362 results about "Current limiting reactor" patented technology

The invention discloses a multi-terminal flexible DC grid DC line quick protection method and system based on single-terminal voltage. The method comprises the following steps: collecting the voltagesignals on the two sides of DC positive and negative line current limiting reactors in real time; forming a low voltage starting criterion; performing wavelet transform on the calculated line mode voltage on the current limiting reactors and calculating the wavelet transform modulus maximum; performing the data validity test and recording the size and the symbol of the first wavelet transform modulus maximum meeting the data validity condition; establishing the fault identification criterion to identify the fault based on the symbol and amplitude difference of the single-terminal voltage traveling wave wavelet transform modulus maximum; and constructing the fault pole identification criterion to identify the fault pole based on the size difference of the voltage traveling wave transient energy. According to the line protection method, the fault direction can be reliably and quickly identified under various initial fault conditions, and the transition resistance, the fault location andthe fault of the AC system and other factors have little influence on the protection criterion so as to have high reliability and sensitivity.

The present invention relates to a complex superconducting fault current limiter which adds a current limiting reactor to a superconductor to protect the power line from a fault current, and more particularly, to a complex superconducting fault current limiter using a minimum number of superconducting fault current limiters, while avoiding series and parallel connections of a plurality of superconductors and coils, in order to economically manufacture the fault current limiter in a small size. A superconductor, a high speed switch, and a circuit breaker are connected in series to each other, and a first reactor with a low impedance and a second reactor with a high impedance are connected in parallel to the power line so as to provide a branch circuit for the current to the series circuit. A semiconductor switch is connected in parallel to the second reactor with a high impedance in accordance with the opening high speed switch. A circuit is breaker trip drive controller is configured so as to be connected to the superconductor and the branch circuit, and when a fault current occurs, the fault current is branched into the branch circuit, so that the second reactor limits the fault current. When a fault current occurs, the circuit breaker trip drive controller provides a trip drive signal to the circuit breaker for tripping in accordance with the voltage of the superconductor or the current of the branch circuit.

The invention discloses a control method and device for an alternating-current solid-state power controller with a current limiting function. Through control over a first power tube and a second power tube of a main branch circuit and a third power tube and a fourth power tube of a current limiting branch circuit, when loads are normally started, natural zero-voltage turn-on and natural zero-current turnoff of the alternating-current solid-state power controller are achieved within a half of power source period, when large-capacity capacitive loads or rectifier bridge loads with large-capacity filtering capacitance are started, current-limiting charging is firstly performed through the current-limiting branch circuit, and then the alternating-current solid-state power controlled is completely turned on; when a short circuit fault happens on the alternating-current solid-state power controller, the main branch circuit is firstly turned off to perform fault current limiting, and then the alternating-current solid-state power controller is turned off. Natural zero-voltage on-off of the alternating-current SSPC is achieved within the half of the power source period, besides, the control method and device have a high-power current-limiting function, capacity for carrying the capacitive loads and the rectifier bridge loads of the alternating-current SSPC is improved, and anti-interference capacity of short circuit protection of the alternating-current SSPC is improved.

The invention relates to a neutral point direct-current restrainer of ground transformer, wherein it comprises blocking capacitor, thyristor solid switch connected to two ends of blocking capacitor, relative trigger unit, limit resistance, and bypass switch. The invention can effective restrain the neutral point direct current caused by high-voltage direct current, etc, to reduce noise, overheat, etc. And it has simple process and low cost, without external reverse voltage.

A zero-loss depth current-limiting protection device relates to the technical field of over-voltage absorption in a power system. The protection device is characterized in that the device comprises a Rogowski coil, a measurement and control unit, a high-speed switch and a depth current-limiting reactor 1, wherein the measurement and control unit is connected with the Rogowski coil and the high-speed switch, and the two ends of the depth current-limiting reactor 1 are connected with the two ends of the high-speed switch respectively. The device is fast in speed, high in breaking capacity and good in current-limiting effect, not only can protect electrical equipment of a transformer and a motor and the like in a system, but also improves service life of a load vacuum circuit breaker at a short circuit point.

