CN110970875A - Combined current-limiting type direct current breaker for direct current power grid - Google Patents
Combined current-limiting type direct current breaker for direct current power grid Download PDFInfo
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- CN110970875A CN110970875A CN201910990997.2A CN201910990997A CN110970875A CN 110970875 A CN110970875 A CN 110970875A CN 201910990997 A CN201910990997 A CN 201910990997A CN 110970875 A CN110970875 A CN 110970875A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
Abstract
The invention discloses a combined current-limiting type direct current breaker for a direct current power grid, which comprises a current-carrying transfer branch and a current-limiting blocking branch; the current-carrying transfer branch is formed by connecting an ultra-fast mechanical switch and a load current conversion switch in series, one end of the current-carrying transfer branch is connected with a direct-current bus of a direct-current power grid, and the other end of the current-carrying transfer branch is connected with a direct-current transmission line of the direct-current power grid in series; the current-limiting blocking branch comprises a current-limiting branch, a main breaker branch and an energy absorption branch, wherein one end of the main breaker branch and one end of the energy absorption branch are connected in series with one end of the current-limiting branch after being connected in parallel, and the other end of the current-limiting branch is connected to a potential point A1; the other end of the main breaker branch and the energy absorption branch are connected in parallel with a potential point A2. The invention can realize the separation and sharing of the expensive breaking assembly from the circuit breaker, and can effectively reduce the investment cost and the occupied area. The thyristor in the current-limiting blocking branch circuit can share the voltage to ground, and the charging resistor and the current-limiting inductor can effectively reduce the speed increase of fault current and reduce the on-off current of the circuit breaker.
Description
Technical Field
The invention relates to the field of power system automation, in particular to a combined current-limiting type direct current breaker for a direct current power grid.
Background
With the increase of new energy grid-connection consumption demand, the high-voltage direct-current power grid is widely concerned as an effective means for new energy grid connection and high-power long-distance power transmission, wherein the modular multilevel converter based on the voltage source commutation technology has the advantages of PQ decoupling control, low harmonic level, capability of supplying power for a passive system and the like, has no troubles of commutation failure, reactive compensation and the like, and becomes a primary choice for forming the multi-terminal direct-current power grid.
However, the MMC has many problems to be solved urgently in the process of constructing the direct-current power grid, such as fault protection. At the moment when a direct current power grid fails, the capacitance of the MMC sub-module is rapidly discharged to a fault point, the fault current can be increased to dozens of kiloamperes within milliseconds, the reliability of a power electronic device is reduced, and the stability of a system is greatly influenced. When in fault, the direct current voltage is rapidly reduced, and fault current has no natural zero crossing point, so that the fault current is difficult to clear, and the direct current network is seriously influenced. In addition, the overhead line fault rate of the possible large-scale application in the future is higher, which puts higher requirements on the effectiveness and the economy of a direct current fault isolation scheme. The hybrid direct current circuit breaker has the advantages of small on-state loss, short on-off time, no arc on-off current and the like, is one of the most potential fault protection elements, but economic benefit limits the popularization and application of the hybrid direct current circuit breaker.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a combined current-limiting type direct current breaker for a direct current power grid, which comprises a current-carrying transfer branch and a current-limiting blocking branch;
the current-carrying transfer branch is formed by connecting an ultra-fast mechanical switch and a load current conversion switch in series, one end of the current-carrying transfer branch is connected with a direct-current bus of a direct-current power grid, and the other end of the current-carrying transfer branch is connected with a direct-current transmission line of the direct-current power grid in series;
the current-limiting blocking branch comprises a current-limiting branch, a main breaker branch and an energy absorption branch, wherein one end of the main breaker branch and one end of the energy absorption branch are connected in series with one end of the current-limiting branch after being connected in parallel, and the other end of the current-limiting branch is connected to a potential point A1; the other end of the main breaker branch and the energy absorption branch are connected in parallel with a potential point A2.
