Apparatus for synchronizing uninterruptible power supplies

An Intelligent Synchronization Module (ISM) for an Uninterruptible Power Supply (UPS) system for servicing a load is disclosed. The UPS system has at least one of a first UPS group and a second separate and independent UPS group, each of the first and second UPS groups having a master UPS. The ISM has a processing circuit and a storage medium, readable by the processing circuit, storing instructions for execution by the processing circuit for: assigning the first UPS group the role of master group and the second UPS group the role of slave group; and, passing phase information relating to the master group to the slave group, thereby enabling the master UPS of the slave group to effect synchronization with the master group.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to uninterruptible power supplies (UPSs), and particularly to synchronization control systems for control thereof.

UPSs are employed in a variety of applications where a constant source of power is desired at a load. A typical UPS system involves an inverter feed path, also generally referred to as the inverter, that is operably connectable in parallel with a bypass feed path, also generally referred to as the mains. The mains may be connected to a utility, but may also receive power from some other supply not connected to a utility electrical grid. The inverter may receive power from the same source as the mains, but may also receive power from some other supply.

There are several types of UPSs depending on their operation mode. Double conversion UPSs offer the maximal protection level as the load is always fed by the inverter. On the other hand, with line-interactive UPSs, the load is fed by the mains and the inverter is used to correct the shape of the load voltage.

There are also several possible UPS configurations to supply a critical load, such as Redundant Parallel Architecture (RPA), Dual Independent Configuration, Load Bus Synchronization, and Power Tie, for example. With the RPA concept, (N+M) UPSs are paralleled to supply a load that can be fed by N UPSs only. This way, a redundancy of M units is achieved. More and more, and for high availability, Dual Independent Configurations are requested by customers. This requires the synchronization of two independent UPS groups and the use of an Intelligent Static Switch (ISS) that automatically switches the critical load from one source to the other. Another concept is Load Bus Synchronization where two independent UPS groups can be temporarily synchronized in order to move the critical load from one side to the other for maintenance purposes. An extension of the Load Bus Synchronization concept is the Power Tie concept, where the two independent UPS groups are permanently synchronized and their load shared as if they were a unique UPS group in a RPA configuration. Finally, and with consideration to the bypass configuration, it is desirable to be able to choose between two different options, a centralized bypass or decentralized bypass.

Accordingly, there is a need in the art for a control system and apparatus that allows multiple configurations of UPSs in critical power management systems.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment of the invention includes an Intelligent Synchronization Module (ISM) for an Uninterruptible Power Supply (UPS) system for servicing a load, wherein the UPS system has at least one of a first UPS group and a second separate and independent UPS group, each of the first and second UPS groups having a master UPS. The ISM has a processing circuit and a storage medium, readable by the processing circuit, storing instructions for execution by the processing circuit for: assigning the first UPS group the role of master group and the second UPS group the role of slave group; and, passing phase information relating to the master group to the slave group, thereby enabling the master UPS of the slave group to effect synchronization with the master group.

Another embodiment of the invention includes an Uninterruptible Power Supply (UPS) system for servicing a load. The UPS system includes a first UPS group and a second UPS group separate from and independent to the first UPS group, each of the first and second UPS groups being configured to service the load, and an Intelligent Synchronization Module (ISM) in signal communication between the two UPS groups. The ISM is configured to assign the first UPS group the role of master group and the second UPS group the role of slave group, and to pass phase information relating to the master group to the slave group, thereby enabling the slave group to effect synchronization with the master group.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention provides an Intelligent Synchronization Module (ISM) for allowing multiple configurations of an Uninterruptible Power Supply (UPS) system that services at least one load. In an embodiment, the UPS system has a first UPS group and a second separate and independent UPS group, with each of the first and second UPS groups having a master UPS. A function of the ISM is to assign the first UPS group the role of master group and the second UPS group the role of slave group, and to pass phase information relating to the master group to the slave group, thereby enabling the master UPS of the slave group to effect synchronization with the master group. In a more general sense, the ISM is a control system that exchanges information, performs synchronizations, and executes control algorithms and commands.

