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This paper is published in the electric power engineering technology, Vol. 39, No. 2, 2020, welcome to read!
Citation information
Cheng Ming, Tian Weijie, Wang Wei, et al. Key technologies and research progress of new synchronous condenser [J]. Electric power engineering technology, 2020, 39 (2): 2-9
CHEGN Ming, TIAN Weijie, WANG Wei, et al. Review on Key Technologies and Latest Development of New Synchronous Condenser[J]. Power Engineering Technology, 2020, 39(2):2-9.
Abstract of this paper
With the development of HVDC transmission system, large capacity new synchronous condenser becomes the best choice to meet the dynamic demand of reactive power, and gradually becomes one of the research hotspots. Based on this, this paper systematically summarizes and summarizes the research status of the new synchronous condenser. Firstly, the paper introduces the operation principle of the new synchronous condenser from three aspects: mathematical model, operation state and control strategy. Secondly, compared with the power electronic reactive power compensation equipment, this paper expounds the superior dynamic reactive power output characteristics of the new synchronous condenser, and studies the influencing factors of its reactive power output characteristics. Finally, the research status of the state monitoring technology of the new synchronous condenser is summarized from two aspects: the existing fault diagnosis technology and parameter identification technology. In this paper, the mathematical model, control strategy and operation state monitoring of the new synchronous condenser are summarized, which can provide reference for the further research of the new synchronous condenser.
Key technology and research progress of new synchronous condenser
Cheng Ming1, Tian Weijie 1, Wang Wei1, Wei Chao 2
1. Southeast University
2. Electric Power Research Institute of State Grid Jiangsu Electric Power Co., Ltd
1. Introduction
Synchronous condenser is a synchronous motor running in the state of generator, providing and absorbing reactive power to the power grid, without mechanical load and prime mover, which is used for reactive compensation of power system and maintaining the stability of power grid. The traditional condenser has been put into operation in the power grid since last century, but because of its high cost, insufficient capacity and other shortcomings, it has been gradually withdrawn from the power grid operation and gradually eliminated. However, in recent years, with the promotion of HVDC transmission project, the new type of condenser used to meet the transmission demand has become one of the research hotspots.
Firstly, this paper introduces the operation principle of the new synchronous condenser, compares it with the traditional one and the power electronic reactive compensator, and expounds the dynamic reactive output characteristics of the new synchronous condenser. Secondly, the influence of the operation conditions and state parameters of the new type of condenser on its dynamic reactive power output characteristics is analyzed. At last, the status quo of condition monitoring technology is summarized from two aspects: fault diagnosis technology and parameter identification technology.
2. Operation principle of new type condenser
The camera system is mainly composed of the camera body, excitation system, step-up transformer system, starting system, cooling system, oil system and protection system. The main body and excitation system are the core parts of the whole system. By adjusting the excitation system, the camera can work in different states to realize the adjustment function of the power grid.
2.1 camera modeling
The condenser can be studied as a special form of synchronous generator, so the motor modeling of the new condenser can refer to the process of synchronous generator modeling.
There are two forms of the model of the condenser: one is the park motor model which takes the flux as the state variable and derives directly from the prototype circuit; the other is the practical model which takes the electromotive force of the motor as the state variable and derives from the park model. In this paper, the park model of generator is introduced in detail, and the practical model of generator is derived under two different assumptions. The transient calculation accuracy of the two practical models is analyzed by comparing the calculation results of the model with that of the time stepping finite element method.
2.2 operation state of condenser
(1) Three operation states of condenser
In terms of operation principle, the condenser is a kind of no-load synchronous generator. Its operation state can be divided into three categories: leading phase operation, late phase operation and no-load operation. The simplified phasor diagram of three typical operation states of the condenser is shown in Figure 1.
(2) The relationship between the position and operation state of the condenser in the power grid
The new type of condenser is generally used in the transmission and receiving end system of the DC UHV transmission system to meet the reactive power demand of the end converter station and maintain the bus voltage stability of the converter station. However, the operation characteristics of the transmission and receiving systems are different, and the demand for reactive power compensation capability is also different.
2.3 control strategy of condenser
The starting control of the condenser includes: direct starting, generator dragging starting and frequency conversion starting of the static frequency converter. Direct starting will produce large starting current in the stator of the condenser, which will cause great impact and damage the motor easily. Due to the limitation of motor output power and coupling facilities, motor drive start is suitable for starting small capacity units. Static frequency converter start is to use static frequency converter to supply variable frequency current to the condenser stator to realize the start of the condenser. Static frequency converter start-up, with many advantages, relatively simple structure, convenient maintenance, wide range of speed regulation, flexible control, has become the primary choice of the starting mode of the condenser.
The excitation control of the condenser can be divided into 2 types: terminal voltage regulation control and grid voltage regulation control. Terminal voltage regulation is a double closed-loop control mode, with the outer loop closed to reactive power and the inner loop closed to terminal voltage. Power grid voltage regulation control is based on the terminal voltage regulation control, adding the power grid voltage closed-loop link to ensure the constant power grid voltage and give full play to the reactive power of the condenser.
