Patent Description:
<CIT> relates to wind power generation and to a soft start circuit of a high-power converter. The high-power converter soft starting circuit includes an inverter loop and a bypass circuit is arranged on inverter loop. The bypass circuit includes a rectifier diode component V, at least one charging resistor R, AC fuse FU1, DC fuse FU2 and contactor KM1. The high-power converter soft starting circuit can reduce surge impact of the power supply to the DC capacitor and realize a soft start of the system.

<CIT> relates to voltage disturbance resisting power system for coal feeder. The voltage disturbance resisting power system comprises an on-off element, a bypass loop, a branch circuit three-phase wire, a storage loop and an inversion loop. The storage loop can charge an energy storage element when auxiliary power is normal, and when the auxiliary power is abnormal, the energy storage element can discharge electricity by means of the inversion loop and carry out power supply compensation for loads. The voltage disturbance resisting power system for the coal feeder ensures that power can be supplied reliably when transient state, dynamic state or long-time power incoming line voltage of a variable-frequency speed control system of the coal feeder is boosted or lowered, and the problem of high and low voltage ride through of a coal feeder system is solved.

<CIT> relates to a power unit test system. The test system can test a single power unit and reduce test costs. An electronic load device is connected with a controller and a measured power unit. The measured power unit is also connected with the controller and a temperature sensor. The controller is configured to start the electronic load device and the measured power unit so that the electronic load device and the measured power unit respectively receive AC power generated by a power grid.

<CIT> discloses uninterruptible power supply systems and methods supporting load balancing.

<CIT> discloses an AC power supply apparatus with economy mode and methods of operation thereof.

The invention is what is claimed in the independent claims.

According to an aspect of the present invention, there is provided a precharge system according to claim <NUM>.

According to another aspect of the present invention, there is provided a system according to claim <NUM>.

According to a further aspect of the present invention, there is provided a method for precharging a DC bus circuit, according to claim <NUM>.

The circuit topologies defined in the independent claims are reflected by <FIG> and <FIG>. The arrangement shown in <FIG> represents an exemplary implementation.

Power conversion systems often use shared DC bus architectures, in which multiple DC loads are connected to a single bus. For example, a shared bus system can accommodate multiple inverters to power respective motors, as well as one more capacitor banks. High capacity systems with high power requirements can include large DC bus capacitors designed to operate at relatively high DC voltages. Upon starting such systems, the DC bus capacitor or capacitors need to be charged to a sufficient voltage for safe operation of the connected inverters or other DC loads. However, charging a capacitive load by simple connection to an input voltage source can lead to excessive inrush current levels. In this regard, the amount of loading and bus capacitance may vary, for example, when certain loads are inactivated and/or disconnected from the shared bus. Limiting the charging current, on the other hand, lengthens the precharging time. Long precharge/startup times are undesirable for industrial installations. Difficulties can occur when a precharge circuit becomes physically too large, complicated and/or exceeds expected precharge times. Problems also occur when the multiple different inverter power ratings exceed the maximum precharge capability of a shared DC bus system.

Referring initially to <FIG>, <FIG> shows a power conversion system <NUM> according to the present invention including a precharging system that uses a VFD in a precharging circuit branch to provide small and scalable precharging solution for a shared bus system. The system <NUM> of <FIG> uses a primary AC to DC converter with an AFE rectifier, and a precharge system with a VFD to precharge the DC bus. <FIG> shows another power conversion system according to the present invention with a shared DC bus, a primary AC to DC converter with a six-pulse bridge rectifier, and a precharge system with a VFD to precharge the DC bus. <FIG> shows an example, in which a precharge system has a VFD and a bridge rectifier to precharge the DC bus.

