Abstract:
An improved UPS/CPS system employs a high-efficiency uninterrupted power supply function integrated with an engine-generator set that combines both short term protection against momentary power interruptions with longer term power generation. Feedback from a controller provides for improved voltage regulation during abrupt load changes and flexibility of application. A number of features and operating modes are disclosed. In one embodiment, the system is useful as a kit to retrofit existing backup power facilities.

Description:
BACKGROUND OF THE INVENTION 
     Many electrical loads can be adversely effected if their supply of electrical power is disrupted for even a fraction of a second. Uninterruptible Power Supply (UPS) systems are commonly used to prevent such disruption when the normal supply of electrical power fails or falters. These UPS systems use a temporary energy source, such as batteries or flywheels, to provide power to their protected load for a limited ride through time of from several seconds to several minutes. Some UPS systems are coordinated with a standby engine-generator set, whereby the gen-set is automatically started in the event that the failure of the incoming utility power line is not quickly restored. Typically, the engine-generator set is electrically connected on the utility side (or normal power supply side) of the UPS system, and a transfer switch is used to connect it to the input of the UPS system once the engine is started and up to normal operating speed. 
     Traditional battery-based static UPS systems, or even flywheel-based rotary UPS systems which use a motor-generator set, completely regenerate their input power in order to protect their loads. Therefore, if the engine-generator set is intended to supply power to the protected load through the UPS system, then it must be sized to provide for the additional parasitic losses of the UPS system that is in series with the protected load. Also, this combination of engine-generator set and UPS system will have a reduced ability to provide short circuit currents needed to isolate faults properly or to regulate voltage during abrupt changes to the protected load (such as from large motor starting or load switching). Control of this combination of gen-set and UPS system is, however, simple. For example, the UPS system has its own utility power disturbance analyzer, and is able to tell when utility power has failed. After a set time delay, it can issue a start command to the engine-generator set to begin its standby operation. The engine-generator set has its own automatic control scheme to start the engine, bring the engine up to normal operating speed, close the transfer switch, and thereafter adjust its own power frequency using a governor to control engine throttle. The engine-generator set governor and the UPS frequency control circuits can work independently. For example, if there is a step change in the protected load, the UPS system will automatically try to maintain load frequency regardless of its input frequency from the gen-set. The governor of the gen-set will respond to the change in electrical load requirement it sees from the UPS system by automatically increasing the engine throttle setting when the engine slows down due to the higher torque required. This is a stable control system. 
     There are three problems associated with the above combination of engine-generator set and traditional UPS system. First, the normal parasitic losses of series in-line UPS systems are inherently high; typically 90 to 92% efficiency. This equates to high operating cost over the life of the equipment. Second is that the standby engine-generator set must be oversized to account for this low operating efficiency. This equates to higher initial capital costs for the gen-set, plus higher operating costs while on standby. Third, the reduced ability to supply short circuit current for proper fault clearing, and to regulate voltage during abrupt load changes creates an application problem for such systems. This last issue is one of the reasons why traditional UPS systems are not normally used to protect industrial process circuits, where loads are constantly changing and large motors are often switched on and off. 
     Integrated rotary UPS and engine-generator systems are available to address some of these problems. Such systems include a synchronous motor generator which can be driven by a temporary energy source (such as a flywheel) for a short duration, or an integrally-mounted engine for long duration. In normal operation, the synchronous motor-generator (SMG) is connected to the utility line as a lightly loaded synchronous motor. When a utility disturbance is detected, a circuit breaker automatically opens to isolate both the SMG and the protected load from the utility supply. The temporary energy supply then mechanically drives the SMG so that it becomes a generator to supply power to the protected load. At the same time, the engine is automatically started, brought up to speed, and a clutch engaged to allow the engine to mechanically drive the SMG after the temporary energy source is depleted. Such an integrated systems is termed a Continuous Power Supply (CPS) system, as it is able to continue to supply power to the protected load long after the temporary energy source of a traditional UPS is exhausted. 
