Method for reducing UPS component stresses during transition from inverter to green/bypass operation

UPS systems, methods, and computer-readable mediums utilizing electromechanical bypass relays to switch from an on-line mode of operation to a green/bypass mode of operation include control logic to adaptively adjust the timing of when an inverter of a UPS turns off to prevent backfeeding a utility. After the UPS is instructed to transition from the on-line mode to the green mode, a monitoring period begins. During the monitoring period, a parameter related to the output current of the inverter is monitored and compared to a predetermined threshold. If the parameter exceeds the predetermined threshold before a fixed period time has elapsed, the inverter is turned off early. If the inverter current does not exceed the predetermined value within the fixed period of time, the inverter is turned off.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates generally to systems and methods for controlling an Uninterruptible Power Supply (UPS).

2. Description of Related Art

The use of power devices, such as uninterruptible power supplies (UPS), to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems and other data processing systems, is known. Known uninterruptible power supplies include on-line UPS's, off-line UPS's, line interactive UPS's as well as others. On-line UPS's provide conditioned AC power as well as back-up AC power upon interruption of a primary source of AC power. Off-line UPS's typically do not provide conditioning of input AC power, but do provide back-up AC power upon interruption of the primary AC power source. Line interactive UPS's are similar to off-line UPS's in that they switch to battery power when a blackout occurs but also typically include a multi-tap transformer for regulating the output voltage provided by the UPS.

SUMMARY

According to one embodiment, an uninterruptible power supply (UPS) comprises an input configured to receive input power, a backup input configured to receive backup power from a backup power source, an output configured to provide output power to a load from at least one of the input power or the backup power, an inverter coupled to the input, the backup input, and the output, and configured to provide inverter-output current, a sensor configured to detect a parameter indicative of the inverter-output current, a relay coupled between the input and the output, and at least one controller coupled to the sensor and configured to determine that the relay has closed, and turn off the inverter based on the determination that the relay has closed.

In one example, the at least one controller is configured to determine that the relay has closed based on the parameter indicative of the inverter-output current.

In another example, the at least one controller is configured to determine that the relay has closed based on a derivative value of the inverter-output current.

In one example, the at least one controller is coupled to the relay and configured to detect that the inverter-output current exceeds a threshold and determine that the relay has closed based on the detection that the inverter-output current exceeds the threshold.

In another example, the at least one controller is configured to determine that the relay has closed based on a lapse of a predetermined period of time after instructing the relay to close.

In one example, the UPS includes a power factor correction (PFC) circuit, and the sensor is configured to detect a current at an input of the PFC circuit as the parameter indicative of the inverter-output current.

In another example the uninterruptible power supply further comprises a second input configured to receive second input power and provide the second input power to the relay.

In one example, the sensor is configured to detect the inverter-output current at an output of the inverter.

In another example, the sensor is configured to detect a current at an input of the inverter as the parameter indicative of the inverter-output current.

According to one embodiment, a method for operating an uninterruptible power supply (UPS) comprises receiving input power at an input, receiving backup power from a backup power source, providing output power to a load from at least one of the input power or the backup power, detecting a parameter indicative of inverter-output current from an inverter of the UPS, determining that a relay has closed, and turning off the inverter included based on the determination that the relay has closed.

In one example, the method further comprises determining that the relay has closed based on the parameter indicative of the inverter-output current.

In another example, the method further comprises detecting that the parameter indicative of the inverter-output current exceeds a threshold in response to instructing the relay to close and determining that the relay has closed based on the detection that the inverter-output current exceeds the threshold.

In one example, the method further comprises determining that the relay has closed based on a lapse of a predetermined period of time.

