CN103346528A - Current limiting type hybrid direct-current circuit breaker based on power electronic combination switch - Google Patents

Abstract

The invention discloses a current-limiting hybrid DC circuit breaker based on a power electronic composite switch. The invention includes a mechanical switch, a power electronic composite switch, a snubber circuit, a current limiting circuit and a freewheeling diode. The mechanical switch is mainly used for the normal working current through _the DC line; the power electronic composite switch is connected in parallel with the mechanical switch, including the IGBT valve group T1 and _the thyristor group T2 ; the buffer circuit is connected in parallel with the power electronic composite switch, including the capacitor C _T , the first resistor R _T and the first diode D _T are used to limit the voltage rise rate at both ends of the power electronic composite switch when the line is broken; the current limiting circuit is connected in series with the mechanical switch, including the inductance L , the second resistor R _L and the second resistor Two diodes D _L ; the freewheeling diode is connected in antiparallel with the DC load, and is used for continuous current of the line inductance and load energy after the line is disconnected. The fault pretreatment control strategy adopted by the invention can significantly shorten the time for disconnection of the line after a fault occurs.

Description

A current-limiting hybrid DC circuit breaker based on power electronic composite switch

technical field

The present invention relates to a DC circuit breaker, especially a current-limiting hybrid DC circuit breaker based on a power electronic composite switch. the

Background technique

In the face of rapid economic and social development, users have put forward many requirements for the power system to be more environmentally friendly, safer and more reliable, more economical and support two-way interaction between users and the power grid. Research data show that the distribution network based on DC has better performance than the existing AC distribution network, and the research on the DC distribution network with reliability, security, stability and economy has huge market value and economic value , It is of great significance to improve the safety, economy, stability and flexibility of my country's power system, and to establish an environmentally friendly and efficient intelligent power system. However, one of the technical bottlenecks restricting the research and application of flexible DC distribution networks is the lack of practical high-voltage DC circuit breakers. Therefore, developing new DC circuit breakers and increasing the capacity of DC circuit breakers has become the goal of the current DC circuit breaker development, and it is also a practical problem that needs to be solved urgently for the safe and stable operation of my country's power system and power construction and development.

Since there is no zero-crossing point for DC current, it is much more difficult to extinguish a DC arc than an AC arc. At present, the design schemes of DC circuit breakers can be mainly divided into two categories: mechanical circuit breakers and solid state circuit breakers. Although the DC circuit breaker technology has made great progress in recent years, judging from the current research situation, most scientific research institutions are still in the stage of prototype trial production and verification of DC circuit breakers, and the capacity is small and concentrated in some Special fields cannot be put into engineering applications.

Contents of the invention

The purpose of the present invention is to address the deficiencies of the prior art, to provide a current-limiting hybrid DC circuit breaker based on a power electronic composite switch, and to provide a fault pretreatment control strategy adopted by it, which can not only control the DC system when the DC system is working normally The on-off of the DC line can also break the short-circuit current when a short-circuit fault occurs in the DC circuit, and significantly shorten the time for the line to be disconnected after the fault occurs.

The current-limiting hybrid DC circuit breaker based on the power electronic composite switch of the present invention includes a DC power supply, a mechanical switch, a power electronic composite switch, a snubber circuit, a current limiting circuit, a freewheeling diode, and a DC load.

The mechanical switch includes only one mechanical switch S , and the freewheeling diode only includes one freewheeling diode D. The positive pole of the DC power supply is connected to one end of the mechanical switch S , the positive end of the power electronic composite switch, and one end of the snubber circuit. The negative end of the switch, the other end of the buffer circuit, and one end of the current limiting circuit are connected, and the other end of the current limiting circuit is respectively connected with the cathode of the freewheeling diode D and the other end of the DC load.

The power electronic composite switch includes IGBT valve group T ₁ (Insulated Gate Bipolar Transistor, insulated gate bipolar transistor) and thyristor group T ₂ , the collector of IGBT valve group T ₁ is the positive terminal of the power electronic composite switch, and the mechanical switch S One end is connected, the emitter _of the IGBT valve group T1 is connected in series with the anode of the thyristor group T2 , and the cathode of _the thyristor group T2 is the negative terminal of _the power electronic composite switch, which is connected to the other end of the mechanical switch S. The IGBT valve group T1 is composed of multiple IGBTs _connected in series, parallel or series-parallel; the thyristor group T2 is composed of multiple thyristors connected in series, _parallel or series-parallel.

The snubber circuit includes a capacitor C _T , a first resistor R _T and a first diode D _T , one end of the first resistor R _T is connected to the cathode of the first diode D T and one _end of the capacitor C _T respectively, and the first resistor R _T The other end is respectively _connected with the anode of the first diode DT and the positive end of the power electronic composite switch, and the _other end of the capacitor CT is connected with the negative end of the power electronic composite switch.