     These CPS systems address some of the problems inherent in the traditional UPS/engine-generator set combination, but add other disadvantages. First, the efficiency of the system is improved over the series in-line type of UPS system (typically 92 to 94%), but still represents a significant operating loss over the life of the equipment. For example, for each percentage point of inefficiency at a utility rate of 5 cents per kilowatt-hour, a 1000 kW machine will cost $35,000 to operate over ten years. Second, the engine and the SMG do not have to be oversized because, during standby, the protected load is supplied directly from the SMG. While the CPS has slightly higher short circuit capability, and therefore slightly better voltage regulation during abrupt load changes, it still does not have the full capability of a typical utility supply. Further, it has several limitations that often make its application impractical. One of these is that it is not practical to retrofit a CPS where there is an existing engine-generator set. The generator cannot be used, there may not be sufficient space to accommodate the larger CPS assembly, and reconnection of the existing engine is expensive. Another problem is lack of flexibility in applying the “UPS” function versus the “engine-generator set” function. For example, it is often desirable to protect only part of a facility&#39;s electrical load with a “UPS” function, but connect a much larger portion of the facility to the emergency standby power of an engine-generator set. These application problems are inherent in the integrated construction of the traditional CPS system. 
     SUMMARY OF THE INVENTION 
     Some aspects of this invention are to provide a high-efficiency UPS function that can be integrated with an engine-generator set resulting in a combination which has improved short circuit capability, improved voltage regulation during abrupt load changes, and flexibility of application. Accordingly, the invention utilizes a high-efficiency synchronous generator connected to a normal source of electrical power, temporary energy storage to drive this generator during initial loss of normal power, and a control system to coordinate power operation with an engine-generator set. 
     The synchronous generator is similar to a traditional generator, but can be rated and sized to carry the full load current need by a load for only a brief period of a few tens of seconds or minutes. Therefore, the generator can be of reduced mass and cost, and does not require the high-capacity fan construction normally needed to provide cooling at full rating for extended operation. The parasitic losses associated with the cooling fan are the largest single element of a generator&#39;s inefficiency; reducing these cooling requirements means that efficiencies of 95 to 97% can be attained under normal operation. A transient, temporary energy storage system is connected to the synchronous generator. In the preferred embodiment, this comprises a continuously variable transmission (CVT) which can transfer energy into and out of a spinning flywheel. This combination of generator, CVT and flywheel forms part of the Ride-Thru Module (RTM). However, as explained herein, other structures may take the place of the generator, CVT and flywheel combination to equivalently perform the functions of the RTM. During normal operation, the RTM is connected to the utility power supply through a disconnecting means such as a circuit breaker (any switch). Most of the power flows directly to the protected load, with only a small amount being used by the RTM to overcome friction, windage, and electrical losses in the generator and to maintain the flywheel at a pre-set rotational speed. When a disturbance in the utility power is detected, the circuit breaker is opened to isolate the RTM and the protected load. A feedback control system maintains the speed of the generator (and therefore the frequency of the protected load) by adjusting the ratio of the CVT to extract power from the spinning flywheel. If the utility power supply quickly returns to a stable condition (e.g., voltage magnitude, frequency and phase angle), the speed and angle of rotation of the generator are adjusted until the voltages of the utility power supply and the RTM are in synchronism, at which time the circuit breaker is closed. The control system then adjusts the CVT to take a small amount of power from the utility supply to accelerate the flywheel back to its preset energy storage value. 
     If the utility power supply does not quickly return to a stable condition, a control command is sent to start the engine-generator set, and a second circuit breaker connects the RTM in parallel with the gen-set as soon as they are in synchronism. An important element of the invention is the control of the two generators, which are now operating in parallel; in effect magnetically linking the flywheel and the engine through two generators and the CVT. When there is an abrupt change in the protected load, both generators will supply power. This effectively doubles the short circuit capability and greatly enhances voltage control. In addition, the flywheel and control system in the RTM are able to maintain frequency control much better than the engine alone. For example, diesel engines typically have a one-second response characteristic, while the flywheel and CVT are about ten times faster, or a complete order of magnitude. The control system of the invention uses the flywheel and CVT to control RTM frequency (and therefore engine-generator set frequency) and uses the engine to provide torque when needed (and therefore the long-term power supply). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of one embodiment of the present invention. 