According to one embodiment, a non-transitory computer-readable medium storing thereon sequences of computer-executable instructions for controlling an Uninterruptible Power Supply (UPS) comprising an input configured to receive input power, a backup input configured to receive backup power from a backup power source, an output configured to provide output power to a load from at least one of the input power or the backup power, an inverter coupled to the first input, the backup input, and the output, a sensor configured to detect a parameter related to inverter-output current of the inverter, a relay coupled between the input and the output and configured to provide output power, and at least one controller coupled to the sensor, the sequences of computer-executable instructions instructing the at least one controller to detect the parameter related to the inverter-output current, determine that the relay has closed, and turn off the inverter based on the determination that the relay has closed.

In one example, the sequences of computer-executable instructions instruct the at least one controller to determine that the relay has closed based on the parameter related to the inverter-output current.

In another example, the at least one controller is coupled to the relay and the sequences of computer-executable instructions instruct the at least one controller to detect that the parameter related to output current of the inverter exceeds a threshold in response to instructing the relay to close, and determining that the relay has closed based on the detection that the inverter-output current exceeds the threshold.

In one example, the UPS includes a power factor correction (PFC) circuit, and the sequences of computer-executable instructions instruct the at least one controller to detect a current at an input of the PFC circuit as the parameter related to the inverter-output current.

In another example, the sequences of computer-executable instructions instruct the at least one controller to determine that the relay has closed based on a lapse of a predetermined period of time.

In one example, the sequences of computer-executable instructions instruct the at least one controller to detect the inverter-output current at an output of the inverter.

In another example, the sequences of computer-executable instructions instruct the at least one controller to detect a current at an input of the inverter as the parameter related to the inverter-output current.

DETAILED DESCRIPTION

In existing UPS systems parallel operation of the inverter and utility power may briefly occur during a transition from an on-line mode of operation to a bypass mode of operation after a bypass relay is closed. After the bypass relay is closed a conductive path may be formed between the inverter, an inverter relay, the bypass relay, and a utility line connected to the bypass relay. During this time of parallel connection of the utility line and the inverter, current in components of the UPS may continue to increase until the inverter is turned off. This can cause stress on the components of the UPS and may result in the use of more robust and expensive components to handle the increase in current. For relays used in the transition from the on-line mode to the bypass mode, the increased current, if not properly controlled, may cause the relays to weld, permanently damaging them. Also, during this time of parallel connection, the inverter may backfeed into the utility.

At least some embodiments of the present disclosure provide methods, UPS's and non-transitory computer-readable media for adaptively adjusting the timing of turning off the output of an inverter of a power supply or UPS after a bypass relay lands (closes). At least some embodiments disclosed herein improve existing UPS systems by enabling them to handle inconsistent relay timing and unit-to-unit timing variations without additional hardware circuits while reducing the time that parallel operation occurs.

One embodiment of an uninterruptible power system100in accordance with the present disclosure will now be described in reference toFIG.1, which illustrates a functional block diagram of a first UPS100. The UPS100is an on-line UPS and includes a controller12, a rectifier/power factor correction (PFC) circuit14, a DC-DC converter16, a battery18, a polarized capacitor20, a DC bus22, an inverter24, an inductor26, a current sensor28, a backfeed line relay32, a backfeed neutral relay34, a bypass relay36, an inverter relay38, input101, a neutral input103, an output104, and a neutral output105. The UPS100supplies power to a load110based on input power received at the input101and/or power from the battery18.

In some embodiments, the inductor26is one of an air core inductor, an iron core inductor, and a ferrite core inductor.

The input101is coupled to the backfeed line relay32, which is coupled to the PFC circuit14. Each output of the PFC circuit14is coupled to the inverter24. The outputs of the PFC circuit14are coupled together by the polarized capacitor20, where the output of the PFC circuit coupled to the anode of the polarized capacitor20forms the DC bus22, which is coupled to the DC-DC converter16. The DC bus22also acts as a backup input that receives backup power from the battery18through the DC-DC converter16. The battery18acts as a backup power source. The cathode of the polarized capacitor20is coupled to the PFC circuit14, the inverter24, and the DC-DC-converter16. One output of the DC-DC converter16is coupled to the anode of the battery18and another output of the DC-DC converter16is coupled to both the cathode of the battery18and ground. The inverter24has an output coupled to the inductor26and another output coupled to the neutral output105. The neutral output105is coupled to the PFC circuit14, the inverter24, the backfeed neutral relay34, and the load110. The inductor26is coupled to the current sensor28, which is coupled to the inverter relay38. The inverter relay38is coupled to both the bypass relay36and the output104, which is coupled to the load110. The load110is coupled between the output104and the neutral output105. The bypass relay36is coupled to the input101and the backfeed line relay32.

