CN104635151A - Cascade full-bridge direct-current circuit breaker low-voltage equivalent test circuit and detection method thereof - Google Patents

Abstract

The invention discloses a low-voltage equivalent test circuit of a cascaded full-bridge DC circuit breaker and its detection method. The equivalent test circuit and its detection method proposed by the invention are based on the time equivalent principle of current transfer and shutdown process, through impedance, etc. Using low-voltage devices and lines to simulate the action process of the cascaded full-bridge HVDC circuit breaker, the equivalent of the two commutation times of the cascaded full-bridge HVDC circuit breaker can be realized, thereby accurately verifying the control and protection equipment For timing requirements, by simplifying the equivalent conditions, under the condition that the voltage ratio, current ratio, and impedance ratio satisfy the proportional relationship, the number of transfer sub-modules can be different, and the equivalent of two commutation times can also be realized. The invention proposes The equivalent experimental circuit realizes the verification of the working principle of the cascaded full-bridge high-voltage DC circuit breaker and the secondary control and protection equipment at a lower cost, and can well represent the working process of the high-voltage DC circuit breaker.

Description

A low-voltage equivalent test circuit of a cascaded full-bridge DC circuit breaker and its detection method

technical field

The invention relates to a power electronic device, in particular to a low-voltage equivalent test circuit of a cascaded full-bridge DC circuit breaker and a detection method thereof.

Background technique

With the development of high-voltage and high-current semiconductor devices and power electronics technology, high-voltage DC transmission technology, flexible DC transmission technology, and DC grid technology have received more and more attention and applications. When a bipolar short circuit occurs on the DC side of the flexible DC transmission line, the DC current rises rapidly, and the current rise rate is as high as 3kA/ms, and the rise of the short-circuit current cannot be effectively prevented by the flexible DC converter valve itself. If no effective measures are taken, it will Cause damage to the diverter valve equipment. The HVDC circuit breaker for flexible DC transmission equipment can cut off the fault current in a short time, isolate the fault, and protect the converter valve.

Since the current rises rapidly when a bipolar short circuit occurs on a flexible direct current transmission line, it is required that the high voltage direct current circuit breaker can act quickly. Therefore, the breaking time of fault current is an important index of DC circuit breaker.

A DC circuit breaker with a combination of mechanical switches and power electronic devices. In normal operation, the mechanical switch passes the current. When a fault occurs, the current of the main branch is transferred to the parallel connected power electronic device branch, and then the power electronic device breaks the current. When this type of circuit breaker is in normal operation, the on-state loss is small and the breaking time is short. The cascaded full-bridge DC circuit breaker disclosed in CN103280763A is an example of this type of DC circuit breaker, and its circuit topology is shown in Figures 1 and 2. Fig. 1 is the circuit topological diagram of the cascaded full-bridge DC circuit breaker in the prior art; Fig. 2 is the internal circuit diagram of the submodule (SM) in Fig. 1; The fault current avoids the problem of uneven device voltage during the breaking process. When the IGBT is turned off, the terminal voltage is zero, which reduces the switching loss.

During the breaking process of the cascaded full-bridge DC circuit breaker, it is necessary to coordinate and control the main branch sub-module, the transfer branch sub-module, and the fast isolation switch according to the control process and timing, and execute different fault handling procedures for faults that occur in different stages. And all detection, processing, and actions need to be completed within 2 to 3 ms, and the cascaded full-bridge DC circuit breaker has strict requirements on the control timing of the control system. Therefore, before the high-voltage test, it is necessary to replace the high-voltage DC circuit breaker equivalently in time under the condition of low voltage and low current, and verify the working principle of the circuit breaker and the secondary control and protection equipment.