     FIG. 2 is a table of different operating modes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A single-line diagram of one preferred embodiment of a system utilizing the teachings of the subject invention is shown in FIG.  1 . In FIG. 1, a UPS/CPS system  10  has the protected load  12  connected to the utility source  14  through circuit breaker (UCB)  16 . A ride-through module (RTM)  20  is connected to the protected load  12  through circuit breaker (SCB)  22 , and is mechanically separate from a second module termed the engine generator set (EGS)  50 . The combination of generator, continuously variable transmission (CVT) and flywheel is termed the Ride-Thru Module (RTM). Other structures, however, may take the place of the generator, CVT and flywheel combination to equivalently perform the functions of the RTM. For example, the CVT, which is controlled by a master controller  70 , is used to control the speed of the synchronous motor-generator (SMG)  24  when driven by the flywheel  38 , and in this way controls the magnitude, frequency and relative phase angle of the voltage and current phasors (i.e., power) generated by SMG  24 . But from the teachings of the present invention a suitable power switching circuit using power semiconductor switches (e.g. SCR&#39;s) could be constructed to control the phasor output of SMG  24 , which could be of an alternate construction. Further, the master controller  70  and voltage regulator  88  can control the electric power output via the field windings  26  as explained further herein. With suitable technology, the flywheel  38  may be replaced by any device to store potential energy, such as a rechargeable battery, fuel cell, compressed gas, or a spring wound device, that provides shaft power to drive SMG  24 . The potential energy storage device in RTM  20  should have the ability to provide energy to the protected load  12  much faster, in response to an interruption of power, than the energy supplied by the engine generator-set (EGS) module  50 . 
     The RTM  20  forms an independent power reserve supply to the power reserve supply formed by the engine generator-set (EGS) module  50 . The two modules, RTM  20  and EGS module  50 , are mechanically separate, each having their own generators, and are only electro-magnetically linked through the master controller  70 . The RTM  20  is continuously connected to the utility source power supply  14 , and, by nature of the transient, temporary energy storage device such as flywheel  38 , has a faster response time in supplying adequate power (e.g. suitably conditioned power that can be used at the protected load) to the protected load  12 , in an event of an interruption of power, than does EGS  50 , which stores a more permanent supply of energy that is not as readily available. However, the EGS  50  is needed because there is greater electric power stored, or more precisely, capable of being generated, by EGS  50  than by RTM  20 . RTM  20  derives the bulk of its energy stored from utility source  14 , and does not generate its power from a reserve (e.g., as in a fossil fuel reserve powering EGS  50 ). As a consequence, RTM  20  is used as a transient source of power until such time that EGS  50  can be activated and become available to supply energy to the protected load  12  (hence the suggestion of the term “ride-through” module). Further, the RTM  20  replenishes its power supply from the utility source, and both inputs and outputs energy from the utility power supply (i.e., acts as a source or a sink), whereas in one embodiment the EGS  50  only outputs energy (acts as a source). Master controller  70  forms the intelligence to link RTM  20  with EGS  50  and with the rest of the circuit components of the system, and to coordinate the components for the smooth transfer of power between components and the monitoring and maintaining of properly conditioned power between the utility source  14  and protected load  12 . 
     The RTM  20  consists of a synchronous motor-generator (SMG)  24  with rotor field  26  and speed sensor or tachometer  28 , pony motor  30  connected to synchronous motor-generator  24  via shaft  32 , continuously-variable transmission  34  connected to synchronous motor-generator  24  through shaft  36 , and flywheel  38  with speed sensor or tachometer  40  connected to shaft  42  of the continuously variable transmission  34 . The synchronous generator  24  is similar to a traditional generator, but can be rated and sized to generate and carry the maximum full load current that may be needed by a protected load  12  for only a brief predetermined period, typically tens of seconds or a few minutes (e.g., less than 10 continuous minutes at any given time). The generator  24  of the RTM  20  would also thus be smaller, and rated to carry less continuous current, than the generator  52  of the gen-set  50 . Therefore, the generator  24  can be of reduced mass and cost, and does not require the high-capacity fan construction normally needed to provide cooling at full rating for extended operation. 