Preferably, the current limiting branch in the current limiting blocking branch comprises a charging capacitor C, charging resistors R1-R2, a current limiting reactor CLR, and current commutation elements T1-T4; t1 is a bridge arm 1 and is connected between potential points D1 and D3, T2 and R1 are connected in series to form a bridge arm 2, the bridge arm 2 is connected in series between potential points D1 and D4, T3 is a bridge arm 3, the bridge arm 3 is connected between potential points D3 and D2, T4 and R2 are connected in series to form a bridge arm 4, the bridge arm 4 is connected between potential points D4 and D2, and a charging capacitor C is connected between potential points D3 and D4; the current phase-change elements T1-T4 are composed of thyristors connected in parallel in the forward and reverse directions; the potential point D1 is connected with a direct current bus, and the potential point D2 is connected with one end of the main breaker branch and one end of the energy absorption branch which are connected in parallel; the potential point D3 is arranged between the bridge arms T1 and T3; the potential point D4 is arranged between the bridge arms T2 and T4.
Preferably, the potential point A1 is connected with a polar direct current bus; the connection mode of the potential point A2 is as follows: if the direct current power grid is a true bipolar system with two poles capable of independently operating, the potential point A2 is connected with a neutral bus of a grounding pole;
if the direct current power grid is a pseudo bipolar system, if the alternating current side of the converter valve is grounded, and a fault current path is formed by the fault point and the alternating current grounding point after the single-pole fault, the connection mode of the potential point A2 is the same as that of the true bipolar system; if the converter valve AC side is not grounded, potential point A2 is connected to the other pole DC bus.
Preferably, the load commutation switch in the current-carrying transfer branch is formed by reverse series-parallel connection of insulated gate bipolar transistors, the main circuit breaker branch in the current-limiting blocking branch is formed by reverse series-parallel connection of insulated gate bipolar transistors, the number of the insulated gate bipolar transistors in the reverse series-parallel connection is determined by the rated voltage and the rated current which are calculated, and the rated voltage, the current and the direction coefficient of the insulated gate bipolar transistor, and the calculation formula is as follows:
n in the above formulaLCS、nMBNumber of insulated gate bipolar transistors, I, of load commutation switch and main breaker branch, respectivelyN_LCS、UN_LCSFor voltage and current at the time of opening and closing of load converter switch, UN、INFor maximum withstand voltage and maximum breaking current of DC circuit breakers, IC、VCESIs rated voltage and rated current, k, of the insulated gate bipolar transistord1Is 2, kd2Is 1.
Preferably, the parameter setting values of the combined current-limiting type direct current circuit breaker are as follows:
(1) the rated voltage of the main breaker branch circuit is the rated voltage of a line, and the rated current of the main breaker branch circuit is the fault current flowing into a bus after the bus bipolar short-circuit fault is quickly cut off;
(2) the initial value of the overvoltage of the load commutation switch is the product of the change rate of the fault current at the action moment of the load commutation switch and the current-limiting reactance current-limiting reactor, and the rated current is 1.1-1.5 times of the rated current of the fault line under the steady-state operation condition;
(3) current commutation element T1～T4The rated voltage is 1/2 of the maximum voltage value generated by the current-limiting reactor at the moment of current reduction, and the rated current is the same as the rated current of the branch circuit of the main circuit breaker;
(4) the value of the current limiting reactor is the same as the reactance value of the current limiting reactor connected in series with the power transmission line; charging resistor R in current-limiting blocking branch1And R2The resistance values of the current limiting reactors are the same, and the value of the resistance values is 1/2 of the ratio of the maximum voltage value generated by the current limiting reactors when the current is reduced to the rated current under the stable operation condition of the system; the value of the charging capacitor C is 1/4000R1(ii) a And the rated voltage of the lightning arrester is set as the rated voltage of the power transmission line in steady operation.
Further, the control method for isolating the direct current line fault and reclosing the combined current limiting type direct current breaker comprises the following steps:
(1)t0at the moment of time, the time of day,failure of the DC line, t1At the moment when the system detects fault current, after time delay, t2Constantly turning on the IGBT in the main breaker branch and triggering T1、T3The load current conversion switch connected in series with the fault line is turned off, a brake-separating signal is applied to the ultra-fast mechanical switch, and current is transferred to the current-limiting blocking branch circuit;
(2)t3time of day, trigger T4When the charging capacitor begins to discharge and the discharge current direction is opposite to the fault current, T flows2Is reduced to 0 and the fault current is transferred to T4Branch, charging resistor R2The voltage on both sides increases until the fault current shifts to T4A branch circuit, wherein the voltage of the charging capacitor is reversed;
(3)t4at all times, triggering the thyristor T2When the discharge direction of the charging capacitor is opposite to the above process, the fault current is from T1Transfer to T2Charging resistor R1The voltage on both sides increases until the fault current is completely transferred to T2A branch circuit;
(4)t5at the moment, the ultra-fast mechanical switch is opened to a rated opening distance, a turn-off signal is applied to the insulated gate bipolar transistor in the branch circuit of the main circuit breaker, fault current is transferred to the metal oxide lightning arrester of the lightning arrester, t6The fault current drops to 0 at this time.