Therefore, it is necessary to propose a low-voltage equivalent method for cascaded full-bridge DC circuit breakers to realize the principle timing and control of cascaded full-bridge DC circuit breakers at low voltage and low current before conducting high-voltage and high-current tests on the actual circuit breaker. methods and troubleshooting measures.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention provides a low-voltage equivalent test circuit for a cascaded full-bridge DC circuit breaker. The equivalent test circuit is composed of DC power supply V _dc , equivalent reactor L _{lv_3} , solid state relay, Inductor L _{lv_1} , main branch low-voltage equivalent sub-module SM _lv , load switching switch S1, load resistor R _L and current limiting resistor R _X form the main circuit, the solid state relay, the inductor L _{lv_} and the main branch A low-voltage equivalent sub-module SM _lv constitutes the main branch, and there are transfer branches and energy consumption branches connected in parallel at both ends of the main branch. The transfer branch is composed of diode D _lv , inductor L _{lv_2} and transfer The branch low-voltage equivalent sub-module is composed, the energy-consuming branch is connected with a varistor, and the two ends of the load resistance RL are connected in parallel with a fault analog switch S2.

Preferably, the voltage level of the test circuit is 100-200V, and the current level is 5-10A.

Preferably, the operating voltage of the varistor is higher than the voltage of the DC voltage source V _dc , and the residual voltage cannot exceed the sum of the rated voltages of the transfer branch sub-modules.

A detection method for a low-voltage equivalent test circuit of a cascaded full-bridge DC circuit breaker, the detection method comprising the following steps:

Opening the main branch low-voltage equivalent submodule in the low-voltage equivalent test circuit of the cascaded full-bridge DC circuit breaker, locking the transfer branch low-voltage equivalent submodule, and opening the solid state relay;

Opening the fault simulation switch S1, closing the load switching switch S2;

Powering on the DC stabilized power supply, adjusting the voltage to 100-200V, the loop current i _{breaker_lv} flows through the main loop, and returns to the DC stabilized power supply;

Closing the fault simulation switch S2;

When the loop current i _{breaker_lv} exceeds the overcurrent limit value I _{lim_lv} , turn on the transfer branch low-voltage equivalent sub-module, block the main branch low-voltage equivalent sub-module, so that the loop current i _{breaker_lv} is controlled by the main branch transfer to transfer branch;

When the main branch current i _{main_lv} drops to 0, disconnect the solid state relay;

After a delay of 2 ms, the low-voltage equivalent sub-module of the transfer branch is blocked, the capacitor C _lv of the low-voltage equivalent sub-module of the transfer branch is charged, and the voltage of the transfer branch and the energy-consuming branch starts to rise;

When the voltage of the energy-consuming branch rises to the operating voltage of the varistor, the varistor is gradually turned on. As the voltage of the energy-consuming branch rises further, the current iMOV_lv flowing through the varistor in the energy-consuming branch rises, and the transfer branch The current icomm_lv gradually drops to 0;

The current iMOV_lv flowing through the piezoresistor in the energy-consuming branch begins to drop after reaching a peak value, and finally reaches 0, and the detection is ended;

Preferably, the detection method includes two commutation time equivalents:

The time of the first commutation is equivalent. After the circuit breaker detects the overcurrent of the line, it starts the first commutation, blocks the main branch, opens the transfer branch, and the current is transferred from the main branch to the The transfer branch described above, the first commutation time of the low-voltage equivalent model is equal to the commutation time of the actual high-voltage DC circuit breaker;

The time of the second commutation is equivalent. After the transfer branch is blocked, the voltage of the sub-module of the transfer branch rises. After rising to the operating voltage of the arrester, the arrester starts to discharge the current. The second commutation time of the low-voltage equivalent model is the same as the actual The commutation time of HVDC circuit breakers is equal.

Preferably, the detection method satisfies the following formula when performing the time equivalent of the first commutation:

(L ₁ +L ₂ )C=(L _{1_lv} +L _{2_lv} )C _lv (1)

In formula (1), C is the capacitance value of the sub-module in the actual circuit breaker, L ₁ and L ₂ are the parasitic inductance of the main branch and the transfer branch in the actual circuit breaker, respectively, and C _lv is the capacitance of the sub-module in the low-voltage equivalent model , L _{lv_1} , L _{lv_2} are the parasitic inductance of the main branch and the transfer branch in the low-voltage equivalent model, respectively.

Preferably, the detection method satisfies the following formula when performing the time equivalent of the second commutation:

CV _MOV I _{break_lv} = C _lv V _{MOV_lv} I _break (2)

In formula (2), V _MOV is the operating voltage of the arrester, I _break is the maximum segment current of the circuit breaker, C is the capacitance value of the neutron module of the actual DC circuit breaker, V _{MOV_lv} is the operating voltage of the varistor in the low-voltage equivalent model, I _{break_lv} is the maximum sub-section current of the circuit breaker, and C _lv is the capacitance value of the sub-module in the low-voltage equivalent model.