An inrush current protection circuit for a polyphase alternating current power system may include a plurality of current limiting resistors, each of which is electrically coupled in series between a respective one of a plurality of phases of the polyphase alternating current power source and a respective input of a polyphase transformer rectifier unit. The circuit may also include a plurality of power switches, each of which is electrically coupled in parallel with a respective one of the current limiting resistors, such that when each of the power switches is closed, essentially no electrical current flows through the respective current limiting resistor. The circuit may also include a power switch controller configured to control each of the power switches to be open such that an inrush current passes through the respective current limiting resistor rather than the power switch, and to be closed after a time delay has passed.

In order to protect Magnetic Energy Recovery Switch (MERS) against an overvoltage and an overcurrent a voltage detection unit is provided for detecting the capacitor voltage of the MERS and control means is provided to control so as to turn ON the switch of the discharge circuit connected in parallel with the capacitor to make the capacitor discharge the electric charge thereof when the output of the voltage detection unit exceeds a predetermined value. Moreover, a current detection unit is interposed between the AC power supply and the load for detecting the current flowing to the load, and the current limiting is carried out by making the duty ratio of the ON / OFF of the pulse of the gate control signals of the MERS switches smaller than 0.5, when the output of the current detection unit exceeds a predetermined value.

This invention relates to a transformer type adjustable reactance, and the static reactive compensator made thereof. The reactance comprises: a parallet transformer unit, voltage source type inverter combniation unit and an adjustable reactance combination unit. Above-said inverter combination unit involves a double arm inverter power source series connected with current-limiting reactance. Adjustable reactane combniation unit involves (N-1) pieces of crystal valve tube throw-over reactance branches, each of which being parallel connection respectively antiparallet connection crystal valve tube T(i1) and a reactance X(i1). The compensator composed by above-said reactance comprises also: a fixed parallet capacitor unit, testing unit and a control unit. Advantages are: no need of special fitler, large capacity, low harmonic wave pollution, quick dynamic response.

The invention discloses a single-phase ground fault positioning method for a ground protective device of a low-current system, which does not affect the protective performance of the ground protective device and can quickly find out a ground fault point. The method comprises the following steps that: a microcomputer intelligent controller judges whether a system is grounded or has a grounded phase difference or not according to the changes of a system line, a phase voltage and a zero sequence voltage; and when a single-phase ground fault happens, the microcomputer intelligent controller sends a switching command to a grounded phase breaker. The method is characterized in that: after being switched on, the grounded phase breaker is connected with a grounding network through a low-impedance current limiting reactor with a secondary coil; at the time, a grounding current of the system flows through a primary coil of the low-impedance current limiting reactor, and after the current is detected, the microcomputer intelligent controller gets into work with a high frequency signal generator connected with the secondary coil of the low-impedance current limiting reactor to inject a high frequency current into the system; and a handheld high frequency signal receiver moves along a fault line for searching the fault point, and the corresponding point where the received signal gradually becomes weak when the handheld high frequency signal receiver departs from some point is the ground fault point.

The invention relates to a detecting device for low voltage ride-through of a grid-connected photovoltaic inverter. The detecting device comprises a current-limiting reactor, a short-circuit reactor and a grounding combination switch, the current-limiting reactor, an intermediate reactor and circuit breaker unit, the short-circuit reactor and the grounding combination switch are sequentially connected in series, the current-limiting reactor is connected into a 10 / 35kV power grid, the intermediate reactor and circuit breaker unit and the grounding combination switch are respectively connected into a neutral point of a high-voltage side of a step-down transformer, and the step-down transformer is connected into the grid-connected photovoltaic inverter. The detecting device has the advantages that the proportion of the current-limiting reactor and the short-circuit reactor is changed by the aid of different on-off modes of a plurality of circuit breakers under the condition that short-circuit capacity is unchanged assuredly, accordingly, the requirement that low voltage ride-through with different dropping amplitudes is detected by the aid of fewer reactors is met, and the detecting device is used for detecting low voltage ride-through ability of the photovoltaic inverter.