(5)t7At the moment, conducting a current-limiting blocking branch of the combined current-limiting type direct current circuit breaker far away from the fault, if fault current is monitored in delta t, judging that the fault type is a permanent fault, and t8The current-limiting blocking branch at the current detection side is turned off again after the moment, the fault line is isolated, and the line is maintained and shut down; if the fault current is not monitored within 2-5 times of delta t time, the fault is judged to be an instantaneous fault, t9And conducting the current-carrying transfer branches at the two ends of the fault line at any moment, and finishing reclosing.
Furthermore, the current-limiting blocking branch is connected in series with a charging resistor, and the voltage drop at two ends of the charging resistor is used for offsetting negative voltage generated by the current-limiting reactor due to fault current reduction.
Further, in the above-mentioned case,during the period of the current-limiting blocking branch circuit being disconnected with the main breaker branch circuit at the moment of conducting the current-limiting blocking branch circuit, the charging resistor is switched between a short-circuited state and a state of being connected in series with the current-limiting blocking branch circuit, and the switching process is as follows: at the moment of conducting the current-limiting blocking branch, the short circuit of the charging resistor reduces the initial value of overvoltage at two ends of the load commutation switch at the moment of segmenting the load commutation switch; controlling the current commutation element T by pre-charging and reverse-charging of the charging capacitor before the main breaker branch is opened1～T4The charging resistor is connected in series with the current-limiting blocking branch circuit.
The invention has the beneficial effects that: 1) by sharing the current-limiting blocking branch, the selective investment is realized according to the requirement of the fault clearing line, so that the investment cost and the occupied area are greatly saved;
2) the combined current-limiting type direct current breaker has obvious current-limiting effect. The current-limiting branch circuit uses a thyristor to switch the charging resistor, the operation is simple, and the economic benefit is obvious.
Drawings
Fig. 1 is a topology diagram of a combined current limiting dc circuit breaker for a dc grid;
FIG. 2 is a schematic diagram of a control sequence of the DC circuit breaker when a fault current is cut off;
FIG. 3 is a schematic diagram of a fault current flow path of the current limiting branch;
FIG. 4 is a schematic diagram of the control sequence of the DC circuit breaker when normal current is cut off;
FIG. 5 is a reclosing flow diagram;
FIG. 6 is an equivalent schematic diagram of the DC side circuit of the MMC;
FIG. 7 is a schematic diagram of the dynamic response of the DC breaker current and voltage when breaking a fault current;
FIG. 8 is a schematic diagram of the dynamic response of the DC breaker current and voltage when normal current is cut off;
figure 9 is a multi-line circuit breaker topology;
figure 10 is a schematic diagram of the opening process of the multi-line circuit breaker;
FIG. 11 is a schematic diagram of the voltage current dynamic response of the multi-line circuit breaker;
fig. 12 is a four-terminal dc grid fault system dynamic response diagram.
Detailed Description
The technical solutions of the present invention are further described in detail below with reference to the accompanying drawings, but the scope of the present invention is not limited to the following.
On the basis of the existing research, the invention provides a combined current-limiting type direct current breaker for a direct current power grid, and the basic topological structure of the direct current breaker is shown in figure 1.
The direct current breaker mainly comprises a current-carrying transfer branch and a current-limiting blocking branch CLB. The load flow transfer branch is formed by serially connecting an ultra-fast mechanical switch UFD and a load flow conversion switch LCS, and the LCS is formed by serially connecting a plurality of reverse serially-connected IGBTs. The current-limiting blocking branch consists of a current-limiting branch, a main breaker branch MB and an energy absorption branch, and the current-limiting branch consists of a charging capacitor C, charging resistors R1 and R2, a current-limiting reactor CLR and four identical thyristors which are connected in series and in parallel with each other and are divided into components T1-T4. A plurality of reverse series-parallel IGBTs constitute MB for switching off fault currents. The energy absorption branch is composed of a metal oxide arrester MOV and used for absorbing residual energy.