Compared with the closest prior art, the beneficial effects of the present invention are:

1Using low-voltage devices and lines to replace the cascaded full-bridge HVDC circuit breaker and the high-voltage line connected to it can realize the working principle and secondary control of the cascaded full-bridge HVDC circuit breaker at low voltage and at a lower cost verification of protective equipment;

2. Equivalently reduce the electrical parameters of the high-voltage DC circuit breaker by a certain ratio, and select low-voltage devices with similar electrical characteristics for the devices in the high-voltage DC circuit breaker to perform equivalents according to the proportional relationship, which can well represent the high-voltage DC circuit breaker working process;

3. The low-voltage equivalent replacement method can realize the equivalent of the two-time commutation time of the cascaded full-bridge high-voltage DC circuit breaker, thereby accurately verifying the timing requirements for control and protection equipment;

4. By simplifying the equivalent conditions, under the condition that the voltage ratio, current ratio, and impedance ratio satisfy the proportional relationship, the number of transfer sub-modules can be different, and the equivalent of two commutation times can also be realized.

Description of drawings

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a circuit topology diagram of a cascaded full-bridge DC circuit breaker in the prior art;

Fig. 2 is the internal circuit diagram of the submodule in Fig. 1;

Fig. 3 is the low-voltage equivalent test circuit diagram of the cascaded full-bridge DC circuit breaker of the present invention;

FIG. 4 is an internal circuit diagram of the equivalent sub-module in FIG. 3 .

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings.

In order to thoroughly understand the embodiments of the present invention, the detailed structure will be set forth in the following description. Obviously, the practice of the embodiments of the invention is not limited to specific details familiar to those skilled in the art. Preferred embodiments of the present invention are described in detail below, however, the present invention may have other embodiments besides these detailed descriptions.

Referring to Fig. 3 and Fig. 4, Fig. 3 is a low-voltage equivalent test circuit diagram of the cascaded full-bridge DC circuit breaker of the present invention; Fig. 4 is an internal circuit diagram of the equivalent sub-module in Fig. 3 . The DC circuit breaker low-voltage equivalent test circuit in Figure 3 is divided into main branch, transfer branch and energy consumption branch. The main circuit is composed of DC power supply V _dc , equivalent reactor L _{lv_3} , solid state relay, and inductor L _{lv_1} , main branch low-voltage equivalent sub-module SM _lv , load switching switch S1, load resistor R _L and current limiting resistor R _X , the main branch is composed of the solid state relay, the inductor L _{lv_} and the main branch The low-voltage equivalent sub _- module SM _{lv is connected in} series, and the transfer branch and the energy consumption branch are connected in parallel at both ends of the main branch. The equivalent sub-module SM _lv is formed, the energy consumption branch is composed of a varistor, and a fault analog switch S2 is connected in parallel at both ends of the load resistance _RL .

In order to make the equivalent experimental circuit operate in a low-voltage environment, the voltage level of the low-voltage equivalent model of the cascaded full-bridge DC circuit breaker is about 100-200V, and the current level is about 5-10A.

The low-voltage equivalent sub-module SM _lv circuit of the cascaded full-bridge DC circuit breaker uses a low-voltage MOSFET to replace the IGBT in the actual sub-module SM (see Figure 2), uses a low-voltage capacitor C _lv to replace the high-voltage capacitor C in the actual device, and uses a low-voltage resistor R _lv replaces the high voltage discharge resistor R in the actual device. Since the turn-on voltage drop of the MOSFET is much smaller than that of the IGBT, even the turn-on voltage drop of multiple series-connected sub-modules in the transfer branch may be smaller than the turn-on voltage drop of the solid state relay in the flow branch. In order to make the commutation process of the low-voltage equivalent model the same as that of the actual device, a diode D _lv is connected in series in the transfer branch as required to increase the conduction voltage drop, so that the relationship between the conduction resistance of the two branches in the low-voltage equivalent model is the same as The actual device is the same.