FIG. 1shows an exemplary block diagram of a typical double conversion UPS system100. The UPS system100consists of two converter blocks, a rectifier130and an inverter140, and energy storage device135such as a battery for example. During normal operation, the rectifier130converts the mains input supply112to regulated DC to charge the energy storage-battery bank135as well as supply power to the inverter140. The inverter140converts the DC to a voltage and frequency regulated AC output at all times. During a stored energy mode, that is, during a condition where the mains input supply112is not available, the inverter140draws power from the energy storage device135, thereby enabling continued supply the output or load105. Bypass operation is possible through a Static Switch Module (SSM)120.

FIG. 2expands on the schematic ofFIG. 1to show further detail of the exemplary UPS system100that services the load105. Here, the UPS100includes a bypass feed path110and an inverter feed path115that are operable in parallel with each other during the transfer of power from one path to the other. The power source112for the bypass feed path110may be a utility or other main power source, as discussed previously. The power source117for the inverter feed path115may be the same as that of the bypass feed path110(as illustrated inFIG. 1for example), or it may be a different power source (as illustrated inFIG. 2for example). The bypass feed path110is engagable with the load105via the SSM120, also herein referred to as a first switch120, to deliver a bypass current to the load105, and the inverter feed path115is engagable with the load105via a second switch125to deliver an inverter current to the load105. In an embodiment, the first switch120is a remote controllable SSM of a type known to one skilled in the art, and the second switch125is a remote controllable contactor of a type known to one skilled in the art. The inverter feed path115includes the rectifier130, the battery135, and the inverter140, and may also include an output isolation transformer145, and filtering capacitors150. Disconnect switches (K4)155, (K6)160, (Q1)165and (Q2)170may be employed for additional protection and/or control and/or maintenance. In an embodiment, switches (K4)155and (K6)160are circuit breakers, and switches (Q1)165and (Q2)170are manual disconnects. The leakage inductance of isolation transformer145and the output capacitors150are used together to filter the inverter output voltage (Uo)200.

WhileFIG. 1depicts a UPS100in one-line diagram form, it will be appreciated that UPS100may have multiple phases, such as three phases for example, and that any reference herein to a current or a voltage in one phase is intended to be a reference to the current and voltage of each phase.

In an embodiment, a control system175, illustrated generally inFIG. 1and more specifically inFIG. 2, includes a processing circuit180and a storage medium185, readable by the processing circuit180, storing instructions for execution by the processing circuit for controlling the UPS100in a manner to be described in more detail below.

In an embodiment, input signals to control system175include inverter bridge currents (Ib)190, inverter load currents (IL)195, inverter output voltages (Uo)200, load voltages (UL)205, bypass load currents (Ibyp)210, and bypass input voltages (Ubyp)215, that are generated by any sensor suitable for the intended purpose. Another input signal to control system175may be (aux)220that is provided by an auxiliary contact (not specifically shown but represented also by reference numeral220) at second switch (K7)125and identifies the on/off state of the main contacts of second switch125. Further input signals to control system175include a Ssyncsignal and a φothersignal, which will be discussed in more detail below.

While reference is made to bypass feed path110and inverter feed path115, it will be appreciated by those skilled in the art that the inverter load currents IL195are not the same as those currents flowing through contactor K4155.

In an embodiment, output signals from control system175include a command signal (S1)225to first switch120, a command signal (S2)230to second switch125, a command signal (S3)235to inverter140, and a command signal (S4)237to rectifier130.

In one embodiment, output signals225,230,235and237may originate from control system175. However, in another embodiment, the same output signals or any combination thereof may be analog, may originate from another source (not shown), and may be monitored and used by control system175.In an embodiment, the following logic is employed:S1=(1, 0): SSM command signal (ON, OFF), respectively;S2=(1, 0): K7command signal (ON, OFF), respectively;S3=(1, 0): Inverter command signal (ON, OFF), respectively; andS4=(1, 0): Rectifier command signal (ON, OFF), respectively.

