This paper analyzes the characteristics of converter transformer and the influence of various operating conditions of EHV DC transmission on converter transformer protection. The overall design idea of converter transformer protection is put forward.

Keywords: converter transformer protection analysis

Introduction to 0

EHV DC transmission has been more and more widely used because of its unique advantages. These advantages include: there is no need to consider stability; Strong line fault recovery ability; Regulation is beneficial to the stability of AC system; Reduce the short-circuit capacity of interconnected AC system; It is more economical to invest in construction beyond a certain distance. Converter transformer is an indispensable and important equipment in DC transmission system. It can provide 12 pulse AC voltage with a phase difference of 30, reducing the harmonic current on the AC side; As an electrical isolation between AC system and DC system, it provides commutation reactance of the valve; The AC voltage can be adjusted in a wide range through the converter transformer, so that the DC system can run in the best state.

1 characteristics of converter transformer

In the DC transmission of 1. 1 short circuit impedance, the commutation process of the valve is actually a two-phase short circuit. In order to limit the current in the commutation process to a certain range, the short-circuit impedance of converter transformer is greater than that of general transformer. If the short-circuit impedance is too large, when the secondary side fails, the short-circuit current of converter transformer will be smaller than that of ordinary transformer, so this aspect should be considered in protection configuration and setting.

When 1.2 DC biased DC system uses earth loop, DC current will flow into the earth under some working conditions, such as bipolar unbalanced operation and unipolar earth loop mode. This will change the ground potential, lead to DC current flowing into the primary winding of the transformer, and lead to DC bias and shift of the operating point of the converter transformer. If this DC current is too large, the iron core of converter transformer will be saturated, and the loss and temperature rise will also increase. Therefore, in this case, it is necessary to configure corresponding protection to prevent the converter transformer from being damaged.

1.3 harmonics due to the nonlinearity of the converter, harmonic voltages and currents will appear in AC and DC systems. For converter transformers, the characteristic harmonic current mainly flows, that is, the harmonic current of order p * n+ 1 (P is the number of pulses and n is any positive integer). In operation, harmonic current will increase the loss and temperature rise of converter transformer, produce local overheating and emit high-frequency noise, and also overheat generators and capacitors in AC network, which will interfere with communication equipment. These harmonic currents should be considered to avoid affecting the protection device.

1.4 voltage regulating tap In order to make the DC system run in the best working condition and reduce the influence of AC system voltage disturbance on the DC system, the converter transformer has the function of tap regulating in a wide range. For example, the gear range of converter transformer on the Three Gorges side of the Three Gorges-Changzhou project is +25/-5, and the adjustment range of each gear is 1.25%. Therefore, the influence of tap adjustment, such as the change of ratio during normal operation, should be considered in the protection design.

Special operating conditions of 1.5 DC system Because of the special regulating function of DC control system, the operating conditions and faults of converter transformer are different from those of ordinary transformer. These differences mainly include the following points:

The fault of 1.5. 1 DC system is equivalent to the out-of-zone fault of converter transformer, and the short-circuit current is generally not too large. For the rectifier side, the current through the converter transformer will increase, but due to the rapid action of DC control and protection system, the current will soon decrease. For the inverter side, the failure of DC system will lead to the failure of DC current transmission to the AC side, but will reduce the ride-through current.

1.5.2 For converter transformer protection, the most serious out-of-zone fault caused by DC system is the short-circuit fault of rectifier side valve, which is equivalent to the two-phase or three-phase short-circuit fault at the outlet of converter transformer. However, due to the intervention of DC protection, only half a period of two-phase short circuit will actually occur. For the inverter side, the current flowing through the converter transformer when the valve is short-circuited is much smaller than that on the rectifier side because of the large trigger angle.

When the converter transformer fails in 1.5.3 area, the DC system generally does not provide short-circuit current. This is caused by the function of DC control system. On the rectifier side, the power is converted from AC side to DC side, and the fault of converter transformer will only stop this conversion, but will not reverse the power, so the DC side will not provide short-circuit current; On the inverter side, when the fault commutation is normal, the DC side will not provide additional short-circuit current due to the constant current control characteristics of the DC system. If the fault is serious, it will inevitably lead to commutation failure (AC voltage reduction), and DC side will not provide short-circuit current.