The current limiting circuit includes an inductor L , a second resistor RL _, and a second diode _{DL .} One end of the second resistor RL is connected in series with the anode of the second diode _DL , and the cathode of the second diode DL is _respectively connected to the inductor L One end is connected to the negative end of the power electronic composite switch, and the other _end of the second resistor RL is respectively connected to the other end of the inductor L and the cathode of the freewheeling diode D.

The working process of the present invention is as follows:

When a short-circuit fault occurs or the line needs to be disconnected, an action command is sent to the mechanical switch and the power electronic composite switch at the same time, so that the mechanical switch is opened and the power electronic composite switch is closed. At this time, the inductance L in the current-limiting circuit hinders the rise of the short-circuit current. Since the on-state resistance of the mechanical switch is much smaller than that of the power electronic composite switch, the power electronic composite switch is still in the off state. The mechanical switch starts to arc. When the voltage across the mechanical switch reaches the conduction voltage of the power electronic composite switch, the power electronic composite switch is turned on, and the short-circuit current flowing through the mechanical switch is gradually transferred to the power electronic composite switch. The mechanical switch is equivalent to The resistance gradually increases and the mechanical switch opens. At this time, a shutdown signal is sent to the power electronic composite switch, the power electronic composite switch is disconnected, the short-circuit current on the line drops rapidly, and the inductance L _{in the current-limiting circuit induces a counter electromotive force, which makes the second diode DL} conduct , The inductance L is bypassed, and the energy in it is released through the second diode DL and the second resistor RL , so as to _{avoid the oscillation caused by series connection with the snubber circuit capacitor CT ; the} energy in the DC load is continued through the freewheeling diode D The current is released, and the short circuit fault is removed.

In addition, the present invention adopts a fault pretreatment control strategy, that is, a "pre-shutdown-shutdown (or recovery)" strategy. When the rate of change of the line current exceeds the set value and the instantaneous value of the DC system current has not exceeded the threshold, it is determined that a fault is about to occur, and the "pre-shutdown" process is started, that is, the line current is commutated to the power electronic composite switch circuit immediately. and start timing. Continue to detect the instantaneous value of the line current. If the current value exceeds the threshold within the set time, it is determined that a fault has occurred, and directly sends a shutdown signal to the power electronic composite switch to continue to complete the circuit breaker shutdown process; if the current If the value does not exceed the threshold value, the line current is commutated to the mechanical switch circuit again, and the DC circuit breaker resumes normal operation.

The beneficial effects of the present invention are:

The load side of the present invention has a freewheeling diode D , which provides a release circuit for the energy stored in the DC line and the load after the DC line is broken, and is safe to operate; its power electronic composite switch is composed of IGBTs and thyristors connected in series (parallel), effectively reducing The number of switch tubes connected in series (parallel) and technical difficulty; combining the current limiting circuit, mechanical switch, and power electronic composite switch, the current limiting circuit and the switch part are connected in series, the structure is simple, and the segmentation speed is fast, so as to limit the short-circuit current rise of the DC line High efficiency, avoiding line voltage and current oscillations, and effectively breaking the short-circuit current of the DC line; adopting a fault pre-treatment control strategy to significantly shorten the time for line disconnection after a fault occurs. The invention provides a set of safe, efficient and convenient methods for solving the problem of breaking the short-circuit current of the DC line.

Description of drawings

Fig. 1 is a schematic structural diagram of the present invention;

Fig. 2 is a waveform diagram of breaking process in the present invention;

Fig. 3 is an equivalent circuit diagram of an energy release process in the present invention;

Fig. 4 is the schematic diagram of the energy release process waveform of each passive element and composite switch in the present invention;

5 is a schematic diagram of a method for adjusting the pressure relationship between the IGBT valve group and the thyristor valve group in the present invention;

Fig. 6 is a schematic diagram of a fault pretreatment control strategy in the present invention;

Fig. 7 is simulation circuit diagram among the present invention;

Fig. 8 is a simulation waveform diagram of breaking process in the present invention;

