Abstract:
The present invention is directed to a power transformer and VSD mounted on the chiller equipment. The transformer is preferably a liquid-filled transformer rated for medium/high voltage input, and at least two output low voltages, one voltage for the VSD and other power equipment, and another voltage for the control panel and control equipment. The placement of the power transformer on the chiller eliminates the need to allocate additional floor space for an auxiliary transformer or medium/high voltage VSD. The transformer also includes cooling means in the form of a heat exchanger in which the liquid is cooled by circulating condenser water, chilled water or refrigerant from the chiller system. The cooling means may also be connected to cooling coils connected to the VSD. The size of the power transformer is significantly reduced by the use of the chiller system to remove heat from the transformer while in operation, thereby maintaining the ambient temperature rise of the transformer within its rated operating temperature range. The transformer and VSD heat is discharged to the outside ambient eliminating additional cooling requirements for the equipment room.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a variable speed drive powered from medium/high voltage mains, and more particularly to a low voltage variable speed drive powered by a medium/high voltage, fluid or gas cooled transformer. 
     BACKGROUND OF THE INVENTION 
     Chiller systems for applications in commercial or industrial building HVAC systems typically include relatively large electric motors for powering a compressor. The motors may range in horsepower from 100 HP up to 5,000 HP or larger. Many of these systems include variable speed drives (VSD) for controlling the speed of the motor in response to cooling demand. Motors and VSDs of this size must be designed for the applicable main distribution voltages. In the case where low voltage mains (600 volts or less) are supplying the chiller system, higher current capacity is required. The relatively high current load requires larger more expensive cables, step-down transformers, and switchgear. 
     Conversely, where the voltage main supplies medium/high voltage (greater than 600 volts) to the chiller system, the current capacity requirements are low relative to the low voltage system requirements. However, other considerations such as equipment costs, complexity and safety must also be taken into account. Medium/high voltage (MV) switchgear, typically 4,160V, requires sophisticated arc suppression, insulation, and safety characteristics. Moreover, because of the greater potential danger of electrocution, only specially trained and qualified maintenance personnel may be permitted to perform the highly specialized maintenance operations of the medium/high voltage distribution system. Corona, flashover and arcing are some of the adverse factors associated with medium/high voltage distribution systems. The presence of moisture and dust particles further contributes to the adverse factors, resulting in increased switchgear costs. Additionally, power semiconductors are not generally available for medium/high voltage applications, and require especially high reverse-voltage characteristics, further adding to the increased cost size and complexity of such equipment. 
     An electric system suitable for a typical low voltage chiller system, due to high current requirements, can include large expensive conductors, switchgear and transformers for a low voltage distribution system. Alternatively, due to the lower current requirements, the components of the medium/high voltage switchgear could be smaller; however, due to the higher voltage significantly greater equipment clearances are required. As a result, there is no space saving advantage associated with the medium/high voltage switchgear and drives. 
     Floor mounted medium/high voltage drives and starters are commercially available. One such motor drive is a model T300 MVi manufactured by the Toshiba International Corporation. The T300 MVi variable speed drive arrangement includes a rather complex 24 pulse input transformer having 12 three-phase secondary windings to supply the drive input rectifiers, plus two secondary windings for two control voltage levels. The floor area required to accommodate most medium/high voltage VSDs, including the T300MVi, is approximately 50 ft. 2  to 100 ft. 2 , which in many instances is equal to or greater than the floor area required for the chiller. Due to the limited floor area allocated for HVAC systems in buildings, it is desirable to minimize the floor area utilized by the chiller system, while maintaining an appropriately sized chiller system. 
     U.S. Pat. No. 5,625,545 discloses an electric drive apparatus and method for controlling medium/high-voltage alternating current motors wherein a multi-phase power transformer supplies multi-phase power to multiple power cells. In one embodiment of the &#39;545 patent which can be applied to 2300 VAC inductive motor loads, three power cells are used for each of the three phase output lines. In another embodiment, which may be applied to a 4160 VAC inductive motor load, five power cells may be used for each of the three phase output lines. Such an embodiment can have eleven voltage states. Such multiple power cell transformers are more costly and consume an excessive amount of space to accommodate the large number of cells and clearances space required for medium/high voltage motors. As a result they require separate mounting cabinets and cannot be mounted directly on the chiller equipment to save floor space. 