     As shown an engine generator-set (gen-set)  50  is connected to the load through circuit breaker (GCB)  51 . The engine generator-set  50  consists of synchronous generator  52  with rotor field  54  and speed sensor or tachometer  56 , engine  58  connected to generator  52  through shaft  60 , starter motor  62  connected to engine  58  through shaft  69 , and governor  66  connected to the throttle of engine  58  and receiving input signals from the speed sensor  56  and master controller  70 . The engine driving the gen-set is preferably an internal combustion motor engine having a throttle but may be any prime mover that produces shaft power. Master controller  70  is able to receive analog and discrete inputs, provide automated logic control, accept manual control through a man-machine interface (not shown), and provide automated closed-loop feedback control as is described herein. Master controller  70  has many of the features of a microprocessor and may be replaced by a microprocessor or other suitable circuitry that emulates its functions. Other functional elements of the system are included to simplify explanation of operation, but are not required to be separate devices. These include potential transformers  72 ,  74 ,  76  and  78 , current transformer  77 , synchronizing relays  82 ,  84  and  86 , voltage regulators  88  and  90 , and utility disturbance analyzer  92 . Refer to FIG. 2 for the operating modes of the system. 
     The normal operating mode of the system is defined by the parameters shown under the column Normal Mode  200  of FIG.  2 . In this mode, the protected load  12  is receiving power from the utility source  14  through circuit breaker  16 , which is closed. The ride-thru module (RTM)  20  is connected to the utility source  14  and the protected load  12  through circuit breaker  22 , which is closed (circuit breaker  22  being downstream of circuit breaker  16 ). The ride-thru module&#39;s synchronous motor-generator  24  is rotating at synchronous speed with the utility source  14  frequency, and the flywheel  38  is rotating at a speed representing its preset energy storage value. The master controller  70  uses input I 2  from speed sensor  40  to determine the speed of the flywheel  38 , and compares this input value to the preset energy storage value in its memory. Any error signal in this comparison is used to control the speed ratio of the continuously-variable transmission  34  through output O 1  to adjust the speed of the flywheel up or down, as needed. The master controller  70  provides an output O 3  to the voltage regulator  88  to indicate that it should operate to regulate voltage of the ride-thru module&#39;s synchronous motor-generator  24  to its preset value. The voltage regulator  88  then measures voltage of the protected load  12  through potential transformer  76 , and adjusts the current to field winding  26  of the ride-thru module&#39;s synchronous motor-generator  24  to maintain that preset value. In this way, voltage is controlled at the protected load  12  to improved accuracy compared to the utility source  14  alone. It as also possible to have the voltage regulator  88  control the current to the rotor field  26  based on the measured or derived power factor of the utility source  14  or power factor of the protected load  12  (use of a synchronous motor-generator in this way is well known in the art per se, as a synchronous condenser). The engine generator-set  50  is not operating, and is isolated from the protected load  12  and the utility source  14  by circuit breaker  51 , which is open. 
     The system will remain in the Normal Mode  200  described above as long as the utility source  14  continues to meet specific performance criteria. The disturbance analyzer  92  monitors the current and voltage of the utility source  14  using current transformer  77  and potential transformer  72 , respectively. Both of these phase and voltage measuring instruments are upstream of circuit breaker  16 , and are ultimately sampled, by the master controller  70  acting through the disturbance analyzer  92  and synchronous relays  82 ,  84 ,  86 . The disturbance analyzer  92  compares utility source  14  voltage to preset high and low limits, and sends a signal to input I 3  of the master controller  70  when voltage is outside these limits. The analyzer  92  may also calculate other utility source  14  power quality indicators from the current and voltage inputs, such as frequency and direction of power flow, and accordingly issue a signal to input I 3  of the master controller  70  when values fall outside of preset limits. 
