AG10 battery.Causes and causes of unbalanced current in transformer differential protection

　　Causes and causes of unbalanced current in transformer differential protection

　　1 Introduction

　　Transformer differential protection is based on the principle of circulating current. Double winding transformer with current transformers installed on both sides. When the current transformers on both sides have the same polarity in the same direction, connect the secondary terminals of different polarities of the current transformers on both sides (if the terminals of the same polarity are placed close to the busbar, the secondary sides will have the same polarity. connected), the working coil of the differential relay is connected in parallel to the secondary terminal of the current transformer. During normal operation or external fault, the secondary currents on both sides are equal in magnitude and opposite in direction. The current in the relay is equal to zero, so the differential protection does not operate. However, due to various unbalanced currents caused by the actual operation of the transformer, the operating current of the differential relay increases, thereby reducing the sensitivity of the protection.

　　2 reasons

　　The generation of unbalanced current has two aspects: steady state and transient state. Reasons for the steady-state unbalanced current: (1) The high and low voltage side windings of the transformer are connected in different ways; (2) The models and transformation ratios of the current transformers on each side of the transformer are different; (3) The transformer transformation ratio is caused by the on-load tap adjustment changes. The transient unbalanced current is mainly caused by the excitation inrush current generated when the transformer is put into power without load or external fault is removed, and the voltage is restored.

　　3Impacts and preventive measures

　　The following explains the causes and preventive measures of unbalanced currents in the above types of transformer differential protection.

　　3.1 The effects and preventive measures of different wiring methods of high and low voltage side windings of transformers:

　　3.1.1 The influence of transformer wiring group on differential protection

　　For the transformer with Y and y0 connections, since the voltages of the corresponding phases of the primary and secondary windings are in the same phase, the phases of the corresponding phases on both sides of the primary and secondary windings are almost identical. For commonly used transformers with Y and d11 connections, since the line voltages on the triangle side are 30° different in phase, the current phase relationship of the corresponding phases is also 30° different, that is, the triangle side current is larger than the same phase current on the star side. It is 30° ahead in phase, so even if the values of the secondary currents of the current transformers on both sides of the transformer are equal, unbalanced currents will appear in the differential protection circuit.

　　3.1.2 Preventive measures for the influence of transformer wiring groups

　　In order to eliminate the influence of unbalanced current caused by the Y and D11 wiring of the transformer, the phase compensation method can be used, that is, the secondary side of the current transformer on the star side of the transformer is connected into a triangle, and the secondary side of the current transformer on the triangle side of the transformer is connected into a triangle. The side is connected in star shape to correct the phase of the secondary current of the current transformer. After phase compensation, in order to make the current values of the two differential arms of each phase approximately equal, when selecting the transformation ratio nTA of the current transformer, the wiring coefficient KC of the current transformer should be considered, that is, the current of the differential arm is KCI1/nTA. . Among them, I1 is the primary current. When the current transformer is connected in star shape, KC=1, and when it is connected in delta shape, KC=√3. For example, when the secondary current of the current transformer is 5A, the transformation ratio of the current transformer on both sides is Choose from the two options below.

　　The current transformer ratio on the star side of the transformer is:

　　nTA(Y)=√3In(Y)/5

　　The current transformer ratio on the delta side of the transformer is:

　　nTA(△)=In(△)/5

　　In the formula, In(Y) transformer winding is connected to the rated current of the star side;

　　In (△) The rated current of the transformer winding connected to the delta side.

　　In fact, when selecting a current transformer, a standard transformation ratio that is close to and slightly larger than the calculated value is determined based on the current transformer finalized product transformation ratio (the following table lists the calculations for a 15MVA38.5kV/6.3kV main transformer in our factory) .

　　3.2 The influence and preventive measures of the current transformer model and transformation ratio on each side of the transformer

　　The rated voltages on both sides of the transformer are different, and the models of current transformers installed on both sides are different. As a result, their saturation characteristics and excitation currents (reduced to the same side) are also different. Therefore, an external short circuit will cause a large unbalanced current, which can only be considered by appropriately increasing the protection operating current. Since current transformers are all standardized products, the actual transformation ratio is generally not completely consistent with the calculated transformation ratio, and the transformation ratios of each transformer cannot be exactly the same. Therefore, it will cause inconsistencies in the differential protection circuit. Balance current. This kind of unbalanced current caused by the improper selection of the transformation ratio can be eliminated by setting up a balance coil in the differential relay using the principle of magnetic balance. Generally, the balance coil is connected to the side of the protection arm with a small current, because the balance coil and the differential The moving coil is wound together on the middle magnetic column of the relay. The number of turns of the balance coil is appropriately selected so that the magnetic potential generated by it offsets the magnetic potential generated by the differential current in the differential coil. In this way, the secondary winding does not The electric potential will be induced, and the actuator of the differential relay will have no current. However, pay attention to the polarity when wiring. The magnetic potential generated by the small current side in the balance coil and the differential current in the differential coil should be opposite.

