CN109217265B - A Method for Clearing DC Faults in a Current-Shifting Multilevel Converter Topology - Google Patents

Abstract

A kind of removing DC Line Fault method of electric current transfevent multilevel converter topology, belong to flexible DC transmission technology field, increase cutout branch, energy absorption branch and bridge arm on semi-bridge type multilevel converter underlying topology and shifts branch three parts branch, establish electric current transfevent modularization multi-level converter topology, disconnect the connection of inverter and faulty line rapidly by the branch that stops, branch is shifted by bridge arm again and energy absorption branch absorbs fault current, to quickly remove DC Line Fault.The present invention shifts by the cooperation of each branch in electric current transfevent modularization multi-level converter topology and removes Converter DC-side short-circuit current, and economy is good, practicability is stronger.

Description

A Method for Clearing DC Faults in a Current-Shifting Multilevel Converter Topology

technical field

The invention belongs to the technical field of flexible direct current transmission, and in particular relates to a topology and method of a current transfer type modular multilevel converter capable of clearing direct current faults.

Background technique

Modular multilevel converter (MMC) is a new type of power transmission technology proposed by German scholars R.Marquardt and A.Lesnicar. It adopts the cascading form of sub-modules and has high modularity and harmonic distortion. Since it was proposed in 2002, it has developed rapidly in the field of flexible direct current transmission. However, the half-bridge type MMC commonly used in practical engineering does not have the ability to block the DC fault current. When a bipolar short circuit occurs on the DC side, even if the converter is blocked, the freewheeling diode connected in antiparallel with the IGBT is still connected to the AC current. The system and fault points constitute a three-phase uncontrolled rectifier bridge, and the short-circuit current cannot be cleared, which seriously threatens the safe operation of the system.

The use of AC circuit breakers is almost the only economically feasible means in the existing MMC-HVDC. However, this method has a slow response speed, a long time for fault clearing and system recovery, and the huge fault current is likely to damage the freewheeling diode in the sub-module. Using new sub-modules (such as full-bridge sub-modules, clamping twin sub-modules, etc.) to self-clear DC faults is an effective means. The MMC based on the full-bridge sub-module has the strongest ability to clear DC faults, and the control technology is simple and mature, but there are too many components, which greatly increases the loss and input cost of the converter. Compared with the full-bridge sub-module, the clamping twin sub-module requires fewer components, but its structure shows a certain degree of coupling, which increases the complexity of control and voltage equalization. Using a DC circuit breaker to quickly clear the fault is an ideal means. The hybrid DC circuit breaker with the characteristics of low on-state loss and fast breaking speed has the most application prospect. However, due to the disadvantages of high cost, immature technology, and limited ability to break current, DC circuit breakers are rarely used in high-voltage and large-capacity applications. Therefore, there is an urgent need for a novel technical solution in the prior art to solve this problem.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for clearing DC faults based on the current transfer type multilevel converter topology. And clear the short-circuit fault current of the DC side of the converter, and has good economy and strong practicability.

A method for clearing a DC fault in a current transfer type multilevel converter topology, characterized in that: it includes the following steps, and the following steps are performed in sequence,

Step 1. Establishment of Current Transfer Modular Multilevel Converter Topology

The current transfer type modular multilevel converter topology includes a converter, a current breaking branch, an energy absorbing branch, and a bridge arm transfer branch. The current breaking branch includes an ultra-fast mechanical switch K and a current breaking switch T , and the ultra-fast mechanical switch K and the cut-off switch T are connected in series, and the cut-off branch is set in series at the DC outlet of the converter; the energy absorbing branch includes a freewheeling diode D _c and a absorbing resistor R _c , and The diode _Dc is connected in series with the absorbing resistor _Rc , the high-voltage end of the energy absorbing branch is connected to the end of the cut-off branch away from the converter, and the low-voltage end is connected to the DC negative line; the bridge arm transfer branch includes a bypass switch _Tb , the grounding switch T _d and two bypass resistors R _b of the same size, and the bypass switch T _b is connected in series with two bypass resistors R _b of the same size, and the lead wire between the two resistors is grounded through the grounding switch T _d , and the bridge The arm transfer branch is connected in parallel with the upper and lower arm inductances of each phase of the converter;

