Patent Application
20070151272
The present invention relates generally to an electronic transformer for supplying control voltage. More specifically, the present invention relates to an electronic control transformer powered by the DC link of a variable speed drive for a chiller system.
In the past, the motors for driving compressors in chiller systems were designed to operate from standard line (main) voltages and frequencies that were available from the power distribution system of the facility where the motor was being operated. The use of line voltages and frequencies typically limited the options for modulating the capacity of the compressors to less efficient mechanical devices such as inlet guide vanes and slide valves, as a result of the motor being limited to one operating speed that was based on the input frequency to the motor. In addition, if the operating speed of the motor was not equal to the desired operating speed of the compressor, a “step up,” or “step down,” gearbox was inserted between the motor and the compressor to obtain the desired operating speed of the compressor.
Next, variable speed drives (VSDs) were developed that could vary the frequency and voltage that was provided to the motors of a chiller system. This capability to vary the input frequency and voltage to the motor resulted in a motor that was capable of providing a variable output speed to the corresponding compressor of the chiller system. The variable speed operation of the motors (and compressors) enabled the chiller system to take advantage of efficiencies that occur during partial loading of the compressors, when operation at a speed lower than full design load speed is desirable. The use of the variable speed drive also permitted the use of other types of motors that required their own electronic drive in chiller systems in addition to the previous motors that were capable of operating directly from a three-phase power line, e.g., induction motors, or synchronous motors.
Generally, chiller systems include a number of auxiliary devices in cooperation with the compressor. These auxiliary devices normally include a control panel, contactors, relays, pumps and fans, that are rated for a fixed voltage different than that of the compressor motor, and that operate at a fixed frequency, e.g., 60 Hz or 50 Hz. Since the auxiliary devices cannot operate on the variable voltage and frequency output of the VSD, the power for the control voltage is usually provided by a conventional wound magnetic control transformer that is connected at the input AC power to the VSD, or from another input power source. The output voltage of the control transformer secondary winding is proportional to the primary winding input voltage, such that a voltage sag or interruption on the primary winding is reflected nearly instantly on the output voltage of the control transformer. The sag or interruption, if sufficient in magnitude and/or duration, may cause the chiller system's contactors or relays to drop out, or cause the control panel or pumps to shut down, resulting in interruption of chiller system operation.
The VSD supplying the compressor is also subject to input voltage sags and interruptions for the same reason. VSDs typically include a converter that converts AC input power to DC voltage for a DC link. An inverter is connected to the DC link and converts the DC link voltage to a variable voltage, variable frequency AC power output. The DC link is normally supported by a capacitor which provides limited stored energy, and in some instances an active rectifier or converter may be employed to boost the input voltage above the peak of the RMS value, i.e., greater than 1.414 times the nominal value of the input voltage source. These components of the VSD provide a measure of ride-through capability. Very often, however, the ride-through capability is insufficient to maintain the chiller operation for input voltage sags or interruptions greater than a few cycles on a 60 Hz utility line. In the case where the VSD is configured with ride-through capability, the control transformer can leave the chiller system vulnerable to a voltage sag, even though the VSD and compressor motor may survive the voltage sag.
One limitation of chiller systems having separate AC input power sources for the compressor and the auxiliary devices is that if either the VSD output voltage or the control voltage sags or experiences a brief interruption, then the chiller system fails. Another limitation of chiller systems having separate AC input control power sources for the compressor and the auxiliary devices is that different control transformers must be used for power systems having different distribution voltages, the design of the control transformer being dependent upon the input AC voltage and frequency supplied to the chiller system.
Therefore, what is needed is a control transformer that is powered by the VSD driving the compressor of a chiller system, and that has improved ride-through capability.
