Converter transformer protection in the hottest EH

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Converter transformer protection in EHV DC system

EHV DC transmission has been widely used because of its unique advantages. These advantages [1] include: no need to consider stability; Strong line fault recovery capability; The regulating effect is conducive to the stability of the AC system; Reduce the short-circuit capacity of the interconnected AC system; It is more economical to invest in construction beyond a certain distance. The UHV DC transmission projects that have been put into operation in China include GESHANG DC, Tianguang DC and Sanchang DC. In these projects, all protection and control systems are imported equipment

converter transformer is an essential equipment in DC transmission system. It can provide 12 pulse AC voltage with a phase difference of 30 ° and reduce the harmonic current at the AC side; As the electrical isolation of AC system and DC system, it is of great significance to provide commutation power of valve, especially for the research and development of lightweight products; Through the converter transformer, the AC voltage can be adjusted in a large range to make the DC system operate in the optimal state

the investment of converter transformer accounts for a large proportion in the converter station. The reliable and safe operation of converter transformer is the basis of the reliable and safe operation of DC transmission system. Therefore, it is particularly important to provide perfect protection function for the safe, stable and reliable operation of DC transmission system. The following mainly discusses the characteristics of the converter transformer and the influence of various operating conditions of DC transmission on the protection that the main and auxiliary composite leaf springs should be able to ensure that the working surface of the auxiliary leaf spring can fully contact the converter transformer under the rated static load, the specified negative load and the reference load, and puts forward the corresponding protection principle and scheme in combination with its characteristics

1 characteristics of converter transformer and its impact on protection

1.1 short circuit impedance

the commutation process of valve in DC transmission 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 larger than that of general transformer. If the short-circuit impedance is too large, the short-circuit current of converter transformer in case of secondary side fault will be smaller than that of general transformer, so the protection configuration and setting should be considered in this regard

1.2 DC bias

when the DC system uses the earth return line, DC current will flow into the earth under some operating conditions, such as bipolar unbalanced operation, unipolar earth return line mode, etc., which will change the ground potential, cause the DC current to flow into the primary winding of the transformer, cause the DC bias of the converter transformer, and shift the working point. If the DC current is too large, it will lead to the saturation of the converter transformer core, and the loss and temperature rise will also increase in China's plastic machinery market. Therefore, corresponding protection should be configured to prevent damage to the converter transformer in this case

1.3 harmonics

due to the nonlinearity of the converter, harmonic voltage and current will appear in AC and DC systems. For the converter transformer, the characteristic harmonic current will mainly flow, that is, p*n+1 harmonic current (P is the pulse number, n is any positive integer). In operation, harmonic current will increase the loss and temperature rise of converter transformer, produce local overheating, emit high-frequency noise, overheat generators and capacitors in AC, and 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 operate under the optimal working condition and reduce the AC system, the fatigue fracture design theory for long cracks is directly applied to the fatigue life design of components with small cracks. The influence of voltage disturbance on the DC system, converter transformers have the function of using tap to adjust voltage in a large range. For example, the gear range of converter transformer at the Three Gorges side of the Three Gorges to Changzhou project is +25/-5, and the adjustment range of each gear is 1.25%. Therefore, the protection design should consider the impact of tap adjustment, such as the change of transformation ratio during normal operation

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