In accordance with an embodiment of the invention, the output signals from control system175serve to synchronize a slave UPS group with a master UPS group, which will be discussed in more detail below.

WhileFIG. 1depicts certain switches (Q1, Q2, K4, K6, K7, SSM) open and others closed, it will be appreciated that control system175may send appropriate control signals to change the state of these switches. As such, it will be appreciated that the state of the switches may differ from the state actually depicted inFIG. 1, but will be discussed in context with reference toFIG. 1.

During inverter feed path115operation, the rectifier130converts the input power supply117to regulated DC to charge the battery135as well as supply power to the inverter140. The inverter140converts the DC to a voltage & frequency regulated AC output at all times. During “stored energy” mode, that is, during an absence of power from power supplies112and117(also referred to as mains failure), the inverter140draws power from the battery135and continues to supply output power. Bypass operation that switches in the bypass feed path110is possible through the first switch120, which may be a Static Switch Module (SSM) for example.

In an exemplary embodiment, the startup of rectifier130is accomplished automatically via switch (K4—circuit breaker)155or manually via switch (Q4—manual disconnect), manual bypass is accomplished via switch (Q2)170, output power is supplied via switch (Q1)165, short-circuit protection of SSM120is provided by switch (K6), and output power from inverter140is provided via switch (K7)125. The transitions from the bypass feed path110to the inverter feed path115and vice-versa are controlled through the SSM120and breaker switch (K7)125.

With an RPA (Redundant Parallel Architecture) concept, (N+M) UPSs are paralleled to supply a load that can be fed by N UPSs only. This way, a redundancy of M units is achieved. To realize the RPA configuration, a communication between the UPSs is required to synchronize the output of each inverter and to share the load among the inverters.

To realize a full digital control for single and parallel UPSs, embodiments of the invention employ dedicated control algorithms and the necessary hardware. In particular, a DSP (digital signal processor) based mother board and a small board for the digital communication between the units is employed. In an embodiment, this communication is made very reliable by doubling the communication channels (redundancy) and by using CRC (cyclic redundancy check) error controls.

This powerful and flexible control board scheme is used on various kinds of LTPSs and power quality systems, such as single and parallel units, with or without an output isolation transformer, with or without an input active filter, and with or without a stand alone active filter, for example. This adaptability is obtained through software configurations.

To allow multiple configurations of uninterruptible power supply in critical power management systems, embodiments of the invention employ an Intelligent Synchronization Module (ISM) that ties one or two groups of UPSs to realize one or more of the following five different functions:

F1) Intelligent Static Transfer Switch

Synchronize permanently the outputs of two separate and independent UPS groups, where an Intelligent Static Switch (ISS) decides where to switch the load;

Command the centralized bypass of a group of UPSs in a RPA configuration;

F3) Load Bus Synchronization

Synchronize temporarily the outputs of two separate and independent UPS groups;

F4) Power Tie

Synchronize permanently and load share the outputs of two separate and independent UPS groups; and

Allow an extension of the maximal distance between the first and the last UPS.

With reference now toFIG. 3, the principle of the ISM300will be discussed. In an embodiment, the ISM300is disposed between a first310and a second320group of UPSs100(separately illustrated as UPS1, UPS2, UPS3and UPS4, for example, but having the configuration illustrated inFIG. 2and discussed above), and an ISS330that decides which UPS group will service the load105. For simplicity,FIG. 3illustrates only the communication lines101between ISM300and the two groups of UPSs, however, it will be appreciated by one skilled in the art that the UPSs also have power lines102connecting them to the ISS330. Each UPS group has a master UPS, such as UPS1for group A310, and UPS3for group B320, for example. The master UPS serves to control the synchronization of the UPSs100within its group. The ISM300controls synchronization between first310and second320UPS groups by providing control commands to the appropriate control system175associated with the UPSs100.

