Pre-charge circuit and control system for power converter

Abstract

The power converter comprises an electrical network comprising a plurality of intermediate nodes. Each intermediate node is configured to be connected to one phase of an AC power source, the electrical network is configured to convert an AC current to a DC current, and the electrical network includes a plurality of electronic switches. The power converter also includes a DC link electrically connected to the electrical network; a pre-charge path electrically connected to the DC link, the pre-charge path including a pre-charge impedance; a control path electrically connected to one of the electronic switches, the control path including a relay; and a control system configured to analyze the plurality of electrical measurements from the power converter to determine a state output; and determining whether to control the relay to change the state based on the state output.

Description

Technical Field

[0001] The present disclosure relates to a pre-charge circuit and a control system for a power converter. Background Art

[0002] A power converter, such as a variable speed drive, an adjustable speed drive, or an uninterruptible power supply, can be connected to an alternating current (AC) high power distribution system, such as an electrical grid. A power converter can drive, power, and / or control, for example, an electric machine or power electronic load. An electrical device includes an electrical network that converts AC power to direct current (DC) power or converts DC power to AC power. Summary of the invention

[0003] In one aspect, a power converter includes an electrical network including a plurality of intermediate nodes. Each intermediate node is configured to be connected to one phase of an alternating current (AC) power source, the electrical network is configured to convert AC current into direct current (DC) current, and the electrical network includes a plurality of electronic switches. The power converter also includes a DC link electrically connected to the electrical network; a pre-charge path electrically connected to the DC link, the pre-charge path including a pre-charge impedance; a control path electrically connected to one of the electronic switches, the control path including a relay; and a control system configured to analyze a plurality of electrical measurements from the power converter to determine a state output; and determine whether to control the relay to change state based on the state output.

[0004] Implementations may include one or more of the following features.

[0005] The plurality of electrical measurements from the power converter may include: an indication of a voltage across a pre-charge impedance, an indication of a voltage across an AC power source, and an indication of a voltage across a DC link; and the control system may be configured to analyze the indication of a voltage across the pre-charge impedance, the indication of a voltage across the AC power source, and the indication of a voltage across the DC link to generate a status output.

[0006] The plurality of electronic switches may include at least one controllable electronic switch electrically connected to each intermediate node; the control path may include a plurality of control branches; each control branch may include a relay; and each control branch may be electrically connected to a control node of a controllable electronic switch. The control system may be configured to determine whether to control all relays to change states based on the state output. The controllable electronic switch may include a thyristor; and the control node may include a gate on the thyristor. The precharge path may be electrically connected to one of the intermediate nodes and the DC link. The precharge path may include a plurality of precharge branches, and each precharge branch may be electrically connected to one of the intermediate nodes and the precharge impedance. The power converter may also include a voltage sensing impedance electrically connected to the input node. The plurality of electrical measurements may include a voltage indication across the precharge impedance, a voltage indication of the AC power source measured at the voltage sensing impedance, and a voltage indication across the DC link.

[0007] The relay may include a contactor.

[0008] The plurality of electronic switches may include diodes, and two diodes may be electrically connected to each intermediate node; a relay may be connected in series with one of the diodes; and a pre-charge impedance may be connected in parallel with the relay.

[0009] The plurality of electronic switches may include diodes, and two diodes may be electrically connected to each intermediate node; the control path may include a plurality of control branches, each control branch may include a relay connected in series with one of the diodes; and the pre-charge path may include a plurality of pre-charge paths, each pre-charge path may include a pre-charge impedance connected in parallel with one of the relays.

[0010] In another aspect, a control system includes: an analysis module configured to analyze a plurality of electrical measurements from a power converter to determine a state output; and a control module configured to determine whether to close a relay in a control path of the power converter based on the state output. When the relay is closed, current is provided to a DC link of the power converter through a pre-charge path, and closing the relay causes one or more electronic switches in the power converter to conduct current, so that when the relay is closed, current is provided to the DC link of the power converter through the power path.

[0011] Implementations may include one or more of the following features.

[0012] To analyze the plurality of electrical measurements, the analysis module may be configured to compare each electrical measurement to a predetermined condition associated with the electrical measurement to generate a result for the electrical measurement; and the status output may include the result for each electrical measurement.

[0013] The control module may be configured to close the relay only when each result satisfies an associated predetermined condition, such that current is provided to the DC link of the power converter through the power path only when each electrical measurement satisfies the associated predetermined condition.

[0014] On the other hand, an apparatus includes: an electrical network comprising: a plurality of intermediate nodes, wherein each intermediate node is configured to be connected to one phase of an alternating current (AC) power source, the electrical network being configured to convert AC current into direct current (DC) current; and an electrical network comprising a phase branch connected to each intermediate node, each phase branch comprising a thyristor and a diode; a DC link electrically connected to the electrical network; a pre-charge path electrically connected to at least one of the intermediate nodes and the DC link, the pre-charge path may include a pre-charge impedance; and a plurality of control paths, each control path being connected to one of the intermediate nodes and a gate of one of the thyristors, and each control path comprising a relay.

[0015] Implementations may include one or more of the following features.

[0016] The precharge path can be connected to all intermediate nodes.

[0017] The device may further include sensing impedances connected to all intermediate nodes, and a voltage across the sensing impedances may provide an indication of a voltage input to the device.

[0018] On the other hand, a method includes: accessing multiple electrical measurements from a power converter, the multiple measurements including: a voltage indication across a pre-charge impedance; a voltage indication of an AC power source connected to the power converter, and a voltage indication across a DC link of the power converter; comparing at least two indications with associated specifications to produce at least two status outputs; analyzing at least two status outputs to determine whether to change a state of a relay in the power converter; if it is determined to turn on the relay, controlling the relay to turn on so that current flows in an electronic switch connected to the relay so that the current flows to the DC link through a power path, the power path including the electronic switch; and if it is determined to turn off the relay, controlling the relay to turn off so that current flows to the DC link through the pre-charge impedance.

[0019] In some implementations, each indication is compared to an associated specification to produce three state outputs, and the three state outputs are analyzed to determine whether to change the state of the relay.

[0020] Implementations of any technology described herein may include devices, equipment, systems, control systems, machine executable instructions, and / or methods. Details of one or more implementations are set forth in the accompanying drawings and the following description. Other features will be apparent from the description and drawings, as well as from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a block diagram of an example power system.