Step 2. Analyzing the current transfer type modular multilevel converter topology to clear DC fault process

A bipolar short circuit occurs on the DC line at time t ₀ , and the short-circuit current rises. The system detects a fault at time t ₁ and blocks the converter. After a delay, all bridge arm sub-modules have been blocked, and the conduction bridge is triggered at time t ₂ The bypass switch T _b and the grounding switch T _d of the arm transfer branch make the AC grid connected to the ground through the bridge arm inductance and the bypass resistance R _b , preparing for the shutdown of the cut-off switch T;

After a delay, the cut-off switch T is turned off at time _t3 , so that the current in the cut-off branch is transferred to other branches, and the current in the cut-off branch drops to 0. After the AC current flows through the inductance of the bridge arm, the branch is transferred to the bridge arm Transfer, flow into the ground through the bypass resistor R _b , the current of the DC fault line is transferred to the energy absorption branch, absorb the remaining energy of the DC line through the absorption resistor R _c , after the current transfer of the cut-off branch is completed, the current flowing through the sub-module of the bridge arm is zero;

After the current of the cut-off branch drops to zero, the ultra-fast mechanical switch K is turned off, and the ultra-fast mechanical switch K breaks the cut _- off branch without arcing, and the action is completed at time t4 _; after a delay, the grounding switch T is removed at time t5 The trigger pulse of the thyristor in _d , the grounding switch T _d is naturally turned off at the zero crossing point of the current, and the three-phase short circuit state of the AC power grid is over. Immediately after the fault has been cleared, it is ready for the next fault current breaking.

The determination condition of the shunt resistance R _b is, assuming t=t _b front bridge arm inductance initial current decays to zero, L ₀ is the bridge arm inductance, R _b is the shunt resistance, then the size of the shunt resistance R _b needs to be Satisfy

The conditions for determining the absorbing resistance R _c are as follows: assuming that the energy absorbing branch current decays to zero before t=t _c , the absorbing resistance is R _c , the DC line resistance is R _L , and the inductance is L _L , then the absorbing resistance R _c The size needs to meet

The condition for determining the rated voltage of the cut-off switch T is that a metal fault occurs at the outlet of the DC side of the topological DC side of the current transfer type modular multilevel converter, and the current of the bridge arm at the moment the cut-off switch T breaks satisfies The maximum withstand voltage of the cut-off switch T is In the formula, I _{bp0_max} and I _{bn0_max} respectively represent the maximum value of the current flowing through the bypass resistor R _b of the upper and lower bridge arms of the converter at the moment when the breaker switch T is turned off, I _m is the amplitude of the AC phase current in the steady state, and I _{0_max} is the DC maximum fault current when the cut-off switch T is broken, and R _c is the absorbing resistance.

Through the above design scheme, the present invention can bring the following beneficial effects: a method for clearing DC faults based on the current transfer type multilevel converter topology, after a DC bipolar short circuit fault occurs, the current transfer type modular multi-level converter is used to The current interruption branch of the flat converter topology quickly disconnects the connection between the converter and the fault line, and the bridge arm transfer branch and energy absorption branch absorb the fault current, thereby quickly clearing the DC fault. It has the advantages of scientific method, strong applicability and good effect.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and specific embodiment:

Fig. 1 is a schematic diagram of the topology structure of the main circuit of the current transfer type modular multilevel converter of the present invention.

Fig. 2 is a schematic diagram of the structure of the current path after the fault current is transferred according to the present invention.

FIG. 3 is a schematic diagram of the bridge arm inductance discharge path of the present invention.