     When master controller  70  senses a signal at its input I 3 , indicating a current or voltage value outside the present limit of the disturbance analyzer  92 , it initiates a control program which does the following. First, a command is given through input/output B 1  to open circuit breaker  16 . At the same time the feedback control of the continuously-variable transmission  34  is reversed so that energy is taken from the flywheel  38 , to drive synchronous motor-generator  24  at a constant speed. This is accomplished by comparing the output from speed sensor  28  at input I 1  of the master controller  70  with a preset value stored in memory. This resulting error signal is then used to drive output O 1  in order to control the speed ratio of continuously-variable transmission  34 . During this mode of operation, the synchronous motor-generator  24  is now acting as a generator, and the voltage regulator  88  continues to adjust the current to field winding  26  in order to maintain the desired output voltage from synchronous motor-generator  24 . Further, it can be appreciated from the teachings herein that the flywheel  38  may be replaced by any transient, temporary energy source, such as a chemical battery, fuel cell, wound spring, compressed gas container or other potential energy storage device, having a response rate for supplying energy to the protected load that is faster, in response to an interruption of power, than the energy supplied by the EGS  50  module. In this way, the ride-thru module  20  is able to continue supplying power to the protected load  12  even though the utility source  14  power supply has become unreliable. This Ride-Thru Mode  300  of operation will continue until one of the below following conditions is met. 
     I. The utility source quickly returns to a stable condition 
     This is sensed by the disturbance analyzer  92 , as measured voltage, within preset high and low limits for a defined amount of time (for example two seconds). When the disturbance analyzer  92  determines that utility source  14  power has returned to normal, it issues a command to input I 3  of master controller  70 . Master controller  70  then initiates a resynchronizing program sequence, which does the following. Synchronizing relay  82  compares the voltage signals from potential transformers  72  and  74 , and provides a signal to input I 4  of the master controller  70  which tells it both the magnitude of the voltage difference, as well as the phase angle difference, between the utility source  14  and the synchronous motor-generator  24  voltage waves. Using this information, the master controller  70  adjusts the speed (and therefore the frequency and relative phase angle) of the synchronous motor-generator  24  by adjusting the speed ratio of continuously-variable transmission  34  through output O 1 . Simultaneously, master controller  70  also adjusts the voltage magnitude of the synchronous motor-generator  24  to match the utility source  14  voltage by issuing control commands to voltage regulator  88  through output O 3 . When the voltage magnitude, frequency and relative phase angle of the synchronous motor-generator  24  and utility source  14  are within preset limits, as computed or as measured directly from instruments, indicating a return to a stable condition of the interrupted utility source, master controller  70  issues a so-called closed loop synchronizing close command to circuit breaker  16  through input/output B 1 . After circuit breaker  16  is closed, the master controller  70  reverts back to the Normal Mode  200  control program which then adjusts the output O 1  to take energy from the utility source  14  to accelerate the flywheel  38  back to its preset energy storage speed. 
     II. The utility source does not return quickly to a stable condition. 
     Master controller  70  monitors the speed of the flywheel  38  using input I 2  connected to speed sensor  40 . If the flywheel  38  speed falls below a preset lower limit before an input from disturbance analyzer  92  is sensed at input I 3  indicating that the utility source  14  has returned to stable conditions, the master controller  70  initiates a logic sequence to start the engine generator set  50 . The flywheel speed lower limit could be set, for example, to allow sufficient remaining temporary energy storage for ten seconds of operation of the ride-thru module  20  at full rated power to the protected load  12 . This is enough time for a diesel engine generator set to start, come up to synchronous speed, and take over the load from the RTM  20 . The logic sequence to start the engine generator-set  50  will now be described. First, a start command is issued from master controller  70  output O 6  to send power to starter motor  62  to turn the engine through shaft  64 . The power source for this starter motor  62  is not shown, but it could be traditional batteries, DC power derived from the ride-thru module  20  using a transformer and rectifier, or a combination of both. Simultaneously, a signal is sent using output O 5  to governor  66  to open the engine&#39;s throttle. The start signal O 6  is shut off when sufficient engine rpm is sensed at input I 7  from speed sensor  56 . The governor  66  is given a setpoint from master controller  70  output O 5  which represents a synchronous speed corresponding to the desired protected load  12  voltage frequency, and the governor  66  then brings the engine up to and maintains this speed. Controller output O 4  then sends a signal to voltage regulator  90  to adjust the current in the rotor field  54  of engine generator  52  to provide a preset voltage measured by potential transformer  78 . When the engine  58  is up to synchronous speed, the master controller  70  then initiates a synchronizing program sequence, which does the following. Synchronizing relay  86  compares the voltage signals from potential transformers  74  and  78 , and provides a signal to input I 6  of the master controller  70  which tells it both the magnitude of the voltage difference, as well as the phase angle difference, between the synchronous motor-generator  24  and the synchronous generator  52  voltage waves. Using this information, the master controller  70  adjusts the speed (and therefore the frequency and relative phase angle) of the synchronous motor-generator  24  by adjusting the speed ratio of continuously-variable transmission  34  through output O 1 . Simultaneously, master controller  70  also adjusts the voltage magnitude of the engine synchronous generator set  52  to match the ride-thru module&#39;s synchronous motor-generator  24  voltage by issuing control commands to voltage regulator  90  through output O 4 . When the voltage magnitude, frequency and relative phase angle of the synchronous motor-generators  24  and  52  are within preset limits, master controller  70  issues a close command to circuit breaker  51  through input/output B 3 . After circuit breaker  51  is closed, the master controller  70  reverts to the Generator Mode  400  control program, which then coordinates the entire system as described below. 