[0022] Figure 2 is a schematic diagram of a system including an example of a power converter.

[0023] Figure 3 is a flow chart of an example of a process that may be used with a power converter.

[0024] Figure 4-8 is a schematic diagram of a system including an example of a power converter.

[0025] Fig. 9 is a schematic diagram of an example of a system including a rectifier and a DC link. DETAILED DESCRIPTION

[0026] Figure 11 is a block diagram of an example of an electric power system 100. The electric power system 100 includes a power converter 110 electrically connected to an electric power distribution network 101 at an input node 111. The power converter 110 includes a filter system 170, a rectifier 117, a DC link 118, a pre-charge path 140 including a pre-charge impedance 142, a control path 150 including a relay 152, and a control system 130. The filter system 170 is located between the input node 111 and the intermediate node 114. The rectifier 117 includes electronic components that are arranged and / or controlled to convert alternating current (AC) or time-varying power from the electric power distribution network 101 into direct current (DC) power provided to the DC link 118. The rectifier 117 is any type of arrangement that converts AC current into DC current. The rectifier 117 includes a plurality of switches (e.g., diodes), and the switches may or may not be controllable switches (such as transistors and thyristors). For example, the rectifier 117 may be a diode bridge rectifier, or a diode-thyristor hybrid rectifier.

[0027] The power distribution network 101 may be, for example, a three-phase power grid that provides power to industrial, commercial and / or residential facilities. The AC power distribution network 101 distributes AC power having a fundamental frequency of, for example, 50 or 60 Hertz (Hz). The distribution network 101 may be a low voltage distribution network with an operating voltage of up to 1 kilovolt (kV).

[0028] exist Figure 1 In the example of FIG. 1 , the power system 100 includes a transformer 103 between an input node 111 and the power distribution network 101. The transformer 103 lowers or reduces the voltage of the distribution network 101 so that the AC voltage at the input node 111 is lower than the voltage of the distribution network 101. For example, the voltage at the input node 111 may be 240 V. The power system 100 may be implemented without the transformer 103.

[0029] The DC link 118 includes one or more energy storage devices, such as, for example, capacitors. During the working use of the power converter 110, the DC link 118 receives DC current from the rectifier 117 and stores electrical energy, so that a potential difference in the form of a voltage is generated across the DC link 118. However, when the power converter 110 is powered on, there is no voltage or almost no voltage across the DC link 118. Therefore, when the power converter 110 is turned on, the voltage across the DC link 118 is much smaller than the voltage at the input node 111, and the voltage difference can cause a transient surge current to flow into the rectifier 117. The surge current may be short-lived, but the amplitude is large enough to damage the components in the power converter 110. In order to reduce these transient surge currents at startup, the DC link 118 is first charged in a pre-charge sequence through the pre-charge path 140. After the pre-charge sequence is completed, the control system 130 commands the relay 152 to close and bypass the pre-charge path 140. After pre-charge path 140 is bypassed, current flows in the power path through the electronic components of rectifier 117 , and power converter 110 operates in a stable state.

[0030] As described below, control system 130 analyzes a plurality of different electrical measurements 131 from power converter 110 to determine when to close relay 152. Electrical measurements 131 include an indication of a voltage across DC link 118, an indication of a voltage across pre-charge impedance 142, and an indication of a voltage at intermediate node 114. The voltage across DC link 118 is the DC bus voltage, and the voltage at intermediate node 114 is the voltage of the AC input source.

[0031] Some conventional power converters include a pre-charge path that is bypassed based solely on an evaluation of the voltage across the DC link. However, such approaches are unable to effectively handle brown-out and brown-in conditions. A brown-out condition occurs when the peak voltage of any phase of the input voltage suddenly drops below the DC link voltage due to, for example, an upstream electrical fault. During brown-out, the DC link remains connected to the power path until the voltage of the DC link is discharged to a predefined value or below. When the DC link is discharged to a predefined value, the DC link is removed from the power path and connected to the pre-charge path.

[0032] However, the time for the DC link to discharge to a predefined value is generally longer than the duration of the undervoltage condition. In this scenario, when the undervoltage condition ends and the input voltage is restored (also known as voltage ramping), the precharge path is not connected to the DC link, and since the voltage across the DC link has not dropped to a predefined value, a precharge sequence does not occur. However, even if the voltage across the DC link has not dropped to a predefined value, the voltage across the DC link will also decrease during the undervoltage period, so that when the input power is restored during the voltage ramping period, there is a voltage difference between the input voltage and the DC link. In addition, during the voltage ramping period, the amplitude of the input voltage may be greater than the typical value and more variable than the typical value. Therefore, even if the DC link is not fully discharged during the undervoltage condition, there may be a relatively large voltage difference between the DC link and the input voltage after the undervoltage. This voltage difference can cause a large surge current.

[0033] On the other hand, control system 130 analyzes each of electrical measurements 131 to determine whether to bypass pre-charge path 140. By analyzing all electrical measurements 131, control system 130 can avoid or mitigate inrush current during power converter 110 startup and during brownout and brownout conditions.

[0034] Figure 2 2 is a schematic diagram of a system 200. The system 200 includes a power converter 210 connected to a load 202 and a three-phase AC power distribution network 201. The load 202 may be, for example, an induction motor or a permanent magnet synchronous machine. The dashed lines are used to illustrate the grouping of components, and the dashed lines do not necessarily represent physical objects. However, the power converter 210 may be located in a housing or enclosure, such as a rack case or cabinet.

[0035] The power distribution network 201 distributes AC power having a fundamental frequency of, for example, 50 or 60 Hertz (Hz). The distribution network 201 may include, for example, one or more transmission lines, distribution lines, cables, and / or any other mechanism for transmitting power. The distribution network 201 includes three phases, referred to as a, b, and c. Each phase a, b, c has a corresponding grid voltage Va, Vb, Vc.

[0036] The power converter 210 includes input nodes 211a, 211b, 211c, each of which is electrically coupled to one of the three phases (a, b, c) of the power distribution network 201. The power converter 210 also includes intermediate nodes 214a, 214b, 214c. The power converter 210 includes a rectifier 217, a DC link 218, and an inverter 219. The rectifier 217 includes an electronic switch that is arranged and / or controllable to convert the AC currents ia, ib, ic at the nodes 214a, 214b, 214c into a DC current Idc flowing to the DC link 218.