The controller12is shown inFIG.1as being coupled to the PFC circuit14, the DC-DC converter16, the inverter24, the current sensor28, the backfeed relay32, the backfeed neutral relay34, the bypass relay36, and the inverter relay38. Each solid line connected to the controller12represents a communication path that can transmit signals from the controller12or receive signals at the controller12from one or more internals components of the UPS100. Each relay32,34,36,38shown inFIG.1is configured to switch between an open position and a closed position when instructed by the controller12. In the closed position, a conductive path is formed between a first terminal and a second terminal of the given relay. For example, when the bypass relay36is open (as illustrated inFIG.1), current does not conduct between input101and the output104within the bypass relay36. Conversely, when the bypass relay36is closed, current conducts at the relay connection to the input101and the relay connection to the output104, when the load110is coupled between the output104and the neutral output105, and power is present at the input101.

The UPS100, as illustrated inFIG.1, is a single-phase UPS with a double conversion (AC to DC, DC to AC) topology. In other embodiments, the UPS100may be a multi-phase UPS, such as a three-phase UPS. The UPS100is illustrated inFIG.1as operating in the on-line mode, where the UPS100is configured to provide output power to the load110utilizing the inverter24. As illustrated inFIG.1, the backfeed line relay32is configured in a closed position to connect the input101with the PFC circuit14. The bypass relay36is configured in an open position. The backfeed neutral relay34and the inverter relay38are illustrated as being in the closed position. When the backfeed line relay32, backfeed neutral relay34, bypass relay36, and the inverter relay38are configured in these positions, the UPS100is configured to operate in the on-line mode of operation. To enter the bypass mode, a higher efficiency operational mode, the controller12activates (closes) the bypass relay36and turns off the inverter24.

In some embodiments, one or more of the backfeed line relay32, the backfeed neutral relay34, the bypass relay36, and the inverter relay38is an electromechanical relay (EMR). Electro-mechanical relays are devices that convert a magnetic flux into a mechanical force which operates the electrical contacts within the relay, often using a spring. Solid state relays (SSR), on the other hand, lack moving parts and achieve their functionality with semiconductors. Due to the mechanical nature of EMRs, the time for the internal switch to leave one contact and land on another can vary depending on the age of the EMR, the type of the EMR, spring strength, contact wear, coil damage, temperature, and other factors. Accordingly, when controlling an EMR to close, it is not entirely predictable how long it will take for the internal switch of the EMR to land from one position to the other. Therefore, if the controller12only used a fixed time to interrupt the output of the inverter24after commanding the bypass relay36to close, the inverter24can provide power to the input101through the bypass relay36.

FIG.2illustrates a functional block diagram of the UPS100transitioning to the bypass mode of operation.FIG.2differs fromFIG.1in that the bypass relay36is closed and a current120is present between the inverter relay38and the bypass relay36. Under normal operation of the UPS100, during transition from the on-line mode of operation to the bypass mode of operation, the controller12instructs the bypass relay36to close and then disables (turns off) the inverter24so that input AC power is provided directly to the output line104via the bypass relay36. After the inverter24is instructed to turn off, the inverter relay38is instructed to open. In some embodiments, after the controller12instructs the inverter24to turn off, the controller12instructs the backfeed line relay32and the backfeed neutral relay34to open to prevent backfeeding power to the utility grid.