The use of a VFD with an AC input provides advantages over previous DC bus precharging systems and techniques. Simple precharge circuits include resistors and contactors that can be used on AC inputs or DC outputs of a rectifier to charge the bus capacitance. The precharge resistors are uncontrolled system component and act as the limiter for the inrush charging currents of the system capacitance, thereby limiting the charging time. When the system capacitors are completely charged, the system control switches the power flow between the precharge and main branches in circuitry. More complicated precharge circuits include AC or DC controlled system components such as SCRs, IGBTs as part of the main rectifier and precharge circuitry controls the inrush charging currents of the system capacitance during precharge operation, and then start conducting the main power flow after precharging is completed. Other precharging circuits suffer from limitations of on-off duty cycling when using precharge resistors, as well as long and fixed precharging time as a function of AC line conditions. In addition, simple precharge resistor approaches do not have current limiting control circuitry. Other solutions, moreover, suffer from limited protection on precharge power-up from short circuit faults in common DC bus system (e.g., fuse protection coordination with precharge resistors is difficult). In addition, simple precharge resistor systems provide only limited protection from ground faults in a common DC bus system on precharge power up, and these approaches typically require custom engineering system design of the precharge circuit component ratings for a given application.

<FIG> shows a power conversion system <NUM> with a shared DC bus, a primary AC to DC converter with an active front end (AFE) rectifier, and a precharge system with a VFD to precharge the shared DC bus. The system <NUM> operates from three phase AC power supplied by an AC power source <NUM>. In other implementations, single phase AC input power can be used, or other multi-phase power having N phases, where N is greater than <NUM>. The system <NUM> includes a precharge system <NUM> with separate first and second circuit branches to enable DC bus precharging in a first operating mode (e.g., PRECHARGE mode) and normal operation in a second operating mode (NORMAL). The first circuit branch includes a first contactor <NUM>, and the second circuit branch includes a second contactor <NUM>. The precharge system <NUM> includes a first input <NUM> configured to be coupled to the AC power source <NUM>. The second circuit branch includes the second contactor <NUM> as well as a disconnect switch <NUM> coupled to the first input <NUM>. In the illustrated example, the first contactor <NUM>, the second contactor <NUM> and the disconnect switch <NUM> are all three-phase components, although not a strict requirement of all possible implementations. The second circuit branch in one implementation further includes an overcurrent protection circuit <NUM>, such as a three-phase resettable circuit breaker or fuses. In other implementations, the overprotection circuit <NUM> is omitted. The first circuit branch in the illustrated example includes an overprotection circuit <NUM>, such as a three-phase circuit breaker or fuses. In other implementations, the overprotection circuit <NUM> is omitted. The second circuit branch further includes a three-phase inductor circuit (hereinafter referred to as the inductor) having three inductors <NUM> in the three respective phase lines of the second circuit branch.

The precharge system <NUM> in one example includes a precharge controller <NUM> with suitable control outputs configured to operate the first and second contactors <NUM> and <NUM> and the disconnect switch <NUM>. In accordance with certain aspects of the present disclosure, the second circuit branch also includes a variable frequency drive (VFD) <NUM> having an AC input and an AC output. In one implementation, the precharge controller <NUM> includes suitable control outputs to selectively operate the VFD <NUM>. In another implementation, the precharge controller <NUM> is implemented in one or more programmed processors of the VFD <NUM>.

In the illustrated example, the VFD <NUM> is an AC to AC converter. The example VFD <NUM> includes a converter <NUM> (e.g., a rectifier) having an AC input coupled to the disconnect switch <NUM> (e.g., directly or through any included fuses <NUM>), as well as an internal (e.g., second) DC bus circuit <NUM> coupled to a DC output of the rectifier <NUM>. In one example, the converter <NUM> is or includes a passive rectifier, such as a six-pulse diode bridge rectifier circuit. In another example, the converter <NUM> is or includes an active front end (AFE) switching rectifier. The second converter <NUM> provides and regulates an internal bus voltage VB across a bus capacitor of the DC bus circuit <NUM>. The VFD <NUM> further includes an inverter <NUM> having a DC input coupled to the second DC bus circuit <NUM> and an AC output coupled to the second contactor <NUM> through the inductor <NUM>.

The system also includes a primary AC to DC converter, in one example having an AFE rectifier <NUM> coupled directly or indirectly to a three phase AC input <NUM>. In the example of <FIG>, the primary AC to DC converter includes an input filter circuit <NUM>. In one example, the filter circuit <NUM> is a three-phase inductor-capacitor-inductor (LCL) filter, including two inductors (not shown) connected in series with one another and each of the three respective phases, as well as three capacitors individually coupled in a Y circuit between a common node (e.g., a local neutral) and a respective joining node that joins the two inductors of a corresponding one of the respective phases. In other implementations, other forms of rectifiers can be used, with or without an input filter circuit. The primary AC to DC converter includes a DC output of the AFE rectifier <NUM>.

