CN110460022A - No circuit breaker removal method for ring DC microgrid based on improved VSC - Google Patents

Abstract

The present invention relates to a kind of annular direct-current micro-grids based on improvement VSC without breaker cutting method, utilize the anti-paralleled diode in three ipsilateral bridge arms in three thyristor substitution VSC inverter three-phases, after the system failure, whole IGBT carries out fault location to provide DC side fault signature in locking improvement VSC first, the Continuity signal of three thyristors is cancelled after fault location, half of power frequency period improves VSC later and realizes automatic shutoff at the latest, after DC side line current continues zero passage, it can tripping faulty line both ends disconnecting switch, realize Fault Isolation.

Description

No circuit breaker removal method for ring DC microgrid based on improved VSC

technical field

The invention belongs to the field of relay protection for power systems, and in particular relates to a VSC topology transformation-based cutting strategy for a ring-shaped DC microgrid without a circuit breaker.

Background technique

With the growth of social economy and the enhancement of human awareness of environmental protection, new energy has become a hot spot of widespread concern in the power and energy industry and has been widely promoted. In addition, the proportion of some DC loads such as electric vehicles, frequency conversion equipment, LED lighting, information equipment, etc. is increasing day by day. However, based on the voltage source converter (Voltage Source Converter, VSC) using pulse width modulation (PWM) to realize the DC microgrid mixed with the AC system, as the main grid, distributed power supply and DC load are higher The efficient access form has the advantages of high efficiency, large power supply capacity, good anti-interference, high reliability, and low power loss, and has attracted extensive attention from scholars at home and abroad. However, as the bottleneck of its application, relay protection technology is still in the stage of theoretical research.

Among the many directions of protection technology research, the fault isolation method has become the focus of scholars' research. Although the traditional fault removal scheme based on the action of DC circuit breakers can achieve fast fault isolation, due to the high development cost of DC circuit breakers at present, large-scale application in low-voltage DC power grids with many branches will increase the construction and operation costs of the system accordingly. Therefore, the study of a fault removal strategy not based on circuit breakers is of great significance for the theoretical design and practical application of relay protection schemes, as well as improving the economy and reliability of system operation, and is an urgent need for the development and promotion of low-voltage DC systems. With the precise fault location method, the relay protection device can quickly realize the fault handling function, thereby improving the economy of system operation and the reliability of power supply.

Contents of the invention

The present invention provides a VSC-based annular DC microgrid without circuit breaker removal strategy for low-voltage DC systems based on two-level VSC operation. Compared with existing protection strategies based on DC circuit breaker removal faults, the faults proposed in the present invention The removal strategy does not need to configure a circuit breaker, and only relies on the fast isolating switches arranged at both ends of the ring DC system line to achieve fault isolation; compared with the existing handshake method based on VSC topology and fast isolating The three same-side bridge arms in the level VSC three-phase bridge arm are transformed, without disconnecting the VSC AC side circuit breaker, and only relying on the fault current self-shutdown capability of its own topology structure, the electrical isolation on both sides of the AC and DC can be realized. , which greatly shortens the time of fault recovery, which is of great significance for improving the reliability of system operation. The technical solution is as follows:

A circuit breaker-free cutting method for annular DC microgrid based on improved VSC, using three thyristors to replace the anti-parallel diodes in the three same-side bridge arms in the three phases of the VSC converter. After the system fails, first block the improved VSC All IGBTs provide fault characteristics on the DC side for fault location. After the fault location is completed, the conduction signals of the three thyristors are canceled. After half a power frequency cycle at the latest, the VSC is improved to realize self-shutoff. The line current on the DC side continues After zero crossing, the isolating switches at both ends of the faulty line can be tripped to realize fault isolation.

2. According to claim 1, a method for removing a ring-shaped DC microgrid without a circuit breaker, its specific implementation is as follows:

(1) After an inter-pole fault occurs in the system, when the DC side outlet capacitor of the improved VSC detects that the current on the capacitor branch reaches the preset threshold, a blocking signal is sent to all IGBTs in the improved VSC at the same time.

(2) Improve the VSC three-phase bridge arm and the IGBT in the DC side capacitor branch and complete the blocking within a few hundred microseconds after receiving the blocking signal, and improve the VSC into a three-phase uncontrolled rectification circuit, after which the DC side line is carried out fault location;

(3) After a fixed delay, cancel the conduction signal of the three thyristors inside the improved VSC. After a period of time, the thyristors of the three bridge arms on the same side in the three phases are all turned off, and the improved VSC is cut off and turned on. The DC side current A continuous zero crossing occurs;

(4) Afterwards, according to the fault location results, the isolating switches on both sides of the fault line are tripped to realize fault isolation, unlock the IGBT and thyristor, and restore the normal operation of the system.

The present invention has following advantage with respect to existing technology:

1. Compared with the existing protection strategy based on DC circuit breaker fault removal, the fault removal strategy proposed in the present invention does not need to configure a circuit breaker, and can realize fault isolation only by fast isolating switches arranged at both ends of the ring DC system line, maximizing It greatly reduces the construction and operation cost of the DC microgrid, which is of great significance for improving the economy of system operation.

2. Compared with the existing handshake method based on VSC topology and fast isolation switch, the fault removal strategy proposed in the present invention only needs to modify the three same-side bridge arms in the traditional two-level VSC three-phase bridge arm, Without disconnecting the VSC AC side circuit breaker, only relying on the fault current self-shutdown capability of its own topology, it can realize electrical isolation on both sides of the AC and DC sides, which greatly shortens the fault recovery time and is important for improving the reliability of system operation. significance.

Description of drawings

Figure 1 is a VSC topology with fault self-shutdown capability;

Figure 2 is a diagram of the VSC self-shutdown process;

Figure 3 is a diagram of the VSC three-phase current change process.