Mechanical switch S , IGBT valve group T ₁ , thyristor group T ₂ , capacitor C _T , first resistor RT , second resistor _RL , line resistance R _s , load resistor R , first diode D _{T ,} second Diode D _L , freewheeling diode D , inductance L , line inductance L _s , mechanical switch (power electronic composite switch) voltage V _S across terminals, IGBT valve group voltage V _{T 1} , thyristor group voltage V _{T 2} , mechanical switch Current i _S , power electronic composite switch current i _T , snubber circuit capacitor current i _C , current limiting circuit inductor current i _L , current limiting circuit diode current i _DL , freewheeling diode current i _D , DC line short-circuit current i , DC power supply Voltage V _dc , line normal working current i _w , time t 17 elapsed from time t 1 to time t ₇ , _time t _sec1 experienced _in stage 1, energy release time t _{sec2 iL} of current-limiting inductor, energy release of short-circuit current Time t _{sec2 i} , time t _SCRoff for thyristor valve group T ₂ to recover forward blocking characteristics, voltage value V _s ( t _SCRoff ) at both ends of the power electronic composite switch at time t _SCRoff , maximum value of current-limiting inductor current i _{L max} , power The maximum voltage V _{s max} at both ends of the electronic composite switch, the time t _{V max} required for the voltage at both ends of the power electronic composite switch to reach the maximum value, the equivalent leakage resistance R ₁ of the IGBT valve group, the equivalent leakage resistance R ₂ of the thyristor valve group, IGBT single-transistor leakage resistance R _IGBT , thyristor single-transistor leakage resistance R _SCR , IGBT single-transistor parallel resistance R ^' _IGBT , thyristor single-transistor parallel resistance R ^' _SCR .

Detailed ways

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

As shown in Figure 1, a current-limiting hybrid DC circuit breaker based on a power electronic composite switch includes a DC power supply 1, a mechanical switch 2, a power electronic composite switch 3, a buffer circuit 4, a current limiting circuit 5, and a freewheeling diode 6 , DC load 7.

The mechanical switch 2 includes only one mechanical switch S , and the freewheeling diode 6 only includes one freewheeling diode D . The positive pole of the DC power supply 1 is respectively connected to one end of the mechanical switch S , the positive end of the power electronic composite switch 3, and one end of the snubber circuit 4, the negative pole is respectively connected to the anode of the freewheeling diode D and one end of the DC load, and the other end of the mechanical switch S is respectively connected to The negative end of the power electronic composite switch, the other end of the snubber circuit, and one end of the current limiting circuit are connected, and the other end of the current limiting circuit is respectively connected with the cathode of the freewheeling diode D and the other end of the DC load.

The power electronic composite switch 3 includes IGBT valve group T ₁ (Insulated Gate Bipolar Transistor, insulated gate bipolar transistor) and thyristor group T ₂ , the collector of IGBT valve group T ₁ is the positive terminal of the power electronic composite switch, and the mechanical switch One end of S is connected, the emitter of IGBT valve group T1 is connected in series with the anode of thyristor group T2 , and _the cathode of thyristor group T2 is the _negative terminal of the power electronic composite switch, which is connected to the other end _of mechanical switch S. The IGBT valve group T1 is composed of multiple IGBTs _connected in series, parallel or series-parallel; the thyristor group T2 is composed of multiple thyristors connected _in series, parallel or series-parallel.

The snubber circuit includes a capacitor C _T , a first resistor R _T and a first diode D _T , one end of the first resistor R _T is connected to the cathode of the first diode D T and one _end of the capacitor C _T respectively, and the first resistor R _T The other end is respectively _connected with the anode of the first diode DT and the positive end of the power electronic composite switch, and the _other end of the capacitor CT is connected with the negative end of the power electronic composite switch.

The current limiting circuit includes an inductor L , a second resistor RL _, and a second diode _{DL .} One end of the second resistor RL is connected in series with the anode of the second diode _DL , and the cathode of the second diode DL is _respectively connected to the inductor L One end is connected to the negative end of the power electronic composite switch, and the other _end of the second resistor RL is respectively connected to the other end of the inductor L and the cathode of the freewheeling diode D.

As shown in Figure 2, the principle and working process of the current-limiting hybrid DC circuit breaker based on the power electronic composite switch are described.

0~ t ₁ (Interval Ⅰ): The DC line supplies power normally, the power electronic composite switch is in the off state, the voltage V _S across the mechanical switch (power electronic composite switch) is zero, and the mechanical switch current i _S is the normal power supply current of the line.

t ₁ ~ t ₂ (interval II): A fault occurs at time t ₁ , and the inductance L of the current-limiting circuit begins to suppress the rise of the short-circuit current i . At time t2 , the circuit breaker sends out the opening signal of the mechanical switch and the conduction signal _of the power electronic composite switch at the same time.

t ₂ ~ t ₃ (Interval Ⅲ): It is the action delay time of mechanical switch S. Since the voltage V _S across the mechanical switch (power electronic composite switch) is less than the conduction voltage of the power electronic composite switch, the power electronic composite switch is still in the off state, and the short-circuit current i continues to rise.

t ₃ ~ t ₄ (interval IV): At t ₃ , the mechanical switch S starts to turn off, and the voltage across it rises.

t ₄ ~ t ₅ (interval Ⅴ): At t ₄ , the voltage V _S across the mechanical switch (power electronic composite switch) reaches the conduction voltage of the power electronic composite switch, and the power electronic composite switch starts to conduct, because the IGBT valve group T ₁ The turn-on time is much shorter than that of the thyristor valve group T ₂ . During the turn-on process of T ₁ , T ₂ is not yet turned on, and the compound switch current i _T is zero. _{At time t5} , the conduction _of IGBT valve group T1 is completed.

