The Protection Of Thyristor Controlled Reactor Based On Third-harmonic

The Static Var Compensator (SVC) is one of the critical equipments which can meet the requirements of long-distance large-capacity power transimission to achive west-to-east power transimission and nationalwide interconnection in China. SVC can improve the dynamic reactive reserve and response speed of EHV transimission system and then enhance system static stability, transient stability and voltage stability, which is very important for the safe and stable operation of EHV transmission system. Thyrisitor controlled reactor (TCR) is widely applied to all types of SVC as a key component. At present, the differential protection and current unbalance protecion are used as main protection of TCR, the overcurrent protection is used as backup protection and the non-electrical parameters based protection is used to prevent reactor from overheating. However, the differential protection can not be applied to the protection of turn-to-turn short circuit of TCR. The current unbalance protection has a large dead zone because the unbanlance current decreases with the the increase of firing angle. The setting value of overcurrent protection, which is usually selected according to3-5times TCR rated current, is also affected greatly by firing angle. Therefore, it is necessary to study new protection methods of TCR to solve the problems existing in traditional protection.These problems of the traditional protection of TCR can result in the protection maloperation or failure to trip, which will affect the safe and stable operation of SVC device, and even cause harmful effects to the power system and other electric elements. This paper is devoted to make thorough reseach to solve these problems. It puts forward new TCR protection methods based on third-harmonic, which overcome the problems existed in the traditional proteciton effectively. The detailed research work is as follows.(1) The concept of quasi-static model for TCR is proposed. The conditons that TCR can be represented by quasi-static model in theoretical calculation are given, and then an explanation is given to illustrate that these conditions are satisfied in practical operation. These works lay a firm foundation for the following analysis on the operation and fault characteristics of TCR.(2) The quasi-static model of a single-phase TCR is used to analyse the current passed through the reactor, the valve voltage on the thyristor and the voltage on the reactor with different firing angles. It also calculates the harmonics of TCR current to analyse the relationship between harmonic current and firing angle. It can be conclude that the variation of firing angle affects the fundamental current more than third-harmonic current.(3) Based on the quasi-static model, the operation characteristic of three-phase TCR is also analysed. Each harmonic of three-phase TCR current meets that the6k+1(k=0,1,2,3,...) harmonic is positive-sequence component, the6k+5(k=0,1,2,3,...) harmonic is negative-sequence component and the3k+3(k=0,1,2,3,...) harmonic is zero-sequence component. Then the conclusion that there is no third-harmonic in the current of TCR under normal operation can be achieved.(4) The fault reasons of TCR used in practical are analysed and classified. The concept of Non-characteristic third-harmonic current is proposed. The theoretical calculation method of Non-characteristic third-harmonic current for all types of TCR internal fault is given.(5) Based on the analysis of Non-characteristic third-harmonic current for every TCR fault, the Non-characteristic third-harmonic current protection method and its setting principle are given. The protection method not only works for the line-to-line short circuit and earth short circuit, but also protects turn-to-turn short circuit. Although the Non-characteristic third-harmonic current protection is also affected by the firing angle, it has higher sensitivity and smaller protection dead zone compared to the tradional protection.(6) A constant current source model of TCR is constructed. Through the theoritical analysis we conclude that the positive-sequence third-harmoinc voltage at TCR access point is lagging the positive-sequence third-harmoinc current for internal fault and is leading the positive-sequence third-harmoinc current for external abnormal operation or when the positive-sequence third-harmoinc current is produced by other electronic devices. The criterion of directional protection based on positive-sequence third-harmonic is proposed. The phase relationship of positive-sequence third-harmonic voltage and current is used to determine whether the TCR is faulty or not, and the maloperation of TCR can be avoid during external abnormal operation without time delay. Simulations show the method has good selectivity and speedness..(7) A static model to verify the protection method based on third-harmonic is constructed and a test prototype is developed. Non-characteristic third-harmonic protection and positive-sequence third-harmonic directional protection are verified by the prototype and the results show that these two new protection methods are feasible and effective.The proposed Non-characteristic third-harmonic protection and positive-sequence third-harmonic directional protection have less influence of firing angle and offer higher sensitivity and smaller protection dead zone compared traditional protection methods. These two new methods supply a new thought for the TCR protection and even other large capacity power electronic equipments. The research results of this paper are important for the safe and stable operation of the SVC in EHV power system, improving power supply reliablility and power quality.