In an ideal scenario, the optimal transition from the on-line mode to the bypass mode would have the inverter24stop at the same time that the bypass relay36lands (closes). However, if the controller12stops the inverter24too early (before bypass relay36lands), the load110might be dropped and/or the load input capacitors could be drained, resulting in a large inrush current as the bypass relay36lands. If the controller12stops the inverter24too late (for a significant time after the bypass relay36lands), the parallel connection of utility power and inverter power may result in the inverter24sourcing or sinking large currents resulting in possible stressing or damage to the internal components of the UPS100. In such a scenario, the inverter24may be attempting to supply power to the connected utility (backfeeding).FIG.3andFIG.4illustrate a second UPS200that is substantially the same as the first UPS100, except that the second UPS200includes a second input102. Common elements in the UPS100and the UPS200are labelled with the same reference numbers. The second input102is coupled to the bypass relay36. As illustrated inFIG.3andFIG.4, the second input102may receive input power and is separate from the input101. In some embodiments, the input101and the second input102are designed to receive power from different power sources to provide additional redundancy. In an example, one of the input101and the second input102receives power from a utility grid and the other receives power from an alternative energy source. In some embodiments the alternative energy source is one or more of solar power, wind power, and hydroelectric power.

Certain embodiments include an optional capacitor connected between neutral output105and the conductive line connecting sensor28and inverter relay38in each ofFIG.1,FIG.2,FIG.3, andFIG.4.

As illustrated inFIGS.1and2, the current sensor28is coupled between the inductor26and the inverter relay38to measure an inverter-output current of the inverter24as a parameter indicative of the current being output by the inverter24. In some examples, the UPS100or the UPS200includes one or more current sensors including the current sensor28. Each of the one or more current sensors may be coupled at a different location in the UPS100or the UPS200. In an example, in addition to or as an alternative to the current sensor28coupled between the inductor26and the inverter relay38, a current sensor is coupled to the DC bus22to measure current received by an input of the inverter24as the parameter being indicative of the output current of the inverter24. In some examples, a scaling factor is applied to the received current to estimate the output current of the inverter24. Other scaling factors can similarly be applied for other locations of the one or more current sensors. In another example, the current sensor28is coupled to an input of the PFC circuit14that is connected to both the PFC circuit14and the backfeed line relay32. A scaling factor or a separate threshold value may be compared to the value of the current sensor28at the input of the PFC circuit14to determine if the bypass relay36has closed. In some embodiments, a plurality of the one or more sensors is used, the plurality including the current sensor28, and the parameter indicative of the output current is a weighted average of each current sensor measurement of current.

InFIGS.1-4, straight lines which intersect and cross over one another are not electrically connected. Solid circles overlapping straight lines indicate the lines beneath the circles are electrically connected. Within each of the backfeed line relay32, backfeed neutral relay34, bypass relay36, and the inverter relay38, there are three electrical contacts represented by circles. The straight lines connecting one of the circles and components outside a respective relay indicate a connection. As an example, inFIG.1, the input101is connected to the PFC circuit14via the backfeed relay32. Discussion of the operation of the first UPS100continues below. The second UPS200operates in substantially the same manner with the exception of having the additional input102for receiving input power herein is intended to apply in every respect to the first UPS100.

The operation of the first UPS100will now be described in greater detail with reference to a method500, which is illustrated as a logic flowchart inFIG.5. The method500includes at least three acts502,506, and510, and two conditions504and508. In some embodiments, the controller12executes each act and condition in method500. Certain embodiments implement the method500as a firmware algorithm stored as program instructions in an internal storage of the UPS100.

In the method500, the current sensor28is utilized to convert the inverter-output current into an ADC measurement value allowing the controller12to detect a rise in the ADC measurement value, indicating the landing of bypass relay36and the beginning of current120backfeeding into the utility.

