Abstract:
A method and system to automatically restart a permanent magnet turbogenerator/motor when a fatal fault is detected. The automatic restart logic includes time constraints and limitations on the number of iterations, and is useful regardless of whether the permanent magnet turbogenerator/motor is in a grid connect mode or a standalone mode, and regardless of how the fault originates or is detected. Additional control logic is utilized to handle grid transients and over load conditions to prevent a fatal fault from occurring by using time constraints and an iterative process, together with a brake resistor to control DC bus voltage.

Description:
This is a division of application Ser. No. 09/444,487, filed Nov. 19, 1999, now U.S. Pat. No. 6,281,596. 
    
    
     TECHNICAL FILED 
     This invention relates to the general field of turbogenerator controls, and more particularly to an improved method and system for automatically restarting the turbogenerator under certain fault conditions. 
     BACKGROUND OF THE INVENTION 
     A turbogenerator with a shaft mounted permanent magnet motor/generator can be utilized to provide electrical power for a wide range of utility, commercial and industrial applications. While an individual permanent turbogenerator may only generate 20 to 100 kilowatts, powerplants of up to 500 kilowatts or greater are possible by linking numerous permanent magnet turbogenerators together. Peak load shaving power, grid parallel power, standby power, and remote location (standalone) power are just some of the potential applications for which these lightweight, low noise, low cost, environmentally friendly, and thermally efficient units can be useful. 
     The conventional power control system for a turbogenerator produces constant frequency, three phase electrical power that closely approximates the electrical power produced by utility grids. Key aspects of such a power generation system are availability and reliability. 
     In grid-connect power generation, lack of availability can result in penalties from the local utility. Since many utility users are charged variable rates depending upon the amount of power drawn during a given period of time, the lowest $/k Wh is charged when power is drawn at levels lower than some negotiated base. Power drawn above the base level will usually have greatly increased fees and sometimes a penalty associated with it. While grid-connect power generation can be used to provide less expensive power when more than the utility base level of power is required, should this grid-connect power generation fail, or otherwise be unavailable, greater costs to the user would ensue. 
     Availability and reliability are even more important in a standalone system in which the turbogenerator itself is providing the entire load for a user. If the turbogenerator is unavailable, lengthy interruptions to all aspects of a user&#39;s business can occur and result in significant financial loss to the user. For remote installations, the turbogenerator could be down for a lengthy period of time since it might take a while for a service person to provide support at the remote site, 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method and system to automatically restart a permanent magnet turbogenerator/motor when a fatal fault is detected. The automatic restart logic includes time constraints and limitations on the number of iterations. If successful, the automatic restarting of the permanent magnet turbogenerator/motor eliminates the costly need for a complete shutdown. The automatic restart is useful regardless of whether the permanent magnet turbogenerator/motor is in a grid connect mode or a standalone mode, and regardless of how the fatal fault originates or is detected. Additional control logic is utilized to handle grid transients and over load conditions to prevent a fatal fault from occurring by using time constraints and an iterative process, together with a brake resistor to control DC bus voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus described the present invention in general terms, reference will now be made to the accompanying drawings in which: 
     FIG. 1 is a perspective view, partially cut away, of a turbogenerator for use in the automatic turbogenerator restarting method and system of the present invention; 
     FIG. 2 is a detailed block diagram of a power controller for use with the turbogenerator of FIG. 1; 
     FIG. 3 is a detailed block diagram of the power controller of FIG. 2 having a dynamic brake resistor; 
     FIG. 4 is a grid transient handling flow diagram in a grid connect mode for the automatic turbogenerator restarting method and system of the present invention; 
     FIG. 5 is an over load handling flow diagram in a standalone mode for the automatic turbogenerator restarting method and system of the present invention; and 
     FIG. 6 is an auto restart flow diagram for automatically restarting the turbogenerator after a fatal fault. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A permanent magnet turbogenerator/motor  10  is illustrated in FIG. 1 as an example of a turbogenerator/motor for use in the automatic restarting method and system of the present invention. The permanent magnet turbogenerator/motor  10  generally comprises a permanent magnet generator  12 , a power head  13 , a combustor  14  and a recuperator (or heat exchanger)  15 . 
     The permanent magnet generator  12  includes a permanent magnet rotor or sleeve  16 , having a permanent magnet disposed therein, rotatably supported within a permanent magnet generator stator  18  by a pair of spaced journal bearings. Radial stator cooling fins  25  are enclosed in an outer cylindrical sleeve  27  to form an annular air flow passage which cools the stator  18  and thereby preheats the air passing through on its way to the power head  13 . 
