Patent Publication Number: US-2023155368-A1

Title: Modular static transfer switches

Description:
BACKGROUND 
     The field of the disclosure relates to static transfer switches, and more particularly, to static transfer switches with modular transfer circuits. 
     A static transfer switch is a device that is designed to transfer from supplying a load via a preferred power source to supplying the load via an alternate power source when the power quality of the preferred power source is deemed unacceptable for the load. Conventional static transfer switches use thyristors as the main solid-state switching device, which suffer from some drawbacks, including the inability to interrupt an output current flowing through the thyristors. Therefore, additional protection devices such as fuses and circuit breakers are included in a conventional static transfer switch that utilizes thyristors for overcurrent fault protection. One result of the inability of thyristors to interrupt the output current is that the thyristors are over-specified in their current handling capability in order to survive the high surge currents that can occur during overcurrent faults at the load. 
     Another problem that arises when using thyristors in conventional static transfer switches is the slow transfer speed when switching the load from being supplied by one power source to another power source. When a power quality issue is detected in the preferred power source and a switchover to the alternate power source is commanded, a zero-crossing load current is needed in order to implement the transfer when the solid-state switches are thyristors. The result of this delay is that the load is subjected to the power quality issues from the preferred power source for up to a half-waveform delay in the current signal before the transfer is made, which can adversely impact the load. 
     Yet another problem that can arise for conventional static transfer switches relates to redundancy and maintenance downtime when thyristors fail. Thyristors in the conventional static transfer switch may exhibit single point failure modes which require the static transfer switch to be de-energized for service, which can be disruptive to the load supplied by the static transfer switch. 
     Thus, it is desirable to improve the operation and performance of static transfer switches, and more specifically, improve the operation and performance of static transfer switches that utilize solid-state switching elements. 
     BRIEF DESCRIPTION 
     In one aspect, a modular static transfer switch is provided. The module static transfer switch includes an output configured to couple to a load, a first input configured to couple to a first power source, and a second input configured to couple to a second power source. The modular static transfer switch further includes a plurality of sold-state switch modules each comprising at least one solid-state switch. A first plurality of the solid-state switch modules are coupled in parallel between the first input and the output, each solid-state switch module of the first plurality configured to selectively couple the first power source to the output using the at least one solid-state switch. A second plurality of the solid-state switch modules are coupled in parallel between the second input and the output, each solid-state switch module of the second plurality configured to selectively couple the second power source to the output using the at least one solid-state switch. 
     In another aspect, a method of operating a modular static transfer switch is provided. The modular static transfer switch includes a plurality of solid-state switch modules installed therein, where each of the solid-state switch modules includes at least one solid-state switch. The method includes operating a first plurality of the solid-state switch modules coupled in parallel between a first power source for the modular static transfer switch and a load, where the at least one solid-state switch for each of the first plurality of the solid-state switch modules selectively connects the first power source to the load. The method further includes operating a second plurality of the solid-state switch modules coupled in parallel between a second power source for the modular static transfer switch and the load, where the at least one solid-state switch for each of the second plurality of the solid-state switch modules selectively connects the second power source to the load. The method further includes collaboratively transferring the load between the first power source and the second power source by coordinating operation of the first plurality of the solid-state switch modules with operation of the second plurality of the solid-state switch modules. 
     In yet another aspect, another modular static transfer switch is provided. The module static transfer switch includes an output configured to couple to a load, a first input configured to couple to a first power source, and a second input configured to couple to a second power source. The modular static transfer switch further includes a plurality of sold-state switch modules each comprising at least one solid-state switch and a controller configured to operate the at least one solid-state switch. A first plurality of the solid-state switch modules are coupled in parallel between the first input and the output, each solid-state switch module of the first plurality configured to selectively couple the first power source to the output using the at least one solid-state switch. A second plurality of the solid-state switch modules are coupled in parallel between the second input and the output, each solid-state switch module of the second plurality configured to selectively couple the second power source to the output using the at least one solid-state switch. Controllers of the first plurality of the solid-state switch modules communicate with controllers of the second plurality of solid-state switch modules to collaboratively transfer the load between the first power source and the second power source. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG.  1    is a block diagram of a modular static transfer switch in an example embodiment. 
         FIG.  2    is a block diagram of a solid-state switching module for the modular static transfer switch of  FIG.  1    in an example embodiment. 
