What are the causes of unbalanced current in transformer
Writer： Hengfeng you electric Time：2023-05-15 views：times
Power Transformer differential protection is the main protection of electric transformer. Generally, longitudinal differential protection shall be installed above 6300kV, which is composed according to the principle of circulating current. Two winding transformers are equipped with current transformers at both ends. Under the condition of ambition, when the same polarity of the two current transformers is in the same direction, the secondary terminals of the two current transformers with different polarities shall be connected. (if the same polarity terminals are placed near the bus side, the secondary side shall be connected with the same polarity.) The working coil of the differential relay is connected in parallel on the secondary terminal of the current transformer. During normal operation or faults outside the area, the secondary currents at both ends are equal in size and opposite in direction, and the current passing through the relay is equal to zero. Therefore, the differential relay does not act, but in practice, due to the existence of various unbalanced currents, The time keeping requirement of differential protection can avoid the unbalanced current flowing through the differential circuit. Therefore, it is necessary to analyze the causes and elimination methods of unbalanced current of differential protection.
1. Causes of unbalanced current:
The main causes of unbalanced current are: ⑴ magnetizing inrush current of transformer. ⑵ the current phases at both ends of the transformer are different. (3) the difference between accounting transformation ratio and practice transformation ratio. (4) the types of current transformers at both ends are different. (5) transformer on load adjustment tap.
2. Influence on differential protection and elimination methods:
(1) unbalanced current generated by transformer inrush current
The inrush current of the transformer only flows through one side of the transformer. Therefore, it can not be balanced in the differential drop through the current transformer. Under normal working conditions, this current is very small and generally does not exceed 2-10 of the rated current. In case of external disadvantages, due to the reduction of voltage and inrush current, its influence is very small. However, when the transformer is put into no-load operation and external defects are removed and restored, the excitation current will increase sharply due to the severe fullness of the iron core of the transformer. At this time, there is a large excitation current, which can reach 5-10 times of the rated current. (commonly known as inrush current) inrush current contains many aperiodic components and high-order harmonic components (mainly second harmonic). It is not a sine wave. It is a spike wave, which tends to bias the current to one side of the time axis. The magnitude and attenuation speed of magnetizing inrush current, the phase of applied voltage at the moment of closing, the magnitude and direction of residual magnetism in iron core, the magnitude of current capacity, loop impedance, transformer capacity and iron core properties are related. For example, just when the instantaneous voltage value is Zui large, there will be no inrush current, but only the normal excitation current. For three-phase transformer, at least two phases have different degrees of magnetizing inrush current at any moment.
Methods to reduce the influence of inrush current in transformer differential protection: to prevent the influence of inrush current, BCH relay with fast full converter is widely used in China. In case of external defects, the Zui large unbalanced current containing aperiodic component can make the iron core of the fast full converter plump on a single surface quickly, making it difficult for the unbalanced current to be transmitted to the differential coil of the differential relay, so as to ensure that the differential protection will not misoperate. The internal disadvantage is that although there are aperiodic components in the primary coil of the fast full converter, its attenuation speed is appropriate and fast. Generally, the attenuation ends after 2 cycles, and all the periodic short-circuit current passes through the converter in the future, so the relay can act sensitively. ⑵ distinguish the difference between short-circuit current and magnetizing inrush current waveform. It uses the existence time of rectified waveform under action setting value to determine whether it is internal defect or magnetizing inrush current. (3) apply the second harmonic braking. When the transformer is put into no-load operation and the external defect is removed and the voltage is restored, the differential protection applies the second harmonic braking. When the internal defect is removed, the share braking circuit is used to avoid the unbalanced current.
⑵ unbalanced current caused by different current phases at two ends of transformer
Since the transformer generally adopts the wiring method of Y and D11, the phase difference of the current at both ends is 30 degrees. Even if the current at both ends of the transformer is equal, there will be unbalanced current reflected in the differential relay. In order to eliminate the influence of this unbalanced current, the phase compensation method can be used. Generally, the three current transformers on the star side of the transformer are connected into a triangle, and the three current transformers on the triangle side of the transformer are connected into a star. After properly considering the connection method, the phase of the secondary current can be corrected. After phase compensation, the current increases by 1.732 times in the differential arm on the triangular side of the transformer. In order to ensure that there is no current in the differential circuit under normal operation and external defects, the transformation ratio of the current transformer on this side must be increased by 1.732 times (in microcomputer protection, it is balanced through the balance coefficient in the program) to reduce the secondary current and make it equal to the current on the other side.
(3) unbalanced current caused by the difference between calculated transformation ratio and practical transformation ratio
Since the current transformers at both ends of the transformer select the standard transformation ratio according to the product catalog, and the transformation ratio of the transformer is also certain, it is difficult for the transformation ratio of the two end transformers and the transformation ratio of the transformer to meet the requirements. At this time, there will be current flowing in the differential circuit. When selecting transformer in practice, it is generally based on the finalized products of transformer to confirm a relatively close transformation ratio. For example, if our company has a transformer with y, D11 wiring, capacity of 31.5MVA and voltage of 110 / 35kV, the transformation ratio of TA on high-voltage side is 300 / 5 and that on low-voltage side is 600 / 5, it shall be calculated first:
Rated current on each side:
The secondary current at each side of the differential relay is: sp; Therefore, the unbalanced current flowing through the differential relay is:
Ibp=I1-I2=4.76-4.33=0.43A。
In order to eliminate the unbalanced current, the differential relay with fast and full iron core is often selected, and its balance coil is used to eliminate the influence of the differential current. Generally, the balance coil is connected to the side with small current of the protection arm, because the balance coil and differential coil are wound on the middle magnetic column of the relay together. Properly select the number of balance turns of the balance coil to make its magnetic potential offset with the magnetic potential generated by the differential current in the differential coil. Therefore, without magnetic flux in the iron core, the relay cannot act. However, in practice, the balance coil can only be selected according to the number of turns, so there will be a residual unbalanced current, which should be considered in setting accounting.
(4) unbalanced current caused by different current types at both ends
Due to the different types of current transformers at both ends of the transformer, their fullness characteristics and excitation current (reduced to the same side) are also different. Therefore, the unbalanced current generated in the external fault TDOA dynamic circuit is large. At this time, the same type coefficient of current transformer shall be selected, and the action current of differential protection shall be properly increased.
(5) unbalanced current generated by on load tap adjustment of transformer
The on load tap changer is a common method to select the on load voltage regulating transformer to adjust the voltage in the power system. In practice, changing the tap changer is to change the transformation ratio of the transformer. If the differential protection has been adjusted according to a certain transformation ratio (such as using the balance coil), when the tap changer is changed, new unbalanced current will flow into the differential circuit, At this time, it is impossible to eliminate the unbalanced current by selecting the balance coil from scratch. In order to prevent the influence of unbalanced current, it should be considered when setting the action current of protection, generally increasing the action setting value of protection.
To sum up, for the unbalanced current generated by the different current phases at both ends of the transformer and the difference between the calculated transformation ratio and the practical transformation ratio, the connection method and transformation ratio of the secondary coil of the current transformer and the method of selecting the balance coil can be properly selected to reduce it to Zui small. However, it is impossible to eliminate the unbalanced current caused by magnetizing inrush current, different types of transformers and on-load adjustment taps. Therefore, the longitudinal differential protection of transformer must avoid the influence of unbalanced current. The smaller the unbalanced current, the higher the sensitivity of the protection, and then ensure the safe and reliable operation of the transformer.
The above is the reason for the unbalanced current of the transformer. If you still don't understand or can't see it, you can call + 86 15853252696 or email hfy@hengfengyou.com Learn more.