The present invention includes an electronic transformer configured to provide single phase AC control power from the DC link of a VSD for powering at least one auxiliary device associated with a chiller system. The electronic control transformer includes an inverter module to convert a DC voltage to an AC voltage to power a load. The inverter module includes: a plurality of pairs of power switches, wherein each pair of power switches includes an insulated gate bipolar transistor connected in anti-parallel to a diode; an input DC connection for connecting the electronic control transformer to a DC link of a variable speed drive; an input AC connection for connecting the electronic control transformer to the AC input to the variable speed drive and an output AC connection for connecting the at least one auxiliary device. The inverter module is controllable to provide a fixed output AC voltage and fixed frequency for powering the at least one auxiliary device.
In a preferred embodiment, the invention is directed to a drive system to power a plurality of components of a chiller system having different voltage requirements than the drive system itself. The drive system includes an active rectifier or converter stage connected to an AC power source to receive an input AC power at a fixed input AC voltage and a fixed input frequency. The active converter or rectifier may also be configured with a pre-charge circuit for controlling the inrush current to the DC link upon start up. The active converter stage is configured to convert the fixed input AC voltage to a boosted DC voltage, the boosted DC voltage being greater than the peak value of the fixed input AC voltage. A DC link is connected to the converter stage. The DC link is configured to filter the boosted DC voltage and store energy from the converter stage. Also, a first inverter stage is connected to the DC link. The first inverter stage is configured to convert the boosted DC voltage from the DC link to provide an output power at a variable voltage and variable frequency to a motor of a chiller system. The variable voltage has a maximum voltage capability greater in magnitude than the fixed input AC voltage. The variable frequency has a maximum frequency capability greater than the fixed input frequency. The active converter provides additional ride-through capability to the extent that the AC input voltage may decrease without a corresponding decrease in the DC link. When the active converter reaches saturation, the inverter is controlled to reverse the flow of energy from the motor and compressor to the DC link stage to transfer electrical power from energy stored in a rotating mass of a motor connected to the first inverter stage to maintain a voltage level of the DC link in response to a decreasing fixed input AC voltage; and an electronic control transformer connected to the DC link, the electronic control transformer being configured to convert the boosted DC voltage from the DC link into an auxiliary output power source having a fixed output AC voltage and a fixed output frequency for at least one component of a chiller system. The fixed output AC voltage is less than the fixed input AC voltage to the converter of the drive. In addition, the fixed output AC voltage and fixed output frequency of the electronic control transformer is maintained in the event of a temporary decrease in the fixed input AC voltage.
In another embodiment, the VSD contains a conventional rectifier or converter at the input end. There is a DC link connected to the converter stage, the DC link being configured to filter the DC voltage and store energy from the converter stage. A first inverter stage is connected to the DC link. The first inverter stage is configured to convert the DC voltage from the DC link into the output power for the motor. The first inverter stage is configured to reverse power flow through the first inverter stage to the DC link stage to transfer electrical power from energy stored in a rotating mass of the motor to maintain the voltage level of the DC link stage in response to a decreasing input AC voltage; and an electronic control transformer connected to the DC link, the electronic control transformer being configured to convert the DC voltage from the DC link into an auxiliary output power source having a fixed output AC voltage and a fixed output frequency for at least one component of a chiller system. The fixed output AC voltage is less than the fixed input AC voltage to the converter of the drive. In addition, the fixed output AC voltage and fixed output frequency of the electronic control transformer is maintained in the event of a temporary decrease in the fixed input AC voltage.
In another embodiment of the present invention there is a chiller system which includes an electronic control transformer. The chiller system includes a refrigerant circuit comprising compressor, a condenser, and an evaporator connected in a closed refrigerant loop. A motor is connected to the compressor to power the compressor. A variable speed drive is connected to the motor. The variable speed drive is configured to receive an input AC power at a fixed input AC voltage and a fixed input frequency and provide an output power at a variable voltage and variable frequency to the motor. The variable voltage has a maximum voltage capability greater in magnitude than the fixed input AC voltage and the variable frequency having a maximum frequency capability greater than the fixed input frequency.