[0037] The power converter 210 also includes a filter system 270. The filter system 270 may be, for example, an inductor (such as an Figure 2 ), and may or may not include additional electronic components. Other implementations of the filter system 270 are also possible, and the filter 270 is not necessarily located between the input nodes 211a, 211b, 211c and the intermediate nodes 214a, 214b, 214c. For example, the filter system 270 can be implemented as a DC link choke between the rectifier 217 and the DC link 218.

[0038] The DC link 218 includes an energy storage device 216. The energy storage device 216 is any component capable of storing electrical energy. The energy storage device 216 may be, for example, a capacitor, or a network consisting of a collection of such devices. In some implementations, the energy storage device 216 includes one or more electrolytic capacitors. The rectified current Idc charges the energy storage device 216, which stores energy in the form of a voltage. Figure 2 In the example shown, energy storage device 216 is a capacitor labeled Cdc.

[0039] The inverter 219 modulates the energy stored in the energy storage device 216 into a three-phase AC voltage and current drive signal 204 provided to the load 202. The inverter 219 includes output terminals 205u, 205v, 205w, each of which is connected to one of the three phases of the load 202. The voltage appearing across the terminals 205u, 205v, 205w becomes the three-phase AC voltage drive signal 204. The inverter 219 includes a network of electronic switches (e.g., power transistors) arranged to generate the AC voltage drive signal 204. The inverter 219 can be controlled based on a pulse width modulation (PWM) control scheme.

[0040] The rectifier 217 is a diode-thyristor hybrid rectifier, which includes thyristors 215-1, 215-3, 215-5 and diodes D4, D6, D2. Thyristor 215-1 and diode D4 are electrically connected to the intermediate node 214a and the DC link 218. Thyristor 215-3 and diode D6 are electrically connected to the intermediate node 214b and the DC link 218. Thyristor 215-5 and diode D2 are electrically connected to the intermediate node 214c and the DC link 218.

[0041] A thyristor includes a cathode, an anode, and a gate. When the cathode is negatively charged relative to the anode, no current flows in the thyristor until current is applied to the gate. When current is applied to the gate, the thyristor conducts current until the voltage between the cathode and the anode reverses or drops below a threshold or holding value. Thyristors have three states: forward conduction (current is applied to the gate and the thyristor conducts forward current), forward blocking (the thyristor blocks the flow of forward current despite the direction of the applied voltage that would cause the diode to conduct current), and reverse blocking (the thyristor blocks the flow of reverse current). Each thyristor 215-1, 215-3, 215-5 can be a silicon controlled rectifier (SCR).

[0042] The power converter 210 also includes relays M1, M2, and M3. Each relay M1, M2, and M3 has two states: on (or closed) and off (or disconnected). The control system 230 controls the states of the relays M1, M2, and M3. The relay M1 is connected to the intermediate node 214a and to the gate of the thyristor 215-1 through a diode D13. The relay M2 is connected to the intermediate node 214b and to the gate of the thyristor 215-3 through a diode D14. The relay M3 is connected to the intermediate node 214c and to the gate of the thyristor 215-5 through a diode D15. When the relays M1, M2, and M3 are turned off (or disconnected), no gate current is provided to the thyristors 215-1, 215-3, and 215-5. When the relays M1, M2, and M3 are turned on, the gate current is provided to the thyristors 215-1, 215-3, and 215-5.

[0043] Relays M1, M2, M3 can be, for example, contactors. The gate current of thyristors 215-1, 215-3, 215-5 is relatively small, for example, in the range of several to tens of milliamperes (mA). Therefore, relays M1, M2, M3 can be relatively low rated current, rather than large and heavy contactors (e.g., single-pole double-throw contactors). Therefore, power converter 210 does not include a separate gate driver board for thyristors 215-1, 215-3, 215-5. This allows power converter 210 to have a smaller overall footprint, lighter weight, fewer parts, and lower cost than a power converter including a separate gate driver board.

[0044] The power converter 210 further includes a pre-charge path 240 including a pre-charge impedance 242 , a voltage divider including resistors 243 and 244 , and a capacitor 245 connected in parallel with the resistor 243 .

[0045] The pre-charge impedance 242 may be a resistor or an inrush current limiter (ICL). The pre-charge path 240 is connected to the intermediate nodes 214a, 214b, and 214c so that all three phases of the input voltage are used to pre-charge the DC link 218. Although not all phases of the input voltage may be used to pre-charge the DC link 218 (such as Figure 4 ), but using all three phases of the input voltage to pre-charge the DC link 218 provides more current and reduces the pre-charging time to the DC link 218. For example, using all three phases of the input voltage to charge the DC link 218 is about three times faster than using one phase of the input voltage.

[0046] Furthermore, using less than all phases of the input voltage to pre-charge the DC link 218 may cause ripple to appear in the voltage across the pre-charge impedance 242. The presence of ripple may cause the voltage indicating V ICL The value of (the voltage across the pre-charge impedance 242) is inaccurate, which in turn may degrade the process 300 (see below). Figure 3 Discussion) indicates V ICL The accuracy of the comparison between the pre-charge impedance voltage condition.

[0047] The pre-charge path 240 includes three branches connected to the pre-charge impedance 242, as follows: a diode D1 connected to the intermediate node 214a and the pre-charge impedance 242, a diode D3 connected to the intermediate node 214b and the pre-charge impedance 242, and a diode D5 connected to the intermediate node 214c and the pre-charge impedance 242. With this configuration, when AC power is applied to the power converter 210, at least one of the diodes D1, D3, D5 is forward biased and conducts current at any given time, so that current flows constantly or nearly constantly through the pre-charge impedance 242 and into the DC link 218 during the pre-charge sequence. Compared to the contactors used in the pre-charge path of some conventional methods, the rated current of the diodes D1, D3 and D5 and the pre-charge impedance 242 is relatively low.

[0048] When the relays M1, M2, M3 are turned off, current flows in the pre-charge path 240. When the relays M1, M2, M3 are turned on, the pre-charge path 240 is bypassed, and the thyristors 215-1, 215-3, 215-5 are in a forward conduction state and can conduct current. Because the pre-charge impedance 242 has a much larger impedance than the thyristors, current flows in the thyristors 215-1, 215-3, 215-5 (in the power path).