Fig. 4 is a schematic diagram of the AC circuit after the fault current is transferred according to the present invention.

Fig. 5 is a schematic diagram of the AC path in the zero initial state of the present invention.

Fig. 6 is a schematic diagram of an equivalent circuit of an energy absorbing circuit of the present invention.

Fig. 7 is a simulation waveform diagram 1 of some parameters of the converter station 1 according to the embodiment of the present invention.

Fig. 8 is a simulation waveform diagram 2 of some parameters of the converter station 1 according to the embodiment of the present invention.

Fig. 9 is a simulation waveform diagram 1 of some parameters of the converter station 2 according to the embodiment of the present invention.

Fig. 10 is a simulation waveform diagram 2 of some parameters of the converter station 2 according to the embodiment of the present invention.

Detailed ways

A method for clearing a DC fault in a current transfer type multilevel converter topology, as shown in Figures 1 to 6, includes the following steps, and the following steps are performed in sequence,

Step 1: On the basis of the topology of the half-bridge multilevel converter, add three branches: the current-transferring branch, the energy absorbing branch and the bridge arm transfer branch, and establish a current-transferring modular multilevel converter topology : The current-breaking branch composed of an ultra-fast mechanical switch K and a current-breaking switch T is connected in series at the topological DC outlet of the modular multilevel converter. converter and DC fault line; on the side of the DC line, an energy absorbing branch composed of a freewheeling diode D _c and an absorbing resistor R _c is set. The terminal is connected to the DC negative line. When the system is operating normally, the diode bears the reverse DC voltage. This branch is in an open circuit state, which does not affect the stable operation of the system. During the fault clearing process, this branch provides discharge for the fault current on the DC line. circuit, to discharge the energy stored in the faulty line; a bridge arm transfer branch is set between the upper and lower bridge arm inductances of each phase of the converter, and the branch is to connect the bypass switch T _b and two of the same size The bypass resistor R _b is connected in series, and the lead wire between the two resistors is grounded through the grounding switch T _d . During normal operation, the switches T _b and T _d are both in the off state, and the bridge arm transfer branch does not work. fault, the transfer branch of the bridge arm is quickly turned on, providing a path for the AC current, and the AC current flows into the ground through the bridge arm inductance and bypass resistor R _b , so that no current flows through the bridge arm sub-module to protect the power electronics in the sub-module The device is not damaged;

Step 2: Analyze the current transfer type modular multilevel converter topology to clear the process of DC faults: a bipolar short circuit occurs in the DC line at time t ₀ , the sub-module capacitors discharge rapidly, the short-circuit current begins to rise sharply, and the system detects at time t ₁ If a fault is detected, the inverter will be blocked immediately. After a short delay, all the sub-modules of the bridge arm have been blocked. At time _t2 , the bypass switch T _b and the grounding switch T _d of the transfer branch of the bridge arm are triggered to make the AC The power grid is connected to the ground through the bridge arm inductance and the bypass resistor R _b to prepare for the shutdown of the cut-off switch T;

After a short delay, turn off the cut-off switch T at time _t3 , forcing the cut-off branch current to transfer to other branches, and the cut-off branch current quickly drops to 0. At this time, after the AC current flows through the bridge arm inductance, Transfer to the bridge arm transfer branch, flow into the ground through the bypass resistor R _b , the DC fault line current transfers to the energy absorption branch, and dissipate the energy stored in the DC line through the heat generation of the absorption resistor R _c , after the current transfer of the cut-off branch is completed , the current flowing through the sub-module of the bridge arm is zero, which ensures the safety of the power electronic devices in the sub-module;

After the current of the cut-off branch drops to zero, the ultra-fast mechanical switch K is turned off. The mechanical switch K needs about 2ms to cut off the cut _- off branch without arcing, and the action is completed at time t4. After a short delay, at time _t5 , the trigger pulse of the thyristor in the grounding switch _Td is removed, and then the grounding switch _Td will be turned off naturally at the current zero crossing point, and the resulting three-phase short-circuit state of the AC power grid ends, when the bridge arm When the inductor current and energy absorbing branch current decay to zero, the DC fault is completely cleared, and immediately prepares for the next fault current breaking after the fault clearing is completed.

