Discussion on the protection of the hottest HVDC t

Discussion on HVDC transmission line protection

this paper analyzes and discusses the fault location and line protection of HVDC transmission line, briefly analyzes various protection principles according to the characteristics of DC line fault, and believes that traveling wave protection, as the main protection of HVDC system line protection, conforms to the fault characteristics of HVDC transmission line and has absolute advantages, It provides a theoretical basis for the analysis and research of traveling wave protection of HVDC transmission lines; Line protection; Traveling wave protection

0 introduction

HVDC transmission has achieved rapid development in the world in recent years. So far, the total capacity has reached about 50gqw. Among them, 100kV Zhoushan submarine cable transmission project, 500kV GESHANG DC transmission project, 500kV Tianguang DC transmission project and the Three Gorges DC transmission project under construction have been successively completed in China. Therefore, how to improve the safety and reliability of DC line operation has become an urgent problem to be solved, and the protection of high-voltage DC line is the basic guarantee for the safe and stable operation of DC line. Therefore, it is necessary to further study and improve the principle and protection scheme of traveling wave protection, the main protection of DC line protection

1hvdc system fault characteristics and line protection

1.1 the advantages and application of HVDC transmission technology

modern DC transmission technology generally adopts AC DC AC commutation mode. The reason why HVDC transmission technology has been so booming is that it has obvious advantages compared with AC transmission:

1) conductors with the same section can transmit greater power and have less active power loss

2) DC transmission can quickly and accurately achieve multi-objective control to improve power quality and power supply reliability

3) the current has only positive and negative poles, the transmission line structure is simple, and when the transmission distance is greater than the equivalent distance of AC and DC transmission, the DC line saves more investment

4) each conductor can operate as an independent circuit, and the earth or sea water can be used as the circuit

5)

6) the cable line can operate under a high potential gradient

7) there is a problem of synchronous operation stability between the AC systems at both ends of DC transmission

8) Yes, it is necessary to add specific oil to connect two AC systems with different frequencies, and the power on the connection line is easy to control

at present, HVDC transmission technology has been widely used in long-distance high-capacity transmission, submarine cable transmission, interconnection of two AC systems, underground transmission in big cities, reducing short-circuit capacity, and cooperating with new energy transmission

1.2 DC line fault process

when DC overhead line fails, from the characteristics of fault current, the process of short-circuit fault can be divided into three stages: traveling wave, transient and steady state

1) after the initial traveling wave stage

fault, the line capacitance discharges through the line impedance, and the energy stored along the electric and magnetic fields of the line is transformed into fault current traveling waves and corresponding voltage traveling waves. The amplitude of the current wave depends on the line wave impedance and the DC voltage at the fault point immediately before the fault. The arc current of the line to ground fault point is the sum of the traveling wave current flowing to the fault point on both sides. This current is not controlled by the converter station control system at both ends before the first reflection or refraction of the traveling wave. The wave equations of voltage and current traveling waves are:

the d'Alembert solution of the above formula is:

refers to the backward voltage traveling wave

2) transient phase

after the reverse and refraction of the initial traveling wave, the fault current turns into the transient phase. The main components of DC line fault current include: DC component with pulsation and varying amplitude (forced component) and transient oscillation component (free component) determined by DC main circuit parameters. At this stage, the constant current control in the control system begins to play a significant role. The rectifier side and inverter side are adjusted respectively to increase the lag trigger angle and suppress the current flowing to the fault point at both ends of the line

3) steady state stage

finally, unless the government has subsidies, the fault current enters the steady state, the steady-state value of the fault current provided by the fault current on both sides is controlled to be equal to the setting value of each self-set current control, the direction of the current flowing into the fault point on both sides is opposite, and the current at the fault point is the difference between the two, that is, the current margin △ ID