The ISM300has the function of assigning one of the UPS groups, such as the first group310for example, the role of master group and the other UPS group, such as the second group320for example, the role of slave group. The ISM300also has the function of passing phase information relating to the master group310to the slave group320, thereby enabling the master UPS (UPS3) of the slave group320to effect synchronization with the master group310. To carry out this synchronization process, the ISM300has a processing circuit302, and a storage medium304, readable by the processing circuit302, storing instructions for execution by the processing circuit302for carrying out the necessary control algorithms, which will be discussed in more detail below.

Referring now toFIG. 4, an exemplary ISM300, illustrated in block diagram form, includes processing circuit302, storage medium304, a DSP board306, and a communication board308having increased capability able to deal with four communication channels. As illustrated, the ISM300is able to exchange information with the two independent UPS groups (group A310, and group B320), interpret the operator commands, and command the power switches. As a result, the five aforementioned functions F1-F5may be realized by software configuration only. To maximize the reliability, an embodiment of the ISM300has a redundant communication and a redundant power supply, fed by the output of both UPS groups. In addition, for very critical applications it is possible to use redundant ISM modules300to assure a full redundancy of the control electronics.

The input signals to ISM300include phase information φ1through φP, or φ1through φQ, from each UPS group A and B, where P represents the number of UPSs in group A and Q represents the number of UPSs in group B. The output signals from ISM300include the Ssyncsignal and the φothersignal. The φothersignal represents the actual phase of the master UPS group, and the Ssyncsignal represents a command signal for the slave UPS group to synchronize with the master UPS group.

With reference now toFIG. 5, the Intelligent Static Transfer Switch Function (F1) will be discussed.

FIG. 3shows the principle of the use of ISM300in combination with ISS330, andFIG. 5expands on this principle. For the configuration illustrated inFIG. 5, four UPSs100(UPS1, UPS2, UPS3and UPS4) are organized in two groups A and B in a 2+2 configuration. The load105is connected to the ISS330, which continuously monitors the two input sources A and B, which may be provided by a utility or other means as discussed previously with reference toFIG. 2, and decides where to switch the load depending on the quality of the two input sources and on its configuration. Typically, if one source fails, the ISS330can be programmed to either stay on the preferred input source or switch, even if the two input sources are in phase opposition. Since an out-of-phase condition is undesirable and dangerous for both the UPS system and the critical load, it is important to keep the outputs of the two UPS groups synchronized. The synchronization function is performed by the ISM300, which synchronizes one group (slave UPS group B for example) to the other group (master UPS group A for example). The master UPS group is also herein referred to as the sync Master. Through a front panel module301of the ISM300, a user may select various working modes, thereby making it possible to keep the two groups isolated, to force the sync master to group A or B, and also to let the ISM300decide which one of the two groups is the sync Master. The ISM300considers, among other things, the availability and the state of the two inputs sources A and B. In addition, the synchronization may be activated continuously or just under certain phase error conditions.

RegardingFIGS. 5-10generally, the lines connecting ISM300to the UPSs100represent communication and control lines, while the lines connecting the UPSs100to the input sources and the load represent power distribution lines. Also, it will be appreciated by one skilled in the art that graphical symbols suggestive of switches, while not specifically identified by a reference numeral, do indeed represent power distribution switches, such as circuit breakers or the like.

With reference now toFIG. 6, the Centralized Bypass Function (F2) will be discussed.

As previously mentioned, there are several possible UPS configurations that may be employed to supply a critical load. With the RPA (Redundant Parallel Architecture) concept, (N+M) UPSs are paralleled to supply a load that can be fed by N UPSs only. This way, a redundancy of M units is achieved. In a RPA configuration, we have decentralized bypasses, that is, each UPS100has its own bypass feed path110and Static Switch Module (SSM)120(seeFIG. 2for example). This improves the reliability of the global power system as a redundancy of M bypasses is also achieved. The situation is even more favorable as these decentralized bypasses are sized for more than the nominal power.