The selection method of the bypass resistor R _b in the present invention is: when the cut-off switch T is disconnected, the AC grid current flows into the ground through the bridge arm inductance and the bypass resistor R _b , and the three phases are independent of each other. Taking the A phase as an example, Suppose u _sa is the AC voltage of phase A, I _b0 is the initial current of the bridge arm inductance of phase A, _{Req and L eq are} the resistance and inductance of the AC side respectively, L ₀ is the inductance of the bridge arm, R _b is the bypass resistance, and i _bpa is The inductor current of the upper bridge arm of phase A, the calculation formula of the inductor current of the upper bridge arm of phase A is: In the formula, the phase angle time constant For the bridge arm inductance discharge circuit, the current generally needs 5 times of the decay time constant to decay to approximately 0. Assuming t = t _b , the initial current of the bridge arm inductance decays to zero, and the size of the bypass resistor R _b needs to satisfy In the 2ms time before the ultra-fast mechanical switch is turned off, the resistance _values of the bypass resistor R _b and the absorbing resistor R _c jointly affect the voltage that the cut-off switch T bears. The larger the withstand voltage, so the appropriate selection of the resistance determines the number and cost of the components required for the cut-off switch T;

The selection method of the absorbing resistance _Rc in the present invention is: when the cut-off switch T is disconnected, the energy absorbing branch, the DC fault line and the short-circuit point constitute the freewheeling circuit of the DC line fault current, and the resistance of the DC line is set as R _L , The inductance is L _L , R _c is the absorbing resistance, and I _c0 is the initial current of the energy absorbing branch, then the formula for calculating the current of the energy absorbing branch is where the time constant Assuming that the current of the energy absorbing branch decays to zero before t=t _c , the size of the absorbing resistance R _c needs to satisfy It can be seen that the attenuation speed of the fault current of the DC line is related to the absorption resistance R _c , the length of the line and the impedance parameters, and the longer the DC line is, the greater the inductance value is, and the longer the fault current decay takes _. When designing the resistance value of the DC line, it should be considered according to the most severe situation of the DC line, that is, the equivalent impedance of the entire line should be considered. The increase of R _c resistance can shorten the fault current clearing time, but with the increase of R _c , the absorption The voltage drop at both ends of the resistor R _c increases, and before the ultra-fast mechanical switch is disconnected, the voltage borne by the cut-off switch T also increases correspondingly, which needs to be weighed and considered;

The method for selecting the rated voltage of the interrupter switch T in the present invention is as follows: when a metal fault occurs at the outlet of the MMC DC side, the current of the bridge arm at the moment when the interrupter switch T is broken meets When , the cut-off switch T withstands the maximum voltage, which is Among them, Ibp0_max and Ibn0_max represent the maximum value of the current flowing through the bypass resistor R _b of the upper and lower bridge arms of the converter at the moment when the cut-off switch T is turned off, Im is the amplitude of the AC phase current in the steady state, and I0_max is the cut-off switch The maximum DC fault current when T is broken. For AC current, the time from the occurrence of the fault to the disconnection of the cut-off switch T is very short (only a few milliseconds). It can be considered that the AC current is the same as the steady-state operation.

Simulation analysis: One embodiment of the present invention is based on the RT-LAB OP5600 simulation platform, and a 51-level double-terminal MMC-HVDC system model is built. The main parameters are shown in Table 1.