The invention relates to a high-voltage and ultrahigh-voltage dry-type air-cored current limiting reactor. A main reactor body (1) comprises a coil (10), a rain hat (20) and a combined lazy arm (11), wherein the main reactor body (1) is supported by an insulated support post (30); electric field shielding parts (40) are arranged at the upper part and the bottom part of the main reactor body (1); and each electric field shielding part (40) comprises an upper main reactor body corona ring (41) and a lower main reactor body corona ring (42) to solve the problem of power generation of the main reactor body and accessories at high voltage. The insulated support post (30) comprises a support post insulator (31) and a metal bracket (33); and the metal bracket (33) is connected with a support post insulator (31) through an epoxy flat plate (73) and insulated bolts. According to the high-voltage and ultrahigh-voltage dry-type air-cored current limiting reactor, the requirements on the ground insulation level and support are met; leakage flux is reduced; the requirements on the temperature of accessories are reduced; and the reactor provided by the invention can meet the requirements of power grids of 220kV and 500kV on voltage and current distribution, insulation level and short-time current.

The invention discloses a device and a method for monitoring and protecting a series reactor. A search coil assembly is arranged inside or outside a dry type air core reactor coil body; one side of the search coil assembly corresponds to the reactor; the other side of the search coil assembly is correspondingly provided with an isolation transformer; one end of the isolation transformer is grounded; the other end of the isolation transformer is connected in series with a filter module; the filter module, a resistor and a TVS (Transient Voltage Suppressor) are connected in series in sequence and then grounded; the filter module and a data acquisition module are connected in series in sequence and then grounded; and the data acquisition module is connected with a control computer. The device and the method for monitoring and protecting the series reactor have the beneficial effects that the occurrence of serious accidents such as reactor firing is prevented, and the operation safety coefficient of a current limiting reactor is improved, so that the safe operation of the transformer is guaranteed.

The invention discloses a breaking character detector of current limiting fuse, which comprises a main power supply, a current meter, a timer, an auxiliary power supply, a surge current generator, a heat engine buffer circuit, a current gate circuit and a fuse breaking locking signal extracting circuit, wherein the surge current generator is connected with the main power supply, the main power supply is connected with the current meter to be connected with the heat engine buffer circuit, the heat engine buffer circuit is connected with the current gate circuit mounted with a detector for detecting the fuse, the detector via the fuse breaking locking signal extracting circuit is connected with the current meter and the timer, the heat engine buffer circuit is connected with a preset trigger button to be controlled, the current gate circuit is connected with a synchronous trigger button to be controlled, the synchronous trigger button is connected with the timer, the auxiliary power supply is connected with the timer, a hand-operated return switch is connected with the timer and the current meter. The invention has simple operation, which can detect the breaking character of general current limiting fuse and can be added with a plug interface to detect other fuses of different types.

The invention discloses a DC circuit breaker and a test method. The DC circuit breaker is formed by connecting two current limiting reactors in series to the two ends of four parallel branches. Of the four parallel branches, the first branch is a main branch, and is composed of a quick isolation switch K and an auxiliary quick switch T0 connected in series. The second branch is a transfer branch, and is composed of a solid state switch. The solid state switch is composed of a plurality of IGBT modules connected in series. Each IGBT module is composed of two IGBTs reversely connected in series. A diode is reversely connected in parallel to each IGBT. The third branch is a buffer circuit which is composed of a resistor and a capacitor connected in series. The fourth branch is an energy absorption circuit composed of a zinc oxide arrester. The DC circuit breaker can be quickly closed in a bidirectional manner, and has the advantages of simple structure, easy manufacture and high practicality.

The invention discloses a power grid current limiting device, a power grid current limiter and a power grid current limiting system. The power grid current limiting device comprises a first current limiting reactor, a first intelligent high speed switch, a current transformer and a controller, wherein the first intelligent high speed switch is parallelly connected with the first current limiting reactor, the current transformer is sleeved on a bus on one side of the parallelly connected first current limiting reactor and the first intelligent high speed switch and is used for monitoring current in the bus in real time, the controller is connected with the current transformer and used for controlling to switch off the first intelligent high speed switch when the current is larger than the first preset value and to switch on the first intelligent high speed switch when the current is smaller than the second preset value, and the first preset value is larger than the second preset value. By the aid of the power grid current limiting device, the power grid current limiter and the power grid current limiting system, running reliability of a power grid is improved, and the power grid current limiting device can be utilized in series to economically and effectively limit short-circuit current level of the power grid smaller than 330 (500)KV.