In the first act502of the method500, the controller12instructs the UPS100to transition from the on-line mode to the bypass mode. To transition to the bypass mode, the controller12instructs the bypass relay36to close. After the bypass relay36is instructed to close, the controller12begins monitoring current detected by the current sensor28. In some embodiments, the controller12waits for a predetermined period of time before monitoring current with the current sensor28. In one example, the predetermined time is in a range of about 0.1 ms to about 5 ms. In another example, the predetermined time is in a range of about 0.01 ms to about 10 ms. In one example, the predetermined time is in a range of about 0.001 ms to about 50 ms.

The controller12then compares the most recent current value detected by the current sensor28to a predetermined value of current as the first condition504. In some embodiments, the predetermined value is about 125% of the nominal current output by the inverter24. In certain embodiments, the predetermined value is a current threshold adjusted for the particular load110. In one example, the load is about 5.5 kW consumed by a server rack, a maximum capacity of the inverter24is about 30 A, and the current threshold is set to about 80% of the maximum capacity, which is about 24 A. In another example, the current threshold is within a range. According to certain aspects, the range is about 3 to about 5 times an RMS value. For example, if the RMS value is about 30 A, then a range around 100 A is reasonable. In some examples, the range is about 30 A to about 100 A. In other examples, the range is about 24 A to about 30 A. In some examples, the range is about 10 A to about 50 A. If the current value detected by the sensor28exceeds the predetermined value (YES), then the first condition504is satisfied and the method500proceed to the second act506. If the current value detected by the sensor28does not exceed the predetermined value (NO), then method500proceeds to evaluate the second condition508. In some embodiments, the first condition504compares a current increase per unit of time detected by the sensor28to a threshold that represents a predetermined derivative value of current.

In the embodiments shown inFIGS.1-4, the current sensor28is electrically coupled to an output of the inverter24. The inverter output is coupled to the inductor26, which is coupled to the current sensor28that is coupled between the inductor26and the inverter relay38, to measure the output current of the inverter224. In other embodiments, the sensor28, or an additional sensor may be coupled to an input of the inverter24. For example, in some embodiments the current sensor28is coupled to the DC bus22. Other locations for current (or voltage) sensors within the UPS100are included within embodiments disclosed herein. While the specific value indicating the presence of the current120monitored by the sensor28may change depending on the location of the sensor28within the UPS100, the acts and conditions of the method500are the same for each location. More specifically, in different embodiments, any sensor that determines parameters related to the output current from the inverter to detect an increase in the output current after switching the UPS100to the bypass mode may be used in addition to or in place of the current sensor28.

The second condition508compares the time elapsed since the controller12instructed the UPS100to close the bypass relay36in the first act502to a predetermined threshold of time. The threshold indicates a maximum fixed period of time that can elapse since the bypass relay36was instructed to close. According to certain aspects, the predetermined threshold is about 1 ms to about 2 ms. In some embodiments, the predetermined threshold is 10 ms. In certain embodiments, the threshold is 13 ms. In some embodiments, the threshold is a value between 10 ms and 13 ms. If the current time (as of the second condition508being currently evaluated) exceeds the predetermined threshold, then the second condition508is satisfied (YES) and method500proceeds to the third act510. If the current time does not exceed the predetermined threshold, then the second condition508is not satisfied (NO) and method500returns to evaluating the first condition504. Certain embodiments use a fixed relay time as the predetermined threshold based on an average relay time (for the type of relay used as the bypass relay36) to close plus a margin to determine the time to interrupt the inverter24after controller12commands the bypass relay36to close.

In some embodiments, when the second condition is not satisfied (NO), the method500returns to evaluating the second condition508. In an example, the method500includes evaluating the first condition504one or more times before proceeding to evaluating the second condition508in the event the first condition504is not satisfied. Some examples include a predetermined waiting period in the first condition504before comparing the parameter indicative of the inverter24current to a predetermined threshold, and then proceeding to evaluating the second condition508.