The variable speed drive includes an active converter or rectifier stage connected to an AC power source providing the input AC power. The converter stage is configured to convert the input AC voltage to a boosted DC voltage. The boosted DC voltage is greater than the fixed input AC voltage. There is a DC link connected to the converter stage, the DC link being configured to filter the boosted DC voltage and store energy from the converter stage. A first inverter stage is connected to the DC link. The first inverter stage is configured to convert the boosted DC voltage from the DC link into the output power for the motor. The first inverter stage is configured to reverse power flow through the first inverter stage to the DC link stage to transfer electrical power from energy stored in a rotating mass of the motor to maintain the voltage level of the DC link stage in response to a decreasing input AC voltage.
An electronic control transformer is connected to the DC link. The electronic control transformer is configured to convert the boosted DC voltage from the DC link into an auxiliary output power source having a fixed output AC voltage and a fixed output frequency, the fixed output voltage being less than the fixed input AC voltage to the active converter or rectifier. The fixed output AC voltage and fixed output frequency of the electronic control transformer is maintained in the event of a temporary decrease in the input AC voltage.
In another embodiment, the present invention is directed to an electronic control transformer for powering at least one auxiliary device associated with a chiller system. The electronic control transfer includes an inverter module to convert a DC voltage to provide a fixed output AC voltage and fixed frequency to power the at least one auxiliary device. The inverter module has a plurality of pairs of power switches, wherein each pair of power switches includes an insulated gate bipolar transistor connected in anti-parallel to a diode. An input DC connection is provided for connecting the electronic control transformer to a DC link of a variable speed drive. An input AC connection is also provided for connecting the electronic control transformer to the AC input of the drive system. An output AC connection is provided for connecting the at least one auxiliary device to the inverter module.
One advantage of the present invention is that the electronic transformer is connected to a variable speed drive with improved ride-through capabilities thereby providing ride-through capability to auxiliary components of the chiller system and providing improved reliability of the chiller system.
Another advantage of the present invention is the use of a single electronic control transformer for powering the chiller controls, that is independent of the input AC voltage magnitude and input frequency to the VSD equipped chiller system.
Another advantage of the present invention is that the electronic transformer eliminates the need for a conventional control transformer and provides a common source of electrical power for the entire chiller system.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
Referring next to
The electronic transformer 108 is also connected to the DC link 204 of the VSD 104. The electronic transformer 108 converts the DC power from the DC link 204 into a fixed AC voltage, preferably 120 VAC, at a fixed frequency, preferably 60 Hz, although the fixed voltage and/or frequency may be changed to suit various localities or operating requirements. Auxiliary devices 110 for the chiller system are connected to the output of the electronic transformer 108, and can include a control panel, contactors, relays, pumps and fans. If required, the electronic transformer 108 may be configured to supply three-phase power to the auxiliary devices. The electronic transformer 108 has the ability to provide control power at the rated voltage of the auxiliary devices 110, as long as the DC voltage of the DC link is maintained at a sufficient level. The benefit of a ride-through capability of VSD 104 is transferred to the auxiliary devices 110 of the chiller system, as discussed below in further detail. The electronic transformer 108 may be built within the same enclosure that houses the VSD 104. Alternately, the electronic transformer 108 may be built into the control panel, or may be housed in an enclosure having distribution circuit breakers and switches to feed the control panel 308 (see
The motor 106 is preferably an induction motor that is capable of being driven at variable speeds. The induction motor can have any suitable pole arrangement including two poles, four poles or six poles. The induction motor is used to drive a load, preferably a compressor of a refrigeration or chiller system as shown in
As shown in
Compressor 302 compresses a refrigerant vapor and delivers the vapor to the condenser 304 through a discharge line. The compressor 302 is preferably a centrifugal compressor, but can be any suitable type of compressor, e.g., screw compressor, reciprocating compressor, etc. The refrigerant vapor delivered by the compressor 302 to the condenser 304 enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser 304 flows through an expansion device (not shown) to an evaporator 306.
