The differential protection of power transformers is the main protection for a 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 dual or triple winding transformers and their outgoing lines, and can also be used to protect single-phase turn to turn short circuit faults in transformers. Current transformers are installed on both sides of the winding transformer, and their secondary side is wired 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 a current relay is connected in series between the two wires.
The main protection of the transformer reflects internal and external faults, and the protection acts on the switch to disconnect the transformer from the system. But the response to a few inter turn short circuits in the winding is not as good as gas protection.
Working principle of the differential protection
The current flowing through the relay coil is the secondary current difference between the current transformers on both sides, which means that the differential relay is connected to the differential circuit. In theory, during normal operation and external faults, the differential circuit current is zero. In fact, due to the fact that the characteristics of the current transformers on both sides cannot be completely consistent, there is still an unbalanced current Iumb flowing through the differential circuit during normal operation and external short circuits. At this time, the current IK flowing through the relay is Ik=I1-I2=Iumb, and the unbalanced current should be as small as possible to ensure that the relay does not malfunction. When a phase to phase short circuit fault occurs inside the transformer, due to I2 changing direction or equal to zero (without power supply side) in the differential circuit, the current flowing through the relay is the sum of I1 and I2, that is, Ik=I1+I2=Iumb, which can ensure reliable operation of the relay. The scope of transformer differential protection includes the electrical equipment between the current transformers that make up the transformer differential protection, as well as the wires connecting these equipment. Due to the fact that differential protection does not act on faults outside the protection zone, it does not need to cooperate with adjacent component protection outside the protection zone in terms of action values and time limits. Therefore, in the event of an internal fault, differential protection can act instantaneously.
Main features of differential protection
The Existence of Transformer Excitation Inrush Current
The excitation current of the transformer only flows through one side of the transformer, so it will react to the differential circuit through the current transformer and form an unbalanced current. During 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 a decrease in voltage. So the unbalanced current generated in both cases is very small and has little impact on the differential protection of the transformer.
However, when the transformer is put into operation without load and the voltage is restored after external faults are removed, there may be a large excitation current, namely excitation inrush current. The existence of this phenomenon is caused by the saturation of the transformer core and the presence of residual magnetism. The specific analysis is as follows:
When the secondary side is open and the primary side is connected to the power grid, the equation for the primary circuit is
U1=umcos (wt+ α)= I1R1+N1d φ/ Dt (1)
U1: Primary voltage,
Um: peak value of primary voltage,
α： The initial phase angle of the voltage at the moment of closing,
R1: Resistance of the primary winding of the transformer,
N1: Number of turns in the primary winding of the transformer,
φ： Transformer primary side magnetic flux.
Due to the relatively small size of i1R1, it can be ignored during the initial stage of analyzing transient processes
therefore
Umcos (wt+ α)= N1d φ/ Dt
D φ= (um/N1) cos (wt+ α) Dt
Integral
φ= (um/N1) sin (wt+ α)+ C
φ=φ M sin (wt+ α)+ C φ M is the peak magnetic flux, and c is the integral constant.
When t=0, there is no residual magnetism in the iron core, φ= 0 so c=- φ MSIN α
So the no-load closing magnetic flux is
φ=φ M sin (wt+ α) -φ MSIN α (2)
The magnitude of no-load closing magnetic flux and the initial phase angle of voltage can be obtained from equation (2) α Regarding considering the most unfavorable situation
When α= At 900, the voltage crosses zero
φ=φ M sin (wt+900)- φ M= φ Mcoswt- φ M
Magnetic flux has two components, the periodic component φ Mcoswt and non periodic components φ m. At this point, the maximum value of magnetic flux is twice that of steady-state magnetic flux. If the influence of residual magnetism is also considered, this value should be even larger.
We know that transformers normally operate near the knee point of the magnetization curve of the iron core, when the iron core is close to or slightly saturated. At this point, the excitation current of the transformer increases significantly, reaching 6-8 times the rated current. Due to the excitation current only appearing on one side of the transformer, a large unbalanced current will be generated in the differential relay. Subsequently, due to the presence of R1, the non periodic component will decay, φ The value will decrease.