The invention belongs to the field of power system relay protection and automation, and particularly relates to a direct-current power grid ground fault ultrahigh-speed non-unit protection method. Themethod includes: considering that no ground fault occurs when current at a ground point on an alternating-current side of a system does not meet fault occurrence and type criteria; considering that there might be a ground fault when the current meets the fault occurrence and type criteria, sending a fault occurrence signal to a higher-level protection device, and sending fault type criteria to the higher-level protection device; measuring voltage at two ends of a current-limiting reactor in real time by each protection device, considering that a ground fault occurs within the protection rangewhen the voltage is greater than a voltage threshold value, and sending a fault positioning signal to the higher-level protection device. By the method, protection correctness when ground faults occur at different positions of the system can be ensured; compared with other protection methods, the method is higher in action speed, only needs to utilize non-unit information and does not depend on inter-station communication devices, so that protection reliability is improved.

The invention provides a defense method for commutation failures of direct-current transmission based on a current limiting method. The method comprises the following steps: I, according to an output signal of failure detection, detecting the failure type of a commutation bus of a direct-current transmission system; II, by using the current limiting method, reducing a direct-current instruction and continuing for 60-100 ms; III, judging whether a failure is eliminated and a turn-off angle of the direct-current transmission system is restored; and IV, determining the direct-current instruction according to a judgment result until the failure is eliminated, and restoring the direct-current instruction. According to the method, adverse effects such as increasing a direct-current, increasing a power factor angle of the direct-current transmission system, and the like caused by advanced triggering are avoided, and a timely and effective response is made to an alternating-current system failure, so that the required commutation area of a thyristor is reduced, thereby facilitating that the direct-current transmission system is prevented from having commutation failures.

The invention relates to a quick fault current restrictor based on a carrier isolator, comprising a carrier isolator, an ignition pulse transformer, an electronic measuring and controlling unit, a sensor, a current-limiting reactor, an MOA, a bypass switch, a first isolation switch, a first grounding switch, a second isolation switch, a second grounding switch and a bus-bar; the sensor, a first isolation switch carrier isolator and a third isolation switch are connected on the bus-bar in sequence; one end of the first grounding switch and one end of the second grounding switch are connected with the bus-bar; the other ends of the first grounding switch and the second grounding switch are grounded; the electronic measuring and control unit is connected with one end of the sensor and one end of the ignition pulse transformer respectively; the other end of the ignition pulse transformer is connected with the carrier isolator; and the bypass switch and the MOA are connected in parallel with the current-limiting reactor in sequence. The device provided by the invention can detect the system current failure in time, can restrict the fault current, does not affect the normal power supply of a user, enhances the power supply reliability, and realizes flexible and self-adaptive fault current treatment and self-healing.

The invention relates to a current limiting device consisting of a current limiting inductor and a high-capacity and high-speed switch which are connected in parallel, in particular to an energy-saving outdoor grid current limiting device, comprising a current limiting inductor and a high-capacity and high-speed switch which are connected in parallel. The energy-saving outdoor grid current limiting device is characterized in that the high-capacity and high-speed switch comprises a tubular insulating shell (8) both ends of which are sealed and are respectively provided with an upper flange (5) and a lower flange (11), and leading-out terminals are respectively arranged on the upper flange (5) and the lower flange (11); a bridge body (1), a fuse and a non-linear resister (7) are installed in the insulating shell (8) which is also provided with a measuring and control unit (2) consisting of a controller and a current inductor (9). The invention can not only be used in a medium-voltage system, but also have obvious effect on short-circuit current limitation of 110kV and 220kV high-voltage grids.

An interlocking vacuum switch and a series compensation type current limiting device and method based on the same are provided. The switch uses an interlocking vacuum interrupter. A fast repulsion mechanism is selected to cooperate with the interlocking vacuum interrupter to realize fast switching of the series compensation type current limiting device between the two functions of series compensation and current limiting. When the line of a traction network runs normally, a series compensation capacitor is accessed, and the device works in a series compensation state. When the line is short-circuited, an interlocking vacuum switch acts to short-circuit the capacitor, and a current limiting reactor is put into use in the system to limit the short circuit current of the line. Inside the sameinterrupter, a main static contact and a main moving contact constitute a first contact group to switch on / off the current limiting reactor, and an auxiliary static contact and an auxiliary moving contact constitute a second contact group to switch on / off the compensation capacitor. A single vacuum interrupter and a set of operating mechanism are adopted to realize the switching of a traction system between the functions of series compensation and current limiting. The volume cost and economic cost of the device can be reduced, and the reliability of the system can be improved.