The time to interrupt the inverter24, after commanding the bypass relay36to close, should not be less than the relay flying time or a large inrush current from utility to the load110could occur possibly resulting in a damaged relay and/or damage to other UPS components.

In some embodiments, the method500results in one of two possible outcomes. In one outcome, the first condition504is satisfied and the controller12instructs the inverter24to turn off early (before the predetermined threshold of time has been exceeded). In the other outcome, the second condition508is satisfied and the controller12instructs the inverter24to turn off as a result of the predetermined period of time elapsing. Certain embodiments include additional conditions or acts. In an example, a third condition505(not shown) may be evaluated between the first and second conditions504and508that evaluates whether the inverter current is a different value than the value checked at the first condition504. In such an example, the first condition504evaluates the current for a first current value, and if that current value is not exceeded, then a second, higher current value is evaluated in the third condition505. If the current in the third condition505is exceeded, then method500proceeds to the second act506and if not, proceeds to the second condition508.

FIG.6shows a timing sequence600of the first UPS100and the second UPS200for the transition between the on-line mode and the bypass mode when the second condition508in the method500is satisfied. In the timing sequence600at a first point in time602, the controller12provides a command to the bypass relay36, instructing the bypass relay36to close. After a fixed period of 13 ms, the controller12commands the inverter24to turn off at a second point in time604. The inverter relay38is then commanded to open at a following zero crossing point606of the output voltage waveform. The illustrated first parallel duration610shows the duration of time beginning when the bypass relay36has landed and ending when the controller12instructs the inverter relay38to open. The amount of time that the first parallel operation610occurs is dependent on the actual closure time of bypass relay36, which can vary between different relay samples. During this parallel duration610, the inverter24and the utility are both connected to the load110, which may result in a current120feeding back to the utility. The resulting current120is related to the connected load110and the difference between the output voltage of the inverter24and utility voltage at the input101. The current120could result in the bypass relay36becoming damaged and/or stressing components of the inverter24or any other component of the UPS100. The scenario depicted inFIG.6for the second condition is similar to the operation of typical UPS's. In embodiments disclosed herein, the operation of UPS's for detecting the first condition, either in conjunction with the second condition or alone, provide improvements in the transition time from online mode to bypass mode.

FIG.7shows the timing sequence700of the first UPS100and the second UPS200for the transition between the on-line mode of operation and the bypass mode of operation when the first condition504of the method500is satisfied. In the timing sequence, as inFIG.6, at the first point in time602, the controller12provides a command to the bypass relay36, instructing the bypass relay36to close. After the controller12instructs the bypass relay36to close, the method500begins. Once the bypass relay36has landed, current in the sensor28begins to rise. The second parallel duration710and the detection period712both begin at the point when the bypass relay36has landed. The detection period712indicates the length of time for the controller12to detect that a value from the sensor28has exceeded the predetermined threshold in the first condition504of the method500. At the end of the detection period712, the first condition504of the method500is satisfied and the controller12instructs the inverter24to turn off in the second act506of the method500at a point in time704that is earlier than the fixed threshold time evaluated in the second condition508. Once the controller12instructs the inverter24to turn off, the controller12then instructs the inverter relay38to open at the next estimated zero crossing point706. As a result of the first condition504being satisfied during the detection period712, the second parallel duration710is shorter than the first parallel duration610.

While one or more embodiments described above pertain to UPS systems, it is to be understood that these and other embodiments may comprise general power supplies instead or in addition to UPS systems. Other embodiments include using techniques described herein in other power systems. Certain embodiments include using techniques described herein in other types of UPS's including, but not limited to, standby UPS's, line interactive UPS's, standby on-line hybrid UPS's, standby-ferro UPS's, delta conversion on-line UPS's, and offline UPS's. Other embodiments include using techniques described herein with devices other than relays.

Having thus described several aspects of at least one embodiment disclosed herein, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the present disclosure. Accordingly, the foregoing description and drawings are by way of example only.