The HVDC circuit breaker combines power electronic devices and is a hvdc circuit breaker used to break the DC circuit. The moment when the DC breaks is realized is the zero-crossing point of the current. HVDC circuit breakers can generally be divided into mechanical HVDC circuit breakers, solid state HVDC circuit breakers and hybrid DC circuit breakers. Among them, the control of power electronic devices and DC arc extinguishing are the key technologies of HVDC circuit breakers.
1. Background and significance of HVDC circuit breaker
In the field of power transmission, in order to adapt to the new energy pattern, the multi-terminal DC transmission system and DC grid technology based on conventional DC and flexible DC will become the future development trend. Multi-terminal DC transmission realizes multi-source power supply and multi-drop power reception. A more flexible and fast power transmission method. On this basis, if the DC transmission lines are interconnected on the DC side to form a DC grid, it can effectively solve the problems of active power fluctuations caused by new energy grid integration. In the future, urban smart distribution networks, micro-grids, etc. And other fields also have great advantages, which is of great significance to the construction and development of my country's future power grid.
The DC side fault is a type of fault that must be considered in the DC transmission system, which affects the calculation of equipment parameters and the design of control and protection strategies. Compared with the AC system, the damping of the DC system is relatively low, the fault penetration speed is faster, the penetration is deeper, and the control and protection are more difficult. With the development of multi-terminal flexible DC transmission systems, how to deal with DC faults has become a key issue that needs to be considered in engineering practice.
In the field of power distribution, driven by modern power electronics technology and distributed power supply, DC distribution network has great development prospects. On the one hand, common distributed power sources can generate DC power or become DC power after rectification. If these power sources are connected to the DC distribution network, the commutation link will be greatly saved; on the other hand, many loads themselves currently use DC power supply. The distribution network can directly supply power to these loads without going through the rectification link, thereby reducing costs and reducing losses. Compared with AC distribution network, DC distribution network has many technical and economic advantages due to its low line cost, small transmission loss and high power supply reliability. As a DC protection device, the HVDC circuit breaker is of great significance to ensure the safe operation of the DC distribution network.
To sum up, in order to effectively suppress the spread of fault current, ensure the safe operation of DC transmission and distribution networks and the normal operation of equipment, HVDC circuit breakers will become an effective or even the only technical means.
2. Domestic and foreign development of HVDC circuit breakers
2.1 Mechanical HVDC circuit breaker
Mechanical HVDC circuit breakers are usually obtained after transformation of AC circuit breakers. According to different arc extinguishing principles, they can be divided into vacuum circuit breakers, sulfur hexafluoride circuit breakers, oil-rich (less oil) circuit breakers, and compressed air circuit breakers. , Magnetic blowing circuit breakers and gas circuit breakers. At present, vacuum circuit breakers and sulfur hexafluoride circuit breakers have replaced other circuit breakers on a large scale and are widely used in power systems. Vacuum circuit breakers use vacuum as the insulation and arc extinguishing medium between contacts. The insulation strength of the contact gap is high, and it has the advantages of safety, reliability, and long life. It is widely used in 10kV and 35kV power distribution systems, and the shutdown current level can reach 20 -63kA; SF6 circuit breaker uses SF6 gas as the medium for insulation and arc extinguishing. Due to the excellent characteristics of SF6 gas, the voltage and current parameters at the fracture are better than oil-less circuit breakers and compressed air circuit breakers, and do not require higher air pressure and The number of serial breaks is large, and the breaking current capacity can reach 40kA in the application of 252kV voltage level.
2.2 Solid State HVDC Circuit Breaker
With the continuous advancement of power electronics technology, solid-state HVDC circuit breakers have gradually emerged. The basic topology is shown in Figure 1. In the 1970s, solid-state HVDC circuit breakers using thyristors to turn off appeared; in the 1980s, with the birth of full-control devices such as gate turn-off thyristors (GTOs) and insulated gate bipolar transistors (IGBTs), solid-state HVDC circuit breakers There are new options for devices used in circuit breakers. In the 1990s, with the advent of new high-power devices such as integrated gate thyristors from ABB and Mitsubishi, more choices were made for solid-state HVDC circuit breaker topologies.
Foreign countries started earlier in the research of solid-state HVDC circuit breakers. In 1987, the 200 V/15 A solid-state HVDC circuit breaker developed in the United States used the gate turn-off thyristor as the main switching device; in 1999, Dr. Jeffrey A. Casey and others elaborated and listed the solid-state HVDC circuit breaker in power distribution Network distribution, cost and engineering application; Houston University subsequently developed a solid-state HVDC circuit breaker prototype with a voltage level of 500V; Circuit breaker prototype and passed the test.