The DC output of the rectifier <NUM> provides a main DC bus voltage VDC to drive multiple loads having DC inputs, in this example including multiple inverters <NUM> and respective driven motors <NUM>, as well as a DC bus capacitor bank <NUM>. The DC output of the rectifier <NUM> and the DC loads <NUM>, <NUM> are coupled to a shared DC bus circuit <NUM> (e.g., DC bus system) that includes a first (e.g., positive or "+") line or node <NUM> and a second (e.g., negative or "-") line or node <NUM>.

The precharge controller <NUM> operates in a first mode to charge the main DC bus circuit <NUM> to a threshold value of the DC bus voltage VDC, and thereafter operates in a second mode for normal operation of the primary AC to DC converter <NUM>, <NUM>. In one example, with the precharge controller <NUM> implemented in one or more processors of the VFD <NUM>, the operating mode of the precharge system <NUM> is controlled at least in part based on a feedback signal VFB that represents the DC bus voltage VDC. <FIG> shows one example in which a feedback connection is provided from the first DC bus line <NUM> to the VFD <NUM> in order to selectively implement the first or second operating modes according to the DC bus voltage VDC of the DC bus circuit <NUM>. The first contactor <NUM> is closed and the second contactor <NUM> is opened during normal operation in the second mode.

As further shown in <FIG>, the disconnect switch <NUM> is coupled between the first input <NUM> and the AC input of the VFD <NUM>, the VFD <NUM> is coupled between the disconnect switch <NUM> and the inductor <NUM>, the inductor <NUM> is coupled between the AC output of the VFD <NUM> and the second contactor <NUM>, and the second contactor <NUM> is coupled between the inductor <NUM> and the primary AC to DC converter <NUM>, <NUM>. In this example, moreover, the outputs of the first contactor <NUM> and the second contactor <NUM> are coupled together and to the input <NUM> of the primary AC to DC converter <NUM>, <NUM>, although not a strict requirement of all possible implementations. In the illustrated example, moreover, the second contactor <NUM> is coupled between the inductor <NUM> and the input <NUM> of the AC to DC converter <NUM>, <NUM>, although not a strict requirement of all possible implementations.

The controller <NUM> in one example is a processor that implements precharge in and normal mode operation according to program instructions stored in an electronic memory, such as a memory of the VFD <NUM>. The controller <NUM> according to the present invention is configured to open the first contactor <NUM> and close the second contactor <NUM> and the disconnect switch <NUM> in the first mode PRECHARGE, so as to prevent direct current flow from the first input <NUM> to the AC to DC converter <NUM> and to allow current flow from the AC output of the VFD <NUM> through the inductor <NUM>. In one implementation, the controller <NUM> implements the first mode to charge up the DC bus voltage VDC until VDC exceeds a threshold value. In the second mode NORMAL, the controller <NUM> according to the present invention operates to close the first contactor <NUM> and open the second contactor <NUM> and the disconnect switch <NUM>, in order to allow direct current flow from the first input <NUM> to the AC to DC converter <NUM>, <NUM> and to prevent current flow from the AC output of the VFD <NUM> through the inductor <NUM>.

In one example, the precharge system <NUM> in <FIG> implements adjustable and scalable precharging functions or options utilizing a standard AC VFD (e.g., variable speed drive) with an adjustable inverter output voltage and frequency (e.g., adjustable voltage and frequency or AVAF control) as a component for precharging the common or shared DC bus circuit <NUM> with multiple inverters <NUM> and potentially large DC bus capacitance via the capacitor bank <NUM>. The VFD <NUM> in one example implements charge rate control according to the feedback voltage signal VFD and current limiting to facilitate both fast precharging and protection against over-currents and/or short circuit protection. The VFD <NUM> and the main AC to DC converter <NUM>, <NUM> operate concurrently during the first mode, with the VFD <NUM> providing input AC power to the input <NUM> of the main AC to DC converter <NUM>, <NUM>, and the AFE provides DC charging current to charge the bus capacitance of the shared DC bus circuit <NUM>. The precharge VFD <NUM> in one example provides precise control of the precharge operation such as charging time, current limiting, short-circuit protection and ground fault protection, and verifies the health of the shared DC bus circuit <NUM> after startup diagnostic checking, as shown, for example, in <FIG> below.