The combined current-limiting type direct current breaker is arranged at an outlet of the converter station, a node A1 is connected with a direct current bus, and the connection mode of a node A2 is related to the direct current converter valve topology and the alternating current valve side grounding mode. If the direct current power grid is a true bipolar system with two poles capable of independently operating, the node A2 is connected with a neutral bus of the grounding electrode, and the positive electrode and the negative electrode of the direct current bus outlet of the converter station are respectively connected with a direct current breaker.
If the dc grid is a pseudo-bipolar system (symmetrical monopole configuration), the connection point of node a2 is related to the grounding mode of the ac side of the valves of the system converter station. If the alternating current side of the converter valve is grounded, a fault point can form a fault current path with an alternating current grounding point after a single-pole fault, the same as the true bipolar situation, the node A2 is connected with a neutral bus of a grounding electrode, and two breakers are respectively arranged at two poles at a direct current outlet of the converter station. If the alternating current side of the converter valve is not grounded, a single-pole fault point cannot form a current path through alternating current grounding, no short-circuit current exists, only a zero potential point of a direct current system is deviated, the voltage of a positive electrode and a normal electrode is increased, and the system can still keep normal operation, so that the system only generates short-circuit current after bipolar fault, A2 is connected with another direct current bus, namely only one direct current breaker is arranged at the outlet of the converter station, and the direct current breaker only responds to the bipolar short-circuit fault.
The specific functions of each branch of the dc circuit breaker are as follows:
1) the current-carrying transfer branch circuit is connected in series with two ends of the direct current circuit and can be switched on and off in a bidirectional mode. When the system normally operates, the system is used for circulating normal working current; and after the circuit breaker detects a line fault, the LCS auxiliary switch is disconnected and a turn-off signal is applied to the UFD, the UFD can cut off the line at zero current, and the non-arc breaking is completed by delaying Tmtd (2 ms) after the circuit breaker starts to act.
2) The current-limiting blocking branch circuit is a core component of the combined current-limiting type direct current circuit breaker. The voltage difference between the nodes A1 and A2 is larger, and considering that the thyristors have the advantages of large capacity, low manufacturing cost and the like, the voltage under the normal operation condition can be shared by a small number of thyristors, so that the operation reliability of the circuit breaker is improved.
The CLR and the charging resistor in the current-limiting blocking branch can increase the impedance value of the branch and reduce the change rate of fault current. However, at the moment of opening the LCS, the larger the branch impedance value is, the larger the initial overvoltage value UN _ LCS on both sides of the LCS is. In order to reduce the impedance value of the current limiting blocking branch circuit at the moment of LCS disconnection, the charging resistor is short-circuited before LCS disconnection, and the charging resistor is put into the branch circuit before MB disconnection, so that the charging resistor needs to be switched between a short-circuit state and a series-connection state.
The thyristor can be used to complete the switching and converting process of the charging resistor, but because the thyristor can not be cut off bidirectionally, an external power supply is required to discharge to force the thyristor to change phases. The four sub-components are switched on and off by adopting simple external circuit pre-charging and discharging and reverse discharging of the charging capacitor C, so that the aim of blocking the branch circuit by the charging resistor in a step-by-step input flow-limiting mode is fulfilled, the voltage of the direct-current bus is gradually recovered, and the time for the bus voltage to drop is shortened. The charging point resistor and the CLR can dissipate a part of residual energy after the MB branch is disconnected.
First, taking the occurrence of a fault in a dc transmission line as an example, a fault handling strategy of a combined current limiting type dc circuit breaker is introduced, wherein a fault current flowing through a current limiting branch is shown in fig. 3.
1) At time t0, the dc line is faulted, and at time t1 the system detects a fault current. After a short time delay, the IGBT in the MB branch is turned on at time T2, T1 and T3 are triggered, the LCS module is turned off, a switching signal is applied to the UFD, and the current is transferred to the current-limiting blocking branch.
2) At time T3, T4 is triggered, the charging resistor R2 is connected in series with the branch, the charging capacitor starts to discharge, and the direction of the discharging current is opposite to that of the fault current. The fault current is gradually transferred to the branch T4 from the branch T3 until the fault current is completely transferred to the branch T4, and the charging capacitor is reversely charged.