In HVDC circuit breakers, fast mechanical switches can break within 2ms. The low-voltage equivalent model is replaced by a solid-state relay with a shorter breaking time to achieve the same effect.

The DC link in the low-voltage equivalent model needs to simulate the DC voltage and the corresponding current rise rate after a short circuit. A DC voltage source is used instead of a DC converter station to provide a DC voltage for the DC line, and a reactor is connected in series in the DC line to limit the fault current rise rate during a short circuit.

Replacement for lightning arrestors. In high-voltage DC circuit breakers, arresters are used to dissipate the energy generated in the reactance after the line is short-circuited. In the low-voltage equivalent model, the varistor is used instead of the arrester. The action voltage of the selected varistor is higher than the voltage of the DC voltage source V _dc , and the residual voltage should not exceed the sum of the rated voltages of the transfer branch sub-modules.

Replacement for DC loads. In a HVDC circuit breaker, the DC load supplies the normal operating current. In the low-voltage equivalent model, a sliding rheostat is used to replace the DC load.

Simulation of fault currents. When bipolar occurs in the actual circuit, the DC current will rise according to a certain slope. In the low-voltage equivalent model, in order to simulate the bipolar short circuit of the line, the sliding rheostat that replaces the DC load is bypassed by a DC air switch. For the safety of the test and to limit the magnitude of the DC current, a current-limiting resistor needs to be fixed in series in the line.

A method for detecting a low-voltage equivalent circuit of a cascaded full-bridge DC circuit breaker, wherein the equivalent circuit is the above-mentioned equivalent circuit, and the method for detecting comprises the following steps:

1) Open the low-voltage equivalent sub-module of the main branch in the low-voltage equivalent test circuit of the cascaded full-bridge DC circuit breaker, lock the low-voltage equivalent sub-module of the transfer branch, and open the solid-state relay to make the circuit breaker in a conducting state;

2) Open the fault simulation switch S1, close the load switching switch S2, so that the load resistance _RL and the current limiting resistance Rx are connected in series into the test circuit;

3) Power on the DC stabilized power supply, adjust the voltage to the test voltage between 100 and 200V, the loop current i _{breaker_lv} flows through the equivalent reactor L _{lv_3} , the inductance L _{lv_1} , the main branch low-voltage equivalent sub-module, and the load switching switch S1, the load resistor _RL , the current limiting resistor Rx, returns to the DC regulated power supply;

4) Close the fault analog switch S2, bypass the load resistor _RL , and the loop current i _{breaker_lv} starts to rise rapidly;

5) When the loop current i _{breaker_lv} exceeds the overcurrent limit value _{Ilim_lv} , the low-voltage equivalent sub-module of the transfer branch is turned on, and the low-voltage equivalent sub-module of the main branch is blocked, so that the loop current is transferred from the main branch to the transfer branch;

6) When the main branch current i _{main_lv} drops to 0, disconnect the solid state relay;

7) After a delay of 2 ms, the low-voltage equivalent sub-module of the transfer branch is blocked, the capacitor C _lv of the low-voltage equivalent sub-module of the transfer branch is charged, and the voltage of the transfer branch and the energy-consuming branch starts to rise;

8) When the voltage of the energy-consuming branch rises to the operating voltage of the varistor, the varistor is gradually turned on. As the voltage of the energy-consuming branch rises further, the current i _{MOV_lv} flowing through the varistor in the energy-consuming branch rises and transfers The branch current i _{comm_lv} gradually drops to 0;

9) The current i _{MOV_lv} flowing through the varistor in the energy-consuming branch begins to decline after reaching the peak value, and finally reaches 0, the test ends;

Among them, the above-mentioned experimental method includes two commutation time equivalents:

The time for the first commutation is equivalent. After the circuit breaker detects the overcurrent of the line, it starts the first commutation, blocks the main branch, opens the transfer branch, and the current is transferred from the main branch to the transfer branch. The low-voltage equivalent model needs to be close to the actual circuit breaker in terms of commutation time.