[0049] The power converter 210 also includes a sensing circuit 225. The sensing circuit 225 includes a low power three-phase diode bridge rectifier connected to the intermediate nodes 214a, 214b and 214c, respectively. The three-phase diode bridge rectifier includes diodes D7, D8, D9, D10, D11 and D12. A voltage divider resistor 226 with a sensing resistor 227 is used to sense the health of the input AC voltage. A filter capacitor Cflt is used across the resistor 227 to reduce electrical noise interference. The negative pole of the external sensor circuit is the same as the negative DC bus of the power converter 210, such as Figure 2 The voltage across resistor 227 provides an indication of the input voltage at the intermediate nodes 214a, 214b, 214c.

[0050] The power converter 210 also includes a control system 230 that controls the relays M1, M2, M3 based on electrical measurements 231. The electrical measurements 231 come from various sensors (not shown) in the power converter 210. The sensors may include voltage sensors and / or current sensors (e.g., Hall effect sensors, current transformers, and / or Rogowski coils). The electrical measurements 231 include an indication of the voltage of the DC link 218 (Vdc), an indication of the input voltage (V SENSE ) and the voltage indication (V ICL ).

[0051] The indication may be a direct measurement of the voltage, or may be data derived from the voltage using assumed and / or known values. For example, the voltage indication (Vdc) of the DC link 218 may be a measurement from a voltage sensor measuring the voltage across the capacitor Cdc. ICL ) may be a measurement from a voltage sensor measuring the voltage across the pre-charge impedance 242, and an indication of the input voltage (V SENSE ) may be a measurement from a voltage sensor at nodes 214a, 214b, 214c. In another example, an indication of the input voltage (V SENSE ) may be a measurement of the current through resistor 227 (which control system 230 multiplies by the known impedance of resistor 227 to determine V SENSE ), or a direct measurement of the voltage across resistor 227. In another example, the voltage indication (V ICL ) may be a measurement of the current through the pre-charge impedance 242 (which the control system 230 uses to derive V ICL ).

[0052] Logic block 237 of control system 230 analyzes electrical measurements 231 to determine whether to close relays M1, M2, M3. Control system 230 also includes electronic processing module 232, electronic memory 234, and input / output (I / O) interface 236. Control system 230 may be implemented as a microcontroller or logic controller.

[0053] The electronic processing module 232 includes one or more electronic processors. The electronic processor of the module 232 may be any type of electronic processor and may or may not include a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a field programmable gate array (FPGA), a complex programmable logic device (CPLD), and / or an application specific integrated circuit (ASIC).

[0054] Electronic memory 234 may be any type of electronic memory. In some implementations, electronic memory may store instructions in the form of a computer program or software. Electronic memory 234 may include volatile and / or non-volatile components. Electronic memory 234 is coupled to processing module 232 so that processing module 232 can access or read data in electronic memory 234 and write data to electronic memory 234. Electronic memory 234 stores instructions that, when executed, cause electronic processing module 232 to analyze data and / or retrieve information.

[0055] The I / O interface 236 is any interface that allows a human operator, another device, and / or an autonomous process to interact with the control system 230. The I / O interface 236 may include, for example, pins, ports, a display (such as a liquid crystal display (LCD)), a keyboard, audio input and / or output (such as a speaker and / or a microphone), a visual output in addition to or in lieu of a display (such as a light, a light emitting diode (LED)), a serial or parallel port, a universal serial bus (USB) connection, and / or any type of network interface (such as, for example, Ethernet). The I / O interface 236 may also allow for physical contact-free communication through, for example, IEEE 802.11, Bluetooth, or near field communication (NFC) connections. For example, the control system 230 and / or information stored on the electronic memory 234 may be operated, configured, modified, or updated through the I / O interface 236.

[0056] The I / O interface 236 may also allow the control system 230 to communicate with components in the system 200 and with systems external and remote to the system 200. For example, the I / O interface 236 may include a communication interface that allows communication between the control system 230 and a remote station (not shown) or between the control system 230 and a separate monitoring device. The remote station or monitoring device may be any type of station through which an operator can communicate with the control system 230 without having to make physical contact with the control system 230. For example, the remote station may be a computer-based workstation, a smart phone, a tablet, or a laptop computer connected to the control system 230 via a service protocol, or a remote control connected to the control system 230 via a radio frequency signal.

[0057] Logic block 237 includes analysis module 238. Logic block 237 can be implemented by a collection of logic gates. A logic gate is a device that performs a logical operation on one or more inputs to produce an output. The logic gates in logic block 237 can be implemented with electronic gates (e.g., transistors and / or operational amplifiers) or software.

[0058] The analysis module 238 analyzes the electrical measurements 231. The analysis module 238 includes a comparator 233 for each electrical measurement. Therefore, for an implementation in which the electrical measurements 231 include three different measurements, the analysis module includes three comparators 233. Each comparator 233 is configured to compare one of the electrical measurements 231 with a corresponding reference value or condition. Each comparator 233 can be an electronic device or a physical logic gate, such as, for example, an operational amplifier, which outputs a binary value by comparing two input values. In some implementations, each comparator 233 is implemented in software as executable instructions stored on an electronic memory 234. For example, the comparator 233 can be a subtraction operation that calculates the difference between the two input values, followed by a comparison operation that compares the difference with a predetermined threshold to produce a binary output value. Regardless of how the comparator 233 is implemented, the two input values ​​of each comparator are electrical measurements (Vdc, V ICL or V Sense ) and the corresponding conditions or reference values.

[0059] The binary output of each comparator 233 is provided to an AND block 235. The AND block 235 can be implemented as a physical logic gate or in software. When the binary outputs of all comparators 233 are high (e.g., 1), the output of the AND block 235 is high. Otherwise, the output of the AND block 235 is low (e.g., 0). The control system 230 uses the output of the AND block 235 to determine whether to operate the relays M1, M2, M3, as described in relation to FIG. Figure 3For example, the control system 230 may include a relay driver that generates a relay control signal and applies the relay control signal to the relays M1, M2, M3 when the output of the AND block 235 is high.

[0060] The above discussion relates to the power converter generating the AC drive signal 204 and providing the AC drive signal 204 to the load 202. However, other configurations and applications are possible. For example, the bidirectional power converter 210 can be implemented without the inverter 219 and configured to drive a DC load.