<FIG> and <FIG> show further non-limiting examples. <FIG> shows a power conversion system <NUM> with a shared DC bus, a six-pulse bridge rectifier <NUM> as the primary AC to DC converter, and a precharge system <NUM> with a VFD <NUM> to precharge the DC bus circuit <NUM> as generally described above. In this example, the bridge rectifier <NUM> includes <NUM> rectifier diodes or SCRs configured in a rectifier circuit with an AC input <NUM> and a DC output coupled to the DC bus circuit <NUM>. <FIG> shows another power conversion system <NUM> with a shared DC bus circuit <NUM> and a precharge system <NUM> having a VFD <NUM> as described above in connection with <FIG>. The precharge system <NUM> in this example includes a comprising a rectifier <NUM> coupled between the inductor <NUM> and the second contactor <NUM>. In one example, the rectifier <NUM> is a six-pulse bridge rectifier having <NUM> rectifier diodes or SCRs configured in a rectifier circuit with an AC input coupled to the inductor, and a DC output coupled through the contactor <NUM> to the DC lines <NUM> and <NUM> of the DC bus circuit <NUM>. The primary AC to DC converter in the system <NUM> is an AFE or bridge rectifier <NUM> with an AC input <NUM> and a DC output coupled to the DC bus circuit <NUM>. The second contactor <NUM> in this example has two contacts to selectively allow DC current to flow between the DC output of the rectifier <NUM> in the first mode to directly precharge the DC bus circuit <NUM>, or to prevent DC current flow from the rectifier <NUM> to the DC bus circuit <NUM> in the second mode. In one example, the primary AC to DC converter <NUM> is a second bridge rectifier having an AC input coupled to the first contactor <NUM> and a DC output coupled to the DC bus circuit <NUM>. In another example, the primary AC to DC converter <NUM> includes an AFE rectifier having an AC input coupled to the first contactor <NUM><NUM><NUM> and a DC output coupled to the DC bus circuit <NUM>.

<FIG> shows a method <NUM> for precharging a DC bus circuit, such as the DC bus circuit <NUM> described above in connection with <FIG>. In one example, the method <NUM> is implemented by the precharge controller <NUM> according to program instructions stored in the electronic memory, such as a memory of the VFD <NUM> in <FIG>. The method <NUM> begins at <NUM> and includes coupling the variable frequency drive VFD <NUM> between the AC power source <NUM> and the shared DC bus circuit <NUM>. In one implementation, the controller <NUM> provides this interconnection by opening the first contactor <NUM> and closing the second contactor <NUM> in <FIG> above and applying input AC power at <NUM> in <FIG>. The method <NUM> n one example also includes the controller <NUM> performing an AC/DC converter health check at <NUM>.

In one implementation, the controller <NUM> makes a determination at <NUM> as to whether the main contactor (e.g., the first contactor <NUM>) is open. If not (NO at <NUM> in <FIG>), the controller <NUM> disables operation and such a system fault at <NUM>. Otherwise (YES at <NUM>), the controller <NUM> makes a determination at <NUM> as to whether the precharge drive (e.g., VFD <NUM> in <FIG>) is ready. If not (NO at <NUM>), the controller <NUM> disables operation and sets the system fault at <NUM>. In addition, the controller <NUM> in one example sets one or more references at <NUM> in <FIG>, such as based on user input or other configuration source, including ramp time and a desire DC bus voltage reference.

If the precharge VFD <NUM> is ready (YES at <NUM>), the controller <NUM> causes the VFD <NUM> to execute a start command at <NUM>, and the controller <NUM> determines whether the VFD drive is faulted at <NUM>. If so (YES at <NUM>), the controller <NUM> checks the fault status at <NUM> to determine whether a hard fault exists at <NUM>. If so (YES at <NUM>), the controller <NUM> disables operation and sets the system fault at <NUM>. If there is no hard fault (NO at <NUM>), the controller <NUM> resets the VFD <NUM> at <NUM>, and the VFD <NUM> again executes the start command at <NUM>.

