4a Shown. One is as follows,

I ay’ = I cy – I ay

I by’ = I ay – I by

I cy’ = I by – I cy

It is obvious, compare with first kind of situation just against the looks.

The third kind of situation, CT receives the law to publish instead, such as Fig. 4b Shown. One is as follows,

I ay’ = I ay – I by

I by’ = I by – I cy

I cy’ = I cy – I ay

It is obvious, what recommended connected the law against the looks correctly with the hereinbefore.

The law summarized is as follows:

Transposition phase place instead Draw the end at the same time conversely ; It needs to adjust the looks A and c, B and a, C and b to happen instead . And then come and see CT polarity is marked, can be regarded as L1 and K1 and can be regarded as L2 and K2. Will according to of the same name to bring in, connect, CT two electric current, against looks, for the intersection of voltage transformer and star-shaped side then, one is as follows,

I ay’ = I ay – I by

I by’ = I by – I cy

I cy’ = I cy – I ay

Can be seen and flow into the electric current of the relay against the looks too, it is certainly against the looks that CT star connects the law. So there are the following conclusions: Of the same name the intersection of end and transposition, instead phase place; Both sides are against the looks, function is the same as. For this reason it is necessary to propose a kind of standard receives the law, uses the above-mentioned laws to carry on the change on this basis. The standard connects the law to narrate as follows:

Connect L1 close to the side of voltage transformer, K1 appears.

The star-shaped side AK1 of voltage transformer is connected with BK2, BK1 connects with CK2, CK1 connects with AK2.