In some critical power management systems, a centralized bypass is required, even if the global reliability is reduced. This feature may be implemented in a RPA configuration using the ISM300as represented inFIG. 6for a group of three UPSs (UPS1, UPS2and UPS3). The ISM300exchanges information with all the UPSs and commands a Centralized Static Switch Module (CSSM)350, which serves as an external (centralized) bypass to all UPSs. In addition, the ISM300measures the bypass voltages and the load currents.

The implementation of a centralized bypass in a RPA system with the ISM300may be realized in two ways. First, and with regard to cost minimization, the internal bypass SSM120(seeFIGS. 1 and 2for example) of each UPS100is removed, which is referred to as the modular concept. Second, and with regard to increasing reliability of the RPA system, the internal bypasses SSM120are used as a backup for the external bypass provided by the CSSM350.

In an embodiment that combines features of both the first and the second implementations, which also strives for cost minimization, the internal bypasses (SSMs) of each UPS may be used in combination with an external centralized breaker that is commanded by the ISM300. Here, the centralized breaker may replace the CSSM350.

With reference now toFIG. 7, the Load Bus Synchronization Function (F3) and the Power Tie Function (F4) will be discussed.

FIG. 7illustrates the principle of using the ISM300to realize a Load Bus Synchronization Function (F3), that is, to synchronize temporarily the outputs of two separate and independent UPS groups A and B. In the embodiment ofFIG. 7, four UPSs (UPS1, UPS2, UPS3and UPS4) are organized into two groups A and B. To carry out maintenance work on one UPS group, all of the load105, depicted as Load A and Load B, has to be transferred to the other UPS group. Consider an example where initially the two systems operate independently, that is, breaker Spis open while breakers SAand SBare closed. If now UPS3needs to be repaired, Load B has to be transferred to input source A. For this, we have first to synchronize UPS group B to UPS group A, then close breaker Sp, and after a short while open the breaker SB. At this time, Load B has been transferred to input source A and the UPS group B can be switched off, disconnected from input source B via switches illustrated, for maintenance. In an embodiment, breakers Sp, SAand SB, may be remotely controlled by means known in the art.

FIG. 7also illustrates the principle of using the ISM300to realize a Power Tie Function (F4), that is, synchronizing permanently and load sharing the outputs of two separate and independent UPS groups. As previously discussed,FIG. 7illustrates four UPSs organized into two groups A and B. Assume for example that Load A is too high for UPS group A, that UPS group A is not redundant, and that UPS group B is only slightly loaded. To make the global system redundant we want to share the global load (Load A plus Load B) between the two UPS groups A and B. For this, we first have to synchronize group B (slave UPS group) to group A (master UPS group), then close breaker Sp, and finally load share the global load. By employing an embodiment of the ISM300disclosed herein, it is possible to realize the Load Bus Synchronization and the Power Tie features in the configuration illustrated by implementing the appropriate control algorithms via ISM300. In addition, a system upgrade that adds more UPSs may be accomplished in a simple and straightforward manner. With reference now toFIG. 8, the Bus Repeater Function (F5) will be discussed.

In an exemplary RPA configuration employing embodiments of the invention, eight UPSs (only six being shown inFIG. 8) may be paralleled within a maximal distance, which represents the distal limit achievable by a given data transmission system. By employing an embodiment of the invention, however, and for those applications where this maximal distance is not enough, ISM300may be employed as a bus repeater to control the desired synchronization function for those UPSs beyond the maximal distance. This is illustrated inFIG. 8by UPSs4,5and6being situated beyond the maximal distance from UPS1.

In an alternative exemplary embodiment, the ISM300may also be used to realize combined functions, such as Load Bus Synchronization and/or Power Tie in combination with Centralized Bypass, which is illustrated byFIG. 9.