The invention relates to a wind power station LVRT (Low Voltage Ride Through) device which comprises a wind power station, electric reactors, diodes, adjustable voltage sources, a circuit breaker and a grid system. One adjustable voltage source is connected in parallel with one diode after being connected in series with one electric reactor to form one group of fault current limiting circuit; and after two groups of fault current limiting circuits are connected in series, one end of the obtained circuit is connected with the wind power station and the other end of the obtained circuit is connected with the grid system by the current breaker. The wind power station LVRT device is low in cost and easy to implement. When the LVRT function is implemented, the short current limiting capacity is improved; and by the non delay input of the current limiting electric reactors connected in series, the large-area talk around accident caused by the short circuit inside the wind power station can be avoided. The wind power station LVRT device has low requirement for the control characteristic of a unit plant; and a power electronic valve string of the wind power station LVRT device adopts a diode set string with low cost, large through-current capacity and strong short-time overload capacity, so that the system cost is reduced.

A current limiting device for a power grid includes a first current limiting reactor; a first smart fast switch connected with the first current limiting reactor in parallel; a current transformer sleeved on a bus bar located on one side of a circuit resulting from the parallel connection of the first current limiting reactor with the first smart fast switch to monitor the current in the bus bar in real time; and a controller connected with the current transformer to control the switch-off of the first smart fast switch when the current in the bus bar is higher than a first preset value and the switch-on of the first smart fast switch when the current in the bus bar is smaller than a second preset value, wherein the first preset value is higher than the second preset value. The current limiting device improves the operational reliability of a power grid.

The invention discloses a DC circuit breaker and a test method. The DC circuit breaker is formed by connecting two current limiting reactors in series to the two ends of four parallel branches. Of the four parallel branches, the first branch is a main branch, and is composed of a mechanical switch. The second branch is a transfer branch, and is composed of a solid state switch. The solid state switch is composed of a plurality of IGBT modules connected in series. Each IGBT module is composed of two IGBTs reversely connected in series. A diode is reversely connected in parallel to each IGBT. The third branch is a buffer circuit which is composed of a resistor and a capacitor connected in series. The fourth branch is an energy absorption circuit composed of a zinc oxide arrester. The DC circuit breaker can be quickly closed in a bidirectional manner, and has the advantages of simple structure, easy manufacture and high practicality.

The invention provides an offshore convertor station for a flexible direct-current power transmission system. The offshore convertor station comprises a closed upper structure and a foundation structure used for supporting the upper structure; the upper structure comprises a large-space main workshop and a small-space auxiliary workshop which are arranged side by side; the main workshop is dividedinto an upper layer and a lower layer, wherein the upper layer is provided with a current-limiting reactor, direct-current field equipment, a converter transformer, high-voltage switch equipment, a workshop electricity utilization system and an emergency diesel engine, and the lower layer is provided with a converter valve and a bridge arm reactor; the auxiliary workshop totally comprises six layers, the first layer is provided with a seawater pump room and valve cooling equipment, the second layer is provided with seawater treatment equipment, the third layer is provided with a valve controlroom, the fourth layer is provided with firefighting equipment, the fifth layer is provided with positive-pressure ventilation equipment, and the sixth layer is provided with a station control room;and the upper structure is overall symmetrically arranged, the foundation structure is a guide pipe frame, and a channel allowing a barge to enter or go out is reserved in the middle of the foundationstructure. The offshore convertor station has the characteristics of being compact and reasonable in arrangement, safe and reliable in operation and suitable for marine environment conditions and offshore construction.

The invention discloses an overhead flexible direct current power grid fault detection method based on current limiting reactor voltage difference. The overhead flexible direct current power grid fault detection method based on the current limiting reactor voltage difference comprises the steps that (1) direct voltage change rates of each of direct current lines in a power grid are continuously monitored, and when the direct voltage change rates are greater than a preset first threshold value, turning into a step (4) is carried out; (2) at one ends of the direct current lines, voltage time products of positive pole current limiting reactors and negative pole current limiting reactors within a target time period are correspondingly obtained and specific values are determined so as to carryout fault pole selection; (3) the sum of the voltage time products of the positive pole current limiting reactors and the negative pole current limiting reactors is obtained, and a second threshold value corresponding to a fault type is obtained, if the sum of the voltage time products is greater than the second threshold value, an occurred fault is determined as a fault inside a zone; if not, theoccurred fault is determined as a fault outside the zone or no fault; and (4) at the two ends of the direct current lines, the step(2)-step(3) are correspondingly carried out to achieve the fault detection. The overhead flexible direct current power grid fault detection method based on the current limiting reactor voltage difference has a high tolerating transition resistance capacity and a highcapacity of resisting disturbance, and can identify various fault types.