     The column Generator Mode  400  in FIG. 2 defines the parameters of the system while the protected load  12  is being supplied power from both the engine generator-set  50  and the ride-thru module  20 . In this mode, the circuit breaker  16  is open, and both circuit breakers  22  and  51  are closed. This puts the synchronous motor-generators  24  and  52  in parallel, with both now able to supply power to the protected load  12 . However, the ride-thru module  20  is only able to supply power for a short duration until the temporary energy stored in its flywheel  38  is exhausted. On the other hand, the ride-thru module  20  is able to react to changes in load levels much more quickly than the engine generator-set  50 . To take advantage of these characteristics, the master controller  70  is designed to control system frequency through the ride-thru module  20 , and long term power through the engine generator-set  50 . This is accomplished as described below. 
     In Generator Mode  400 , the master controller  70  monitors system frequency through input I 1  from speed sensor  28  on synchronous motor-generator  24 , and compares this value with a preset value stored in memory. This resulting error signal is then used to drive output O 1  in order to control the speed ratio of continuously-variable transmission  34 . The speed of flywheel  38  is monitored using speed sensor  40  through input I 2 , and master controller  70  compares this value to the preset energy storage value stored in its memory. The resulting error signal is then used to drive output O 5  to adjust the setpoint of governor  66  to increase or decrease throttle position of engine  58 . When the flywheel falls below its setpoint speed, this error signal causes the governor  66  to increase the throttle of engine  58  to supply torque to the system, and allows energy to be put back into the flywheel  38  of the ride-thru module  20 . Thus, RTM  20  can either act as an energy source or act as an energy sink, dependent on whether the flywheel has sufficient stored energy. 
     Conversely, when the flywheel  38  speed exceeds the preset value, the resulting error signal will cause the output O 5  to decrease the throttle position of the governor  66 , thereby reducing energy input to the flywheel  38 . This automatic control of continuously-variable transmission  34  and governor  66  will continue as long as the system is required to provide standby power. 
     The system will continue to operate in Generator Mode  400  until the utility source  14  returns to a stable condition and the engine  58  has run long enough to reach a stable operating condition (typically 20 minutes for a diesel engine). The master controller  70  will first set a timer for a preset delay to allow the engine to reach the desired operating condition, after which it will look to input I 3  from disturbance analyzer  92  to see if the utility source  14  has returned to normal. When a signal is then received from disturbance analyzer  92 , the master controller  70  will initiate a synchronizing sequence as follows. Synchronizing relay  82  compares the voltage signals from potential transformers  72  and  74 , and provides a signal to input I 4  of the master controller  70  which tells it both the magnitude of the voltage difference, as well as the phase angle difference, between the utility source  14  and the synchronous motor-generator  24  voltage waves. 
     Using this information, the master controller  70  adjusts the speed (and therefore the frequency and relative phase angle) of the synchronous motor-generator  24  by adjusting the speed ratio of continuously-variable transmission  34  through output O 1 . Simultaneously, master controller  70  also adjusts the voltage magnitude of the synchronous motor-generators  24  and  52  to match the utility source  14  voltage by issuing control commands to voltage regulators  88  and  90 , respectively, through outputs O 3  and O 4 , and adjusts output O 5  to governor  66  to increase or decrease torque from the engine  58  to maintain the temporary energy storage level in flywheel  38 . 