　　3.3 The impact and preventive measures of changing the tap during operation with load voltage regulation

　　In power systems, the method of adjusting the transformer tap is usually used to maintain a certain voltage level (due to the change of the tap, the transformation ratio of the transformer also changes). However, the selection of the current transformer ratio in differential protection and the determination of the differential relay balance coil can only be calculated and adjusted based on a certain transformer ratio to balance the differential circuit. When the transformer tap is changed, the balance is destroyed and a new unbalanced current appears. This unbalanced current is proportional to the primary current, and its value is

　　Ibp=±△UID.max/nTAA

　　In the formula, ±△U——the maximum variation range of the voltage regulating tap relative to the rated tap position

　　ID.max——The maximum external fault current through the voltage regulating side.

　　In order to avoid the influence of unbalanced current, corresponding consideration should be given when setting the operating current of the protection, that is, increasing the operating setting value of the protection.

　　3.4 Effects and preventive measures of transformer excitation inrush current

　　3.4.1 Impact of transformer excitation inrush current on differential protection

　　The high and low voltage sides of the transformer are connected electromagnetically, so the excitation current only exists on one side of the power supply, which forms part of the unbalanced current in the differential circuit through the current transformer. Under normal operating conditions, its value is very small, generally not exceeding 3% to 5% of the rated current of the transformer. When an external short-circuit fault occurs, since the bus voltage on the power side decreases and the excitation current is smaller, the impact of the unbalanced current on differential protection in these cases generally does not need to be considered. When the transformer is put into power without load or the voltage is restored after an external fault is removed, the magnetic flux in the transformer core increases sharply, causing the core to be instantly saturated. At this time, a large impact excitation current (up to 5 to 10 times the rated current), usually called magnetizing inrush current. The waveform of the excitation inrush current is as shown below:

　　It can be seen from the figure that the excitation inrush current IE contains a large number of non-periodic components and high-order harmonics. Therefore, the excitation inrush current is no longer a sine wave, but a spire wave, and is completely biased to one side of the time axis at the first instant. The size and attenuation speed of the excitation inrush current are related to the phase of the applied voltage at the moment of closing, the size and direction of the residual magnetism in the iron core, the power supply capacity, the capacity of the transformer and the iron core material and other factors. For a single-phase double-winding transformer, under the same conditions, when the instantaneous value of the voltage is zero, the excitation current is the largest; if the instantaneous value of the voltage is the maximum, there will be no excitation inrush current, but only Normal excitation current. For a three-phase transformer, no matter it is closed at any moment, at least two phases will have varying degrees of excitation inrush current. At the initial moment, the excitation inrush current attenuates very quickly. For ordinary small and medium-sized transformers, its value does not exceed 0.25 to 0.5 times the rated current after 0.5 to 1S. The attenuation speed of the excitation inrush current in large power transformers is slower, and it attenuates to The above value is about 2~3S. That is to say, the larger the transformer capacity, the slower the attenuation, and it will take tens of seconds for complete attenuation. According to experimental and theoretical analysis results, it is known that the excitation inrush current contains a large number of high-order harmonic components, of which the second harmonic component accounts for the largest proportion, about 60% or more. The harmonic components above the fourth are very small. In the first few cycles, the waveform of the excitation inrush current is discontinuous (that is, there is an interruption angle between the two waveforms), with an angle of 120 in each cycle. ~180. The minimum discontinuity angle is not less than 80. ~100. [See lower left image (b)]. In addition, the multiple of the excitation inrush current to the rated current amplitude is related to the transformer capacity. The larger the capacity, the smaller the inrush current multiple of the transformer.

　　3.4.2 Measures to reduce the influence of excitation inrush current in transformer differential protection

　　To prevent the influence of excitation inrush current, the use of BCH type relays with fast saturation converters is a method widely used in China. When an external fault occurs, the maximum unbalanced current containing non-periodic components can unilaterally saturate the iron core of the fast saturation converter quickly, causing the transmission performance to deteriorate, making it difficult for the unbalanced current to be transmitted to the differential relay. on the differential coil to ensure that the differential protection will not malfunction. During an internal fault, although the current in the primary coil of the fast saturation converter also contains a certain non-periodic component, it attenuates quickly. Generally, it will be attenuated after 1.5 to 2 cycles. After that, the current passing through the primary coil of the fast saturation converter will It is a completely periodic short-circuit current, so a large induced electromotive force is generated in the secondary coil, and the corresponding current in the actuator is also large, so that the relay can act sensitively. The fast saturation converter uses its easy saturation performance to avoid the influence of the non-periodic component of the external short-circuit unbalanced current of the transformer and the no-load closing excitation inrush current.

　　In addition, the following measures can be taken to reduce the excitation inrush current:

　　3.4.3 Use the difference between the internal short-circuit current and the excitation inrush waveform (with or without discontinuity angle) to avoid the excitation inrush current.

　　That is, the discontinuity angle identification method. This method differentiates the differential current, and then performs full-wave rectification on the differentiated current. The length of time the rectified waveform exists at the action setting value is used to determine whether it is an internal fault or an excitation inrush current.

　　3.4.4 Use second harmonic braking.

　　The protection device uses the second harmonic component to brake when the transformer is put into operation without load and the external fault removal voltage is restored; when there is an internal fault, it uses the fundamental wave; when there is an external fault, it uses the proportional braking circuit to avoid the unbalanced current.

　　4 Conclusion

　　To sum up, in order to ensure the selectivity of differential protection action, the operating current of the differential relay must avoid the maximum unbalanced current. The smaller the unbalanced current, the higher the sensitivity of the protection device, thereby ensuring the safe and stable operation of the transformer.

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