t ₅ ~ t ₆ (interval Ⅵ): Thyristor valve group T ₂ starts to conduct, the power electronic composite switch current i _T increases, the mechanical switch current i _S decreases, and the short-circuit current gradually transfers from the mechanical switch circuit to the power electronic composite switch circuit. At _time t5 , the mechanical switch current i _S drops to zero, _and the conduction of the thyristor valve group T2 is completed.

t ₆ ~ t ₇ (interval VII): The mechanical switch S enters the zero-current opening process, the short-circuit current is completely borne by the power electronic composite switch circuit, and the power electronic composite switch current i _T continues to rise. At time t7 , the mechanical switch S completes opening, and at this _time sends a shutdown signal to the IGBT valve group.

t ₇ ~ t ₈ (interval Ⅷ): During the turn-off process of IGBT valve group T ₁ (interval Ⅶ), the power electronic composite switch still flows current, thyristor valve group T ₂ cannot recover the forward blocking characteristic, and still In the conduction state, the voltage at both ends is still its conduction voltage, the voltage V _{T 1} of the IGBT valve group rises, the current i _T of the power electronic composite switch decreases, and the voltage V _S across the mechanical switch (power electronic composite switch) rises.

t ₈ ~ t ₉ (Interval Ⅸ): At t ₈ , IGBT valve group T ₁ is completely turned off, thyristor valve group T ₂ begins to recover its forward blocking capability, the voltage at both ends rises, and the IGBT valve group voltage V _{T 1} It also continued to rise after falling. At time t9 , the thyristor valve group T ₂ resumes its forward blocking capability, _and the power electronic composite switch is completely turned off.

After time t ₉ (interval X): Due to the existence of capacitance in the snubber circuit, although the power electronic composite switch is completely turned off, the inductor current i _L and short-circuit current i of the current-limiting circuit will still rise temporarily. Then the current starts to drop, the back electromotive force induced by the inductance L of the current limiting circuit turns on the diode D _L , the back electromotive force generated by the inductance of the DC line makes the freewheeling diode D turn on, and the energy in the line and the inductance L of the current limiting circuit begins to release. At a certain moment, the drop rate of the inductor current i _L of the current limiting circuit reaches the maximum value, and the voltage V _S across the mechanical switch (power electronic composite switch) rises to the maximum value (that is, the sum of the power supply voltage and the back EMF generated by the inductor L ), and the limit The diode current i _DL of the flow circuit and the freewheeling diode current i _D also rise to the maximum value.

As shown in Figures 2 to 4, the current limiting process, energy release process and parameter design principles of the current limiting hybrid DC circuit breaker based on the power electronic composite switch are deduced and explained.

The overvoltage and overcurrent suffered by the device usually occurs when the DC circuit breaker breaks the short-circuit current. Assuming that the ground fault resistance is 0, and the fault point is located at the outlet of the circuit breaker, that is, the line impedance is ignored.

From the time when the fault occurs ( t ₁ ) to the time when the power electronic composite switch is turned off ( t ₇ ), the power supply voltage V _dc is equal to the voltage across the inductance of the current-limiting circuit, as shown in Figure 2. In this process, the rate of rise of the inductor current in the current limiting circuit can be expressed as

(1)

In the formula, L is the inductance of the current-limiting circuit, i _L is the inductance current of the current-limiting circuit, and V _dc is the DC power supply voltage. At time t ₇ i _L rises to

                                         （2）

(2)

In the formula, i _{Lt 7} is the inductor current of the current-limiting circuit at time t ₇ , i _w is the normal operating current of the line, and t ₁₇ is the time elapsed from time t ₁ to time t ₇ .

From t7 _to t8 , the IGBT valve group T1 is disconnected, but the thyristor valve group T2 is still in _the conduction _state . At this time, _the snubber circuit starts to play a _role . Before the thyristor valve group T2 recovers the forward blocking capability, the limit The voltage rise rate during the disconnection process of the IGBT valve group prevents the power supply voltage from being directly applied to the IGBT valve group, thereby damaging the device. Neglecting the disconnection time _of T1 , that is _, T1 is disconnected instantaneously at time t7 , the equivalent circuit of the DC circuit breaker is shown in Figure ₃ . At time t7 , the line current (that is, the inductor _current of the current-limiting circuit) is suddenly commutated from the power electronic composite switch to the snubber circuit, the capacitor current i _C changes from 0 to the inductor current i _L of the current-limiting circuit, and the capacitor of the snubber circuit begins to charge. During this process, the inductor current i _L of the current-limiting circuit continues to rise, and the thyristor of the current-limiting circuit is not turned on, so that this stage is stage 1; when the inductor current i _L of the current-limiting circuit rises to the maximum value, di _L / dt = 0, the After time , i _L starts to decrease, let it be stage 2.