     Therefore, there is a need for a low voltage chiller system that can be connected to a medium/high voltage AC power source with a step-down transformer that is sufficiently compact to permit the transformer to be mounted directly on the chiller, integral with the VSD. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a power transformer and VSD mounted on the chiller equipment. The transformer is preferably a liquid-filled transformer rated for medium/high voltage input, and at least two output low voltages, one voltage for the VSD and other power equipment, and other voltages for the control panel and control equipment. The placement of the power transformer on the chiller eliminates the need to allocate additional floor space for an auxiliary transformer. The transformer also includes cooling means in the form of a heat exchanger in which the liquid is cooled by circulating condenser water, chilled water or refrigerant from the chiller system. The cooling means may also be connected to cooling coils connected to the VSD. The size of the power transformer is significantly reduced by the use of the chiller system cooling system to remove heat from the transformer while in operation, thereby maintaining the ambient temperature rise of the transformer within its rated operating temperature range. 
     One embodiment of the invention is directed to a chiller system. The chiller system includes a refrigerant circuit. The refrigerant circuit includes a compressor, a condenser, and an evaporator connected in a closed refrigerant loop. The refrigerant circuit is configured and disposed as a unit. A transformer is configured for connection to medium AC voltage input mains and low AC voltage output. The transformer has a fluid path therethrough in fluid communication with a cooling circuit. A variable speed drive is connected to the output of the transformer. The variable speed drive is configured to power a motor of the compressor. The transformer and the variable speed drive are mounted on the unit. 
     An advantage of the present invention is the ability to provide a refrigeration system having low cost low-voltage electrical components, which can be connected directly to a medium/high-voltage electrical distribution system. 
     Another advantage of the present invention is that it provides a compact MV transformer that may be mounted on the chiller shells together with the VSD minimizing chiller floor space. 
     A further advantage of the present invention is the elimination of a power conduits, e.g., between the transformer and VSD, and between the VSD and motor. 
     Yet another advantage of the present invention is the elimination of the field wiring bending space and the incoming disconnect switch or circuit breaker within the VSD, to provide a less expensive, more compact VSD. 
     Still another advantage of the present invention is the direct transfer of the power transformer and VSD heat losses into the chiller system, avoiding rejection of the losses into the equipment room, thus maintaining lower ambient temperature of the environment. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically a general system configuration of the present invention. 
         FIG. 2  illustrates schematically an embodiment of variable speed drive of the present invention. 
         FIG. 2A  illustrates schematically a transformer with a tertiary winding connected to a control panel. 
         FIG. 3  illustrates schematically a chiller cooling system that can be used with the present invention. 
         FIG. 4  illustrates an elevational view of a chiller system arrangement. 
         FIG. 5  illustrates a plan view of a chiller system arrangement. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates generally the system configuration of the present invention. An AC power source  102  supplies AC power to a medium/high voltage (MV) transformer  100 . The MV transformer  100  supplies low voltage AC power to a variable speed drive (VSD)  104 , which in turn, supplies AC power to a motor  106 . The motor  106  is preferably used to drive a corresponding compressor of a refrigeration or chiller system (see generally,  FIG. 3 ). The AC power source  102  provides multi-phase (e.g., three phase), fixed medium/high voltage (e.g., greater than 600 volt), and fixed frequency AC power to the VSD  104  from an AC power mains or distribution system that is present at a site. The AC power mains can be supplied directly from an electric utility or can be supplied from one or more transforming substations between the electric utility and the AC power mains. The AC power source  102  can preferably supply a three-phase AC medium/high voltage or nominal line voltage of greater than 600V, preferably 3300V or 4160V, at a nominal line frequency of 50 Hz or 60 Hz to the primary of the MV transformer  100 . The MV transformer  100  in turn supplies a fixed secondary voltage of 200 V, 230 V, 380 V, 460 V, or 600 V to the VSD  104  at the corresponding primary frequency, depending on the corresponding AC power mains. It is to be understood that while the AC power source  102  can provide any suitable fixed nominal line voltage or fixed nominal line frequency to the MV transformer  100  depending on the configuration of the AC power mains, the embodiments of the present invention are applicable in general to an AC power source having a nominal voltage greater than 600V, and preferably greater than 2000V. 