In China, the research mainly focuses on the circuit topology of HVDC circuit breakers, and the test prototypes have a small capacity and are concentrated in special fields such as aerospace and ship systems. The research on solid-state circuit breakers applied to naval systems carried out by the Naval Engineering University in China focuses on breaking and current limiting under low voltage and high current, and the application occasions are limited. The 20 kV thyristor solid-state switch developed by the China Institute of Engineering Physics focuses on the research of thyristor series technology, and the current level is low.
2.3 Hybrid HVDC circuit breaker
In order to make full use of the advantages of small on-state voltage drop of mechanical switches and fast turn-off speed of power electronic devices, hybrid HVDC circuit breakers have become a current research hotspot. Hybrid HVDC circuit breakers can be obtained through a reasonable combination of mechanical switches and power electronic devices. Common topologies include direct parallel connection of mechanical switches and power electronic devices, series connection of mechanical switches and power electronic devices, and other topologies derived from this.
3. Key technology of HVDC circuit breaker
3.1 Series and parallel technology of power electronic devices
In high-voltage and high-current applications, power electronic devices need to be connected in series to improve the withstand voltage and parallel to improve the flow capacity. The dynamic and static voltage and current sharing problems caused by the differences in the device's own parameters and the influence of peripheral circuits are particularly prominent. When the power electronic device is used as the main branch of the HVDC circuit breaker, it generally does not need to be connected in parallel to meet the requirements of the cut-off current. However, in order to withstand a high breaking overvoltage, it is often necessary to use a large number of devices in series. This section will focus on the analysis Series voltage equalization problem.
The series voltage unevenness of power electronic devices is generally divided into two situations: static voltage unevenness and dynamic voltage unevenness. During the operation of the device, it will experience four working states: turn-on transient, turn-on steady state, turn-off transient and turn-off steady state. In the turn-on steady state and turn-off steady state, the voltage of each device in series remains basically stable, which is a static voltage balancing problem; in the turn-on transient state and turn-off transient state, the voltage of each device in series changes dynamically, which is a dynamic voltage balancing problem. Due to the complex factors affecting the voltage equalization in series, different voltage equalization strategies should be adopted under different working conditions.
3.1.1 Cause Analysis of Static Voltage Unevenness and Voltage Equalization Measures
When the devices in series are in the on-state steady state, they only bear a very low on-state voltage drop, and the voltage unevenness caused by the difference in device parameters has little impact on their safe operation, and can generally be ignored; when the devices in series are in the off-state steady state At this time, each device can be equivalent to a resistor with a large resistance, and only a small leakage current passes through it. At this time, the voltage at both ends of the device is generally high, and measures must be taken to solve the problem of uneven voltage.
In order to achieve static voltage equalization, firstly, devices with the same parameters and characteristics should be selected as far as possible. In addition, voltage equalization resistors can be connected in parallel between the collectors and emitters of each device in series. When the resistance is much smaller than the leakage resistance of the device, the voltage distribution mainly depends on The value of the equalizing resistance, which should usually be much lower than the off-state equivalent resistance of the device, and should be as large as possible.
3.1.2 Cause Analysis of Dynamic Voltage Unevenness and Voltage Equalization Measures
The factors that affect the dynamic voltage unevenness of series devices are mainly divided into two categories: the parameters of the device itself and the parameters of the peripheral circuit. Among them, the parameters of the device itself mainly include inter-electrode parasitic capacitance, tail current, gate internal resistance and stray inductance, etc., and the peripheral circuit mainly includes gate drive resistance, drive loop parasitic inductance, drive signal delay and absorption circuit, etc.
3.2 DC arc extinguishing technology
Compared with AC current, DC current has no natural zero-crossing point. Under high voltage level and high fault current level, if the DC current is forcibly disconnected with a mechanical switch, on the one hand, an arc with huge energy will be generated, which will seriously threaten the safety of equipment. On the other hand, it often takes tens of milliseconds from the action of the mechanical switch to the recovery of its reliable withstand voltage capability, which is difficult to meet the requirements of quick action. At present, the following strategies are adopted to deal with the arc extinguishing problem:
3.2.1 Study the characteristics of the DC arc and establish an accurate arc model, and develop a mechanical switch with stronger arc extinguishing ability and faster speed.