1.5.4 Due to the rapid adjustment of the DC control system, the power transmission can be quickly reversed from one direction to the other when necessary. For converter transformers, there will be a rapid trend reversal.

1.5.5 When a grounding fault occurs in the converter transformer protection zone, it is actually a valve short circuit. Due to the unilateral conduction of the valve, the half-cycle current of the fault current is large and the half-cycle current is small, which leads to the large second harmonic in the differential current.

1.5.6 For the local fault of converter transformer on the inverter side, commutation failure often occurs, resulting in great harmonics in the current flowing, but the differential current (that is, the current supplied to the fault point) is still mainly the power frequency component.

1.5.7 Due to the special operation mode and large leakage reactance of converter transformer (as commutation reactance), the secondary side fault generally does not cause the saturation of each side TA, even if the saturation causes the "misoperation" of protection, it is correct (the fault outside the converter transformer area, that is, the valve area, will cause DC outage). However, for a half-switch connection mode, when the AC system fails outside the area, the high-voltage side TA may be saturated. . Misoperation in this case is unacceptable and must be prevented.

1.5.8 Before the valve is unlocked, when the AC connection line at the valve side has a grounding fault, it will not generate grounding current and cause no damage to the transformer. But in this case, no fault is found, and once the valve is unlocked, it will cause a short circuit of the valve. Therefore, in this case, protection should be set to detect ground fault.

2 protective measures for converter transformer

2. 1 protection configuration principle In order to ensure reliability and safety, converter transformer protection can be configured in a simple and economical way: each converter transformer protection is equipped with two protection devices, and the power supply and input of each protection device are independent, and the output of each device can reach two tripping coils of the circuit breaker and two systems controlled by DC. Each device takes measures to prevent its own misoperation, and relies on the or outlets of the two devices to prevent malfunction. 2.2 Configuration and Principle of Protection In order to avoid the influence of harmonics unique to converter station on protection, the protection device should take measures from hardware and software to make the protection only aim at power frequency components.

Main protection includes steady-state ratio differential, differential quick break, power frequency rate differential, zero-sequence ratio differential and over-excitation protection. Backup protection includes overcurrent, zero-sequence overcurrent, overvoltage, zero-sequence overvoltage and saturation protection.

2.2. 1 Steady-state ratio differential protection Due to different ratios and wiring groups, when the power transformer is running, the current magnitude and phase on each side are also different. These influences must be eliminated before the relay is formed. TA of converter transformer is usually installed on each winding, so the current phase of primary winding and secondary winding is the same, so only the influence of ratio change needs to be compensated. The premise of the following statement is that the amplitude and phase differences on each side of the transformer have been eliminated.

Steady-state proportional differential protection is used to distinguish whether the sensed differential current is caused by internal fault or output imbalance (especially when external fault occurs). The device adopts variable slope ratio braking characteristics with initial braking, and the steady-state ratio differential element consists of two elements: low-value ratio differential (sensitive) and high-value ratio differential (insensitive). In order to ensure the rapid removal of faults in the area, TA saturation criterion is only set for low-value ratio differential components (sensitive), and TA saturation criterion is not set for high-value ratio differential components (insensitive).

There are three ways to solve the imbalance of differential current caused by the adjustment of tap-changer of converter transformer: one is to avoid it by setting value; The second is to use the floating threshold to adjust adaptively; The third is to adjust with tap position. Method 1 and method 2 are simple and practical, and method 3 is more complicated to implement.

2.2.2 The differential protection device of power frequency change rate discriminates in turn according to the phase, and when certain conditions are met, the differential protection device of power frequency change rate acts. The power frequency change rate differential protection exits after being locked by the inrush current identification element and the over-excitation locking element.

Because the braking coefficient of power frequency change rate differential can be higher, its own characteristics have strong transient and steady-state saturation resistance to faults outside the area. The power frequency ratio differential element improves the sensitivity of the device to slight turn-to-turn fault when the transformer is in normal operation. Power frequency change rate differential protection is not affected by differential current imbalance caused by tap-changer adjustment of converter transformer.

2.2.3 backup protection backup protection includes overcurrent, zero-sequence overcurrent, overvoltage, zero-sequence overvoltage and saturation protection.

3 abstract

This paper analyzes the characteristics of converter transformer relative to the main transformer of AC system, expounds the influences of these characteristics and various special operating conditions of DC transmission on the protection of converter transformer, and advances some corresponding protection schemes.

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