3. Analysis of reactive power characteristics of condenser
The reactive power characteristics of the condenser are divided into steady-state reactive power characteristics and dynamic reactive power characteristics. The steady-state reactive power characteristic is the characteristic that the excitation system controls the output reactive power of the condenser when the system is in steady-state; the dynamic reactive power characteristic is the regulation of the condenser to the power grid when the power grid voltage suddenly changes, including the spontaneous reactive power characteristic and the regulation reactive power characteristic. The spontaneous reactive power characteristic of the condenser is generated at the moment of voltage sudden change and attenuated with time, which is the physical characteristic of the condenser. When the voltage changes suddenly, the excitation system regulates the excitation current to control the reactive power characteristic of the condenser, which is affected by the regulation characteristic of the excitation system.
3.1 comparison of reactive power characteristics between new type condenser and power electronic compensation device
There are essential differences between the condenser and SVC, STATCOM devices in principle. In terms of dynamic response capability, all three have response capability, while SVC and STATCOM devices need to go through electronic circuit calculation before reactive response, so the condenser has faster response speed. In terms of reactive capacity, the late phase capability of SVC device is better than STATCOM, and the condenser is far better than the former two.
3.2 influencing factors of reactive power characteristics of condenser
Compared with the power electronic compensation device, the condenser has a stronger dynamic reactive power output capability. In the steady-state operation of the power grid, there is enough reactive power in the power grid, and in case of grid failure, the transient output capacity of the condenser is the main problem. Like most synchronous generators, the transient output capability of the condenser is related to the design parameters of the motor, the additional equipment of the condenser and the characteristics of the power grid.
(1) The influence of the design parameters of the condenser
The existing research results show that the transient reactive power output capacity of the condenser mainly depends on the direct axis secondary transient reactance and the short-circuit reactance of the main transformer, and the transient reactive power characteristics are negatively related to the sum of the above two parameters. However, if the direct axis reactance is too small, the output of reactive power will be suppressed for a short time. The temperature rise, electromagnetic characteristics, insulation and other characteristics of the motor will also have an impact on the reactive power output characteristics of the condenser.
(2) Power grid characteristics and influence of excitation system
The dynamic reactive power support effect of the condenser is positively related to the fault degree of the power grid, but its reactive power output ability is limited by the parameters of the condenser. The deeper the fault degree of power grid, the better the reactive power support effect of the condenser. The strong excitation ability and under excitation ability of the excitation system will affect the dynamic reactive power output ability of the condenser.
4. Condition monitoring technology of condenser
The state monitoring technology of the condenser is to check and monitor the operation state of the condenser to determine whether it is in normal operation state. The purpose of condition monitoring technology is to determine the abnormal state of the condenser or the damaged information of the parts in advance, so as to take measures to adjust in advance to avoid the occurrence of serious shutdown fault.
The condition monitoring technology of the condenser can be divided into operation characteristic monitoring and fault diagnosis. According to the mode of condition monitoring, it can be divided into online monitoring and offline monitoring. As the camera needs to be out of operation during off-line monitoring, the motor shall be overhauled in the shutdown state, which will not be discussed here.
4.1 operation characteristic monitoring technology
It can be seen from the above analysis that when there is a fault in the power grid, the condenser will show superior dynamic regulation ability. The accuracy of the model is directly related to the monitoring level of the dynamic output capability of the camera, so the accurate motor parameters are the basis of the analysis of the operation characteristics of the camera.
At present, the most commonly used method to estimate motor parameters is parameter identification. The parameter identification method is based on the detection of the operation parameters of the camera, using the monitoring information to identify the body parameters (such as resistance, reactance, etc.) of the camera, according to the identification results and theoretical analysis and comparison, judge the operation characteristics of the camera. In time domain identification method, state variables that cannot be measured need to be iterated repeatedly.
4.2 online fault diagnosis technology
Online fault diagnosis technology is to monitor the operation parameters (such as voltage, current, torque, etc.) of the camera on-line, directly process the detected signal, analyze the signal characteristics, and judge the operation status of the camera. At present, the research on fault diagnosis of the camera is just beginning. There is no specific research on various types of faults of the camera.
The stator turn to turn short circuit fault is one of the common faults of the motor, and also one of the most serious faults. If the turn to turn short circuit fault is not found and properly handled in time, it will gradually evolve into stator phase to phase short circuit, resulting in shutdown accident. Therefore, the diagnosis of stator turn to turn short circuit is essential to the fault diagnosis of the condenser.
Rotor turn to turn short circuit fault is also a common fault in phase modulation unit, and slight turn to turn short circuit is often ignored. If the fault continues to develop, the rotor current will increase significantly, the winding temperature will increase, the reactive power will decrease, and other faults will occur, resulting in major accidents.
Due to the influence of machining and operation environment, the long-term operation of the condenser will make the rotor eccentric fault. Rotor eccentricity fault will produce unbalanced electromagnetic force on stator and rotor, which will cause bearing wear and rotor surface cracking. Therefore, it is necessary to diagnose the rotor eccentricity.
5. Conclusion
With the application of new type of condenser in DC UHV project, the research on reactive power characteristics and condition monitoring of the condenser is of great significance. Firstly, the operation principle of synchronous condenser is introduced, and the relationship between its operation state and power grid position is analyzed. After that, the reactive power output characteristics of the new type of condenser are summarized, compared with the power electronic compensation device, and the advantages of the new type of condenser are analyzed. Finally, the research status of the state monitoring technology of the new type of condenser is analyzed and summarized. Run the original from the condenser