3 Main Measures for Differential Protection! Beginners' Notes

Differential protection is to input the current vector difference at both ends of CT (current transformer), and start the action element when the set action value is reached. The protection range is between the two ends of the input CT (it can be electrical equipment such as lines, generators, motors, transformers, etc.).

Current differential protection is a kind of protection in relay protection. The positive phase sequence is that A leads B, and B leads C by 120 degrees each. The anti-phase sequence (that is, the anti-phase sequence) is that A leads C, and C leads B by 120 degrees each. The reverse of the active direction is only the angle between the voltage and the current plus 180 degrees, which is the reversed phase power, not the reversed phase sequence.

Differential protection is based on the principle that "the sum of the currents flowing into the nodes in the circuit is equal to zero".

Differential protection regards the protected electrical equipment as a node, then the current flowing into the protected equipment is equal to the current flowing out normally, and the differential current is equal to zero. When the equipment fails, the current flowing into the protected equipment is not equal to the current flowing out, and the differential current is greater than zero. When the differential current is greater than the setting value of the differential protection device, the upper computer will alarm and protect the outlet, and trip the circuit breakers on each side of the protected equipment, so that the faulty equipment will be disconnected from the power supply.

1. Working principle of differential protection

Differential protection works by using Kirchhoff's current theorem. When the transformer works normally or has an external fault, it is regarded as an ideal transformer, and the current flowing into the transformer and the outgoing current (converted current) are equal. The differential relay no action. When the transformer fails internally, both sides (or three sides) provide short-circuit current to the fault point, and the sum of the secondary currents sensed by the differential protection is proportional to the current at the fault point, and the differential relay operates.

The principle of differential protection is simple, the amount of electricity used is simple, the protection range is clear, and the action does not need to be delayed. It has always been used for the main protection of transformers. In addition, differential protection includes line differential protection, bus differential protection and so on.

Transformer differential protection is the main protection against internal transformer faults. The wiring method, according to the principle of the loop current method, connects the secondary coils of the current transformers on both sides of the transformer to form a circulating current. If the transformer is in normal operation or an external fault, if the unbalanced current is ignored, there is no difference in the secondary loop arms of the two transformers. Current flows into the relay, ie: iJ=ibp=iI-iII=0.

If there is an internal fault, as shown in the "Differential Protection" point ZD is short-circuited, the current flowing into the relay is equal to the total current of the short-circuit point. Namely: iJ=ibp=iI2+iII2. When the current flowing into the relay is greater than the operating current, the protective operating circuit breaker trips.

2. The function of differential protection

Differential protection is the main protection of the transformer, which is installed according to the principle of circulating current.

It is mainly used to protect various phase-to-phase short-circuit faults that occur inside the windings of double-winding or three-winding transformers and their lead-out lines, and can also be used to protect single-phase inter-turn short-circuit faults of transformers.

The current transformers are installed on both sides of the winding transformer, and the secondary side is wired according to the circulating current method, that is, if the same polarity ends of the current transformers on both sides are facing the bus side, the same polarity terminals are connected, and in A current relay is connected in parallel between the two wires. The current flowing in the relay coil is the difference between the secondary currents of the current transformers on both sides, that is to say, the differential relay is connected to the differential circuit.

Theoretically speaking, during normal operation and external faults, the differential loop current is zero. In fact, due to the fact that the characteristics of the current transformers on both sides cannot be completely consistent, during normal operation and external short circuit, there is still an unbalanced current Iunb flowing in the differential circuit, and the current IK flowing through the relay at this time is Ik=I1 -I2=Iunb

It is required that the unbalanced current should be as small as possible to ensure that the relay will not malfunction.

When a phase-to-phase short circuit fault occurs inside the transformer, in the differential circuit because I2 changes direction or is equal to zero (no power supply side), the current flowing through the relay is the sum of I1 and I2, that is

Ik=I1+I2=Iunb

It can make the relay operate reliably.

The scope of transformer differential protection is the electrical equipment between the current transformers that constitute the transformer differential protection, and the wires connecting these equipment. Since the differential protection will not act on faults outside the protection zone, the differential protection does not need to cooperate with the protection of adjacent components outside the protection zone in terms of action value and action time limit, so it can act instantaneously when there is a fault in the zone.

Differential protection is a protection device that operates by reflecting the current difference on both sides of the protected element (or area). Differential protection is to protect the internal short-circuit fault of the transformer. The current transformer is installed on both sides of the transformer. In normal load conditions or when an external short circuit occurs, the current flowing into the differential relay is an unbalanced current. When the current mutual inductance on both sides is properly selected Under the conditions of the transformation ratio of the transformer and the wiring method, the unbalanced current value is very small and is smaller than the operating current of the differential protection, so the protection does not operate; when a short circuit occurs inside the transformer, the current flowing into the relay is greater than the operating current of the differential protection Operating current, differential protection operates on tripping.

Because the primary and secondary currents and voltages of the transformer are different, the phases are different, the characteristics of the current transformer are different, and the excitation current on the power supply side will cause unbalanced current to flow through the relay, corresponding measures must be taken to eliminate the influence of unbalanced current.

3. Main measures of differential protection

3.1 Reduce the unbalanced current in steady state
For the current transformers used on each side of the differential protection of the transformer, the D-class current transformers dedicated to the differential protection of the transformer are selected; when the external maximum steady-state short-circuit current passes, the secondary load of the differential protection circuit must meet the tolerance of 10% error Require.

3.2 Reduce the secondary load of the current transformer
This is actually equivalent to reducing the terminal voltage on the secondary side, and correspondingly reducing the excitation current of the current transformer. Common ways to reduce the secondary load include: reducing the resistance of the control cable (properly increasing the cross-section of the wire and shortening the length of the control cable as much as possible); using a current transformer for weak current control (the secondary rated current is 1A), etc.

3.3 Using a current transformer with a small air gap
The remanence of the iron core of this current transformer is small, and the current transformer is not easy to saturate under the condition of large primary side current. Therefore, the excitation current is small, which is beneficial to reduce the unbalanced current. At the same time, the transient characteristics of the current transformer are also improved.

Ratio differential protection is a type of differential protection.

Differential protection needs to adopt the principle of ratio differential: to prevent the inconsistency of CT transmission characteristics on the high and low voltage sides when the transformer is faulted outside the area (through fault), resulting in the generation of differential current, and the action exceeds the fixed value. After the differential protection of ratio braking, with the increase of the through current, the threshold of differential start will be raised to ensure that the through fault will not malfunction.