3) At time T4, T2 is triggered, and the charging resistor R1 is connected in series with the branch circuit, where the discharging direction of the charging capacitor is opposite to that of process 2). The fault current is gradually diverted from T1 to T2 until the fault current is completely diverted to the T2 branch. The duration of the thyristor commutation process is about 2ms, which is the same as the time for opening the UFD to the rated open distance.
4) At the time t5, a turn-off signal is applied to the IGBT in the MB branch circuit, the fault current is immediately transferred to the arrester MOV, the arrester absorbs residual energy, and the fault current is reduced to 0 at the time t 6.
5) After the fault is removed, the charging capacitor is precharged again after tens of milliseconds, so that the charging capacitor is restored to the original state capable of converting current and is ready for the next conversion.
The control sequence of each element of the combined current-limiting type direct current breaker is shown in figure 2. During the above operation, t0～t1For fault detection time, it is generally less than 1ms, t1～t2Interval 0.05ms, t2～t3At an interval of 0.5ms, t3～t4At intervals of 1ms, t4～t5At intervals of 1ms, t5～t6The interval is 1.5ms, and the total time from the occurrence of the fault to the isolation of the fault by the DC circuit breaker is about 5 ms.
Under the non-fault condition, when the line is overhauled or the grid structure is changed, the direct current circuit breaker can be used for switching on and switching off the normal direct current, and the power transmission line is isolated. At this time the current does not increase rapidly in a short time like the fault short-circuit current,the LCS has higher voltage bearing level and the capability of breaking normal current, thereby simplifying the breaking process: t for DC circuit breaker at two ends of line1、T3And applying a trigger signal to conduct the MB branch, applying a turn-off signal to the LCS and the UFD, and transferring the current to the MB loop. The UFD is opened to a rated opening distance, the MB branch is disconnected, the MOV absorbs residual energy, and the control sequence of the circuit breaker is shown in a figure 4.
In the future, most of direct-current power grid transmission lines are overhead lines, and the fault type is that the instantaneous fault probability is high, so that reclosing needs to be considered in the fault strategy of the direct-current circuit breaker. If the fault is superposed on the permanent fault, the system may be subjected to the impact of short-circuit current again, the service life of the circuit breaker is influenced, and the fault type needs to be accurately judged before the superposition. The strategy provided by the scheme temporarily adopts a simpler current rapid detection method, firstly, a far side of a fault point is set as a current detection side, and the other side of the fault point is set as a closing side. After a current-carrying transfer branch of the circuit breaker on the reclosing current detection side, a fault current change value is detected: h-didt
1) After the circuit breaker is disconnected due to a fault, in order to ensure that the circuit breaker can recover the insulation performance, the trip free time at least longer than 100ms needs to be waited, and the trip free time is set to 200 ms. When the circuit breaker is overlapped with a permanent fault, the fault current continuously rises, and the detection time is 0.5ms for reducing the breaking current when the circuit breaker acts again and shortening the development time of the fault current. To improve reliability, the detection time of the instantaneous fault may be set to 2.5 ms. t is t7Starting a reclosing program at the moment, if the change value H of the current is greater than a preset threshold value H within 0.5ms of a branch at the reclosing current detection side, judging that the fault is a permanent fault, and t8And breaking a current-carrying transfer branch of the circuit breaker at any time, isolating a fault line, and overhauling and stopping the line. If the current change still does not reach the threshold value H within 2.5ms, the fault type is judged to be an instantaneous fault, t9And the current-carrying transfer branch of the circuit breaker on the other side is overlapped at any moment, the reclosing is finished, and the system recovers to normal operation. The reclosing flow diagram is shown in figure 5.
2) After the breaker is disconnected under normal conditions, the fault type does not need to be detected in principle, and the UFD and LCS switches on two sides can be closed when needed. For the sake of safety, the transmission line can still be detected according to the above process, and the current detection side can be any side.
The direct current fault strategy provided by the invention only aims at line faults, and in addition, when the direct current circuit breaker is used for isolating faults, in order to avoid the converter station from being locked due to overlarge bridge arm current, the on-off current of the combined current limiting type direct current circuit breaker needs to be coordinated with the converter station.
In a direct-current power grid formed by half-bridge converter stations, by taking the occurrence of unipolar metal ground faults as an example, the fault current i is solveddcThe circuit equivalent diagram of (1) is shown in fig. 6. R0For losses of switching devices and bridge arm reactors, L0Is a bridge arm reactance, C0Is the sub-module capacitance value, N is the sub-module number, RdcEquivalent resistance of overhead line of DC power grid, LdcEquivalent reactance, U, of neutral impedance and current-limiting reactance of DC network framedcAnd I0The dc side voltage and the line current are respectively in normal operation.