The evaporator 306 can include connections for a supply line and a return line of a cooling load. A secondary liquid, e.g., water, ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator 306 via return line and exits the evaporator 306 via supply line. The liquid refrigerant in the evaporator 306 enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator 306 undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator 306 exits the evaporator 306 and returns to the compressor 302 by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser 304 and evaporator 306 can be used in the system 300, provided that the appropriate phase change of the refrigerant in the condenser 304 and evaporator 306 is obtained.
The HVAC, refrigeration or liquid chiller system 300 can include many other features that are not shown in
Preferably, a control panel 308, microprocessor or controller can provide control signals to the VSD 104 to control the operation of the VSD 104 (and possibly motor 106) to provide the optimal operational setting for the VSD 104 and motor 106 depending on the particular sensor readings received by the control panel 308. For example, in the refrigeration system 300 of
Referring back to
Furthermore, the active converter or rectifier 202 can be used to improve the ride-through capabilities of the VSD 104 during a decrease of the AC input voltage, also referred to as a voltage sag. The active converter or rectifier 202 can be controlled to provide a desired or predetermined output voltage to the DC link 204 that is independent of the AC input voltage. By providing a DC voltage that is not dependent on the AC input voltage, the active converter or rectifier 202 (and VSD 104) is unaffected by voltage sags in the AC input voltage and thereby provides improved ride-through performance for the VSD 104. The active converter or rectifier 202 can continue to provide the desired DC voltage to the DC link 204, even though the AC input voltage has sagged. This capability of the active converter or rectifier 202 enables the VSD 104 to continue to operate without interruption, or ride through, during times when the AC input voltage sags.
As a further means of improving the ride through in the chiller system, when the active converter or rectifier 202 reaches its current limit during a voltage sag, a compressor control unit (not shown) actuates a mechanical unloading device of the compressor 302 to minimize the power consumed by the mechanical load from the DC link capacitors 208. Also, at approximately the same time that the mechanical load is uncoupled by the compressor control unit, an inverter control unit (not shown) assumes control of the DC link voltage and controls the motor speed in order to cause energy to be transferred to the DC link capacitors 208 from the energy stored in the rotating mass of the motor 106. The motor speed decreases during the ride-through, while the DC link voltage is maintained at or near rated voltage. If the energy stored in the rotating mass is sufficiently depleted before the restoration of the line input voltage to normal range, the system will eventually shut down to prevent damage to the chiller system components. If the line input AC voltage recovers to within a predetermined range of the nominal input AC voltage, then the inverter 206 resumes controlling the speed of the motor 106 as required by the HVAC & R's control system and the converter 202 resumes regulating the DC voltage of the DC link 204, until the next voltage sag occurs.
As discussed in greater detail below, the electronic transformer 108 is comprised of four power semiconductor switches (not shown), such as insulated gate bipolar transistors (IGBTs) or MOSFETs. Preferably, the electronic transformer 108 is configured to convert the DC voltage from the DC link to provide single phase, 120 volt, 60 Hz. AC control power to the auxiliary devices 110. If the auxiliary devices 110 include three phase motors for the pumps and fans, the electronic transformer 108 may be configured to provide three-phase power and if desired, a different fixed voltage and fixed frequency. The electronic transformer 108 is set for a fixed output voltage and frequency and does not adjust the voltage or frequency of the output power once it has been set.