Once the VFD drive has started with no faults (NO at <NUM>), the controller <NUM> runs the precharge operation in the first mode at <NUM> to charge the DC bus circuit <NUM> with the VFD <NUM> until a voltage VDC of the DC bus circuit <NUM> reaches a threshold value or a preset maximum ramp time reference value has been exceeded. In one example, the controller <NUM> operates the VFD <NUM> at or near a line frequency of the AC power source <NUM>, although not a strict requirement of all possible implementations. In another example, the controller <NUM> operates the VFD <NUM> at a higher frequency than the line frequency, for example, two or three times the line frequency of the AC power source <NUM>. In the illustrated example, the controller <NUM> determines at <NUM> whether a predetermined reference time has been exceeded (e.g., determines whether the ramp time is less than a reference time). If so (NO at <NUM>), the controller <NUM> disables operation and sets the system fault at <NUM>. If the reference time has not been exceeded (YES at <NUM>), the controller <NUM> makes a determination at <NUM> as to whether the DC bus voltage VDC is reached or exceeded a threshold value. If the threshold DC bus voltage has not yet been reached (NO at <NUM>), the method <NUM> continues at <NUM> and <NUM> to continue precharging the DC bus circuit <NUM>.

In response to the voltage VDC of the DC bus circuit <NUM> reaching the threshold value without exceeding the predetermined reference time limit (YES at <NUM>), the method proceeds to <NUM> where the controller <NUM> stops running the VFD <NUM> and opens the second contactor <NUM> to disconnect the VFD <NUM> from the DC bus circuit <NUM>. At <NUM>, the controller <NUM> closes the first (e.g., main) contactor <NUM> to couple the primary AC to DC converter (e.g., <NUM>, <NUM> in <FIG>) to the DC bus circuit <NUM> and checks the DC bus capacitance health. In one example, the controller <NUM> controls the timing between opening the second contactor <NUM> and closing the first contactor <NUM> to provide a non-zero (e.g., break before make) switching time delay. This advantageously avoids synchronizing the VFD frequency (e.g., the output frequency of the inverter <NUM>) with the frequency and phase of the AC power source <NUM>. The controller <NUM> in one example verifies at <NUM> whether the first contactor is closed. If not (NO at <NUM>), the controller <NUM> disables operation and sets the system fault at <NUM>. If so (YES at <NUM>), the controller <NUM> determines that the primary AC to DC converter (e.g., the AFE rectifier <NUM> in <FIG>) is ready at <NUM>, and the AFE rectifier <NUM> regulates the voltage VDC of the DC bus circuit <NUM>. At <NUM> in <FIG>, the controller <NUM> determines that the precharge operation is complete and issues a run permissive to the inverters <NUM> coupled to the shared DC bus circuit <NUM>, and the method <NUM> and at <NUM>.

<FIG> show example waveforms illustrating the controlled precharge in operation of the above described precharge system <NUM> time of <FIG> with a DC bus system capacitance of approximately 2f (e.g., capacitor bank <NUM> in <FIG>) in less than <NUM> seconds. <FIG> is a graph <NUM> with a curve <NUM> to showing RMS AC voltage at the LCL filter input during DC bus precharging in the system <NUM> of <FIG>, in which the precharge and begins at time T0, and the filter input voltage reaches <NUM> VRMS in the controller <NUM> opens the second contactor <NUM> at time T1, approximately <NUM> seconds after the precharging began at T0 in one example. A short time later at T2, the controller <NUM> closes the main first contactor <NUM>, by which the AC power source <NUM> begins providing AC input voltage at the input to the filter circuit <NUM>. <FIG> shows a graph <NUM> with a curve <NUM> of AC input current at the LCL filter input during DC bus precharging in the system of <FIG> corresponding to the example of <FIG>. In this example, the precharging current increases from zero to a peak of approximately <NUM> A RMS during precharging in the first mode. In this regard, the use of the VFD <NUM> to provide AC input power to the primary AC to DC converter <NUM>, <NUM> facilitates current limit control, in addition to short circuit protection. <FIG> shows a graph <NUM> with a curve <NUM> that illustrates the DC bus voltage VDC during precharging in the system of <FIG> for the example shown in <FIG> and <FIG>. In one example, the DC bus voltage VDC begins at <NUM> V and the controller <NUM> continues the precharging until the DC bus voltage VDC reaches a predetermined threshold value, such as <NUM> V. Unlike other approaches, the use of the VFD <NUM> for precharging facilitates precharge operations were the initial DC bus voltage VDC is non-zero.