[0061] Figure 3 2 is a flow chart of process 300. Process 300 is implemented by logic block 237 of control system 230. Process 300 is discussed with respect to power converter 210 to provide an example. However, process 300 may be used with other power converters. For example, process 300 may be used with power converter 410 ( Figure 4 )、510( Figure 5 )、610( Figure 6 )、710( Figure 7 ) or 810( Figure 8 ) are used together with the electrical measurements of . In addition, process 300 can be used with system 900 ( Fig. 9 )

[0062] When AC power is applied to power converter 210, process 300 begins (305). Electrical measurements 231 are accessed (310). As discussed above, electrical measurements 231 include an indication of the voltage across DC link 218 (Vdc), an indication of the voltage across pre-charge impedance 242 (V ICL ), and a voltage indication (V SENSE ). These indicators are called indicators Vdc, V ICL 、V SENSE .

[0063] Each indicator Vdc, V ICL 、V SENSE The status of each indicator is determined by analysis relative to the specification, reference value, or condition associated with that indicator. If the condition is met, the status is positive or high ( Figure 3 If the condition is not met, the status is negative or low ( Figure 3 The state may be a binary value, where 1 represents a positive (or high) state and 0 represents a negative (or low) state.

[0064] The specifications or conditions are predetermined and stored on electronic storage 234. Each specification or condition may be a threshold value specifying a minimum or maximum voltage value, or a range of voltage values ​​associated with acceptable performance. Figure 3 In the example of , electronic memory 234 stores three conditions: a DC link condition, a pre-charge voltage condition, and an input voltage condition.

[0065] When power is supplied to the power converter 210 at (305), the relays M1, M2, M3 are turned OFF, and gate current is not supplied to the thyristors 215-1, 215-3, 215-5. Current flows into the pre-charge path 240 to the DC link 218 through any forward biased diode among the diodes D1, D3, D5 and the pre-charge impedance 242. The current charges the capacitor Cdc and returns through the diode D2, D4 or D6. The voltage (Vdc) across the DC link 218 begins to increase.

[0066] The control system 230 analyzes the electrical measurements 231 to determine whether to turn on the relays M1, M2, M3 as follows. An indicator (Vdc) of the voltage across the DC link 218 is compared to a DC link condition (320). The DC link condition is a predefined voltage value. The DC link condition can be, for example, a voltage value known to be associated with an undervoltage condition. If the measured or calculated voltage (Vdc) across the DC link 218 is greater than the DC link condition, the DC link condition is met and the state of the voltage (Vdc) indicator is positive. If the measured or calculated voltage (Vdc) is less than or equal to the DC link condition, the DC link condition is not met and the state of the voltage (Vdc) indicator is negative.

[0067] The voltage (V ICL ) indicator is compared to a pre-charge voltage condition (330). The pre-charge voltage condition is a voltage value. If the measured or calculated voltage (V ICL ) is less than the pre-charge voltage condition, the pre-charge voltage condition is met, and the voltage (V ICL ) indicator is positive. Otherwise, the precharge voltage condition is not met and the indicator (V ICL ) is negative.

[0068] Input voltage (V SENSE ) is compared with an input voltage condition (340). The input voltage condition is a voltage value. If the input voltage (V SENSE ) is greater than the input voltage condition, the input voltage condition is met, and the input voltage (V SENSE ) indicator is positive. Otherwise, the input voltage condition is not met and the input voltage (V SENSE ) is negative.

[0069] If the state of any indicator is negative, the process 300 determines whether the relays M1, M2, M3 are off (345). If the relays M1, M2, M3 are off, the process 300 returns to (310) and continues to monitor the electrical measurement 231. If the relays M1, M2, M3 are on, the control system 230 issues a command to the relays M1, M2, M3 to turn them off (347), and the process 300 returns to (310). The relays M1, M2, and M3 are off, and current continues to flow in the pre-charge path 240, and the process 300 continues to monitor the electrical measurement 231.

[0070] If the state of all indicators is positive, all conditions are satisfied, and the control system 230 issues a turn-on command to the relays M1, M2, M3 (350). When the relays M1, M2, M3 are turned on, the gate current flows into the thyristors 215-1, 215-3, and 215-5, and the thyristors 215-1, 215-3, and 215-5 are turned on and enter the forward conduction state when experiencing the forward bias condition. The current flows through the thyristors 215-1, 215-3, 215-5 and enters the DC link 218. The current does not flow in the pre-charge path 240 because the pre-charge impedance 242 is greater than the impedance of the thyristors 215-1, 215-3, 215-5. The voltage of the capacitor Cdc across the DC link 218 reaches a maximum value, and the power converter 210 operates in a stable state.

[0071] Process 300 returns to (310) via decision path 351, and relays M1, M2, and M3 remain on, with power converter 210 operating in a steady state. Control system 230 continues to monitor electrical measurements 231 and check conditions at (320), (340), (350).

[0072] Continuous monitoring during steady-state operation of the power converter 210 allows the control system 230 to effectively react to the undervoltage condition. When the peak voltage provided by the power grid 201 is lower than the voltage across the DC link 218, the undervoltage condition occurs. During the undervoltage condition, the voltage across the DC link 218 also drops. However, due to the relatively large time constant of the capacitor Cdc, the voltage across the DC link 218 changes much slower than the input voltage. Only monitoring the voltage across the DC link 218 may delay the detection of the undervoltage condition, so that the pre-charge path cannot be used for current flow before the undervoltage condition ends. In this case, when the undervoltage condition ends, there is a voltage difference between the nodes 214a, 214b, 214c and the DC link 218, and since the current cannot flow in the pre-charge path, the difference may cause an inrush current. Therefore, delaying the detection of the undervoltage condition may result in an uncontrolled and / or higher inrush current.

[0073] However, in process 300, if any indicator V SENSE 、Vdc、V ICL If the state of does not satisfy its corresponding condition, the relays M1, M2 and M3 are turned off (345) and (347). For example, if the input voltage (V SENSE ) drops below the input voltage condition value, the indicator V SENSE The state of is negative, and the control system 230 turns off the relays M1, M3, M3. The input voltage dropping below the input voltage condition value is a sign that the undervoltage condition has begun. The time constant of the filter capacitor Cflt is much smaller than the time constant of the DC link capacitor Cdc. Therefore, after the undervoltage condition begins, the voltage across the sense resistor 227 in the sense circuit 225 changes faster than the voltage across the DC link 218. In another example, the undervoltage condition can reverse the polarity of the voltage across the pre-charge impedance 242, so that the pre-charge impedance voltage indicator (V ICL ) no longer meets the precharge impedance condition. When the precharge impedance voltage indicator (V ICL ) does not meet the pre-charge impedance condition, the control system 230 turns off the relays M1, M2, and M3.

