Theory and simulation analysis of parallel operation after main transformer replacement

As one of the most important main equipment in the power system, the safe and stable operation of transformer plays a vital role in ensuring the reliability of power supply. Because the three-phase parameters of the main transformer are not the same, the parameters of each phase and the three-phase load of the power grid are also different, resulting in a certain degree of asymmetry between the three-phase current and voltage of the transformer during normal operation. There will be negative sequence and zero sequence current components. The degree of asymmetry is generally small for the high-voltage power grid, but it has a large impact on the low-voltage side of the main transformer. In this paper, the parameters of two main transformers in a substation are quite different, but when they need to run in parallel, whether they can run in parallel, what are the conditions of parallel operation, these problems have been studied in theory and simulation. The results show that the two main transformers can be operated in parallel under suitable conditions. The actual operation result of a substation’s main transformer is normal, which further verifies the correctness of theoretical calculation and simulation.


Introduction
As one of the most important main equipment in the power system, the safe and stable operation of transformer plays a vital role in ensuring the reliability of power supply.Ideally, the three-phase parameters of the transformer are the same, and the three-phase current should be symmetrical during normal operation, that is, there is only positive sequence current.However, the three-phase parameters of the transformer are not exactly the same, and the parameters of each phase of the power grid are also different.In addition, the three-phase load of the power grid is unbalanced, so that there will be a certain degree of asymmetry between the three-phase current and voltage of the transformer during normal operation.There will be negative sequence and zero sequence current components.The degree of asymmetry is generally small for the high-voltage power grid, but it has a large impact on the low-voltage side of the main transformer.Compared with the separate operation mode, parallel operation can improve the reliability and economy of power supply, completely balance the load of main transformer, and then improve the power supply capacity [1][2][3][4][5][6].A substation originally has two main transformers, # 1 main transformer and # 2 main transformer.The # 1 main transformer cannot operate normally due to failure, so it is required to replace the standby transformer (the standby transformer parameters are the # 1 main transformer parameters replaced in the Table 1 below).There is a big difference between the # 2 main transformer and the replaced # 1 main transformer.What are the conditions for parallel operation.Under the condition of large mode in summer, can the lowvoltage side operate with bus?These problems need to be confirmed.In order to ensure the safe and stable operation of a substation, the parallel operation conditions of the # 2 main transformer and the replaced # 1 main transformer of a substation are analyzed theoretically and simulated.The analysis results show that the # 1 main transformer and # 2 main transformer can operate in parallel when the tap position of the main transformer is appropriate.In addition, under the condition that the lowvoltage side of the main transformer is in bus-connection operation, the medium-voltage side is in operation and the 110kV load does not exceed the summer large-mode operation, the bushing current at the low-voltage side is less than the rated current at the low-voltage side of the main transformer, and the current at the high-voltage side of the main transformer does not exceed the rated value.

Maximum load during peak season (110kV denominator operation)
The maximum load of 110kV # 4 bus and 110kV # 5 bus is 122000 KW and 88400 KW respectively during the peak season of a substation (110kV denominator operation).

Analysis of parallel operation conditions of two main transformers at low voltage side
The most ideal operation condition for parallel operation of transformers is that each transformer can reasonably distribute the load, that is, it can share the load according to its own capacity proportion, and there is no circulating current between the transformers at this time.However, in actual operation, there will always be circulating current between transformers operating in parallel.In order to ensure normal and effective operation of transformers, the circulating current should be minimized and the corresponding conditions must be met.According to the relevant standards and guidelines, this paper analyzes them from the following four aspects [7][8][9][10][11][12][13]: (1) The connection group label is the same; (2) The voltage ratio shall be the same, and the difference shall not exceed ± 0.5%; (3) The impedance voltage deviation is less than 10%; (4) For a transformer with a capacity ratio between 0.5 and 2, an unequal voltage ratio or a unequal short-circuit impedance, except to meet the standards and manufacturer's requirements, the circulating current of each transformer winding running in parallel shall meet the requirements of the manufacturer.

Analysis of parallel operation at low voltage side of transformer
For transformers with different connection group numbers, although the rated voltage at the primary side and secondary side are the same, the phase of the voltage phasor at the secondary side has obvious deviation.Take the Y0/y0 and Y0/d11 connection transformers as an example, the phase difference of the transformer's secondary voltage is 30 °, and the voltage value difference is: Because the short circuit impedance of the transformer is very small, the voltage difference will generate a large circulating current in the two transformer windings; Assuming that the short-circuit impedance of both transformers is 0.12 and the circulating current reaches 2.17 times of the rated current of the transformer, this is absolutely not allowed.Therefore, the connection group number of parallel operation transformers must be the same.
The parameters of # 1 main transformer (after replacement) and # 2 main transformer in a substation are shown in Table 1 and Table 2 above.The connection group of # 1 main transformer is Yg/Yg/d1 connection mode, and the connection group of # 2 main transformer is Y/Yg/d1 connection mode, which meets this condition.