Neglecting line impedance and fault resistance, stage 1 can be expressed by the following differential equations

(3)

solution available

                       （4）

(4)

It can be seen from formula (4) that if there is no energy release loop composed of diode DL and resistor RL in the current limiting circuit, the energy in _the inductor will oscillate with the _capacitance of the snubber circuit. In fact _, when the inductor current i _L of the current limiting circuit reaches the maximum value and starts to drop, the diode DL is turned on, and stage 1 ends. At this time, di _L / dt = 0, that is, the voltage across the inductor of the current-limiting circuit is zero, V _s = V _dc , the time elapsed in stage 1

                            （5）

(5)

where t _sec1 is the time elapsed in stage 1, and

                                 （6）

(6)

Where i _{L max} is the maximum value of current-limiting inductor current. It can be known from formula (5) that t _sec1 is an increasing function of the capacitance C _T of the snubber circuit, that is, the larger the value of C _T is, the longer the time elapsed in stage 1 is.

It can be known from formula (4) that after the end of stage 1 _, i _L begins to drop, the diode DL of the current limiting circuit is turned on, and stage 2 begins. Phase 2 can be expressed by the following system of differential equations

                                  （7）

(7)

In order to achieve the purpose of current limiting, the

                                        （8）

(8)

In the formula, α is a temporary variable set. Solve equation (7) to get

                         （9）

(9)

From formula (9), it can be obtained that when

                                        （10）

(10)

When , the voltage V _s across the power electronic composite switch reaches the maximum value

                           （11）

(11)

In the formula, t _{V max} is the time required for the voltage at both ends of the power electronic composite switch to reach the maximum value, and V _{s max} is the maximum value of the voltage at both ends of the power electronic composite switch. Define t _{sec2 iL} as the energy release time of the current-limiting inductor, that is, the time for its current i _L to drop from the maximum value to the normal operating current i _w of the line. This time value can be obtained by setting i _L = i _w in formula (9).

Starting from stage 2, the short-circuit current i also enters the energy release stage. The short-circuit current i of stage 2 is expressed as

(12)

When i = i _w , the short-circuit current energy release phase ends, and when the voltage V _s across the power electronic composite switch reaches the maximum value, the short-circuit current drops to zero. Define t _{sec2 i} as the energy release time of the short-circuit current, which can be solved by setting i = i _w in formula (12). From the disconnection of the IGBT valve group to the end of the energy release process, the working waveforms of the passive components and composite switches of the DC circuit breaker are shown in Figure 4. It can be seen from Figure 4 that the short-circuit current cut-off time t _i of the DC circuit breaker is shown in formula (13)

                                     （13）

(13)

l Selection of buffer capacitor C _T : When the thyristor valve group T ₂ has not recovered its forward blocking capability, the voltage borne by the IGBT valve group T ₁ is determined by the buffer capacitor C _T , the larger the value of the buffer capacitor C _T , the IGBT valve The voltage withstood _by group T1 is smaller. In addition, the time t _sec1 experienced in stage 1 is mainly determined by C _T , as shown in formula (5), so the value of the snubber capacitor C _T should not be too large, so as not to greatly increase the short-circuit current cut-off time of the DC circuit breaker.

l Selection of snubber circuit resistance R _T : snubber circuit resistance R _T is used to limit the discharge current of snubber capacitor C _T when the DC circuit breaker is closed. When the DC circuit breaker is closed, since _{the turn-on time of the mechanical switch is much longer than the turn-on time of the power electronic composite switch, the power electronic composite switch is first turned on, and the snubber capacitor CT is} discharged through the snubber circuit resistor RT . Due to the large inductance of the current limiting circuit, it can be considered that i _L =0 during the discharge _of CT . The process is described as

(14)

Solutions have to

                                          （15）

(15)

Therefore, the maximum value of the power electronic composite switching current i _T

                                     （16）

(16)

According to the formula (16) to choose R _T , should make the power electronic composite switch current i _T not exceed the allowable value.

l Selection of current-limiting circuit inductance L : It can be seen from formula (6) that the larger the value of current-limiting circuit inductance L , the smaller the maximum value of current-limiting inductance i _{L max} , and i _{L max} is also equal to the maximum value of short-circuit current i value. However, the larger the value of the inductance L of the current-limiting circuit, the smaller the current drop rate during the energy release process, that is, the longer the energy release time, and it may have a certain impact on the control speed of the DC system. Therefore, after selecting the inductance L of the current-limiting circuit according to the allowable value of the short-circuit current i , its energy release time t _{sec2 iL} must also be verified.

l Selection of current-limiting circuit resistance RL : The larger the value of current-limiting circuit resistance RL , the greater the _current drop rate during _the energy release process of the current-limiting circuit inductance L , that is, the faster the energy release speed. However, the value of the resistance RL of the current limiting circuit may also affect the maximum value V _{s max} of _the voltage V _s across the power electronics composite switch. Therefore, after selecting the current-limiting circuit inductance L and the current-limiting circuit resistance R _L according to the energy release speed and energy release time of the current-limiting circuit inductance L, the maximum value V _{s max} of the voltage at both ends of the power electronic composite switch must be calibrated test.