[0074] Since the input voltage (V SENSE ) indicator (such as measured indirectly or directly at resistor 227) and the pre-charge impedance voltage (V ICL ) indicator changes and reacts to an input voltage drop to an undervoltage condition faster than the voltage across the DC link 218, so monitoring all indicators (V SENSE )、(Vdc)、(V ICL ) allows the control system 230 to detect and respond to undervoltage conditions more quickly than conventional systems that use only voltage measurements across the DC link 218.

[0075] After the relays M1, M2, M3 are turned off, the thyristors 215-1, 215-3, 215-5 are turned off when encountering a reverse blocking condition, and current begins to flow in the precharge path 240 instead of flowing through the thyristors 215-1, 215-3, 215-5. SENSE ) recovers from a brownout condition and has a peak voltage greater than the voltage across the DC link 218, current flows in the pre-charge path 240 and avoids or minimizes uncontrolled and / or large inrush currents that may occur when the pre-charge path 240 is unavailable. In this way, the control system 230 can effectively and quickly handle brownout and brownout conditions.

[0076] When the input voltage (V SENSE ) is greater than the DC link voltage, the voltage across the pre-charge impedance (VICL ) is less than the pre-charge voltage impedance condition value, and the voltage (Vdc) across the DC link 218 is greater than the DC link condition, the control system 230 turns on the relays M1, M2, and M3. After the relays M1, M2, and M3 are turned on, the gate current flows into the thyristors 215-1, 215-3, and 215-5, which are turned on when they encounter a forward bias condition. Therefore, after the undervoltage and voltage ramping conditions are over, the power converter 210 resumes steady state operation.

[0077] Figures 4 to 9 Other power converters that may be used with control system 230 and process 300 are contemplated.

[0078] Figure 4 4 is a schematic diagram of a system 400 including a power converter 410. The power converter 410 is connected to the power converter 210 ( Figure 2 ), except that the power converter 410 includes a pre-charge path 240 ( Figure 2 ) A different pre-charge path 440. In power converter 410, pre-charge path 240 is connected to only a single phase, and only a single phase of the input voltage is used to pre-charge DC link 218. Although using more than one phase to pre-charge DC link 218 results in a faster pre-charge sequence and a smaller voltage variation across the pre-charge impedance, Figure 4 The single-phase pre-charging arrangement uses fewer components and is potentially cheaper.

[0079] exist Figure 4 In the example of , the pre-charge path 440 includes a diode D1 connected in series with a pre-charge impedance 442. The pre-charge impedance 442 can be a resistor or an inrush current limiter (ICL). The diode D1 is connected to the intermediate node 214c, and the pre-charge impedance 442 is connected to the DC link 218. Compared with the contactors used in the pre-charge path of some conventional methods, the rated current of the diode D1 and the pre-charge impedance 442 is relatively low. When the relays M1, M2, and M3 are turned off, the current flows in the pre-charge path 440. When the relays M1, M2, and M3 are turned on and the thyristors are in the forward conduction state, the current flows in the thyristors 215-1, 215-3, and 215-5 (in the power path) because the pre-charge impedance 442 has a much larger impedance than the thyristors.

[0080] The system also includes a sensing circuit 425. The sensing circuit 425 includes diodes D7, D8, and D9, which are connected to the intermediate nodes 214a, 214b, 214c, respectively, and connected to a voltage divider including a resistor 426, a resistor 427, and a filter capacitor Cflt in parallel with the resistor 427. The voltage across the resistor 427 provides an indication of the input voltage (VSENSE Although system 400 is shown with sensing circuit 425, system 400 may also be used with sensing circuit 225 ( Figure 2 )

[0081] Figure 5 is a schematic diagram of a system 500 that includes a power converter 510 that can be used with a control system 230 and controlled by a process 300. The power converter 510 is similar to the power converter 210 ( Figure 2 ), except that power converter 510 does not include sensing circuit 225 ( Figure 2 Instead, the input voltage (V SENSE ) and provide the direct measurement to the control system 230 for use in the process 300. The direct measurement of the input voltage may include measurement circuitry (not shown), such as a voltage divider at each intermediate node 214a, 214b, 214c, to facilitate the measurement of the input voltage.

[0082] The power converter 510 includes a pre-charge path 540 including a diode D1 in series with a pre-charge impedance 242, wherein the diode D1 is electrically connected to the intermediate node 214a, and the pre-charge impedance 242 is electrically connected to the DC link 218. Figure 5 In the example shown in , only one phase (phase a) is used to precharge the DC link 218. However, other implementations are possible. For example, the power converter 210 can be implemented to include connections to all three intermediate nodes 214a, 214b, 214c (such as Figure 2 ) or a precharge path connected to two of the three intermediate nodes 214a, 214b, 214c.

[0083] Figure 66 is a schematic diagram of a system 600 including a power converter 610. The power converter 610 may be used with the control system 230 and the process 300. The power converter 610 includes a rectifier 617, a DC link 218, and an inverter 219. The rectifier 617 is a diode-diode rectifier including diodes D1 and D4 connected to an intermediate node 614a, diodes D3 and D6 connected to an intermediate node 614b, and diodes D5 and D2 connected to an intermediate node 614c. Diodes D1, D3, and D5 are directly connected to the DC link 218. Diode D4 is connected to the DC link 218 through a first soft charge circuit SC1, diode D6 is connected to the DC link 218 through a second soft charge circuit SC2, and diode D2 is connected to the DC link 218 through a third soft charge circuit SC3. The soft charge circuits SC1, SC2, and SC3 allow the DC link 218 to be pre-charged at startup.

[0084] The soft charging circuit SC1 includes a relay M1 and a first soft charging impedance Rsc1 connected in parallel with the relay M1. The soft charging circuit SC2 includes a relay M2 and a soft charging impedance Rsc2 connected in parallel with the relay Rrsc2. The soft charging circuit SC3 includes a relay M3 and a soft charging impedance Rsc3 connected in parallel with the relay M3. The relays M1, M2, and M3 are contactors controlled by the control system 230 based on the process 300. The soft charging impedances Rsc1, Rsc2, and Rsc3 can be resistors or inrush current limiters.