     The power head  13  of the permanent magnet turbogenerator/motor  10  includes compressor  30 , turbine  31 , and bearing rotor  36  through which the tie rod  29  passes. The compressor  30 , having compressor impeller or wheel  32  which receives preheated air from the annular air flow passage in cylindrical sleeve  27  around the permanent magnet generator stator  18 , is driven by the turbine  31  having turbine wheel  33  which receives heated exhaust gases from the combustor  14  supplied with air from recuperator  15 . The compressor wheel  32  and turbine wheel  33  are rotatably supported by bearing shaft or rotor  36  having radially extending bearing rotor thrust disk  37 . 
     The bearing rotor  36  is rotatably supported by a single journal bearing within the center bearing housing while the bearing rotor thrust disk  37  at the compressor end of the bearing rotor  36  is rotatably supported by a bilateral thrust bearing. The bearing rotor thrust disk  37  is adjacent to the thrust face of the compressor end of the center bearing housing while a bearing thrust plate is disposed on the opposite side of the bearing rotor thrust disk  37  relative to the center housing thrust face. 
     Intake air is drawn through the permanent magnet generator  12  by the compressor  30  which increases the pressure of the air and forces it into the recuperator  15 . In the recuperator  15 , exhaust heat from the turbine  31  is used to preheat the air before it enters the combustor  14  where the preheated air is mixed with fuel and burned. The combustion gases are then expanded in the turbine  31  which drives the compressor  30  and the permanent magnet rotor  16  of the permanent magnet generator  12  which is mounted on the same shaft as the turbine wheel  33 . The expanded turbine exhaust gases are then passed through the recuperator  15  before being discharged from the turbogenerator/motor  10 . 
     The system has a steady-state turbine exhaust temperature limit, and the turbogenerator operates at this limit at most speed conditions to maximize system efficiency. This turbine exhaust temperature limit is decreased at low ambient temperatures to prevent engine surge. 
     Referring to FIG. 2, the power controller  40 , which may be digital, provides a distributed generation power networking system in which bi-directional (i.e. reconfigurable) power converters are used with a common DC bus  54  for permitting compatibility between one or more energy components. Each power converter operates essentially as a customized bi-directional switching converter configured, under the control of power controller  40 , to provide an interface for a specific energy component to DC bus  54 . Power controller  40  controls the way in which each energy component, at any moment, with sink or source power, and the manner in which DC bus  54  is regulated. In this way, various energy components can be used to supply, store and/or use power in an efficient manner. The energy components include an energy source  42  such as the turbogenerator  10 , utility/load  48 , and storage device  50  such as a battery. 
     In the case of a turbogenerator  10  as the energy source  42 , a conventional system regulates turbine speed to control the output or bus voltage. In the power controller  40 , the bi-directional controller functions independently of turbine speed to regulate the bus voltage. 
     FIG. 2 generally illustrates the system topography with the DC bus  54  at the center of a star pattern network. In general, energy source  42  provides power to DC bus via power converter  44  during normal power generation mode. Similarly, during power generation, power converter  46  converts the power on DC bus  54  to the form required by utility/load  48 . During utility start up, power converters  44  and  46  are controlled by the main processor to operate in different manners. For example, if energy is needed to start the turbogenerator  10 , this energy may come from load/utility  48  (utility start) or from energy source  50  (battery start). During a utility start up, power converter  46  is required to apply power from load/utility  48  to DC bus for conversion by power converter  44  into the power required by the turbogenerator  10  to start up. During utility start, the turbogenerator  10  is controlled in a local feedback loop to maintain the turbine revolutions per minute (RPM). Energy storage or battery  50  is disconnected from DC bus while power converter  46  regulates VDC on DC bus  54  using the load/utility  48  as an energy source/sink. 
     Similarly, in a battery start, the power applied to DC bus  54  from which turbogenerator  10  may be started, may be provided by energy storage  50 . Energy storage  50  has its own power conversion circuit in power converter  52 , which limits the surge current into the DC bus  54  capacitors, and allows enough power to flow to DC bus  54  to start turbogenerator  10 . 
     A more detailed description of the power controller can be found in U.S. patent application Ser. No. 207,817, filed Dec. 8, 1998 by Mark G. Gilbreth et al, entitled “Power Controller”, assigned to the same assignee as this application and hereby incorporated by reference. 
     FIG. 3 illustrates a power controller of FIG. 2 having a dynamic brake resistor and associated controls. The turbogenerator  10  produces three phase AC power which is fed to AC to DC converter  144 , referred to here as the engine control module. The DC voltage is supplied to DC bus  54  which is connected to DC to AC converter  146 , referred to here as the load control module, which is connected to the load  48 , such as the utility grid. 