     The MV transformer  100  receives AC power having medium/high voltage and fixed line frequency from the AC power source  102 . The MV transformer  100  may preferably be a liquid-filled transformer, in which the liquid acts as an insulator and cools the unit, while transferring heat to the chiller system. The MV transformer  100  provides low voltage output or secondary AC power (e.g., 200 V, 230 V, 380 V, 460 V, or 600 V), to the VSD  104 , and auxiliary control equipment. The MV transformer  100  (see  FIG. 2A ) may also include one or more tertiary windings for single-phase or three-phase low voltage (less than 600V AC) control power. The VSD  104  provides AC voltage to the motor  106  at a desired voltage and desired frequency, both of which can be varied to satisfy particular requirements. Preferably, the VSD  104  can provide AC power to the motor  106  having higher voltages and frequencies or lower voltages and frequencies than the fixed voltage and fixed frequency received from the MV transformer  100  secondary voltage. The motor  106  may have a predetermined rated voltage and frequency that is greater than the MV transformer  100  fixed secondary voltage and frequency, however the rated motor voltage and frequency may also be equal to or lower than the MV transformer  100  fixed secondary voltage and frequency. 
       FIG. 2  illustrates schematically some of the components in one embodiment of the VSD  104 . The VSD  104  can have three stages: a converter stage  202 , a DC link stage  204  and an inverter stage  206 . The converter  202  converts the fixed line frequency, fixed line voltage AC power from the transformer  100  into DC voltage. The DC link  204  filters the DC voltage from the converter  202  and provides energy storage components such as capacitors and/or inductors. Finally, the inverter  206  converts the DC voltage from the DC link  204  into variable frequency, variable voltage AC power for the motor  106 . The VSD  104  may be a conventional VSD with a non-boosted DC link voltage, i.e., the maximum output voltage/frequency is equal to the input line voltage/frequency. Alternately, the VSD  104  may be a VSD that includes an active converter, i.e., the active converter provides a boosted DC link, wherein the boosted DC link has a value greater than the peak of the input AC low voltage into the VSD and the VSD&#39;s maximum output voltage/frequency is greater than the input AC voltage/frequency of the low voltage secondary of the transformer. A more detailed explanation of the operation of the active converter configured to boost the DC link voltage of the VSD is contained in U.S. patent application Ser. No. 11/218,757 filed Sep. 2, 2005, entitled “A Ride-Through Method And System For HVAC&amp;R Chillers”, and in U.S. patent application Ser. No. 11/123,685 filed May 6, 2005, entitled “Variable Speed Drive For A Chiller System”, both of which patent applications are commonly assigned, and are hereby incorporated by reference. 
     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 as shown in  FIG. 3 . In one embodiment of the present invention, the system and method of the present invention can be used to drive a compressor of a refrigeration system. 
       FIG. 3  illustrates generally the system of the present invention connected to a refrigeration system. As shown in  FIG. 3 , the HVAC, refrigeration or liquid chiller system  300  includes a compressor  302 , a condenser  304 , an evaporator  306 , and a control panel  308 . The control panel  308  can include a variety of different components such as an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board, to control operation of the refrigeration system  300 . The control panel  308  can be used to control the operation of the VSD  104  and the motor  106 , as well as other components of the chiller system  300 . In a preferred embodiment, the chiller structure permits the stacking or vertical arrangement of the major components on top of the chiller cooling system  300  to provide a prepackaged unit that occupies less floor space with a smaller footprint than a field fabricated unit where the components are arranged horizontally. 
