Causes of Misoperation of Differential Protection of Transformer

The differential protection of the transformer 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 connected according to the circulating current method, that is, if the same-level terminals of the current transformers on both sides are facing the bus side, the same-level terminals are connected, and in A current relay is connected in series between the two wires.

The main protection of the transformer reflects the internal and external faults of the transformer, and the protection acts on the switch to separate the transformer from the system. However, it is not as good as gas protection for a small number of inter-turn short-circuits in the winding.

1. The principle of differential protection of transformers

The differential protection of the transformer 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 connected according to the circulating current method, that is, if the same-level terminals of the current transformers on both sides are facing the bus side, the same-level terminals are connected, and in A current relay is connected in series between the two wires. The current flowing in the relay coil is the secondary current difference 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 Iumb flowing in the differential circuit. At this time, the current IK flowing through the relay is Ik=I1 -I2=Iumb requires 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=Iumb energy 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 of transformer will not act on faults outside the protection zone, the differential protection of transformer 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.

2. Main features of differential protection of transformers

Existence of transformer excitation inrush current

The transformer excitation current (excitation current) only flows through one side of the transformer, so it will react to the differential circuit through the current transformer to form an unbalanced current. In steady state operation, the excitation current of the transformer is not large, only 2-5% of the rated current. When a fault occurs outside the differential range, the excitation current decreases due to the voltage drop. Therefore, the unbalanced current formed in these two cases is very small, and has little influence on the differential protection of the transformer.

However, when the transformer is turned on with no load and the voltage recovers after the external fault is removed, a large excitation current, that is, an excitation inrush current, may appear. The existence of this phenomenon is caused by the saturation of the transformer core and the existence of residual magnetism. The specific analysis is as follows:

When the secondary side is open and the primary side is connected to the grid, the equation of the primary circuit is

u1=umcos(wt+α)=i1R1+N1dφ/dt (1)

u1: primary voltage,

um: peak value of primary voltage,

α: The initial phase angle of the voltage at the instant of closing,

R1: The resistance of the primary winding of the transformer,

N1: the number of turns of the primary winding of the transformer,

φ: Transformer primary flux.

Since i1R1 is relatively small, it can be ignored in the initial stage of analyzing the transient process

so

umcos(wt+α)= N1dφ/dt

dφ= (um/ N1) cos(wt+α) dt

points, get

φ=(um/ N1) sin(wt+α)+c

φ=φm sin(wt+α)+c φm is the main flux peak value, and c is the integral constant.

Assuming that the iron core has no residual magnetism, when t=0, φ=0, so c=-φmsinα

So the no-load closing flux is

φ=φm sin(wt+α) -φmsinα (2)

From the formula (2), it can be obtained that the size of the no-load closing magnetic flux is related to the initial phase angle α of the voltage. Considering the most unfavorable situation

When α=900, the voltage crosses zero

φ=φm sin(wt+900) -φm=φmcoswt-φm

The magnetic flux has two components, the periodic component φmcoswt and the non-periodic component φm. At this time, the maximum value of the magnetic flux is twice that of the steady-state magnetic flux. If the influence of remanence is considered at the same time, this value will be even greater.

We know that the transformer normally works near the knee point of the magnetization curve of the iron core, and the iron core is close to or slightly saturated at this time. At this time, the excitation current of the transformer increases greatly, which can reach 6~8 times of the rated current. Since the excitation current only appears on one side of the transformer, a large unbalanced current will be generated in the differential relay. After that, due to the existence of R1, the non-periodic component will attenuate, and the value of φ will decrease.

To sum up, the magnitude and decay time of the inrush current are related to the applied voltage, the magnitude and direction of the remanence of the iron core, the loop impedance, the capacity of the transformer and the nature of the iron core. For the three-phase AC transformer, since the phase difference between the three phases is 120°, at least two phases have different excitation inrush currents at any moment of closing.

The wiring of the windings on each side of the transformer is different

Among the five standard connection groups of transformers stipulated in our country, 35kV Y/D-11 double-winding transformers are often used. The current difference between the two sides of the transformer in this connection method is 30°. In order to prevent the differential protection of transformer from malfunctioning, it is necessary to try to adjust the wiring and transformation ratio of the CT secondary circuit so that the phase difference between the CT secondary current on the power supply side and the load side 180° and equal in size. In this way, the influence of Y/D-11 transformer wiring on differential protection of transformer can be eliminated.

In order to achieve the above purpose, the TA used for differential protection of the transformer should be connected as shown in Figure 3

The calculated transformation ratio of the current transformer is different from the actual transformation ratio
Since the current transformers on both sides of the transformer are selected according to the standard transformation ratio of the product catalog and the transformation ratio of the transformer is also fixed, the three cannot accurately meet the requirements of nLy/nLd=nT. At this time, there is an unbalanced current flowing in the differential circuit, which causes the protection device to malfunction. Therefore, the balance coil of the differential relay is usually used to eliminate or reduce this difference. That is, the balance coil is used to make up the difference between the actual transformation ratio and the ideal value, so that the current difference between the two arms is close to zero, thereby eliminating or minimizing the unbalanced current.

Different types of current transformers on both sides
If the types of transformers on both sides of the transformer are different, their saturation characteristics and excitation current (referred to the same side) will also be different. Therefore, the current difference generated in the two arms is relatively large, which will affect the action of the protection, so a transformer with the same type coefficient of the current transformer as 1 should be used.

Transformer load adjustment tap
The tap of the transformer with load adjustment is a method of adjusting the voltage by using the transformer with load voltage regulation in the power system. To change the tap is to change the transformation ratio of the transformer, which will generate a large unbalanced current for the adjusted differential protection of transformer device. Since the on-load voltage regulation of the transformer is continuously adjusted with load, and the differential protection of transformer cannot be adjusted with power on, so this factor must be taken into account when setting.

3. Causes of misoperation of differential protection of transformers

3.1 The secondary side load cannot meet the requirements of the CT10% error curve when the short-circuit current flows. When the capacity of the current transformer connected to the system changes or the newly installed protection is put into operation, the maximum short-circuit current passing through the transformer during a short-circuit fault in the differential protection of transformer zone and the measured secondary load of the differential circuit cannot be ignored. Whether the % error curve meets the requirements. Make sure CT is within 10% error. If the 10% error curve requirement of CT is not met, since the capacity of CT is not enough to provide the requirements required by the secondary load, the differential protection of transformer may refuse to operate in the event of a fault, and the malfunction will directly affect the reliability of the differential protection of transformer. At this time, the CT transformation ratio should be appropriately increased, and the 10% error curve of nuclear CT should be re-compare until it meets the requirements.

3.2 Improper grounding method of the secondary current loop of differential protection
When the secondary current loop of the differential protection of transformer is grounded, the secondary current loops of TA on each side must be connected to the ground network through one point, because the grounding networks of the substation are not absolutely equipotential, and there is a certain potential difference between different points. When a short-circuit fault occurs, a large current flows into the ground grid, and the potential difference between each point is large. If the secondary current loop of the differential protection of transformer is connected to different points of the ground grid, the current generated by the potential difference between them will flow into the protection device, affecting the accuracy of the differential protection of transformer device's action and even causing it to malfunction. Therefore, the secondary current loops of CTs on each side should be connected in parallel and then connected to the differential current loop of the protection device, and all current loops must be grounded at the parallel common point.