Table 1 Main parameters of 51-level MMC-HVDC system

The DC bipolar short-circuit fault occurs at t ₀ =2.0s, the fault resistance is 0.01Ω, the short-circuit point is 1km away from converter station 1, and 100km away from converter station 2. 0.1ms after the fault occurs, block the converter station. The shunt resistor R _b is 6Ω, and the absorption resistor R _c is 6Ω. Refer to Fig. 7 and Fig. 8 for the simulation waveforms of various parameters of converter station 1, and Fig. 9 and Fig. 10 for the simulation waveforms of various parameters of converter station 2.

It can be seen from Figure 7(a) that after the fault occurs, the DC current rises rapidly. At time _t1 , the converter station is locked, and the sub-module capacitor stops discharging. After a short delay, a conduction signal is applied to the bypass switch and the grounding switch on the transfer branch of the bridge arm at time _t2 , and the switch is quickly closed. At this time, the fault current flows through the bypass branch of the bridge arm, the current of the energy absorption branch and the fault point at the same time, and the distribution of the current of each branch is inversely proportional to the resistance of each branch. In this calculation example, since the resistance value of the bypass resistor R _b (or absorbing resistor R _c ) is 600 times that of the short-circuit point, there is only a small current flows.

The cut-off switch T is opened at time _t3 , as shown in Figure 7(a), the current of the cut-off branch drops to 0 rapidly, after that, the cut-off switch T bears a large voltage, observe Figure 7(b) and Figure 9 (b) It can be seen that the peak voltages of the cut-off switches T of the converter station 1 and the converter station 2 are about 28kV and 35kV respectively. At the same time, it can be seen from Figure 7(c) that the current of the sub-module of the bridge arm also drops to 0 rapidly, which ensures the safety of the power electronic devices in the sub-module. From Figure 8(c) and Figure 8(d), it can be observed that the fault current on the MMC bridge arm is transferred to the bridge arm transfer branch, that is, the AC current flows into the ground through the bridge arm inductance and bypass resistance R _b , which is quite Short circuit in three phases. From Figure 8(a), it can be observed that the fault current on the DC line side is transferred to the energy absorbing branch, and the energy stored in the DC line is dissipated through the absorbing resistor _Rc . After the current of the cut-off branch drops to zero, the ultra-fast mechanical switch is turned off, and the physical isolation of the faulty line and the converter is realized after about 2ms.

The trigger pulse of the grounding switch is turned off at time t5, as shown in Figure ₇ (d), the thyristor is naturally turned off when the current at the AC side port of the MMC crosses zero, and the three-phase short-circuit state of the AC grid that lasts for about 13ms ends. After the AC three-phase short circuit ends, there is still current in the bridge arm inductance, as shown in Figure 8(c) and Figure 8(d), this energy is gradually dissipated by the heat generated by the bypass resistor R _b , and the converter station can be calculated by the formula The time required for the bridge arm inductance currents of converter station 1 and converter station 2 to decay to 0 is about 50ms. For this reason, the entire fault clearing process is about 51ms.

The simulation analysis of each parameter of converter station 2 is the same as that of converter station 1, the difference lies in the attenuation speed of the energy absorption branch current. Since the fault point is only 1 km away from converter station 1, as shown in Figure 8(a), the fault current rapidly decays to 0, while the fault point is 100 km away from converter station 2, as shown in Figure 10(a), the fault current decays The speed is relatively slow, and the DC fault current decay time of converter station 1 and converter station 2 can be calculated by the formula to be 0.67ms and 50ms respectively.

Apparently, the above examples are only examples for clear illustration, rather than limiting the implementation. For those of ordinary skill in the art, on the basis of the above description, other changes or changes in different forms can also be made. It is not necessary and impossible to exhaustively list all the implementation manners here. And the obvious changes or changes derived therefrom are still within the scope of protection of the present invention.