     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, flowing through a heat-exchanger coil connected to a cooling tower (not shown). The refrigerant vapor in the condenser  304  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  includes connections for a supply line  310  and a return line  312  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  312  and exits the evaporator  306  via supply line  310 . 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 condenser fluid, preferably water, exits the condenser  304  via a return line  314  connected to a cooling tower (not shown), and is circulated from the cooling tower to the condenser  304  via supply line  316 . In a preferred embodiment, condenser water is supplied to the low voltage VSD  104  and MV transformer  100  from the condenser  304  via a supply line  318  connected to supply line  316 . Supply line  318  supplies cooling water to the VSD  104  and to the MV transformer  100 . Piping is preferably installed within each of VSD  104  and the MV transformer  100  for circulation of the condenser water, which absorbs heat generated by the electrical components. A return line  320  is connected to the return line  314  to complete the condenser water circuit from the VSD  104  and the MV transformer  100 . The heated condenser water from the VSD  104  and the MV transformer  100  is mixed with the condenser water in return line  314  and pumped to the cooling tower. Alternatively, chilled water, refrigerant or similar fluids may be used instead of condenser water to cool the MV transformer  100 , VSD  104 . 
     A medium/high voltage disconnect switch  322  is disposed ahead of the MV transformer  100  to disconnect the MV transformer  100  from the input AC power line  102 . The purpose of the disconnect switch  322  is to comply with applicable electrical or fire codes that require a local disconnect means for MV equipment. Preferably, the MV disconnect switch  322  is integrated with or attached to the MV transformer  100  as a single package. 
     The HVAC, refrigeration or liquid chiller system  300  can include many other features that are not shown in  FIG. 3 . These features have been purposely omitted to simplify the drawing for ease of illustration. Furthermore, while  FIG. 3  illustrates the HVAC, refrigeration or liquid chiller system  300  as having one compressor connected in a single refrigerant circuit, it is to be understood that the system  300  can have multiple compressors, powered by a single VSD or multiple VSDs, connected into each of one or more refrigerant circuits. 
     Preferably, a control panel, microprocessor or controller can provide control signals to the VSD  104  to control the operation of the VSD  104  (and thereby the 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. For example, in the refrigeration system  300  of  FIG. 3 , the control panel  308  can adjust the output voltage and frequency of the VSD  104  to correspond to changing conditions in the refrigeration system, i.e., the control panel  308  can increase or decrease the output voltage and frequency of the VSD  104  in response to increasing or decreasing load conditions on the compressor  302  in order to obtain a desired operating speed of the motor  106  and a desired load output of the compressor  302 . 
     Referring next to  FIGS. 4 and 5 , an exemplary physical layout of the MV transformer  100 , the MV disconnect switch  322  and VSD  104  is provided. MV transformer  100  and disconnect switch  322  are disposed within an enclosure  200 . The enclosure  200 , control panel  308  and the VSD  104 , are all mounted on top of the chiller cooling system  300 . The condenser  304  has an exterior shell and the evaporator  306  has an exterior shell. The compressor  302  and the motor  106  are integrally mounted on at least one of the condenser and evaporator shells. A low voltage VSD  104 —e.g., rated less than 600VAC input—is mounted on at least one of the exterior shells and physically attached directly to the motor,  106 , thereby eliminating the need for a motor terminal box and an electrical power conduit connecting the VSD  104  and motor  106 . The MV transformer  100 , is also mounted on at least one of the condenser and evaporator shells. The MV transformer  100  is physically directly attached to the VSD  104 . By directly attaching the MV transformer to the VSD  104 , the space requirement is reduced, because the incoming field wire and conduit bending space requirement, the VSD input disconnect switch or circuit breaker, and the electrical power conduit between the transformer and VSD are all eliminated. By arranging the MV transformer  100 , disconnect switch  322 , and other components on top of the chiller system  300 , it is possible to mount all of the components of the system on a single packaged system, as discussed above. There is a window  323  provided in the cabinet of the disconnect switch  322  to allow viewing of the disconnect switch blades so that the technician can be assured they are open. This window feature is standard on all medium/high voltage equipment. 
     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.