Wherein:
generally, R when the metal is groundeddcLess than R at non-metal grounding (small resistance, large resistance grounding) fault at same positiondc. Compared with the metal grounding, the non-metal grounding has smaller fault current and larger voltage value of a fault point. The influence of non-metallic grounding on the circuit breaker is: u shapeN_LCSNumerical value is moreAnd when the current is large, the fault current of the cut-off of the current-limiting blocking branch is smaller, and the voltage of other elements is influenced to a certain extent but is within the rated range. Therefore, when the design parameters of the direct current circuit breaker are calculated, the maximum breaking current I of the circuit breaker required by the direct current power grid is calculated by taking metal grounding as an exampleNCalculating the rated voltage U of the breaker required by the DC power grid by taking large-resistance grounding as an exampleN。
Theoretically, the larger the CLR value, the better the current limiting effect, but at the moment of the current limiting blocking branch (current carrying transfer branch is disconnected), the U isN_LCSMagnitude and current limiting blocking branch impedance value (L)CLR) The CLR is not connected in series with the direct current line, the value of the CLR is not restricted by the stability of the system, but the larger the CLR value is, the more energy is stored in the CLR value, the dissipation speed is influenced, and the flexibility of the current-limiting blocking branch is influenced. But the voltage of the MB branch is greater than UN_LCSCompared with the reduction of the number of IGBTs connected in series by the LCS, the reduction of the number of IGBTs connected in parallel by the MB is higher in economic benefit. Thus LCLRIt should not be too large or too small. The value is set here to be the same as the transmission line inductance value:
LCLR＝Ldc
from the foregoing analysis, UN_LCSAnd is positively correlated with the impedance value of the current-limiting blocking branch. At the moment of LCS disconnection, the charging resistor is short-circuited, and the impedance in the branch consists of IGBT, thyristor and CLR with small on-state loss, so that U is formedN_LCSMainly determined by the voltage on both sides of the CLR at the moment of switching on and off:
UN_LCS＝LCLRdidc/dt|t＝t2
the size of the charging capacitor C determines the discharging speed and the time for the charging resistor to be put into the current-limiting blocking branch circuit. t is t3Trigger T4When passing through T4Current of (I)4：
In the formula Idc1Is t ═ t3Time idcNumerical value of (t), UCFor charging electricityPre-charge voltage of capacitor C with time constant tau-R2C. The combined current-limiting type direct current circuit breaker requires the capacitor C to complete the charging and discharging process within 1ms, and the capacitor can complete the discharging process within 4 time periods, namely the value of the capacitor C meets the following expression:
t＝4τ＝4R2C≤1ms
the maximum value of the C value is:
C＝14000R2
formula (III) C, R2The units of (a) and (b) are F and omega, respectively.
When R is2When C is a fixed value, if U isCSmaller, too small discharge current may result in I2、I4The current can not be reduced to 0, so that the problem of abnormal current conversion is caused, and the current limiting effect is influenced. In order to ensure the normal operation of the commutation process, the pre-charging capacitor should not be too small:
UC≥R2Idc1
although the current limiting effect is more obvious when the charging resistance value is larger, R is shown by the above formula2The larger the value, the smaller the C value, UCThe larger the voltage resistance of the charging capacitor, the larger the price and volume of C. So R2Cannot be too large, considering the voltage-sharing control of the thyristor, R1、R2Should be the same, here it is set as:
in the formula of UCLRThe maximum voltage across CLR after MB is turned off.