[0085] See also Figure 3 , when AC power is initially applied to the power converter 610, the relays M1, M2, M3 are turned off, and the diodes D4, D6, D8 are connected to the DC link 218 through the corresponding soft charging impedances Rsc1, Rsc2, Rsc3. Current flows through the pre-charging path (soft charging impedances Rsc1, Rsc2, Rsc3), and the voltage across the DC link 218 increases.

[0086] The control system 230 has access to electrical measurements 631. The electrical measurements 631 are measurements from voltage and / or current sensors (not shown) in the power converter 610. The electrical measurements 631 include an indication of the voltage across the DC link 218 (Vdc), an indication of the voltage across the soft charging impedances Rsc1, Rsc2, Rsc3, and an input voltage at the intermediate nodes 614a, 614b, 614c (V SENSE ) instructions. Figure 6 In the example, the input voltage (V SENSE ) is a direct measurement of the voltage at nodes 614a, 614b, 614c. Other implementations are also possible. For example, power converter 610 may include a sensing circuit such as Figure 2The sensing circuit 225 is used to measure an indication of the input voltage.

[0087] The indication (Vdc) is compared to the DC link condition (320), the voltage indication across the soft charge impedances Rsc1, Rsc2, Rsc3 is compared to the pre-charge voltage condition (330), and the input voltage (V SENSE ) is compared with the input voltage condition (340). If all indications have a positive state, the control system 230 issues a command to turn on the relays M1, M2, M3, thereby bypassing the soft charging impedances Rsc1, Rsc2, Rsc3. The power converter 610 begins steady state operation.

[0088] Since the rectifier 617 is a diode-diode rectifier, gate control is not included. However, the state of the relays M1, M2, M3 still determines whether the pre-charge path through the soft charging impedances Rsc1, Rsc2, Rsc3 is bypassed, and the undervoltage condition can be detected faster than the traditional method of using only the voltage across the DC link 218 to determine whether to turn off the relay.

[0089] Figure 7 and Figure 8 Other implementations of system 600 are shown. Figure 7 is a schematic diagram of a system 700 including a power converter 710. The power converter 710 may be used with the control system 230 and the process 300. The power converter 710 may be used with the power converter 610 ( Figure 6 ), except that power converter 710 includes only soft charge circuits SC1 and SC3, but not soft charge circuit SC2. In other words, power converter 710 includes two soft charge impedances instead of three. The magnitude of the soft charge (or pre-charge) current is the same as that in power converter 610, but the soft charge time in system 700 is longer because there is no pre-charge path connected to intermediate node 214b.

[0090] Figure 8 8 is a schematic diagram of a system 800 including a power converter 810. The power converter 810 may be used with the control system 230 and the process 300. The power converter 810 may also be used with the power converter 610 ( Figure 6), except that power converter 810 includes only soft charge circuit SC1, and does not include soft charge circuits SC2 and SC3. In other words, power converter 810 includes one soft charge impedance instead of three, and only one phase of the input voltage (phase a in this example) is used to pre-charge DC link 218. The amplitude of the soft charge (or pre-charge) current is the same as that in power converter 810, but the soft charge time in system 800 is longer because there is no pre-charge path connected to intermediate node 214b or intermediate node 214c. For example, the soft charge time of system 800 may be three times that of system 600. Nevertheless, power converters 710 and 810 can still be used with control system 230 and process 300 to effectively and quickly detect and handle undervoltage conditions.

[0091] Fig. 9 is a schematic diagram of system 900. System 900 includes a diode-diode rectifier 617 and a DC link 918. Diode-diode rectifier 617 converts AC current from power source 201 into a rectified current I DC , the rectified current I DC Provided to DC link 918. DC link 918 includes series capacitors Cdc1 and Cdc2 between the positive (P) bus and the negative (N) bus. Each capacitor Cdc1 and Cdc2 is connected in parallel with a corresponding resistor Rc1, Rc2. Indicator Vdc is the voltage between the positive bus and the negative bus (the voltage across capacitors Cdc1 and Cdc2). DC link 218 can be used in place of DC link 918 in system 900. In addition, in any of systems 200, 400, 500, 600, 700, or 800, DC link 918 can be used in place of DC link 218.

[0092] The DC link 918 is connected to a load 991 through a load switch 990 (eg, a contactor). The load 991 may be, for example, an inverter, such as inverter 219, or may be a DC load.

[0093] System 900 includes a three-phase contactor 952 and inrush current limiters 942-1, 942-2, and 942-3. The three-phase contactor 952 includes three contactors, one of which is connected in series with each diode D4, D6, and D2 at respective nodes 949-1, 948-2, 949-3. ICL 942-1 is connected in parallel with the contactor connected in series with diode D4, ICL 942-2 is connected in parallel with the contactor connected in series with diode D6, and ICL 942-3 is connected in parallel with the contactor connected in series with diode D2.

[0094] Each node 949-1, 949-2, 949-3 is connected to a voltage divider via a corresponding diode, the voltage divider comprising resistors R1 and R2, with a capacitor C2 connected in parallel with the resistor R2. Fig. 9 In the embodiment shown in FIG. 1 , V ICL is the voltage across resistor R2, which tracks the voltage across ICLs 942-1, 942-2, 942-3.

[0095] The system 900 also includes a sensing circuit 925 that monitors the input voltage from the power supply 201. The sensing circuit 925 is a voltage divider having series resistors R3, R4, R5, and a resistor R6, with a capacitor C1 connected in parallel with the resistor R6. The resistor R3 is connected to the intermediate node 618a through a diode D7, to the intermediate node 618b through a diode D8, and to the intermediate node 618c through a diode D9. The indicator V SENSE is the voltage across resistor R6. V SENSE The value of tracks the voltage of the intermediate nodes 618a, 618b, 618c and is used to monitor the voltage of the AC source 202.

[0096] Also refer to Figure 3 , when AC power is initially applied to the rectifier 617, the three-phase contactor 952 is turned off, and the diodes D4, D6, D8 are connected to the DC link 918 through the ICLs 942-1, 942-2, 942-3. Current flows through the pre-charge path (ICLs 942-1, 942-2, 942-3), and the voltage across the DC link 918 increases. Indicator V SENSE The voltage of the AC source 202 is tracked and rises as AC power is applied to the rectifier 617. The contactor 990 closes and applies power to the load 991.