The described examples provide a variety of advantages, in which the VFD <NUM> can be used with shared DC bus systems that have uncontrolled rectifier diode primary rectifiers (e.g., <FIG> and <FIG> above), as well as in systems having controlled rectifier (e.g., SCR) front ends. In addition, the example precharge systems <NUM>, <NUM> can be used with systems having LCL filters and IGBT AFEs (e.g., <FIG> and <FIG>). Moreover, the use of the VFD <NUM> tt* precharge and facilitates control of capacitance charging time, independent of the initial DC bus voltage when the precharge operation began, and the described precharge systems <NUM> and <NUM> facilitate limiting of the charging current. Moreover, the VFD-based precharge systems <NUM> and <NUM> advantageously provide short-circuit protection of the charging circuitry and DC bus components, as well as the capability to provide ground fault protection of the charging circuitry and DC bus components. In addition, the example precharge systems <NUM> and <NUM> provide diagnostics of the system capacitance to increase its reliability. Additional benefits include the ability to optimize the size and cost of the VFD inductor <NUM> in view of the controlled maximum precharge ramp time, without the need to oversize these components to operate for much longer times. For example, pre-charging in only <NUM> seconds facilitates reduced system downtime, and the inductor <NUM> need not be sized to operate for <NUM> minutes in one example. The disclosed examples also facilitate elimination of internal precharge DC link inductors sometimes used in other precharge in systems, thereby providing additional cost savings and reduced harmonics. In practice, once the precharge operations are completed, the VFD <NUM> of the precharge system <NUM>, <NUM> can be taken out of service to improve long time reliability.

Claim 1:
A precharge system, comprising:
a first input (<NUM>) configured to be coupled to an AC power source (<NUM>);
a first circuit branch having a first contactor (<NUM>) configured to be coupled between the first input (<NUM>) and an AC to DC converter (<NUM>, <NUM>);
a second circuit branch having a variable frequency drive, VFD, (<NUM>), which is configured as an AC to AC converter comprising a converter (<NUM>), an internal DC bus circuit (<NUM>) and an inverter (<NUM>), which is connected through an inductor (<NUM>) and a second contactor (<NUM>) to an AC input (<NUM>, <NUM>) of the AC to DC converter (<NUM>, <NUM>), and being connectable through
a disconnect switch (<NUM>) and the second contactor (<NUM>) respectively to both sides of the first contactor (<NUM>) of the first circuit branch, with one side configured to be connected to
the AC power source (<NUM>) and another side configured to be connected to the AC to DC
converter (<NUM>, <NUM>), wherein the inductor (<NUM>) is connected between the VFD (<NUM>) and the second contactor (<NUM>); and
a controller (<NUM>) configured to: in a precharge operation mode, open the first contactor (<NUM>) and close the second contactor (<NUM>) and the disconnect switch (<NUM>) to connect the VFD (<NUM>) to said both sides of the first contactor (<NUM>) of the first circuit branch for connecting the one side to the AC power source (<NUM>) and for connecting the other side to the AC to DC converter (<NUM>, <NUM>), and to prevent direct current flow from the
first input (<NUM>) to the AC to DC converter (<NUM>, <NUM>) and allow current flow from
the AC output of the VFD (<NUM>) through the inductor (<NUM>); and in a normal operation mode, close the first contactor (<NUM>) and open the second contactor (<NUM>) and the disconnect switch (<NUM>) to connect the AC power source (<NUM>) to the AC to DC converter (<NUM>, <NUM>), and to allow direct current flow from the first input (<NUM>) to the AC to DC converter (<NUM>, <NUM>) and prevent current flow from the AC output of the VFD (<NUM>) through the inductor (<NUM>).