The invention provides a method for automatically resetting the grounding protection device of a small-current system, which has the function of automatically resetting. The method comprises the following steps: an intelligent controller of a microcomputer judges whether the small-current system is grounded and further judges the grounding phase if the system is grounded; when the system is subjected to a grounding fault, a switching-on instruction is given to a fault-phase circuit breaker, and the fault-phase circuit breaker is rapidly switched on; the fault phase is connected with a grounding grid via a low-impedance current-limiting reactor provided with a secondary coil; then, a system grounding current flowing through the primary coil of the low-impedance current-limiting reactor is detected by a zero-sequence current transformer, and a high-frequency voltage signal generator connected with the secondary coil of the low-impedance current-limiting reactor is put into operation, so a high-frequency voltage and a high-frequency current are injected into the fault phase of the system; the high-frequency impedance of the fault phase of the system can be calculated by the intelligent controller of the microcomputer, so as to obtain the grounding resistance of the faulted return circuit; and the grounding resistance thereof is further compared with that in the normal system to judge whether the grounding fault disappears, if so, an automatic resetting instruction is given, so as to switch off the fault-phase circuit breaker, thereby automatically resetting the device.

The invention relates to an overvoltage protection device for a variable impedance transformer. The overvoltage protection device is characterized by comprising a resistance-capacitance absorption device connected with a selector switch (K) and an air reactor in parallel, wherein the resistance-capacitance absorption device comprises a resistor R and a capacitor C1 connected in series, and arresters (MOV) are connected with two ends of the capacitor (C1) in parallel. When in use, the air reactor is placed in the box of a transformer, and a high-speed switch is positioned on the outer side of an oil tank and connected between the high-voltage winding and the neutral point of the transformer in series. When the variable impedance transformer works normally, the high-speed switch is closed, a current-limiting reactor is short-circuited, and the reactor is prevented from causing energy loss; when the system is short-circuited, the high-speed switch is open, the current-limiting reactor is available, then the transformer is equal to a high impedance transformer at the moment, and the short circuit current is inhibited to a certain extent. The device provided by the invention is compact in structure and small in size, and can effectively reduce overvoltage amplitude and gradient caused during an impedance conversion process of the variable impedance transformer.

The invention provides an over-current turn-off test circuit, method and device of a converter valve. The circuit is composed of a power supply, an auxiliary valve, a sample valve, a load reactor, a current limiting reactor and a thyristor valve. The auxiliary valve is formed by series connection of m MMC sub modules. The sample valve is formed by series connection of n MMC sub modules. The load reactor is connected between the auxiliary valve and the sample valve. The current limiting reactor and the thyristor valve are connected in series and the series unit is connected in parallel with thesample valve; and an anode of the thyristor valve is connected with the current limiting reactor and a cathode of the thyristor valve is connected with a negative electrode of the sample valve. M pairs of independent output ports at the output terminal of the power supply are connected with the MMC sub module capacitors of the auxiliary valve to charge the auxiliary valve. Therefore, a problem that the steady-state junction temperature of the IGBT can not be reached by means of water temperature heating during the over-current turn-off testing process of the converter valve or different reactor parameters need to be configured when over currents are formed by an added short-circuit current branch in the prior art is solved.

In order to protect Magnetic Energy Recovery Switch (MERS) against an overvoltage and an overcurrent a voltage detection unit is provided for detecting the capacitor voltage of the MERS and control means is provided to control so as to turn ON the switch of the discharge circuit connected in parallel with the capacitor to make the capacitor discharge the electric charge thereof when the output of the voltage detection unit exceeds a predetermined value. Moreover, a current detection unit is interposed between the AC power supply and the load for detecting the current flowing to the load, and the current limiting is carried out by making the duty ratio of the ON / OFF of the pulse of the gate control signals of the MERS switches smaller than 0.5, when the output of the current detection unit exceeds a predetermined value.