The configuration principle of the power electronic composite switch of the current-limiting hybrid DC circuit breaker based on the power electronic composite switch is deduced and explained below in conjunction with Fig. 5 .

Before the IGBT valve group is disconnected, the voltage V _s across the power electronic composite switch is approximately zero, and the maximum current i _T is

                                         （17）

(17)

In the formula, i _{T max} is the maximum value of i _T. After the IGBT valve group is disconnected, i _T drops to 0, and V _s changes according to the laws shown in formula (4) and formula (9), namely

            （18）

(18)

Since the power electronic composite switch is composed of IGBT and thyristor, it can be seen from Figure 3 that the voltage that the IGBT valve group needs to withstand

(19)

In the formula, V _IGBTn is the voltage borne by the IGBT valve group, R ₁ is the equivalent leakage resistance of the IGBT valve group, and R ₂ is the equivalent leakage resistance of the thyristor valve group. Since the withstand voltage value, current withstand value and single-transistor leakage resistance of thyristors are significantly higher than those of IGBTs, the number of parallel connections of IGBTs is generally greater than that of thyristors. However, in practical applications, since the single-transistor leakage resistance of each power electronic device is different and difficult to measure, it is necessary to connect a large resistor in parallel at both ends of each device to achieve the purpose of voltage equalization, as shown in Figure 5. In the power electronic composite switch designed in the present invention, connecting resistors in parallel for each device can not only achieve the purpose of voltage equalization, but more importantly, the voltage at both ends of the IGBT valve group can be reduced as much as possible by properly selecting the resistance value of the parallel resistors, increasing The voltage withstood by the thyristor valve group can achieve the purpose of reducing the number of series and parallel connections of devices and reducing the technical difficulty of series and parallel connections. When selecting each resistance value, the following relationship should be satisfied:

                                         （20）

(20)

then there is

                                     （21）

(twenty one)

In the formula, n _IGBTc is the number of IGBTs in series, n _IGBTb is the number of IGBTs in parallel, n _SCRc is the number of thyristors in series, n _SCRb is the number of thyristors in parallel, where

(twenty two)

And n _IGBTb ≥ n _SCRb , where ceil () is a function of rounding up, i _IGBT is the rated current of IGBT, and i _SCR is the rated current of thyristor.

In addition, when the values of the passive components of the DC circuit breaker are determined, according to formula (18), V _s ( t _SCRoff ) is a fixed value, and according to formula (19), when reducing the number of IGBTs, the IGBT valve group needs The minimum voltage V _IGBTn can withstand is V _s ( t _SCRoff ), so set

                                             （23）

(twenty three)

but

                   （24）

(twenty four)

Where n _SCRc is the number of thyristors connected in series, V _IGBT is the rated voltage of IGBT, and V _SCR is the rated voltage of thyristors. Solve formula (24) to get

                              （25）

(25)

（26）

(26)

Where n is the total number of devices required by the power electronic composite switch. From n _IGBTb ≥ n _SCRb , R ^' _SCR > R ^' _IGBT , it can be obtained that n is an increasing function of V _s ( t _SCRoff ), that is, the larger V _s ( t _SCRoff ), the number of series-parallel devices required by the power electronic composite switch more. In addition, from formula (26) we can get

                       （27）

(27)

Substituting formula (27) into formula (23), the condition for formula (23) to be established is

                         （28）

(28)

That is, when formula (28) is established, formula (26) can be used to design the power electronic composite switch. When formula (28) does not hold, still make

(29)

                            （30）

(30)

Then substitute formula (21) into formula (30), and solve it to get

                         （31）

(31)

         （32）

(32)

Equations (26) and (32) represent the calculation methods for the minimum number of series and parallel connections of power electronic composite switches under different circumstances, but a certain margin is required for the number of series and parallel connections of devices.

The schematic diagram of the fault pretreatment control strategy of the current-limiting hybrid DC circuit breaker based on the power electronic composite switch will be described below in combination with FIG. 6 .

(1) When the instantaneous value of the DC system current does not exceed the threshold and the rate of change of the line current does not exceed the set value, it is determined that the fault has not occurred, so the DC circuit breaker does not perform the commutation process in advance. If the upper control system sends a shutdown signal at this time, the DC circuit breaker will start the shutdown process after receiving the signal.