3.2.2 Learn from the working principle of the AC circuit breaker, artificially create the current zero-crossing point by means of an oscillating circuit.
3.2.3 A hybrid topology including mechanical switches and power electronic devices is adopted, and a reasonable off-time sequence control strategy is used to make the mechanical switch break under extremely small or even zero current.
3.2.4 Use an all-solid-state topology containing only power electronics to avoid arcing.
The above strategies have their own advantages and disadvantages. How to choose a reasonable arc extinguishing method is an important problem faced by HVDC circuit breakers.
4. Control and protection of HVDC circuit breaker
4.1 Fault current detection and judgment
As the key equipment for breaking the fault current, the HVDC circuit breaker's control system should accurately detect the dynamic changes of the loop current, reliably identify various working conditions such as short-circuit faults, current swells, and current fluctuations, and respond quickly.
The current protection strategies mainly include over-current quick-break protection, current increment protection, and current rate-of-change protection. The over-current quick-break protection trips immediately after detecting that the current exceeds the set value. It is generally used for quick removal of faults, and its accuracy and reliability are poor; Different protections have higher requirements for protection verification. In practice, the two protections use the same current rise rate setting value as the starting condition, and enter their respective delay stages after starting, and cooperate with each other to complete the protection function. The action process of the two protections is as follows:
The current rising rate protection continuously detects the current rising rate, and when it is higher than the protection threshold, the protection starts and enters the delay stage. In the delay period, if the current rising rate is always higher than the setting value, the protection will act, otherwise, the protection will return.
The current increment protection and the current rate of rise protection start at the same time, and the relay calculates the relative current increment based on the current at the start time. When the current rising rate is always higher than the slope set by the current incremental protection and the value reaches the closing value of the action after a delay, the protection operates. During the current increment calculation process, the current rising rate is allowed to drop below the set value in a short time. If this period of time does not exceed the return delay set value, the protection will not return; otherwise, the protection will return.
4.2 Opening and closing sequence control strategy
As the HVDC circuit breaker topology continues to evolve, its various parts often contain different types of components, such as fully-controlled devices, half-controlled devices, mechanical switches, and passive and active circuits. During the operation process of HVDC circuit breaker, the reliable timing coordination control strategy among various components determines whether the commutation process and shutdown process can be carried out normally, and is also of great significance to the operation of power electronic devices in a safe working area.
Generally speaking, for a HVDC circuit breaker topology with multiple parallel branches, a reasonable opening and closing sequence should meet the following basic requirements:
4.2.1 The action time is set according to the closed value of the current and the action delay of each component is considered, and the action duration meets the thermal design requirements.
4.2.2 Ensure that the mechanical switch breaks under zero arc or small arc.
4.2.3 When a branch is disconnected, it should be ensured that the previous flow branch has been completely commutated and reliably disconnected.
4.2.4 Avoid overvoltage and overcurrent of power electronic devices.
4.2.5 Prevent misoperation of energy absorbing branch surge arresters.
5. Application of HVDC circuit breaker
5.1 Application in multi-terminal flexible DC transmission system
DC side faults affect the calculation of equipment parameters and the design of control and protection strategies, and are a type of fault that must be considered in DC transmission systems. Although there are three types of methods to deal with DC side faults, due to technical constraints, AC side circuit breakers are still used to isolate faults in actual projects. In this processing mode, the system locks the converter station after detecting a fault to prevent bridge arm overcurrent, and then trips the AC side circuit breaker of each converter station to cut off the AC side current feed, and the DC current follows the bridge arm reactance energy The release tapers off to zero and finally trips the fast DC switches at both ends of the faulty line. And other converter stations need to close the AC circuit breaker again to restart. This processing strategy needs to trip the AC circuit breaker every time there is a fault, so that the DC system is disconnected from the external AC system. Taking into account fault detection, identification, AC circuit breaker action and fast DC switching action, the entire fault clearing process takes up to Hundreds of milliseconds, which will reduce the availability of the DC transmission system.
Since there are various short-circuit types and short-circuit points in the multi-terminal flexible HVDC transmission system, and the short-circuit current changes complexly, in order to make the HVDC circuit breaker topology proposed in this paper can deal with the DC side faults more reliably when used in the multi-terminal flexible HVDC transmission system, it is necessary to control the DC side The fault mechanism and the current change law under various fault types should be studied. At the same time, the original DC side fault protection method of the system and the HVDC circuit breaker control strategy should be combined to reasonably set the HVDC circuit breaker action sequence to ensure the safe and reliable operation of the entire system.
