Schematic diagram of double-loop current balance protection

Current balance protection is another form of cross-link differential protection, which works by comparing the absolute value of the current in the double circuit, as shown in Figure 18. The current balance relay KBL1, KBL2 each has a working coil turn Nw, a brake coil turn NB and a voltage coil turn Nv.

The working coil of the KBL1 is connected to the secondary side of the line L-1 current transformer, generating action moment Mw1, The brake coil is connected to the secondary side of the line L-2 current transformer, and the action moment is generated by the current I1 MB1.

The working coil of KBL2 is connected to the secondary side of the line L-2 current transformer, and the I2 generates action torque Mw2, The brake coil is connected to the secondary side of the line L-1 current transformer and the action moment MB2. by I1  The voltage coils of the KBL1, KBL2 are connected to the secondary side of the bus voltage transformer. The operating condition of the relay is the Mw> MB + Mv (the Mv is the torque generated in the voltage coil).

During normal operation and external short circuit, the II=I2,KBL1, KBL2 keeps the contact disconnected due to its reaction moment Mv and the spring reaction moment Ms in the relay.

When a primary line fails (e. g. the K point of the line L-1), Due to the II> I2, And due to a greatly reduced voltage, The reaction torque of the voltage coil is significantly reduced, Thus the sum of the operating moment Mw1 generated by II in KBL1 is greater than the braking torque MB1 generated by I2 and the braking torque Mv generated by the voltage, So KBL1 action, Remove the fault line L-1;

For the KBL2, Because the current II flowing through its brake coil is greater than the current I2, flowing through the working coil That is, the braking moment is greater than the action moment, So it doesn't move.

It must be noted that on the double circuit of single-end power supply, balance protection can only be installed on the power transmission side and the receiving side. For any short circuit, the current flowing through the two balance relays on the receiving side is equal and the protection will not work.

Due to the double loop parallel line cross link differential protection and balance protection, in the short circuit near the opposite exit, the current, the current difference value of the current is not enough to start protection.

Only by waiting for the opposite protection action, removing the fault, the non-fault line current on the side drops to zero, can the fault line current start the side of the protection, remove the fault line. This situation is known as a successive action. The size of the successive action area on the line is related to the protection setting value and the short-circuit current.

Cross connection differential protection, the direction relay is connected to the bus voltage. When the three-phase short circuit at the parallel line outlet, the voltage is zero. If the voltage circuit of the direction relay has no good memory effect, it will be mistakenly moved, called the voltage dead area.

Figure 18 Schematic diagram of current balance protection

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