Fig. 4 is a time sequence diagram of the excision strategy.

Detailed ways

The present invention will be described in further detail below in conjunction with accompanying drawing and example.

Figure 1 shows the topological structure of VSC, the core component in the fault removal scheme proposed by the present invention. It can be seen that compared with the traditional VSC topology, the present invention only uses three thyristors to replace the traditional VSC on the basis of the traditional VSC topology. The anti-parallel diodes in the three bridge arms on the same side (same as above or below) in the three phases save the construction cost of the VSC to the greatest extent on the premise of ensuring their reliable shutdown.

Figure 2 is a diagram of the self-shutdown process of the new VSC topology. Figure 3 is a diagram of the change process of the new VSC three-phase current. Fig. 4 is a time sequence diagram of the excision strategy.

A circuit breaker-free cutting strategy for a ring-shaped DC microgrid based on VSC topology transformation. The specific shutdown process is as follows:

1. When the system fails, when the VSC DC side outlet capacitor detects that the current on the capacitor branch reaches its preset threshold, it sends a blocking signal to all IGBTs and thyristors in the VSC, and the VSC starts the self-shutdown process. The process 1～ It takes about 2ms to complete.

2. The VSC three-phase bridge arm and the IGBT in the capacitor branch of the DC side complete the blocking within a few hundred microseconds after receiving the blocking signal, and the capacitor branch is cut off, avoiding the discharge and energy loss of the DC side capacitor. At this time, the VSC becomes a three-phase controlled rectification circuit, and then the fault location is performed on the DC side line.

3. After 10ms, the fault location is over, and after a period of delay, the VSC will block the internal thyristor conduction signal. Assuming that the current directions of the three phases a, b, and c on the AC side of the VSC before the start of the fault isolation phase are the directions shown in Figure 2, this paper uses three thyristors to replace the three upper bridge arms of the three phases a, b, and c as an example. Detailed description:

For the two thyristors of only the upper bridge arm in the two phases b and c in Figure 2, they are in the reverse cut-off state before the system failure, and after the system failure, the two branches of the two phases b and c are canceled Only the thyristor of the upper bridge arm will no longer conduct, and because the b-phase current first decays to zero, then the b-phase will no longer conduct, at this time, as shown in Figure 2 process 1; after that, the a and c two phases will form a single The phase uncontrolled rectifier circuit continues to feed current to the DC side. At this time, as shown in Figure 2, process 2, during this process, the thyristor of the upper bridge arm of phase c is always on, and the current is continuously attenuated. When the inductor current of the AC side of phase a decreases At time 0, the thyristor of the upper bridge arm of phase a will be turned off immediately because the conduction signal has been withdrawn, as shown in process three in Figure 2 at this time. At this time, the thyristors of the upper bridge arms of the three phases a, b, and c are all turned off and turned on, and the self-shutdown process of the VSC is over, and the VSC no longer feeds the DC side. The duration of this process will be less than 20ms.

It should be noted that when the number of anti-parallel diodes replaced by thyristors is less than 3, at least two diodes in the three-phase six bridge arm branches of the VSC can form a circuit with the AC power supply, and cannot perform the function of self-shutoff , and then continue to feed current to the fault point on the DC side; and when using thyristors to replace the number of anti-parallel diodes is greater than 3, compared with the topology proposed by the present invention, it will not only increase the on-state loss of VSC operation, but also consider the thyristor The cost is higher than that of the diode, which will increase the construction and operation cost of the system. Therefore, in this paper, the number of thyristors is finally taken as three, and in order to ensure the reliable shutdown of the VSC, the installation position is selected on the same side of the three-phase bridge arm (both as the upper bridge arm or as the same lower bridge arm).

4. Afterwards, according to the fault location results, trip the isolating switches on both sides of the fault line to realize fault isolation, unlock the IGBT and thyristor, and restore the normal operation of the system.

In summary, the self-shutdown process diagram of the VSC topology can be shown in Figure 2, the VSC three-phase current change process diagram is shown in Figure 3, and the fault removal strategy action sequence diagram is shown in Figure 4.

Claims (2)

1. A ring-shaped DC microgrid cut-off method without circuit breaker based on improved VSC, using three thyristors to replace the anti-parallel diodes in the three same-side bridge arms in the three phases of the VSC converter. After the system fails, the improvement is blocked first All IGBTs in the VSC provide fault characteristics on the DC side for fault location. After the fault location is completed, the conduction signals of the three thyristors are canceled. After half a power frequency cycle at the latest, the VSC is improved to realize self-shutdown. In the DC side line After the current continues to cross zero, the isolating switches at both ends of the faulty line can be tripped to realize fault isolation. 2. A method for removing a circuit breaker in a ring-shaped DC microgrid according to claim 1, which is specifically implemented as follows: (1) After an inter-pole fault occurs in the system, when the DC side outlet capacitor of the improved VSC detects that the current on the capacitor branch reaches the preset threshold, a blocking signal is sent to all IGBTs in the improved VSC at the same time. (2) Improve the VSC three-phase bridge arm and the IGBT in the DC side capacitor branch and complete the blocking within a few hundred microseconds after receiving the blocking signal, and improve the VSC into a three-phase uncontrolled rectification circuit, after which the DC side line is carried out fault location; (3) After a fixed delay, cancel the conduction signal of the three thyristors inside the improved VSC. After a period of time, the thyristors of the three bridge arms on the same side in the three phases are all turned off, and the improved VSC is cut off and turned on. The DC side current A continuous zero crossing occurs; (4) Afterwards, according to the fault location results, the isolating switches on both sides of the fault line are tripped to realize fault isolation, unlock the IGBT and thyristor, and restore the normal operation of the system.