In the preferred embodiment of the invention, the VSD 104 includes a precharge configuration for precharging the capacitors connected to the DC link. Referring to
In the active converter or rectifier module 202, one of the power switches in each pair of power switches is an IGBT 450 connected to an inverse or anti-parallel diode 452. The inverse or anti-parallel diode 452 is used to conduct current after the other power switch, IGBT 454, is turned off when the VSD 104 is operated in a pulse width modulation mode. As shown in
The other power switch in the pair of power switches is a reverse blocking IGBT 454, i.e., the IGBT 454 is capable of blocking voltages in the reverse as well as the forward direction. The reverse blocking IGBT 454 is connected to an inverse or anti-parallel IGBT 456, which anti-parallel IGBT 456 is also a reverse blocking IGBT. The anti-parallel IGBT 456 is then preferably controlled during the precharge operation to permit only small pulses of inrush current to reach the DC link. After the precharge operation is completed, the anti-parallel IGBT 456 can be controlled to conduct at all times, similar to the anti-parallel diode 452. The reverse blocking IGBT 454 blocks a positive emitter-to-collector voltage that is approximately equal to the peak line-to-line voltage that appears across the IGBT 454 for as long as the conduction of the anti-parallel IGBT 456 is delayed for the purpose of precharge.
In an alternate embodiment of the active converter 202, as shown in
Referring to
When precharging the DC link, in the preferred embodiment, the electronic transformer 108 includes a pair of diodes 430 connected to any two of the three phases L1, L2, L3 of the input AC power 102. This arrangement provides power immediately to the control circuits when the active converter or rectifier is not operating or is precharging the DC link 204. Diodes 430 are connected to the DC bus 440 of the electronic transformer to convert input AC voltage 102 to the DC voltage of DC bus 440 while the DC link 204 of the converter 202 is precharging. Diode 432 blocks current flow from the DC bus 440 to the DC link 202 during precharging. After the precharge of the DC link 204 is completed, and the Active Converter 202 is enabled, the boosted DC voltage of the DC link 202 is greater than the DC voltage provided by diodes 430, causing DC current to flow to the DC bus 440 from the DC link 204. Thus, diodes 430 are reverse biased when the voltage of the DC link 204 exceeds the peak of the input AC voltage, and they stop conducting current to the DC bus 440. Diodes 430 also isolate the DC link from the input AC line 102. Preferably, a capacitor 442 is connected across the DC bus 440 to filter the DC power and store energy for the electronic transformer 108. An additional diode 430 not shown, may be included to allow a three phase AC power source to be provided to the electronic control transformer with voltage when the active converter or rectifier is not operating.
The electronic transformer 108 is in essence an inverter that converts the DC voltage of the DC bus 440 to the control voltage for the auxiliary devices 110 described above, typically 120 Volt, 60 Hz power. The electronic transformer 108 has four pairs of power switches or transistors. One of the power switches in each pair of power switches is an IGBT 444 connected to an inverse or anti-parallel diode 446. The inverse or anti-parallel diode 446 is used to conduct current after the other power switch, IGBT 444, is turned off when the electronic transformer 108 is operated in a pulse width modulation mode.
As shown in
A capacitor C2 may be connected across the load terminals of inductors 460 to filter the output voltage of the electronic transformer 108, and to store electrical energy for loads that require inrush current when energized, such as solenoid valves and relays. Optionally, an isolation transformer 464 may be connected across the capacitor 462 at the output of electronic transformer 108, to provide electrical isolation, for example, to ground the control circuit. If the isolation transformer 464 is required, the inductor 460 and capacitor C2 may be connected on the load side of the transformer 464. This results in a higher frequency being applied at the primary of the transformer 464, and permits the use of a smaller and less expensive transformer 464.
In another aspect of the invention, no precharge configuration is provided in the VSD 104. Where the inrush current to the DC link is not significant, there may be no need to control the precharge of the capacitors of the DC link. Referring to
In the converter module 202 shown in
The configuration of the electronic transformer 108 in
Connecting the electronic transformer to the DC link 204 provides additional ride-through capability to chiller system auxiliary devices that is not available when using a conventional wound transformer. If a voltage sag occurs on the input AC power source 102, the output voltage of the electronic transformer 108 is maintained at its rated output voltage so long as the DC link 204 is supported by the energy stored in the rotating mass of the motor 106 and the compressor 302 and/or the active converter 202, as described above. Therefore, the operation of the chiller system is not compromised by the failure of auxiliary devices, such as the control panel, pumps, relays and contactors during voltage sags and disturbances.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