In summary, the magnitude and decay time of excitation inrush current are related to the applied voltage, the magnitude and direction of residual magnetism in the iron core, circuit impedance, transformer capacity, and the properties of the iron core. For three-phase AC transformers, due to a 120 ° difference between the three phases, at least two different excitation inrush currents may occur during any instantaneous closing.
The wiring methods of the windings on each side of the transformer are different
Among the five standard connection groups for transformers specified in China, 35kV Y/D-11 dual winding transformers are often used. The current difference on both sides of the transformer in this connection method is 30 °. To ensure that the differential protection does not malfunction, it is necessary to adjust the wiring and transformation ratio of the CT secondary circuit, so that the difference in CT secondary current on the power and load sides is 180 ° and of equal magnitude. This can eliminate the impact of Y/D-11 transformer wiring on differential protection.
To achieve the above purpose, the TA used for transformer differential protection should be wired as shown in Figure 3
The calculated transformation ratio of the current transformer is different from the actual transformation ratio
Due to the fact that the current transformers on both sides of the transformer are selected according to the standard conversion ratio in the product catalog, and the conversion ratio of the transformer is also certain, the three cannot accurately meet the requirement of nLy/nLd=nT. At this point, unbalanced current flows through the differential circuit, causing the protection device to malfunction. So usually, the balance coil of the differential relay is used to eliminate or reduce this difference. By using a balance coil to compensate for the difference between the actual transformation ratio and the ideal value, the current difference between the two arms is close to zero, thereby eliminating or minimizing the unbalanced current as much as possible.
The models of current transformers on both sides are different
If the transformer models on both sides of the transformer are different, their saturation characteristics and excitation current (reduced to the same side) will also be different. Therefore, the current difference generated between the two arms is relatively large, which will affect the operation of the protection. Therefore, a current transformer with the same type coefficient of 1 should be used.
Transformer load adjustment tap
The tap of a load regulating transformer is a method of adjusting voltage in the power system by using a load regulating transformer. Changing the tap is changing the transformer’s ratio, which will generate significant unbalanced current for the adjusted differential protection device. Due to the fact that transformer on-load voltage regulation is continuously regulated with load, and differential protection cannot be adjusted with power, this factor must be taken into account during the tuning process.
Main reasons of differential protection
1. The secondary side load cannot meet the requirements of the CT10% error curve when flowing through short-circuit current. When the capacity of the current transformer is changed or the newly installed protection is put into operation, it cannot be ignored whether the 10% error curve of the CT used for nuclear protection meets the requirements based on the maximum short-circuit current passing through the transformer during a short-circuit fault in the differential protection zone and the measured secondary load of the differential circuit. Ensure that the CT is within a 10% error range. If the 10% error curve requirement of CT is not met, due to the insufficient capacity of CT to provide the required secondary load, the differential protection may refuse to operate or malfunction during faults, which directly affects the reliability of the differential protection. At this point, the CT transformation ratio should be appropriately increased and the 10% error curve of the nuclear CT should be recalculated until it meets the requirements.
2. Improper grounding method of differential protection secondary current circuit
When the secondary current circuit of differential protection is grounded, the secondary current circuits of TA on each side must be connected to the grounding grid through one point, because the grounding network of the substation is not absolutely equipotential and there is a certain potential difference between different points. When a short circuit fault occurs, a large amount of current flows into the grounding grid, and the potential difference between each point is significant. If the secondary current circuit of differential protection is grounded at different points in the grounding grid, the current generated by the potential difference between them will flow into the protection device, affecting the accuracy of the differential protection device’s operation and even causing it to malfunction. So the secondary current circuits of each side CT should be connected in parallel to the differential current circuit of the protection device, and all current circuits must be grounded at the common point of the parallel connection.
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