Abstract:
When a power system experiences a disruptive event, conditions may exist that threaten the survival of power devices used in the system. Embodiments described herein provide an improved means of protecting these devices under such circumstances.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power system protection systems. 
     2. Discussion of the Background 
     In systems that use a power system to provide power to a load, it is often desirable to have a protection system in place for protecting the various components of the power system (e.g., the transistors and other components of the power system). For example, a shorted output or mismatched load can cause damage to the power system components. Many power systems are designed with power handling components whose characteristics have been selected to insure reliable operation under normal operating conditions, and to insure survivability under some extremes of temporary conditions. But there are trade-offs made between cost and the margin on such characteristics. In some types of power systems, the available margin is limited, in the sense that more capable devices are not available. 
     Often, protection systems can sense temporary unusual operating conditions and take protective action. Common solutions include non-resettable fuses, circuit breakers and self resetting components. These solutions may reduce fire hazard, but are often too slow to protect high speed power components from damage. Another common solution is to enter a non-operational fault mode by electronic shutdown and await manual reset. Some systems may have an automatic reset, one or more times. 
     SUMMARY OF THE INVENTION 
     The present invention provides a protection system configured to sense a condition that may threaten reliable operation of a power system (i.e., a “threat condition”) and then take corrective action (e.g., disable the power system output) upon sensing such a threat condition. The protection system may employ an adaptable algorithm to adapt a power system reset condition to assure the briefest possible interruption to service consistent with reliable operation. For example, based on recent operational characteristics of the power system, the protection system may vary the time between reset attempts and/or vary the duration that it will disable the power system output. 
     A complex model of thermal or other overload characteristics of power components may be employed to aid the protection system in selecting appropriate reset conditions. The protection system may also maintain a reset history log, locally or remotely, to aid in the selection of delay before the next reset. In systems that allow a non-operating protective mode, the protection system may also use this available information to determine when an automated reset is no longer appropriate, and to enter into a persistent protective mode. The protection system may also maintain operating logs to be used to establish if the power system has an exceptional history that may need attention. Further, details of operational policies may be maintained as a configurable set of rules and kept locally or maintained at a remote site. 
     In some embodiments, the protection system may include the following temperature sensors: a sensor to measure ambient temperature, a sensor to measure switching device temperature and a sensor to measure the temperature of water (if any) that is used to cool components of the power system. Additionally, the protection system may monitor the operating frequency of the power system, the magnitude of overload experienced by switching devices within the power system, AC mains voltage, and other information collected about the power system. All or some of these characteristics may be used to enhance reliability of a power system (e.g., a power system used in an induction heating system). For example, the protection system could provide a short disable interval of the power system on sensing a first threat condition, and then provide a longer or shorter disable interval on sensing a subsequent threat condition depending on the characteristics mentioned above (i.e., temperature, operating frequency, etc.). In systems that allow a non-operating protective mode, such a mode could be a condition of last resort and require manual intervention after the cause has been investigated. 
     In the case of an RF heating system (e.g., and RF induction heating system or an RF dielectric heating system), which typically includes a power system coupled to a radio frequency (RF) field generator (e.g., a coil or electrodes for producing an RF field) that is used to heat a work piece, the threat conditions being monitored by the protection system may include inadvertent shorting of the RF field generator. 
     An RF heating system, according to one particular embodiment of the invention, includes: a radio frequency (RF) field generator; a power system coupled to the RF field generator and configured to provide power to the RF field generator; and a protection system coupled to the power system, the protection system being configured to: (a) monitor the power system for the presence of a threat condition; (b) automatically reduce the amount of power delivered to the RF field generator by the power system for a determined amount of time in response to detecting a threat condition; and (c) automatically increase the amount of power delivered to the RF field generator by the power system after the determined amount of time has elapsed, wherein the determined amount of time is based, at least in part, on one or more of the following: (a) a sensed temperature, (b) the number of threat conditions that have occurred (i) within the last X seconds, wherein X is greater than zero, and/or (ii) since the occurrence of a certain event, (c) the specific threat condition that was detected, (d) an operating frequency of the power system, (e) a magnitude of overload experienced by switching devices within the power system, and (f) a set of rules. 
     A method, according to one particular embodiment of the present invention, includes: monitoring a power system for the presence of a threat condition; reducing the output of the power system (e.g., shutting down the power system or otherwise reducing the output of the power system) if a threat condition is detected; selecting a disable interval for the power system; and after waiting the determined disable interval, increasing the output of the power system (e.g., increasing the output to the output level that existed immediately prior to the detection of the threat condition), wherein the selection of the disable interval is based, at least in part, on one or more of the following: (a) a sensed temperature, (b) the number of threat conditions that have occurred (i) within the last X seconds, wherein X is greater than zero, and/or (ii) since the occurrence of a certain event, (c) the specific threat condition that was detected, (d) an operating frequency of the power system, and (e) the magnitude of overload experienced by switching devices within the power system. 