Transformer ratio analysis
When the connection group number of two Transformers is the same, but the ratio of transformer is not equal, the secondary voltage of the transformer running in parallel is taken as the basis, and the circulating current of the transformer is calculated as follows: Where, U 1 is the transformer primary voltage, k 1 , k 2 and Z 1 , Z 2 are the transformation ratio and short-circuit impedance of the two transformers respectively.Because the value of transformer shortcircuit impedance is very small, even if the ratio difference is very small, it can produce large circulating current.For example, if the short-circuit impedance of two transformers is 0.12 and the transformation ratio is 5% different, the circulating current will reach 0.21 times of the rated current of the transformer, which will have a great impact on the operation of the transformer.Therefore, the regulation stipulates that the transformation ratio error of power transformer should be controlled within 0.5%, and the circulating current should not exceed 5% of the rated current.
The tap information of # 1 and # 2main transformer in a substation is shown in Table 3 and Table 4.By adjusting the tap of the transformer, at tap position 3 of # 1 main transformer, tap position 5 of # 2 main transformer, tap position 5 of # 1 main transformer and tap position 5 of # 2 main transformer, the calculated transformation ratio error of the power transformer is within 0.5%, and the circulating current is not more than 5% of the rated current, and the # 1 and # 2 main transformers operate in the 1-6 gears (456 maximum) for a long time, so this condition is met.

Analysis of transformer short-circuit impedance deviation
The load factor of each transformer in parallel operation is inversely proportional to its short-circuit impedance, and the transformer with small short-circuit impedance has large load.In parallel operation, in order not to waste equipment capacity, it is generally required that the capacity ratio of two transformers is 0.5~2, and the difference of leakage impedance per unit value is less than 10%.
The short circuit impedance difference at the low voltage side of # 1 and # 2 main transformers in a substation meets the above conditions.

Capacity ratio deviation
When the capacity ratio of parallel main transformer is different, the main transformer will not be used effectively, resulting in low operation efficiency.Generally, the capacity ratio of two transformers is required to be between 0.5 and 2.
The capacity of high voltage and middle side of #1 main transformer is 180MVA, the capacity of phase A high voltage and medium voltage side of #2 main transformer is 150MVA, the capacity ratio of high voltage side and medium voltage side of two transformers is between 0.5 and 2, the capacity of low voltage side of # 1 main transformer is 90MVA, the capacity of low voltage side of # 2 main transformer is 75MVA, and the capacity ratio of low voltage side of two transformers is between 0.5 and 2.
It can be seen from the above analysis that the low voltage side of the replaced # 1 main transformer and # 2 main transformer meet the conditions for parallel operation.In order to further analyze the impact of parallel operation, the bushing current at the low voltage side of the transformer is analyzed in detail.

Simulation method
The simulation analysis is based on the power grid digital laboratory power grid digital real-time simulation system (ADPSS).ADPSS can carry out electromechanical simulation, electromagnetic simulation and electromechanical-electromagnetic hybrid simulation.In this paper, the bushing current on the low-voltage side of the transformer during asymmetric operation is simulated, so the equivalent model can be used to simulate the external power grid of the substation and the electromagnetic transient model can be established in ADPSS.
The schematic diagram of the electromagnetic transient model of two main transformers running side by side in a substation is shown in Figure 1.The electromagnetic transient model includes some equipment of a certain station, including 220kV bus, 110kV bus, 10kV bus, # 1 main transformer, # 2 main transformer, power network equivalent model and load equivalent model.

Analysis content
Based on the verification of the simulation model, aiming at various typical operating conditions, the simulation model is used to analyse the bushing current at the low voltage side when the two main transformers operate in parallel.

Operation of denominator on low voltage side of two main transformers.
When the denominator at the low voltage side of the two main transformers operates, the windings between the main transformers are symmetrical and will not generate circulation, so no simulation analysis will be conducted.

Low voltage side joint bus operation of two main transformers (medium voltage side denominator operation).
(1) When the denominator at the medium-voltage side of the two main transformers operates and the low-voltage side operates in parallel, the following four working conditions are simulated

Figure 1 .
Figure 1.Schematic diagram of electromagnetic transient model when two main transformers operate in parallel.

Table 1 .
Parameters of # 1 main transformer after replacement.

Table 4 .
Tap information of # 2 main transformer a .