     When the voltage magnitude, frequency and relative phase angle of the synchronous motor-generator  24  and utility source  14  are within preset limits, master controller  70  issues a close command to circuit breaker  16  through input/output B 1 . As soon as circuit breaker  16  is closed, the master controller  70  issues an open command to circuit breaker  51 , and reverts back to the Normal Mode  200  control program which then adjusts the output O 1  to take energy from the utility source  14  to maintain the flywheel  38  at its preset energy storage speed. Simultaneously, a command is given to the engine governor  66  through output O 5  to reduce throttle position to idle. After a preset cool-down time, another signal is sent to the governor to shutdown the engine  58 . The system is now back in the Normal Mode  200 . 
     The ride-thru module  20  can also be taken in and out of service. FIG. 2 shows the conditions for this Out of Service Mode  500 . Thus, in the Out of Service mode the circuit breaker  16  is closed and circuit breakers  22  and  51  are open, and the generator/engine  52 , SMG  24  and flywheel  38  are at an idle speed of zero. For example, from the Normal Mode  200  of operation, a command can be given to the controller through its man-machine interface (not shown) to open circuit breaker  22  through input/output B 2 , and switch off voltage regulator  88  through output O 3 . This will cause the ride-thru module  20  to lose power from the utility source  14 , and it will simply coast down to the Out-of-Service Mode  500 . To restart the system, a start command can be given to the master controller  70 , which will cause the following sequence. 
     First, power is delivered to pony motor  30  through output O 2  to accelerate the synchronous motor-generator  24  to normal operating speed. Master controller  70  then adjusts the speed ratio of continuously-variable transmission  34  through output O 1  to bring flywheel  38  up to a preset start-up operating speed while pony motor  30  maintains the speed of the synchronous-motor generator  24 . Note that this preset start-up speed does not need to equal the full temporary storage value, so the pony motor  30  does not have to be sized to provide the full parasitic losses of the ride-thru module  20  in Normal Mode  200 . Once the flywheel  38  is at its preset start-up speed, the controller initiates a synchronizing sequence, which does the following. Synchronizing relay  84  compares the voltage signals from potential transformers  74  and  76 , and provides a signal to input I 5  of the master controller  70  which tells it both the magnitude of the voltage difference, as well as the phase angle difference, between the utility source  14  and the voltage waves of synchronous motor-generator  24 . 
     Using this information, the master controller  70  adjusts the speed (and therefore the frequency and relative phase angle) of the synchronous motor-generator  24  by adjusting the speed ratio of continuously-variable transmission  34  through output O 1 . Simultaneously, master controller  70  also adjusts the voltage magnitude of the synchronous motor-generator  24  to match the utility source  14  voltage by issuing control commands to voltage regulator  88  through output O 3 . When the voltage magnitude, frequency and relative phase angle of the synchronous motor-generator  24  and utility source  14  are within preset limits, master controller  70  issues a close command to circuit breaker  22  through input/output B 2 . After circuit breaker  22  is closed, the master controller  70  reverts to the Normal Mode  200  control program which then shuts off power to the pony motor  30  and adjusts the output O 1  to take energy from the utility source  14  to accelerate the flywheel  38  up to its full preset energy storage speed. 
     Many variations to the preferred embodiment will be evident to those skilled in the art. For example, the functions of the synchronizing relays and voltage regulators can be integrated into the master controller  70 , as could the engine governor function. While circuit breakers are shown to provide connection to certain circuit elements, contactors or static switches could also be used. A bypass switch could be added to allow testing and maintenance of the entire system while still providing utility power to the load. 
     In addition, the foregoing preferred embodiment is particularly useful as a retrofit kit of parts comprising the components of the invention discussed herein, for retrofitting exiting facilities having backup power using getsets, because less disruption of the existing backup power facilities is required. Further, clearly separating the functions associated with “ride-hrough” (via RTM  20 ), from the functions associated with long-term standby power generation (via generator-set  50 ), results in long term savings as discussed above, and can further facilitate retrofitting. 
     Thus, the foregoing description of the invention is for illustration purposes, and changes in the details of the system may be made without departing from the true spirit of the invention. Therefore, the invention should only be limited by the following claims.