(2) When the instantaneous value of the DC system current exceeds the threshold, but the rate of change of the line current does not exceed the set value, it is determined that a fault has occurred. In this case, the DC circuit breaker cannot perform the commutation process in advance, and can only start the shutdown process after judging that the current exceeds the limit.

(3) When the line current change rate exceeds the set value and the instantaneous value of the DC system current has not exceeded the threshold, it is determined that a fault is about to occur, and the "pre-shutdown" process is started, that is, the mechanical switch shutdown signal is sent and the power electronic composite switch Turn on the signal to commutate the line current to the power electronic composite switch circuit, and start timing. Continue to detect the instantaneous value of the line current. If the current value exceeds the threshold within the set time, or receives a shutdown signal from the upper control system, it is determined that a fault has occurred, and a shutdown signal is sent directly to the power electronic composite switch to continue to complete the circuit breaker. Shutdown process; if the current value does not exceed the threshold within the set time, or does not receive the shutdown signal from the upper control system, a mechanical switch conduction signal is sent, and the power electronic composite switch circuit is disconnected after the mechanical switch is closed. The line current is commutated to the mechanical switch circuit again, and the DC circuit breaker resumes normal operation.

The fault pretreatment control strategy proposed by the present invention can perform current commutation in advance when a fault occurs, thereby shortening the time for disconnection of the line after the fault occurs. If the fault does not occur after commutation in advance, the loss of the DC circuit breaker will increase from the conduction loss of the mechanical switch to the conduction loss of the power electronic composite switch within the set time, although the loss of the DC circuit breaker has increased , but this measure of commutation in advance will not affect the normal operation of the line.

However, when the situation (2) occurs, the DC circuit breaker cannot perform the commutation process in advance. This situation can be avoided by reducing the setting value of the rate of change of the line current. Assuming that the rated DC voltage of the DC line is Vdc , when the equivalent load resistance R of the DC line suddenly decreases, the process _can be expressed by the following differential equation

                                           （33）

(33)

In the formula, i is the instantaneous value of DC current. solution

                                      （34）

(34)

In the formula, I is the rated value of DC current. From formula (34) it can be known that when R suddenly decreases, the DC current rises from I to V _dc / R . During this process, the DC current rate of change is continuously reduced to zero. The maximum value of the DC current change rate appears at the moment when R suddenly increases, and its value is

                                    （35）

(35)

It can be seen from formulas (34)~(35) that when L is determined, the smaller R is, the greater the rate of change of DC current and its final value will be. Let the allowable threshold of DC current be 1.25 I , then the allowable value of R is 0.8 V _dc / I . Substitute it into formula (35) to get

(36)

If formula (36) is used as the set value of the line current change rate, when the detected current change rate is greater than this value, the final value of the line current will be greater than 1.25 I , so a "pre-shutdown" process is required; when the current change rate When it is less than this value, the final line current value will not exceed 1.25 I , so no fault pretreatment is performed.

According to the above analysis, the line breaking time that can be shortened by the fault pretreatment control strategy is the time it takes for the DC current to rise above the current threshold. When a serious ground fault occurs in the DC system and the ground fault resistance is 0, according to formula (2), it can be calculated as

(37)

In the formula, Δt is the line disconnection time that can be shortened by the fault pretreatment control strategy. When the ground fault resistance is not 0, according to formula (34), it can be calculated

                                      （38）

(38)

The following is a simulation of the breaking process of the current-limiting hybrid DC circuit breaker based on the power electronic composite switch in combination with Figures 7-8.

The simulation circuit diagram is shown in Figure 7, and its simulation parameters are as _follows : the DC power amplitude is 110kV; the arc model of the mechanical switch _S adopts the Cassie model; The value of the inductance L is 100mH, the value of the second resistance R _L is 2Ω; the conduction time of the thyristor group T2 is 10us, the recovery time _of the forward blocking ability under reverse voltage is 50us, and the recovery time of the forward blocking ability under zero current is 400us; the line The value of resistance R _s is 0.5Ω, the value of line inductance L _s is 5mH, and the value of load resistance R is 55Ω.

The simulation waveform diagram of the breaking process of the current-limiting hybrid DC circuit breaker based on the power electronic composite switch is shown in Figure 8. At 0.5s, the line load has a grounding short-circuit fault. Due to the fault pretreatment control strategy, the fault judgment time is almost negligible. Therefore, it is assumed that the mechanical switch opening signal and the power electronic composite switch conduction signal are issued at 0.5s. According to the Cassie model, the arcing time of the mechanical switch S is 0.3ms, and the power electronic composite switch is turned on at 0.5003s. At 0.503s, the mechanical switch S completes the opening process under zero voltage, and at this time sends a shutdown signal to the power electronic composite switch. The IGBT valve group is disconnected quickly, and after about 400us, the thyristor group T2 also recovers the forward blocking capability, the power electronic composite switch is completely turned off, the short circuit fault _is removed, and the circuit breaker enters the energy release stage. As shown in Figure 8, it takes about 5ms for the circuit breaker set according to the given parameters to clear the short-circuit fault (the fault judgment time is negligible), that is, the short-circuit current i is lower than the normal operating current at 0.505s. Due to the large value of the inductance L of the current limiting circuit, when the value of the second resistor RL is 2Ω, the energy stored in the inductance L can be completely _released in 0.8s, but the current drops to the normal operating current of the line in 0.56s the following.