In order to realize the equivalence of the first commutation time, it is necessary to make the first commutation time of the low-voltage equivalent model equal to the actual high-voltage DC circuit breaker, which should satisfy:

(L ₁ +L ₂ )C=(L _{1_lv} +L _{2_lv} )C _lv (1)

In formula (1), C is the capacitance value of the sub-module in the actual circuit breaker, L ₁ and L ₂ are the parasitic inductance of the main branch and the transfer branch in the actual circuit breaker, respectively, and C _lv is the capacitance of the sub-module in the low-voltage equivalent model , L _{lv_1} , L _{lv_2} are the parasitic inductance of the main branch and the transfer branch in the low-voltage equivalent model, respectively.

The time of the second commutation is equivalent. After the transfer branch is blocked, the voltage of the sub-module of the transfer branch rises to the operating voltage of the arrester, and the arrester starts to discharge the current. This process is the capacitor charging process of the transferred DC sub-module. In order to realize the equivalent of the second commutation time, it is necessary to make the second commutation time of the low-voltage equivalent model equal to the actual high-voltage DC circuit breaker, which should satisfy:

CV _MOV I _{break_lv} = C _lv V _{MOV_lv} I _break (2)

In formula (2), V _MOV is the operating voltage of the arrester, I _break is the maximum segment current of the circuit breaker, C is the capacitance value of the neutron module of the actual DC circuit breaker, V _{MOV_lv} is the operating voltage of the varistor in the low-voltage equivalent model, I _{break_lv} is the maximum sub-section current of the circuit breaker, and C _lv is the capacitance value of the sub-module in the low-voltage equivalent model.

Define the voltage ratio, current ratio, and impedance ratio of the actual circuit breaker and the low-voltage equivalent model as:

${\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; MOV</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; MOV</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; lv</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; break</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; brak</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; lv</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; lv</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; lv</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; lv</span>}}}{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

Then when satisfying:

${\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

When , formulas (1) and (2) are also satisfied, therefore, It is a sufficient condition for two commutation times to be equivalent. Therefore, according to The proportional relationship of the sub-module, the loop inductance, the sub-module voltage and the circuit breaker breaking current and other parameters are selected.

Equation (6) does not include the number of transfer branch sub-modules. Therefore, the number of transfer branch sub-modules in the low-voltage equivalent model can be different from that of the actual high-voltage DC circuit breaker, and the commutation process can also be equivalent in time.

The equivalent experimental circuit proposed by the present invention uses relatively low cost to realize the verification of the working principle and secondary control and protection equipment of the cascaded full-bridge high-voltage DC circuit breaker, and can well represent the working process of the high-voltage DC circuit breaker. The present invention proposes The equivalent detection method is based on the time equivalent principle of current transfer and shutdown process, through impedance equivalent, using low-voltage devices and lines to simulate the action process of cascaded full-bridge high-voltage DC circuit breakers, which can realize the cascaded full-bridge high-voltage The equivalent of the two commutation times of the DC circuit breaker, so as to accurately verify the requirements for the timing of the control and protection equipment, by simplifying the equivalent conditions, under the condition that the voltage ratio, current ratio, and impedance ratio satisfy the proportional relationship, the number of sub-modules can be transferred It can be different, and the equivalent of the two commutation times can also be realized.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art can still implement the present invention Any modification or equivalent replacement that does not deviate from the spirit and scope of the present invention is within the protection scope of the pending claims.

Claims (7)