     A method, according to another particular embodiment of the present invention, includes: (a) using a power system comprising switching devices to provide power to an RF field generator; (b) while the power system is providing power to the RF field generator, automatically detecting a condition that may be harmful to the switching devices; (c) in response to detecting the condition, automatically causing the power system to reduce the amount of power provided to the RF field generator; (d) after causing the power system to reduce the amount of power provided to the RF field generator, waiting for a determined amount of time to elapse; and (e) immediately after the determined amount of time has elapsed, causing the power system to increase the amount of power provided to the RF field generator. 
     The above and other features of embodiments of the present invention are described below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, help illustrate various embodiments of the present invention. In the drawings, like reference numbers indicate identical or functionally similar elements. 
         FIG. 1  illustrates a system according to an embodiment of the invention. 
         FIG. 2  illustrates a process according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an induction heating system  100  according to an embodiment of the invention. System  100  includes a power system  102 , a work coil  104 , which is coupled to the power system and configured to produce an RF field for heating a work piece  106  when power is supplied to the coil, and a protection system  108  for protecting the various components of power system  102 , including the switching devices (e.g., transistors)  190  of the power system  102 . 
     As illustrated in  FIG. 1 , protection system  108  may include a data processing unit  121  (e.g., one or more microprocessors), a storage unit  122  for storing software  123  that is configured to be executed by the data processing unit  121 , thereby causing the data processing unit to perform the operations specified by the software, and a plurality of sensors  131 - 133 . In some embodiments, sensor  131  is configured to sense ambient temperature, sensor  132  is configured to sense the temperature switching devices  190 , sensor  133  is configured to sense the temperature of the water (if any) that is used to cool components of the power system. Additionally, the protection system  108  is in communication with power system  102  such that protection system  102  may monitor the operating frequency of the power system and may determine whether a threat condition is present and the magnitude of the threat condition. 
     Referring to  FIG. 2 ,  FIG. 2  is a flow chart illustrating a process, according to one embodiment, that is defined by software  123 . Process  200  may begin in step  201 , where protection system monitors power system for the presence of a threat condition (e.g., a shorted output or mismatched load). If protection system  108  senses a threat condition, then process  200  may proceed to step  202 , where protection system  108  reduces the output of power system  102  (e.g., causes power system  102  to cease providing power to work coil  104 ). Next (step  203 ) protection system may log the threat condition to an operating log  192  (e.g., a reset history log). As an example, protection system  108  may record an identifier representing the sensed threat condition and the time the condition was sensed (the time could be a relative time (e.g., 5 minutes after the beginning of operation) or an absolute time (e.g., 1:35 pm)). 
     Next (step  204 ), protection system  108  determines whether it should restart power system  102  or enter a non-operating protective mode. If the latter, then process  200  may end, otherwise process  200  may proceed to step  206 . 
     In step  206 , protection system  108  determines a length of time that it should wait before attempting to restart power system  102  (i.e., a “disable interval”). In step  208 , after waiting the determined disable interval (e.g., a 0.1 second interval, a 0.5 second interval, a 1 second interval, a 2 second interval, etc), protection system  108  restarts power system  102  (e.g., causes power system  102  to resume providing power to work coil  104  or other RF field generator). After step  208 , process  200  may return to step  202 . 
     Referring to step  204 , in determining whether to restart power system or enter the non-operating protective mode, protection system  108  may consider one or more of the following factors: (1) the temperature sensed by one or more of sensors  131 - 133 , (2) the number of threat conditions that have occurred with the last X amount of time (e.g., the last 5 minutes) (X can be configurable) (this information can be determined from the reset history log) or since the occurrence of a certain event, (3) the specific threat condition that was sensed, (4) the operating frequency of the power system, (5) the magnitude of overload experienced by switching devices within the power system, etc. 
     Similarly, referring to step  206 , in determining the disable interval, protection system  108  may consider one or more of the same factors listed immediately above. 
     As an example, in step  206 , protection system  108  may determine the length of the waiting period based, at least in part, on a determination of the number of threat conditions that have occurred within the last X amount of time (X can be some predetermined period) or the number of threat conditions that have occurred since some predetermined event (e.g., the number of threat conditions that have occurred since initialization of power system  102 ). As a more specific example, upon detecting the first threat condition since initialization of power system  102 , protection system may select a disable interval of 0.1 seconds, and upon detecting the second threat condition since initialization of power system  102 , protection system may select a disable interval of 0.3 seconds. The disable interval may continue to increase for each subsequently detected threat condition. After detecting some number of threat conditions since initialization, protection system  108  may determine to enter the non-operating protective mode. As another specific example, in some embodiments, the disable interval is initially selected to be 0.1 seconds and is not increased unless 3 or more threat conditions occur within a period of 30 seconds. 
     While various embodiments/variations of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     Additionally, while the process described above and illustrated in the drawings is shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed simultaneously.