The simulation results show that the current-limiting hybrid DC circuit breaker based on the power electronic composite switch can not only limit the short-circuit current of the DC line while avoiding the oscillation of the line voltage and current, effectively break the short-circuit current of the DC line, but also can break the DC line after the DC line is broken. The energy stored in the line and load provides a release circuit, and reduces the number of series (parallel) connections of power electronic composite switches and technical difficulty. In addition, the fault pretreatment control strategy adopted in the present invention can significantly shorten the time for disconnection of the line after a fault occurs. The invention provides a set of safe, efficient and convenient methods for solving the problem of breaking the short-circuit current of the DC line.

Claims (6)

1. A current-limiting hybrid DC circuit breaker based on a power electronic composite switch, which is characterized by including a DC power supply (1), a mechanical switch (2), a power electronic composite switch (3), a snubber circuit (4), a current limiting Circuit (5), freewheeling diode (6) and DC load (7); The mechanical switch (2) includes a mechanical switch S , the freewheeling diode (6) includes a freewheeling diode D , the positive pole of the DC power supply (1) is connected to one end of the mechanical switch S , the positive end of the power electronic composite switch (3) and the buffer One end of the circuit (4) is connected, the negative pole is connected to the anode of the freewheeling diode D and one end of the DC load (7), and the other end of the mechanical switch S is respectively connected to the negative end of the power electronic composite switch (3) and the other end of the buffer circuit (4) And one end of the current limiting circuit (5) is connected, and the other end of the current limiting circuit (5) is respectively connected to the cathode of the freewheeling diode D and the other end of the DC load (7); The power electronic composite switch (3) includes IGBT valve group T ₁ and thyristor group T ₂ , the collector of IGBT valve group T ₁ is the positive terminal of the power electronic composite switch (3), which is connected to the end of the mechanical switch S , and the IGBT valve group The emitter _of T1 is connected in series with the anode of the thyristor group T2 , and the cathode of _the thyristor group T2 is the negative terminal of _the power electronic composite switch (3), which is connected to the other end of the mechanical switch S ; The snubber circuit (4) includes a capacitor C _T , a first resistor R _T and a first diode D _T , one end of the first resistor R _T is connected to the cathode of the first diode D _T and one end of the capacitor C _T respectively, and the first The other end of the resistor R _T is respectively connected to the anode of the first diode DT and the positive end of the power electronic composite switch (3), and the other _end of the capacitor C _T is connected to the negative end of the power electronic composite switch (3); The current limiting circuit (5) includes an inductor L, a second resistor RL and a second diode DL, one end of the second resistor RL is connected in series with the anode of the second diode DL _, and the _{cathode of} the _second diode _DL is respectively It is connected with one end of the inductance L and the negative end of the power electronic composite switch (3), and the other end of the second resistor RL is respectively connected with the other end of the inductance _L and the cathode of the freewheeling diode D. 2. The current-limiting hybrid DC circuit breaker based on power electronic composite switch according to claim 1, characterized in that the current-limiting circuit, mechanical switch, and power electronic composite switch are combined to limit the short-circuit current rise rate of the DC line, effectively Break the short-circuit current of the DC line. 3. The current-limiting hybrid DC circuit breaker based on power electronic composite switch according to claim 1, characterized in that its load side has a freewheeling diode D , which provides energy stored in the DC line and the load after the DC line is broken. Release loop. 4. The current-limiting hybrid DC circuit breaker based on power electronic composite switch according to claim 1, characterized in that _its power electronic _composite switch is composed of IGBT valve group T1 and thyristor group T2 connected in series, and IGBT valve group T ₁ is composed of multiple IGBTs connected in series, parallel or series-parallel; thyristor group T ₂ is composed of multiple thyristors connected in series, parallel or series-parallel. 5. The current-limiting hybrid DC circuit breaker based on the power electronic composite switch according to claim 1, the parameter selection method and the configuration method of the power electronic composite switch are determined according to the formula. 6. The current-limiting hybrid DC circuit breaker based on power electronic composite switch according to claim 1, characterized in that it adopts a fault pretreatment control strategy, that is, when the line current change rate exceeds the set value, the DC system current instantaneous When the value has not exceeded the threshold, it is determined that a fault is about to occur and the commutation process begins.

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