1. A low-voltage equivalent test circuit of a cascaded full-bridge DC circuit breaker, characterized in that, the equivalent test circuit consists of successively connected DC power supply V _dc , equivalent reactor L _{lv_3} , solid state relay, inductor L _{lv_1} , The main branch low-voltage equivalent sub-module SM _lv , load switching switch S1, load resistance R _L and current limiting resistor R _X form the main circuit, and the solid state relay, the inductor L _{lv_} and the main branch low-voltage equivalent The sub _- module SM _lv constitutes the main branch, and there are transfer branches and energy consumption branches connected _in parallel at both ends of the main branch. A varistor is connected to the energy-consuming branch, and a fault analog switch S2 is connected in parallel to both ends of the load resistance _RL . 2. The low-voltage equivalent test circuit of cascaded full-bridge DC circuit breaker according to claim 1, characterized in that, the voltage level of the test circuit is 100-200V, and the current level is 5-10A. 3. cascaded full-bridge DC circuit breaker low-voltage equivalent test circuit according to claim 1, is characterized in that, the operating voltage of described varistor is higher than the voltage of described DC voltage source V _dc , and residual voltage can not exceed The sum of the rated voltages of the transfer branch sub-modules. 4. A detection method of a low-voltage equivalent test circuit of a cascaded full-bridge DC circuit breaker, said equivalent circuit being the equivalent test circuit described in any one of claims 1-3, characterized in that said detection method Include the following steps: Opening the main branch low-voltage equivalent submodule in the low-voltage equivalent test circuit of the cascaded full-bridge DC circuit breaker, locking the transfer branch low-voltage equivalent submodule, and opening the solid state relay; Opening the fault simulation switch S1, closing the load switching switch S2; Powering on the DC stabilized power supply, adjusting the voltage to 100-200V, the loop current i _{breaker_lv} flows through the main loop, and returns to the DC stabilized power supply; Closing the fault simulation switch S2; When the loop current i _{breaker_lv} exceeds the overcurrent limit value I _{lim_lv} , turn on the transfer branch low-voltage equivalent sub-module, block the main branch low-voltage equivalent sub-module, so that the loop current i _{breaker_lv} is controlled by the main branch transfer to transfer branch; When the main branch current i _{main_lv} drops to 0, disconnect the solid state relay; After a delay of 2 ms, the low-voltage equivalent sub-module of the transfer branch is blocked, the capacitor C _lv of the low-voltage equivalent sub-module of the transfer branch is charged, and the voltage of the transfer branch and the energy-consuming branch starts to rise; When the voltage of the energy-consuming branch rises to the operating voltage of the varistor, the varistor is gradually turned on. As the voltage of the energy-consuming branch rises further, the current i _{MOV_lv} flowing through the varistor in the energy-consuming branch rises, and the transfer branch The circuit current i _{comm_lv} gradually drops to 0; The current _{iMOV_lv} flowing through the piezoresistor in the energy-consuming branch begins to drop after reaching a peak value, and finally reaches 0, and the detection ends. 5. The detection method of the low-voltage equivalent circuit of the cascaded full-bridge DC circuit breaker according to claim 4, wherein the detection method comprises two commutation time equivalents: The time of the first commutation is equivalent. After the circuit breaker detects the overcurrent of the line, it starts the first commutation, blocks the main branch, opens the transfer branch, and the current is transferred from the main branch to the The transfer branch described above, the first commutation time of the low-voltage equivalent model is equal to the commutation time of the actual high-voltage DC circuit breaker; The time of the second commutation is equivalent. After the transfer branch is blocked, the voltage of the sub-module of the transfer branch rises. After rising to the operating voltage of the arrester, the arrester starts to discharge the current. The second commutation time of the low-voltage equivalent model is the same as the actual The commutation time of HVDC circuit breakers is equal. 6. The detection method of the low-voltage equivalent circuit of the cascaded full-bridge DC circuit breaker according to claim 5, wherein the detection method satisfies the following formula when performing the time equivalent of the first commutation: (L ₁ +L ₂ )C=(L _{1_lv} +L _{2_lv} )C _lv (1) In formula (1), C is the capacitance value of the sub-module in the actual circuit breaker, L1 and L2 are the parasitic inductance of the main branch and the transfer branch in the actual circuit breaker, respectively, and Clv is the capacitance of the sub-module in the low-voltage equivalent model, Llv_1, Llv_2 is the parasitic inductance of the main branch and the parasitic inductance of the transfer branch in the low-voltage equivalent model, respectively. 7. The detection method of the low-voltage equivalent circuit of the cascaded full-bridge DC circuit breaker according to claim 6, wherein the detection method satisfies the following formula when performing the time equivalent of the second commutation: CV _MOV I _{break_lv} = C _lv V _{MOV_lv} I _break (2) In formula (2), V _MOV is the operating voltage of the arrester, I _break is the maximum segment current of the circuit breaker, C is the capacitance value of the neutron module of the actual DC circuit breaker, V _{MOV_lv} is the operating voltage of the varistor in the low-voltage equivalent model, I _{break_lv} is the maximum sub-section current of the circuit breaker, and C _lv is the capacitance value of the sub-module in the low-voltage equivalent model.