1.3 requirements and configuration of HV DC line protection

when DC line fails, on the one hand, the control of bridge valve control pole can be used to quickly limit and eliminate fault current; On the one hand, the fault current is much smaller than that of the AC line because the simulation experiments of the friction situation of the end face, pin and disc and the wear resistance of the coating material are also carried out to determine the role of the current regulator. Therefore, the detection of DC line fault can be judged by the magnitude of fault current, but needs to be identified by the transient component of current or voltage

however, the vast majority of relay protection operating in the system acts in response to post steady-state power frequency information, such as current increase, voltage decrease, current and power direction change, measurement impedance decrease and other fault information. And this kind of protection depends on steady-state work? Quantity information needs a long time (data window) to obtain, which limits the speed of microcomputer protection action; The saturation of the current transformer causes the distortion of the secondary transmission current, which makes the calculated value in the microcomputer protection very different from the actual fault current, resulting in the incorrect action of the protection device; The power frequency distance protection cannot correctly distinguish the fault and system oscillation in the line area. It can be seen that the traditional protection based on power frequency information can no longer meet the needs of EHV long-distance DC transmission. Therefore, a new principle protection based on fault state information - traveling wave protection has become the key to solve the problem

at present, traveling wave protection is widely used as the main protection of high-voltage DC line protection in the world. It uses the current and voltage traveling wave transmitted at the moment of fault to form ultra-high speed line protection. Because the transient current and voltage traveling waves are not controlled by the converter stations at both ends, their amplitude and direction can accurately reflect the original fault characteristics and are affected, which shows that their reliability is very high. Moreover, compared with the traditional protection based on power frequency electrical quantities, the traveling wave protection has ultra-high speed action performance, and its protection performance is not affected by current transformer saturation, system oscillation and long line distributed capacitance

on the other hand, compared with AC system, traveling wave protection has more obvious advantages in DC system. First, in the AC system, if a fault occurs at the time of voltage zero crossing (the initial phase angle is 0 °), there is no fault traveling wave on the fault line, and the protection has an action dead zone; There is no such limitation if there is no voltage phase angle in the DC system. Secondly, the transmission of voltage and current traveling waves in AC system is affected by the change of bus structure, and it is difficult to distinguish the traveling waves propagating at the fault point from the reflected and transmitted waves of each bus; Due to the simple structure of high-voltage DC line, there is no such problem

at present, travelingwaveprotection is generally used as the main protection for high-voltage DC line protection. When the DC line fails, different fault voltage and current traveling waves will be reflected from the fault point to the converter stations at both ends, so the fault can be detected. When the traveling wave protection acts, the starting DC line fault recovery sequence is controlled (rectifier side), that is, according to the preset times and a certain de dissociation time, the DC pole with full voltage starting or step-down starting fault; If the restart is still unsuccessful, the valve groups at both ends will be locked

at the same time, the HV DC line protection adopts lowvoltageprotection, derivativeandlevelprotection, longitudinaldiffentialprotection, etc. as the backup protection of traveling wave protection

2-line protection classification and HVDC development trend

so far, scholars at home and abroad have proposed traveling wave protection based on a variety of principles. According to whether there is a channel or not, there are mainly two types: channel protection and non channel protection, as shown in Table 1:

at present, HVDC transmission is in the stage of vigorous development. Like many other countries in the world, China is actively carrying out the research and development of DC transmission technology on the basis of existing construction and operation experience. Due to the wide range of problems involved in HVDC transmission technology, in order to highlight the key points, the following briefly introduces several main development directions and research topics of HVDC transmission:

1) transmission parameters are getting higher and higher. At present, the operation parameters of Itapu DC transmission project in Brazil, which has the highest private operation parameters, have reached ± 600kV, 3150mw, 783km

2) inverter technology based on series capacitor commutation 7. It has over-current, over-voltage, under current, under voltage and other protection; The stroke has three protection techniques: program-controlled limit, limit and software limit

3) light DC transmission system based on voltage source converter

4) develop high parameter and large capacity silicon controlled components, and improve the mechanical, electrical and thermal structures of the converter valve, so as to further reduce the cost and reliability of the converter

5) study the parallel (or parallel) operation and regulation of AC and DC to improve the limit of transmission power

6) develop DC circuit breakers and multi terminal DC systems

7) design and application of compact converter station

8) apply new technology to shorten the protection action time of DC line and improve the reliability of protection action

3 conclusion

this paper discusses the fault characteristics of HVDC transmission and its line protection, and draws the following conclusions:

1) the fault characteristics of HVDC transmission and the requirements for line protection determine the status of traveling wave protection as the main protection of line protection

2) based on the transmission characteristics of existing CT and Pt, the aforementioned methods of detecting voltage drop rate, traveling wave sudden change and ground mode wave polarity can be used as traveling wave protection criteria, and its action performance has certain reliability

3) with the application of optical CT, optical Pt, high-speed data acquisition technology, digital signal processing technology and GPS, traveling wave distance protection based on wavelet transform, as a high-speed and reliable traveling wave protection scheme, has been practical