     A brake resistor  170  is connected across the DC bus  54 . Power in the DC bus can be dissipated in brake resistor  170  by modulation of switch  172 . A voltage sensor  174  is also connected across the DC bus  54  to produce a DC bus voltage feedback signal  176  which is compared in comparator  178  with a brake resistor turn on voltage signal  180  to produce a DC bus error signal  182 . The brake resistor turn on voltage signal  180  is adjustable by CPU  62 . 
     The DC bus error signal  182  from comparator  178  is used to control the modulation of switch  172  after being conditioning through a proportional compensator  184 , a brake resistor power limit  186  based on the measured or estimated temperature of the brake resistor  170 , a pulse width modulator  188  and gate drive  190 . The switch  172  may be an IGBT switch although conventional or newly developed switches can be utilized as well. The switch  172  is controlled in accordance with the magnitude of the voltage on DC bus  54 . The generator signal processor  192 , connected to the switch  172  and to the engine control module  144 , or the inverter signal processor  192 , connected to the load control module  146 , typically maintains the DC bus voltage. If a rise in voltage on the DC bus is detected, the brake resistor  170  is modulated on and off until the bus voltage is restored to it desired level. 
     The brake resistor  170  can absorb any amount of power, from zero to greater than the full rated output of the permanent magnet turbogenerator/motor  10  for short periods of time. It is both a fast and reliable place to dissipate power and to stabilize the DC bus  54 . It not only can prevent the permanent magnet turbogenerator/motor  10  from experiencing an overspeed condition, but also protects the system electronics, such as the IGBTs, from damage. 
     As previously mentioned, the permanent magnet turbogenerator/motor  10  can be operated in a grid parallel mode in which permanent magnet turbogenerator/motor  10  is connected to a utility grid, or in a standalone mode in which the permanent magnet turbogenerator/motor  10  supplies all of the power to a load. Each of these modes of permanent magnet turbogenerator/motor  10  operation includes challenges in maintaining system reliability and availability. 
     In the grid connect mode, FIG. 4 illustrates a flow diagram for handling a grid transient. These grid transients can cause either over-currents or loss of control of the output current or DC bus voltage and these effects are monitored as a means to detect grid transients. 
     If an output over-current is detected, block  200 , the number of over-current events within the last second is determined in block  202 . If there has been too many over-current events a warning or fatal fault must be reported, as determined in block  218 . If there has not been too many over-current events, the output inverter  146  is disabled by turning off the IGBT switches, see block  204 . If, at this point, the output current level is normal in all phases, block  206 , the output inverter  146  is enabled by turning on the IGBT switches, block  208 , and normal operation is continued, block  210 . 
     If, however, the output current level is all phases, block  206 , is not normal, block  212  determines if the DC bus  54  voltage level is below the turn-on point of the brake resistor  170 . If the voltage level is below the brake resistor turn-on point, the brake resistor  170  is modulated on, block  214 , to apply control to the DC bus voltage. The loop between blocks  206 ,  212 , and optionally  214  (if the DC bus voltage level is above the turn on point of the brake resistor  170 ) continues until the output current is at a normal level on all phases. 
     If an output over-current is not detected in block  200 , block  216  serves to detect loss of output current control or DC bus voltage control. If a loss of control is not detected in block  216 , normal operation is continued in block  210 . If a loss of output current control or DC bus voltage control is detected in block  216 , a warning or fatal fault must be reported, as determined in block  218 . 
     If block  218  detects too many warning faults within the last minute (including too many over-current events within the last second from block  202 ), block  220  reports a grid fail fatal fault and shutdown is initiated. If there has not been too many warning faults in the last minute, a grid unbalance warning fault is reported in block  222  which disables the output inverter  146  by turning off the IGBT switches, see block  224 . The grid voltage magnitude and frequency is analyzed, block  226  and if acceptable for connection, block  228 , normal operation is continued, block  210 , after the output inverter  146  is enabled, block  208 . 
     If the grid is not acceptable for connection, block  228 , and the maximum allowed reconnection time has expired, block  230 , a grid fatal fault is reported and shutdown is initiated, block  220 . If the maximum allowed reconnection time has not expired, block  230 , and the DC bus voltage level is above the turn-on point of the brake resistor  170 , block  232 , the brake resistor  170  is modulated on to control DC bus output voltage, block  234 . The loop between blocks  228 ,  230 ,  232 , and optionally  234  (if the DC bus voltage level is above the turn on point of the brake resistor  170 ) continues until the grid is either acceptable for connection or the maximum allowed reconnection time has expired. 