In another alternative exemplary embodiment, the ISM300may also be used to realize the combined functions of Load Bus Synchronization and/or Power Tie in combination with Centralized Bypass with Redundant ISM, which is illustrated inFIG. 10. To maximize system reliability, especially when two functions are combined, it is possible to use more then one ISM module300. As an example,FIG. 10shows the use of two ISM modules300to implement the Load Bus Synchronization and/or Power Tie Function combined together with the Centralized Bypass Function. In this case, not only is the power supply of each ISM redundant, but the control electronics of each ISM are also redundant.

Algorithms for implementing the Intelligent Static Transfer Switch Function (F1) will now be discussed with reference toFIGS. 11-13.

Referring now toFIG. 11, a control algorithm400is depicted for execution by the control system175of the master UPS of the slave UPS group for implementing the aforementioned synchronization function. As previously discussed, the ISM300and the UPSs100of both groups A and B are all connected through redundant communication cables, thereby enabling the ISM300to see the phase (Pi of all the UPSs of both groups A and B. It is possible to keep the two groups isolated, to force the synchronization master UPS group to be group A or B, and finally, based on the availability and quality of the input mains, to let the ISM300decide which one of the two groups is going to be the master UPS group. The slave UPS group will receive the command to synchronize onto the master group, which is achieved by applying the SSYNCsignal, and by passing the actual phase information φotherof the master group, through to the slave group.

In and exemplary embodiment, the function of the ISM300is to assign a group the role of master UPS group, and then to pass the phase information to the slave UPS group. The flow diagram ofFIG. 11shows this synchronization process400. However, it is first noted that the synchronization algorithm is to be executed by the master UPS of the slave UPS group, and in the following discussion, reference will be made to this master UPS. After power-up, the UPS will first synchronize to its mains (process loop defined by reference numeral410). When a SSYNCsignal is triggered, via an external command or an automatic command from control system175, the master UPS will start the synchronization process to the master UPS group (process loop defined by reference numeral420). Process loop420first starts with a slow synchronization algorithm430, where the frequency and phase of the master UPS of the slave UPS group will be moved close to the reference of the master UPS group. At the end of process430and440, that is, when frequency and phase differences are smaller than the defined thresholds of Δωto1SLOWand Δφto1SLOW, respectively, the fast synchronization algorithm450will start and the UPS will then be stiff in phase with the master UPS group, resulting in the two UPS groups A and B being synchronized (illustrated by reference numeral460).

The slow synchronization algorithm430is illustrated inFIG. 12with the control variables illustrated. As depicted, algorithm430primarily consists of four modules: a fast frequency observer (FFO)431, which is basically a fast phase control; a phase & frequency error computation module432; a slow phase control433with its own control parameters Kφsand Kωs; and, an oscillator (O)434.

The slow synchronization algorithm430is used to move the phase of the output of a slave UPS group, which is already supplying the load, toward that of the master UPS group. This slow synchronization has to be slow enough to guarantee the safety of the critical load. The input to the slow synchronization algorithm430is φISM—other, which is the actual phase of the master UPS group. The outputs of the slow synchronization algorithm430are the phase and frequency parameters (α0and ω0of the oscillator434, which refer to the master UPS of the slave UPS group.

The fast synchronization algorithm450is illustrated inFIG. 13with its control variables illustrated. As depicted, algorithm450primarily consists of three modules: a phase & frequency error computation module451; a fast phase control452with its own control parameters; and, an oscillator (O)453. This fast synchronization algorithm450serves to keep the two UPS groups A and B synchronized. Similar to the slow synchronization algorithm430, the input to the fast synchronization algorithm450is φISM—other, which again is the actual phase of the master UPS group, and the outputs are the phase and frequency parameters α0and ω0of the oscillator434, which again refer to the master UPS of the slave UPS group.

In an embodiment, the slow430and fast450synchronization algorithms are implemented in firmware, having algorithms driven by control equations, which will now be discussed with reference toFIGS. 12 and 13.

