Causes and Solutions for Unbalanced Current in Transformer Differential Protection
Causes and Solutions for Unbalanced Current in Transformer Differential Protection
During transformer operation, the main protection system is primarily equipped with differential protection. However, when calculating the differential current, the input and output currents are not always equal, resulting in unbalanced current. What causes this unbalanced current?
The main causes of unbalanced current in transformer differential protection are as follows: 1. Different current transformer models on different sides of the transformer: Due to differences in voltage levels and rated currents on different sides of the transformer, the current transformer models on each side are different. Their saturation characteristics and excitation current (referred to the same side) are also different, resulting in a larger unbalanced current in the differential circuit.
2. Difference between calculated and actual turns ratio: Standardization of turns ratio leads to inconsistencies between the actual and calculated turns ratios, resulting in unbalanced current.
3. Changes in transformer tap changers: Adjusting the transformer taps under load is a method of voltage regulation in power systems; changing the taps changes the transformer turns ratio. In setting calculations, differential protection can only be set according to a certain transformation ratio. Selecting an appropriate balancing coil reduces or eliminates the influence of unbalanced current. After the differential protection is put into operation, it is generally impossible to reoperate the current loop of the differential protection when the voltage regulating tap is changed, thus a new unbalanced current will appear. The magnitude of the unbalanced current is related to the voltage regulation range.
4. Steady-state unbalanced current: ① Different phases of currents on both sides of the transformer: Power systems often use a Y,d11 connection method. Therefore, the phase difference between the currents on both sides of the transformer is 30°, with the Y-side current lagging the Δ-side current by 30°. If the current transformers on both sides use the same connection method, the secondary currents of corresponding phases on both sides will also differ by about 30°, resulting in a large unbalanced current. ② Difference between calculated and actual transformation ratios: Due to the standardization of transformation ratios, the actual transformation ratio is inconsistent with the calculated ratio, thus generating unbalanced current. ③ Different current transformer models on each side of the transformer: Due to the different voltage levels and rated currents on each side of the transformer, the current transformer models on each side are different. Their saturation characteristics and excitation currents (referred to as the same side) are also different, resulting in a larger unbalanced current in the differential circuit.
5. Transient unbalanced current: This mainly refers to the non-periodic component of the short-circuit current, primarily the excitation current of the current transformer, which saturates the iron core and generates an unbalanced current.
In summary, the unbalanced current in transformer differential protection mainly includes steady-state unbalanced current and transient unbalanced current. For steady-state unbalanced current, appropriate methods can be used to reduce or eliminate its impact; for transient unbalanced current, corresponding measures need to be taken according to the specific circumstances.
What are the methods for eliminating unbalanced current in transformer differential protection? The methods for eliminating unbalanced current in transformer differential protection include the following: 1. Compensation using an autotransformer. 1. Typically, an autotransformer is installed on one side of the transformer current transformer (or on both sides for a three-winding transformer). The output terminal of the LH transformer is connected to the input terminal of the autotransformer. When the turns ratio of the autotransformer is changed, the output current of the autotransformer can be made equal to the secondary current of the LH transformer without the autotransformer, thus making the current flowing into the differential relay zero or close to zero.
2. Magnetic compensation is achieved using the balancing coil of an intermediate converter. Typically, a main coil (differential coil) is wound on the core of the intermediate converter, connected to the differential current. Additionally, a balancing coil and a secondary coil are wound, connected to the side with the smaller secondary current. By appropriately selecting the number of turns of the balancing coil, the magnetic potential generated by the balancing coil can completely cancel the magnetic potential generated by the differential coil. Therefore, no electromotive force is induced in the secondary coil, and thus no current flows through the differential relay. When using this method, the number of turns of the balancing coil calculated by the formula is generally not an integer, but in practice, the balancing coil can only be selected as an integer. Therefore, a residual unbalanced current will still exist, which should be considered when calculating the setting value of the longitudinal differential protection.
3. Unbalanced current introduced by external faults in the transformer can be addressed using a BCH-Z type differential relay. The BCH-Z type differential relay has a balancing coil; when an external short circuit occurs in the transformer, the external fault current flows through the braking coil, causing the core to become extremely saturated, thus generating a braking effect.
4. Unbalanced current caused by the difference between the calculated and actual turns ratio of the current transformer can be overcome using two methods: ① compensation using an autotransformer; ② magnetic compensation using the balancing coil of an intermediate converter.