     It should be recognized that in grid connect mode, grid transients are but one of the type of disturbances that can cause grid fail fatal faults and initiate shutdown. Examples of disturbances that can cause grid fail fatal faults and initiate shutdown are: voltage sags, voltage surges, voltage interruptions, single phase failures, phase to phase faults and phase to ground faults. 
     In the standalone mode, FIG. 5 illustrates a flow diagram for handling over load. If an output over-current is detected, block  240 , the number of over-current events within the last second is determined in block  242 . If there has not been too many over-current events, the output inverter  146  is disabled by turning off the IGBT switches, see block  244 . If, at this point, the output current level is normal in all phases, block  246 , the output inverter  146  is enabled by turning on the IGBT switches, block  252 , and normal operation in continued, block  254 . 
     If, however, the output current level in all phases, block  246 , is not normal, block  248  determines if the DC bus  54  voltage level is below the turn-on point of the brake resistor  170 . If the voltage level is below the brake resistor turn-on point, the brake resistor  170  is modulated on, block  250 , to apply control to the DC bus voltage. The loop between blocks  246 ,  248 , and optionally  250  (if the DC bus voltage level is above the turn on point of the brake resistor  170 ) continues until the output current is at a normal level on all phases. 
     If too many over-current events within the last second are detected in block  242 , block  256  detects whether too many warning faults within the last minute have occurred. If too many warning faults within the last minute have occurred, block  256 , block  258  reports a failure fatal fault and shutdown is initiated. 
     If there has not been too many warning faults in the last minute, a grid unbalance warning fault is reported in block  256  which disables the output inverter  146  by turning off the IGBT switches, see block  262 . The output voltage control ready is reset for soft-start, block  256  and normal operation is continued (block  254 ) after the output inverter  146  is enabled, block  252 . 
     It should be recognized that in standalone mode, over load is but one of the types of disturbances that can cause failure fatal faults and initiate shutdown. Other examples of disturbances that can cause failure fatal faults and initiate shutdown are phase to phase faults, phase to ground faults, and connection of an out of synchronism generator to the output. 
     Regardless, of how the fatal fault occurs and shutdown is initiated either in grid connect mode or in standalone mode, the automatic restart flow diagram of FIG. 6 comes into play. Block  270  represents the turbogenerator in the process of shutting down with a fatal fault present. If five or more unsuccessful restart attempts have been made since the turbogenerator reached the load state, block  272 , the turbogenerator will continue shutdown, block  274 . The load state is the normal operating state of the turbogenerator, where power is being delivered to the grid or in standalone mode to the load. If less than five unsuccessful restart attempts were made since the turbogenerator reached load state, block  272 , the turbogenerator will proceed with the attempted restart. 
     If the turbogenerator (standalone mode only) is in recharge state, block  276 , or if the turbogenerator is in cooldown state and below cooldown restart temperature, block  278 , or if the turbogenerator is in fault state, block  280 , the logic proceeds to block  282  to determine if more than one minute has elapsed since the previous attempt to clear the fault. If more than one minute has elapsed since the previous attempt to clear the fault, block  284  attempts to clear the fault. If the fault is successfully cleared, block  286 , a restart command is issued, block  288 , and the turbogenerator can continue normal operation, block  290 . 
     If less than a minute has elapsed since the previous attempt to clear the fault, block  282 , and the fault was not cleared successfully, block  286 , the shutdown will continue, block  274 . If the turbogenerator is not in fault state, block  280 , but rather in standby state, block  292 , the restart command can be issued, block  289  and normal operation continued, block  290 . If the turbogenerator is not in standby, block  292 , the shutdown will continue, block  274 . 
     The transient control logic of FIGS. 5 and 6 exists in the inverter signal processor  194  except for the brake resistor controls which exist in the generator signal processor  192 . The automatic restart logic of FIG. 6 exists in the main CPU  62 . 
     The above method and system greatly increase the availability of the permanent magnet turbogenerator/motor  10  during load transients and other fault conditions. By temporarily disconnecting the permanent magnet turbogenerator/motor  10  from the grid during large grid transients of short duration with power absorbed in the brake resistor  170 , the grid can be quickly reengaged without a lengthy shutdown. 
     Unrecoverable grid/load faults will, however, require the shutdown process to commence. The automatic restart logic will allow the system to reset appropriate fault conditions and commence a restart on its own. For safety reasons, the number of retries is limited and delay timers permit a period of time to lapse between tries. 
     While specific embodiments of the invention have been illustrated and described, it is to be understood that these are provided by way of example only and that the invention is not to be construed as being limited thereto but only by the proper scope of the following claims.