The FFO431depicted inFIG. 12includes an internal oscillator having a phase angle φOSCand an angular frequency ωOSC. This internal oscillator is controlled to track the phase angle (φISM—other. Once the oscillator is synchronized to φISM—otherthe oscillator angular frequency ωOSCis a measure for the unknown angular frequency ωISM—other. This is why this block is called fast frequency observer. The equations describing the above algorithm are:
ΔφOSC=φISM—other−φOSCEqua.-1
ΔωOSC=(ΔφOSC−ΔφOSCold)/TEEqua.-2
ΔφOSCold=ΔφOSCEqua.-3
ΔωOSCcom=ΔωOSC* KωFFO+ΔφOSC* KφFFOEqua.-4
ωOSC=ωOSC+ΔωOSCcom* TEEqua.-5
φOSC=φOSC+ωOSC* TEEqua.-6
ωother=ωOSCEqua.-7

where TEis the sampling time (100 μs) and KωFFO, KωFFOare the feedback gains for the angular frequency and phase angle errors of the FFO431. Equation-2 is a simple numerical derivation of ω=dφ/dt, where the value of ΔφOSColdis the previous sampling (100 μs before) of ΔφOSC. Equations-5 and 6 implement the digital oscillator of the FFO431. In addition, the internal variables are limited to their normal variation range (for example, ΔφOSCbetween 0 and 2π) as known by one skilled in the art.

The phase & frequency error computation block432, which is used for the slow synchronization algorithm, may be described by following difference equations:
ΔφISM=φISM—other−αoEqua.-8
ΔωISM=ωother−ωoEqua.-9

Referring now toFIG. 13, the phase & frequency error computation block451, which is used for the fast synchronization algorithm, may be described by following difference equations:
ΔφISM=φISM—other−αoEqua.-10
ΔωISM=(ΔφISM−ΔφISMold)/TEEqua.-11
ΔφISMold=ΔφISMEqua.-12

The slow phase control block433(with reference toFIG. 12and Equations-8 and 9) may be described by the following equation:
Δω=ΔφISM* KφS+ΔωISM* KωSEqua.-13

The fast phase control block452(with reference toFIG. 13and Equations-10 and 11) may be described by the following equation:
Δω=ΔφISM* KφF+ΔωISM* KωFEqua.-14

In a classical digital PLL (phase lock loop) scheme, the synchronization precision of 10 bits of digital phase information would be too a low resolution for proper synchronization control. However, with the slow and fast synchronization algorithms disclosed herein, a synchronization precision of less than 1 μs may be achieved.

Since only the digital phase information is sent on the communication bus between the ISM and the UPS groups, it is possible to optimize the bandwidth of the transmission. Also, since the slow synchronization algorithm needs the frequency information (seeFIG. 12for example), this information may be extracted from the phase information through an FFO (fast frequency observer)431. In an embodiment, the slow and fast synchronizations depicted inFIGS. 12 and 13are implemented using the same algorithm, with the only difference being the feedback gains obtained by assigning different poles to the phase control. Accordingly, different feedback gains lead to different synchronization speed and stiffness.

To determine an appropriate feedback gain, the static stiffness may be defined with respect to the phase and frequency errors. Exemplary relationships are as follows. For slow synchronization, the static stiffness with respect to a frequency error is 16 Hz/sec correction for an error of 1 Hz, and the static stiffness with respect to the phase error is 18/(2π)≈3 Hz/s correction for an error of 1 rad, for example. For fast synchronization, the static stiffness with respect to the frequency error is 20 Hz/s correction for an error of 1 Hz, and the static stiffness with respect to the phase error is 100/(2π)≈16 Hz/s correction for an error of 1 rad, for example. By assigning different poles to the phase control, it is possible to design a fast and a slow phase control, thereby obtaining two sets of feedback gains.