         FIG.  3    is a flow chart of a method of operating the modular static transfer switch of  FIG.  1    in an example embodiment. 
         FIG.  4    is a flow chart of a method of servicing the modular static transfer switch of  FIG.  1    in an example embodiment. 
         FIG.  5    depicts additional details of the method of  FIG.  4    in an example embodiment. 
         FIG.  6    is a flow chart of another method of servicing the modular static transfer switch of  FIG.  1    in an example embodiment. 
         FIG.  7    depicts additional details of the method of  FIG.  6    in an example embodiment. 
     
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of this disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of this disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     As used herein, the terms “processor” and “computer,” and related terms, e.g., “processing device,” “computing device,” and “controller” are not limited to just those integrated circuits referred to in the art as a computer, but broadly refers to a microcontroller, a microcomputer, an analog computer, a programmable logic controller (PLC), an application specific integrated circuit (ASIC), and other programmable circuits, and these terms are used interchangeably herein. In the embodiments described herein, “memory” may include, but is not limited to, a computer-readable medium, such as a random-access memory (RAM), a computer-readable non-volatile medium, such as a flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, additional input channels may be, but are not limited to, computer peripherals associated with an operator interface such as a touchscreen, a mouse, and a keyboard. Alternatively, other computer peripherals may also be used that may include, for example, but not be limited to, a scanner. Furthermore, in the example embodiment, additional output channels may include, but not be limited to, an operator interface monitor or heads-up display. Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor, processing device, or controller, such as a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an ASIC, a programmable logic controller (PLC), a field programmable gate array (FPGA), a digital signal processing (DSP) device, and/or any other circuit or processing device capable of executing the functions described herein. The methods described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processing device, cause the processing device to perform at least a portion of the methods described herein. The above examples are not intended to limit in any way the definition and/or meaning of the term processor and processing device. 
     As discussed previously, thyristor-based static transfer switches present a number of disadvantages, including problematic overcurrent protection capabilities (due to an inability to terminate a fault current), switching delay due to zero-crossing current transfer requirements, and common single point of failure modes, any of which can result in removing the static transfer switch from operation during maintenance. 
     In the embodiments described herein, a modular static transfer switch is described which includes a plurality of solid-state switching modules electrically coupled in parallel. The parallel nature of the modules enables a more finely tuned design option whereby the number of modules arranged in parallel for each branch of a multi-branch source-to-load switching scheme can be adjusted based on the desired output current criteria. Further, the modular nature of the modular static transfer switch enables a hot swap capability for individual modules if a module fails, which allows the transfer switch to continue supplying electrical power to the load during maintenance. In addition, the parallel nature of the modules for each source branch in the transfer switch provides redundancy whereby a failed module can be taken offline during operation without adversely impacting the ability of the transfer switch to continue to supply electrical power to the load. Further still, the modules utilize non-thyristor based solid-state switching elements, which can be switched during non-zero current conditions, thereby improving the source transfer time for the transfer switch. 
       FIG.  1    is a block diagram of a Modular Static Transfer Switch (MSTS)  100  in an example embodiment. In this embodiment, MSTS  100  selectively supplies a load  102  with electrical power from either a first power source  104  or a second power source  106  depending on various criteria. For example, MSTS  100  may supply electrical power to load  102  primarily from first power source  104  unless the electrical power delivered by first power source  104  falls outside of a desired range of values (e.g., first power source  104  has a voltage and/or a harmonic distortion that varies from target values by a threshold amount). If, for example, first power source  104  is either incapable of supplying electrical power to load  102  (e.g., first power source  104  fails or is incapable of supplying electrical power to load  102  at a desired power quality), then MSTS  100  switches load  102  from first power source  104  to second power source  106 . In this regard, first power source  104  may operate as a preferred power source for load  102 , with second power source  106  operating as a backup or alternate power source for load  102 . Although only two power sources are depicted in  FIG.  1   , MSTS  100  selectively couples load  102  to any number of power sources in other embodiments. Further, although MSTS  100  is depicted as switching single phase power in  FIG.  1   , MSTS  100  switches 3-phase Alternating Current (AC) power in other embodiments. In 3-phase embodiments, first power source  104 , second power source  106  are 3-phase sources, and load  102  is a 3-phase load. In other embodiments, first power source  104  and second power source  106  are Direct Current (DC) sources, and load  102  is a DC load. In other embodiments, first power source  104  and second power source  106  are 3-phase sources, and MSTS provides a plurality of sign-phase loads (e.g., load  102  is a plurality of single-phase loads). 