The number of main power electronic devices IGBT and thyristor of the combined current-limiting type direct current breaker is calculated. Calculating the parameters of DC breaker needed by DC network in advance, such as initial value overvoltage U of breaker LCS at fault momentN_LCSLCS switching-on/off instantaneous current IN_LCSMB with a maximum open-end current of INRated voltage U of DC circuit breakerNThyristor-shared voltage to ground UN_TAnd the like. The rated voltage and rated current of IGBT are respectively VCES、 ICRated voltage of thyristor is VSCRRated current of ITR. The number of IGBTs required by the LCS is:
wherein the LCS is connected in series with the transmission line and can be opened and closed bidirectionally, so that the direction coefficient kd＝2；
The total number of the current-limiting blocking branch thyristors is as follows:
in order to achieve the purpose of current conversion, thyristors need to be connected in series in a reverse direction, and the direction coefficient k isd＝2。
The direct current breaker only needs the fault current which is cut off in one direction, and the direction coefficient k isd＝1。
The maximum voltage stress across the arrester, i.e. the level of protection of the arrester, is generally the rated voltage E of the arresterMOV1.5-2 times of the total amount of the active component. After the MB branch is disconnected, in order to avoid the lightning arrester from being broken down due to overhigh voltage, the rated voltage of the lightning arrester is set as the rated voltage of the line:
UMOV＝UN
the foregoing is illustrative of the preferred embodiments of this invention, and it is to be understood that the invention is not limited to the precise form disclosed herein and that various other combinations, modifications, and environments may be resorted to, falling within the scope of the concept as disclosed herein, and that changes may be made by those skilled in the art or by those who are already aware of the above teachings or by those skilled in the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (8)
1. A combined current-limiting type direct current breaker for a direct current power grid is characterized by comprising a current-carrying transfer branch and a current-limiting blocking branch;
the current-carrying transfer branch is formed by connecting an ultra-fast mechanical switch and a load current conversion switch in series, one end of the current-carrying transfer branch is connected with a direct-current bus of a direct-current power grid, and the other end of the current-carrying transfer branch is connected with a direct-current transmission line of the direct-current power grid in series;
the current-limiting blocking branch comprises a current-limiting branch, a main breaker branch and an energy absorption branch, wherein one end of the main breaker branch and one end of the energy absorption branch are connected in series with one end of the current-limiting branch after being connected in parallel, and the other end of the current-limiting branch is connected to a potential point A1; the other end of the main breaker branch and the energy absorption branch are connected in parallel with a potential point A2.
2. A combined current-limiting dc breaker for dc networks according to claim 1, wherein the current-limiting branch of the current-limiting blocking branch comprises a charging capacitor C, charging resistors R1-R2, a current-limiting reactor CLR, current commutation elements T1-T4; t1 is a bridge arm 1 and is connected between potential points D1 and D3, T2 and R1 are connected in series to form a bridge arm 2, the bridge arm 2 is connected in series between potential points D1 and D4, T3 is a bridge arm 3, the bridge arm 3 is connected between potential points D3 and D2, T4 and R2 are connected in series to form a bridge arm 4, the bridge arm 4 is connected between potential points D4 and D2, and a charging capacitor C is connected between potential points D3 and D4; the current phase-change elements T1-T4 are composed of thyristors connected in parallel in the forward and reverse directions; the potential point D1 is connected with a direct current bus, and the potential point D2 is connected with one end of the main breaker branch and one end of the energy absorption branch which are connected in parallel; the potential point D3 is arranged between the bridge arms T1 and T3; the potential point D4 is arranged between the bridge arms T2 and T4.
3. A combined current-limiting dc breaker for a dc network according to claim 1, wherein said potential point a1 is connected to a dc bus; the connection mode of the potential point A2 is as follows: if the direct current power grid is a true bipolar system with two poles capable of independently operating, the potential point A2 is connected with a neutral bus of the grounding electrode;
if the direct current power grid is a pseudo bipolar system, if the alternating current side of the converter valve is grounded, and a fault point and an alternating current grounding point form a fault current path after the single-pole fault, the connection mode of a potential point A2 is the same as that of a true bipolar system; if the converter valve AC side is not grounded, potential point A2 is connected to the other pole DC bus.
4. The combined current-limiting type direct current breaker for the direct current power grid according to claims 1 and 2, wherein a load commutation switch in the current-carrying transfer branch is formed by reverse series-parallel connection of insulated gate bipolar transistors, a main breaker branch in the current-limiting blocking branch is formed by reverse series-parallel connection of insulated gate bipolar transistors, the number of the reverse series-parallel connected insulated gate bipolar transistors is determined by the calculated rated voltage, rated current and rated voltage, current and direction coefficient of the insulated gate bipolar transistors, and the calculation formula is as follows:
n in the above formulaLCS、nMBRespectively representing the number of the insulated gate bipolar transistors of the load commutation switch and the main breaker branch circuit, IN_LCS、UN_LCSFor voltage and current at the time of opening and closing of load converter switch, UN、INFor maximum withstand voltage and maximum breaking current of DC circuit breakers, IC、VCESIs rated voltage and rated current, k, of the insulated gate bipolar transistord1Is 2, kd2Is 1.