[0097] The control system 230 accesses the indication Vdc, V ICL and V SENSE The indication (Vdc) is compared to the DC link condition (320), the voltage indication across VCL 942-2, 942-2, 942-3 is compared to the pre-charge voltage condition (330), and the input voltage (V SENSE ) are compared with the input voltage condition (340). If all indications have a positive state, the control system 230 issues a command to close the three-phase contactor 952. When the three-phase contactor 952 is closed, the ICLs 942-1, 942-2, 942-3 are bypassed, and the system 900 operates in a stable state.

[0098] Undervoltage condition occurs, V SENSEdrops below the input voltage condition and the control system 230 opens the three-phase contactor 952, allowing current to flow through the ICLs 942-1, 942-2, 942-3. The load 991 remains connected to the DC link 918, the ICLs 942-1, 942-2, 942-3 limit the current to the load, and Vdc drops.

[0099] After the AC voltage is restored, V SENSE indicator rises and then exceeds the input voltage condition, and Vdc rises. When V ICL When it drops below the pre-charge voltage condition, all three conditions are met and the three-phase contactor 952 is closed. The contactor 990 is closed and power is applied to the load 991.

[0100] These and other implementations are within the scope of the claims. For example, system 200 ( Figure 2 ) may include a sensing circuit 425 ( Figure 4 ), rather than the sensing circuit 225.

Claims

1. A power converter, comprising: An electrical network comprising: a plurality of intermediate nodes, wherein each intermediate node is configured to be connected to one phase of an AC power source, the electrical network is configured to convert AC current into DC current, and the electrical network comprises a plurality of electronic switches; a DC link electrically connected to the electrical network; a pre-charging path, electrically connected to the DC link, the pre-charging path comprising a pre-charging impedance; a control path electrically connected to one of the electronic switches, the control path comprising a relay; and The control system is configured as: analyzing a plurality of electrical measurements from the power converter to determine a status output; and A determination is made based on the status output whether to control the relay to change state.

2. The power converter according to claim 1, wherein: The plurality of electrical measurements from the power converter include: an indication of a voltage across the pre-charge impedance, an indication of a voltage across the AC power source, and an indication of a voltage across the DC link; and The control system is configured to analyze the voltage indication across the pre-charge impedance, the voltage indication of the AC power source, and the voltage indication across the DC link to generate the status output.

3. The power converter according to claim 1, wherein: The plurality of electronic switches include at least one controllable electronic switch electrically connected to each intermediate node; the control path includes a plurality of control branches; each control branch includes a relay; and each control branch is electrically connected to a control node of a controllable electronic switch; and among them The control system is configured to determine whether to control all of the relays to change states based on the status output.

4. The power converter according to claim 3, wherein: The controllable electronic switch comprises a thyristor; and the control node comprises a gate on the thyristor.

5. The power converter according to claim 4, wherein: The pre-charging path is electrically connected to one of the intermediate nodes and the DC link.

6. The power converter according to claim 4, wherein: The precharge path includes a plurality of precharge branches, and each precharge branch is electrically connected to one of the intermediate nodes and the precharge impedance.

7. The power converter of claim 6 further comprising a voltage sensing impedance electrically connected to the input node, and wherein, The plurality of electrical measurements include an indication of a voltage across the pre-charge impedance, an indication of a voltage of the AC power source measured at the voltage sensing impedance, and an indication of a voltage across the DC link.

8. The power converter according to claim 1, wherein: The relay includes a contactor.

9. The power converter according to claim 1, wherein: The plurality of electronic switches include diodes, and two diodes are electrically connected to each intermediate node; the relay is connected in series with one of the diodes; and the pre-charge impedance is connected in parallel with the relay.

10. The power converter according to claim 1, wherein: The plurality of electronic switches include diodes, and two diodes are electrically connected to each intermediate node; the control path includes a plurality of control branches, each control branch includes a relay connected in series with one of the diodes; and the pre-charge path includes a plurality of pre-charge paths, each pre-charge path includes a pre-charge impedance connected in parallel with one of the relays.

11. A control system comprising: an analysis module configured to: analyze a plurality of electrical measurements from the power converter to determine a status output; as well as A control module is configured to determine whether to turn on a relay in a control path of the power converter based on the status output, wherein when the relay is turned off, current is provided to a DC link of the power converter through a pre-charge path, and turning on the relay causes one or more electronic switches in the power converter to conduct current, so that when the relay is turned on, current is provided to the DC link of the power converter through a power path.

12. The control system according to claim 11, wherein: To analyze the plurality of electrical measurements, the analysis module is configured to compare each electrical measurement with a predetermined condition associated with the electrical measurement to generate a result for the electrical measurement; and the status output includes the result for each electrical measurement.

13. The control system according to claim 12, wherein: The control module is configured to close the relay only when each result satisfies the associated predetermined condition, so that current is provided to the DC link of the power converter through the power path only when each electrical measurement value satisfies the associated predetermined condition.

14. An apparatus comprising: Electrical network, including: a plurality of intermediate nodes, wherein each intermediate node is configured to be connected to one phase of an AC power source, the electrical network being configured to convert AC current into DC current; and an electrical network comprising a phase branch connected to each intermediate node, each phase branch comprising a thyristor and a diode; a DC link electrically connected to the electrical network; a pre-charging path electrically connected to at least one of the intermediate nodes and the DC link, the pre-charging path comprising a pre-charging impedance; and A plurality of control paths are provided, each control path being connected to one of the intermediate nodes and a gate of one of the thyristors, and each control path comprising a relay.

15. The device according to claim 14, wherein: The precharge path is connected to all of the intermediate nodes.

16. The apparatus of claim 15, further comprising a sensing impedance connected to all of the intermediate nodes, wherein: The voltage across the sensing impedance provides an indication of the voltage input to the device.

17. A method comprising: accessing a plurality of electrical measurements from a power converter, the plurality of measurements comprising: an indication of a voltage across a pre-charge impedance; an indication of a voltage of an AC power source connected to the power converter, and an indication of a voltage across a DC link of the power converter; comparing the at least two indications to associated specifications to produce at least two status outputs; analyzing the at least two status outputs to determine whether to change a state of a relay in the power converter; If it is determined to turn on the relay, controlling the relay to turn on so that current flows in an electronic switch connected to the relay, so that the current flows to the DC link through a power path including the electronic switch; and If it is determined that the relay is to be turned off, the relay is controlled to be turned off so that current flows to the DC link through the pre-charging impedance.

18. The method according to claim 17, wherein: Each indication is compared to an associated specification to produce three state outputs, and the three state outputs are analyzed to determine whether to change the state of the relay.

Images