5.2 Mechanisms of different types of DC side faults
In a modular multilevel multiterminal flexible DC transmission system using bipolar symmetrical transmission, the DC side faults are generally divided into three categories: single-pole short-circuit faults, bipolar short-circuit faults, and disconnection faults. In the event of a unipolar short-circuit fault, since the DC side is grounded through a large resistance, which is approximately an open circuit, the sub-module capacitor has no discharge path, and the capacitor voltage remains basically stable; in the case of a bi-polar short-circuit fault, the sub-module capacitor passes through the upper fully-controlled The device forms a discharge circuit, and at the same time, the AC system forms an energy feed circuit through the lower diode of the sub-module, which is equivalent to a three-phase short circuit. At this time, the current of the sub-module is composed of the superposition of the two. Continue to feed current through the circuit until the AC side circuit breaker acts to cut off the feeding circuit.
5.3 HVDC circuit breaker demand analysis and parameter configuration
In the application environment of HVDC circuit breakers, the main fault types of the multi-terminal flexible DC transmission system can be divided into countermeasure internal short circuit, countermeasure external short circuit and overhead line short circuit. Each short circuit is further divided into unipolar short circuit and bipolar short circuit. Among various types of short-circuit faults, the short-circuit of the inner and outer double poles of the near-end parallel resistance of the converter station is a more serious type of fault, and the double-pole short-circuit fault of the inner parallel resistance of the converter station is the most serious. The fault current can reach 7kA within 2ms, and the peak current is 17kA.
The HVDC circuit breaker shall meet the operation requirements under the most serious fault mentioned above. Recently, the DC side fault protection methods of the multi-terminal flexible DC system mainly include bridge arm overcurrent protection and valve DC overcurrent protection. At this time, the converter starts blocking protection, and at the same time, the circuit breaker on the AC side acts to cut off the feeding of AC current. In order to minimize the fault development and enable the system to quickly re-establish the DC voltage after the fault recovers, the HVDC circuit breaker should operate before the converter is blocked. Action before action. At the lower threshold protection level, the converter will be blocked within 1ms, so the HVDC circuit breaker should also operate within 1ms, and it is difficult for the HVDC circuit breaker to meet this requirement when other parameters of the system remain unchanged.
In order to ensure the reliable operation of HVDC circuit breakers, for fault types with low fault current levels, the original protection current setting threshold can be increased to meet the requirements; for serious fault types near the large-capacity converter station, in addition to increasing the current threshold, A current-limiting inductor should also be configured for the DC circuit breaker to limit the rate of current rise.
As the key equipment to quickly and effectively deal with DC faults, high-voltage HVDC circuit breakers will play a key role in the development of multi-terminal DC transmission and DC grid technology in the future. The research on HVDC circuit breaker theory and topology has been carried out for a long time, but a series of problems such as DC arc extinguishing, series voltage sharing and energy absorption of power electronic devices still need to be solved. The rapid development of HVDC power grid has also put forward more and more urgent demands on HVDC circuit breakers. From the technical characteristics of various HVDC circuit breakers and the current research level, the hybrid DC breaking technology and the DC breaking technology based on artificial zero crossing are the most potential solutions for engineering applications.
Theoretical research and prototype development of high-voltage HVDC circuit breakers are not yet mature, and have not been widely used in actual engineering. There are still many deficiencies in the research on its key technologies:
The modeling of the DC network is based on a simplified unidirectional power flow model. The actual structure of the DC transmission and distribution network is complex, and the direction of the power flow is also uncertain. How to establish a more detailed system model and describe the fault current characteristics more accurately on this basis to provide a more reliable basis for the design of HVDC circuit breakers is the focus of follow-up research.
In the study of power electronic device series voltage balancing, the focus is on the dynamic voltage balancing problem during the turn-off process and the load-side snubber circuit. The voltage balancing problem in other states and the gate active voltage balancing strategy are still to be studied.
No detailed research has been done on the lightning arrester of the energy absorbing branch of the HVDC circuit breaker, and the general model is used in the simulation, but the parameter setting of the lightning arrester plays a key role in the reliable operation of the HVDC circuit breaker in practice; in addition, the research in this paper is still in the stage of theoretical analysis and simulation. The next step will be to carry out prototype design and test verification.