As used herein, the following variable definitions apply:
φISM—other=phase angle to synchronize to (phase angle of the “supermaster”)
ωother=angular frequency of the “supermaster” (φand ωare linked by ω=dφ/dt)
ωo=angular frequency of the oscillator (of the master of the “superslave”group)
αo=phase angle of the oscillator (of the master of the “superslave” group)
ΔφISM=phase angle error (between the “supermaster” and “superslave” groups)
ΔωISM=angular frequency error (between the “supermaster” and “superslave”groups)
Δω=angular frequency correction to be applied to the oscillator
KφS=feedback gain for the phase angle error−slow synchronization
KωS=feedback gain for the angular frequency error−slow synchronization
KφF=feedback gain for the phase angle error−fast synchronization
KωF=feedback gain for the angular frequency error−fast synchronization.

While embodiments of the invention have been disclosed combinable to provide only certain combinations of functions, it will be appreciated that the possible combinations of ISM modules and functions are not confined to only the above-described examples.

While embodiments of the ISM concept may have been described in relation to a RPA configuration of a particular UPS system, it will be appreciated that the same ISM concept may be applied to other single and parallel UPS systems. It will also be appreciated that the ISM concept presented herein may be applied to UPSs of any power level with or without an isolation transformer.

As suggested in the aforementioned description of the various embodiments, an embodiment of the invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention may also be embodied in the form of a computer program product having computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, USB (universal serial bus) drives, or any other computer readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention may also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. The technical effect of the executable instructions is to exchange information, perform synchronization, and execute control algorithms and commands between two separate and independent UPS groups thereby allowing multiple configurations of uninterruptible power supply in critical power management systems.

As disclosed, some embodiments of the invention may include some of the following advantages: an Intelligent Synchronization Module (ISM) that allows multiple configurations of uninterruptible power supply (UPS) in critical Power management systems; the availability of five different functions F1-F5that may be realized by software configuration only; the ability to combine and realize more functions by a single ISM module; an exemplary ISM may have a redundant communication and a redundant power supply in order to maximize system reliability; the possibility of using redundant ISM modules for very critical applications to assure a full redundancy of the control electronics; an Intelligent Synchronization Module (ISM) that ties one or two groups of UPSs to realize five different functions F1-F5; for the function of “Intelligent Static Transfer Switch” (F1), an embodiment of the ISM that synchronizes the outputs of two separate and independent UPS groups; for the function of “Centralized Bypass” (F2), an embodiment of the ISM that commands the centralized bypass of a group of UPSs in a RPA configuration, where in a first embodiment, the ISM doesn't use the internal UPS bypasses any more, in a second embodiment, the internal bypasses of the UPSs are used as a backup for the external centralized bypass, thereby providing the maximal reliability even higher than the one of a RPA configuration, and in a third embodiment, using the internal bypasses of each unit in combination with an external centralized breaker commanded by the ISM; for the function of “Load Bus Synchronization” (F3), an embodiment of the ISM that synchronizes temporarily the outputs of two separate and independent UPS groups; for the function of “Power Tie” (F4), an embodiment of the ISM that synchronizes permanently and load shares the outputs of two separate and independent UPS groups; for the function of “Bus Repeater” (F5), an embodiment of the ISM that allows extension of the maximal distance between the first and the last UPS in a RPA configuration; an ISM that provides a flexible multi-functional product, where the five aforementioned functions F1-F5may be realized by software configuration only; the ability to include in the ISM a redundant communication feature and a redundant power supply; the ability to use redundant ISM modules for very critical applications to assure a full redundancy of the control electronics; an ISM capable of implementing more than one function at the same time; the ability to combine functions via the ISM; slow and fast synchronization algorithms that allow the ISM to synchronize two groups of UPSs already supplying their critical load; the ability to send a low resolution signal (10 bits) of digital phase information through the communication bus connecting the ISM to the UPS groups, thereby enabling the precision of the synchronization algorithms to be less than 1 μs; and, the ability to send only the digital phase information on the communication bus, thereby enabling optimization of the bandwidth of the transmission, and since the slow synchronization algorithm also needs the frequency information, this information may be extracted from the phase information through an FFO (fast frequency observer).