Claims (4)

1. a kind of removing DC Line Fault method of electric current transfevent multilevel converter topology, it is characterized in that: include the following steps, And following steps sequentially carry out,

Step 1: the foundation of electric current transfevent modularization multi-level converter topology

Electric current transfevent modularization multi-level converter topology includes inverter, cutout branch, energy absorption branch and bridge arm Branch is shifted, the cutout branch includes supper-fast mechanical switch K and cut-off switch T, and supper-fast mechanical switch K and cutout are opened T series connection is closed, cutout branch is arranged in series in the direct current exit of inverter；The energy absorption branch includes sustained diode_c With absorption resistance R_c, and sustained diode_cWith absorption resistance R_cSeries connection, the high-voltage end and cutout branch of energy absorption branch are separate The connection of inverter one end, low-pressure end are connect with direct current anode circuit；The bridge arm transfer branch includes by-pass switch T_b, ground connection open Close T_dBypass resistance R identical with two sizes_b, and by-pass switch T_bBypass resistance R identical with two sizes_bSeries connection, and two The grounded switch T of lead-out wire between resistance_dGround connection, bridge arm shift the upper and lower bridge arm inductance in parallel of branch and each phase of inverter；

Step 2: analysis electric current transfevent modularization multi-level converter topology removes DC Line Fault process

DC line is in t₀Bipolar short circuit occurs for the moment, and short circuit current rises, and system is in t₁Moment detects failure and is latched the change of current Device guarantees that whole bridge arm submodules have been latched completion, in t by delay₂The bypass of bridge arm transfer branch is connected in time trigger Switch T_bWith earthing switch T_d, so that AC network passes through bridge arm inductance, bypass resistance R_bIt is connected to the earth, is cut-off switch T's Shutdown is prepared；

By delay, in t₃Moment turns off cut-off switch T, shifts cutout branch current to other branches, stops under branch current 0, after alternating current flows through bridge arm inductance is dropped to, to bridge arm transfer branch transfer, through bypass resistance R_bFlow into the earth, DC Line Fault Line current is shifted to energy absorption branch, passes through absorption resistance R_cDC line dump energy is absorbed, cutout branch current turns After shifting, the electric current for flowing through bridge arm submodule is zero；

After cutout branch current drops to zero, supper-fast mechanical switch K, supper-fast mechanical switch K thick-less vane cutout branch are turned off Road, in t₄Moment movement is completed；By delay, in t₅Moment removes earthing switch T_dThe trigger pulse of middle thyristor, ground connection are opened Close T_dIt is turned off naturally in current zero-crossing point, AC network three-phase shortcircuit state terminates, when bridge arm inductive current, energy absorption branch When electric current all decays to zero, DC Line Fault is fully erased, and DC Line Fault is immediately ready for fault current point next time after the completion of removing It is disconnected.

2. a kind of removing DC Line Fault method of electric current transfevent multilevel converter topology according to claim 1, It is characterized in: the bypass resistance R_bReally fixed condition is, if t=t_bPreceding bridge arm inductance initial current decays to zero, L₀For bridge arm electricity Sense, R_bFor bypass resistance, then bypass resistance R_bSize need to meet

3. a kind of removing DC Line Fault method of electric current transfevent multilevel converter topology according to claim 1, It is characterized in: the absorption resistance R_cReally fixed condition is, if energy absorption branch current is in t=t_cBefore decay to zero, absorption resistance For R_c, DC line resistance is R_L, inductance L_L, then absorption resistance R_cSize need to meet

4. a kind of removing DC Line Fault method of electric current transfevent multilevel converter topology according to claim 1, Be characterized in: the voltage rating of the cut-off switch T determines that condition is, electric current transfevent modularization multi-level converter topology direct current Metallic fault occurs at side outlet, cut-off switch T disjunction moment bridge arm current meetsCut-off switch T bears voltage maxIn formula, I_{bp0_max}、I_{bn0_max}Cutout is illustrated respectively in open Pass T disconnection moment flows through the upper and lower bridge arm bypass resistance R of inverter_bCurrent maxima, I_mAC phase currents amplitude when for stable state, I_{0_max}Direct current maximum fault current when for cut-off switch T disjunction, R_cFor absorption resistance.