5. A combined current-limiting dc breaker for a dc grid according to claim 1, wherein the parameter settings of the combined current-limiting dc breaker are:
(1) the rated voltage of the main breaker branch circuit is the rated voltage of a line, and the rated current of the main breaker branch circuit is the fault current flowing into a bus after the bus bipolar short-circuit fault is quickly cut off;
(2) the initial value of the overvoltage of the load commutation switch is the product of the change rate of the fault current at the action moment of the load commutation switch and the current-limiting reactance current-limiting reactor, and the rated current is 1.1-1.5 times of the rated current of the fault line under the steady-state operation condition;
(3) current commutation element T1～T4The rated voltage is 1/2 of the maximum voltage value generated by the current-limiting reactor at the moment of current reduction, and the rated current is the same as the rated current of the branch circuit of the main circuit breaker;
(4) the value of the current limiting reactor is the same as the reactance value of the current limiting reactor connected in series with the power transmission line; charging resistor R in current-limiting blocking branch1And R2The resistance values of the current limiting reactors are the same, and the value of the resistance values is 1/2 of the ratio of the maximum voltage value generated by the current limiting reactors when the current is reduced to the rated current under the stable operation condition of the system; the value of the charging capacitor C is 1/4000R1(ii) a And the rated voltage of the lightning arrester is set as the rated voltage of the power transmission line in steady operation.
6. The combined current-limiting type direct current circuit breaker for the direct current power grid according to claim 1, wherein the combined current-limiting type direct current circuit breaker is used for isolating direct current line faults and controlling reclosing, and the control method comprises the following steps:
(1)t0at the moment, the DC line fails, t1The system detects the fault current at the moment, and after time delay, t2Constantly turning on the IGBT in the main breaker branch and triggering T1、T3The load current conversion switch connected in series with the fault line is turned off, a switching-off signal is applied to the ultra-fast mechanical switch, and current is transferred to the current-limiting blocking branch;
(2)t3time of day, trigger T4When the charging capacitor begins to discharge and the discharge current direction is opposite to the fault current, T flows2Is reduced to 0 and the fault current is transferred to T4Branch, charging resistor R2The voltage on both sides increases until the fault current shifts to T4A branch circuit, wherein the voltage of the charging capacitor is reversed;
(3)t4at all times, triggering the thyristor T2When the discharge direction of the charging capacitor is opposite to the above process, the fault current is from T1Transfer to T2Charging of electricityResistance R1The voltage on both sides increases until the fault current is completely transferred to T2A branch circuit;
(4)t5at the moment, the ultra-fast mechanical switch is opened to a rated opening distance, a turn-off signal is applied to the insulated gate bipolar transistor in the branch circuit of the main circuit breaker, fault current is transferred to the metal oxide lightning arrester of the lightning arrester, t6The fault current is reduced to 0 at the moment;
(5)t7at the moment, conducting a current-limiting blocking branch of the combined current-limiting type direct current circuit breaker far away from the fault, if fault current is monitored in delta t, judging that the fault type is a permanent fault, and t8The current-limiting blocking branch at the current detection side is turned off again after the moment, the fault line is isolated, and the line is maintained and shut down; if the fault current is not monitored within 2-5 times of delta t time, the fault is judged to be an instantaneous fault, t9And conducting the current-carrying transfer branches at the two ends of the fault line at any moment, and finishing reclosing.
7. A combined current-limiting dc breaker for a dc network according to claim 1, characterized in that the current-limiting blocking branch is connected in series with a charging resistor, the voltage drop across which is used to counteract the negative voltage generated by the current-limiting reactor due to a fault current drop.
8. A combined current-limiting dc breaker for a dc network according to claim 1, wherein the charging resistor is switched between short-circuited and series-connected to the current-limiting blocking branch during the instant of switching on the current-limiting blocking branch and disconnecting the main breaker branch, and the switching process is as follows: at the moment of conducting the current-limiting blocking branch, the short circuit of the charging resistor reduces the initial value of overvoltage at two ends of the load commutation switch at the moment of segmenting the load commutation switch; controlling the current commutation element T by pre-charging and reverse-charging of the charging capacitor before the main breaker branch is opened1～T4The charging resistor is connected in series with the current-limiting blocking branch circuit.
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