     In this embodiment, first power source  104  is electrically coupled to MSTS  100  at a first input  108  and second power source  106  is electrically coupled to MSTS  100  at a second input  110 . Load  102  is electrically coupled to an output  112  of MSTS  100 . First input  108  is electrically coupled to a first input bus  114  of MSTS  100  and second input  110  is electrically coupled to a second input bus  116  of MSTS  100 . Output  112  of MSTS  100  is electrically coupled to an output bus  118  of MSTS  100 . 
     MSTS  100  in this embodiment includes a plurality of solid-state switch modules  120 , which operate to selectively couple first power source  104  or second power source  106  to load  102 . Modules  120  include any component, system, or device which selectively couples load  102  between first power source  104  and second power source  106 . Each of modules  120  in this embodiment includes a one or more solid-state switches  122 , which provide a selective electrical path between first power source  104  and load  102  or second power source  106  and load  102 . Solid-state switches  122  may include any number of series-parallel combinations of power electronic devices in order to implement the functionality described herein for solid-state switches  122 . In some embodiments, solid-state switches  122  include a pair of anti-series connected power semiconductor devices. In these embodiments, solid-state switches  122  are Silicon-Carbide Metal-Oxide-Semiconductor Field-Effect Transistors (SiC MOSFETs). 
     In this embodiment, modules  120  are segmented into a first plurality of modules  124 , which are electrically coupled in parallel between first input bus  114  and output bus  118 . Modules  120  are further segmented into a second plurality of modules  126 , which are electrically coupled in parallel between second input bus  116  and output bus  118 . Although only three modules  120  are depicted for each of first plurality of modules  124  and second plurality of modules  126  in  FIG.  1   , first plurality of modules  124  and second plurality of modules  126  may include any number of modules  120  electrically coupled in parallel in other embodiments. Further, modules  120  include 3-phase inputs and 3-phase outputs in other embodiments, depending on whether first power source  104 , second power source  106 , and load  102  are 3-phase sources/loads. 
     In some embodiments, MSTS  100  further includes a back panel  125 , and modules  120  are removably mounted to back panel  125 . When removably mounted, modules  120  may be easily replaced during maintenance, even if MSTS  100  remains energized by first power source  104  or second power source  106  and is supplying electrical power to load  102 . Back panel  125  includes, in some embodiments, a communication bus  128 , which enables modules  120  to communicate with each other. Communication bus  128  includes any component, system, or device that provides wired or wireless communication capability to a user, modules  120 , and MSTS  100 . During operation, modules  120  may utilize communication bus  128  to coordinate their activities during transfer events. For instance, first plurality of modules  124  and second plurality of modules  126  may communicate with each other utilizing communication bus  128  to collaboratively transfer load  102  between first power source  104  and second power source  106 . This will be discussed in more detail below. 
     In some embodiments, MSTS  100  includes a user interface  130 , which includes any component, system, or device which allows a user (not shown) to interact with MSTS  100 . Some examples of user interface  130  include keyboards, mice, display devices, etc. User interface  130  may be used by a user to configure modules  120  for operation after installation at MSTS  100 . For example, user interface  130  may be used to assign first plurality of modules  124  to a first group IDentifier (ID) and assign second plurality of modules  126  to a second group ID, which enables modules  120  to recognize their electrical configuration within MSTS  100 . However, other addressing schemes may be used, such as individual ID assignments for modules  120  with a mechanism to correlate the ID of a module  120  with its electrical configuration within MSTS  100 . 
     MSTS  100  includes, in some embodiments, a management controller  132 . Management controller  132  includes any component, system, or device that provides management functions for MSTS  100 . In some embodiments, management controller  132  communicates with modules  120  via communication bus  128  to manage transfers of load  102  between first power source  104  and second power source  106 . Management controller  132  therefore may perform any functionality described herein for MSTS  100  either alone or in combination with one or more of modules  120  in order to perform the functionality described herein for MSTS  100 . 
       FIG.  2    is a block diagram of module  120  in an example embodiment. In this embodiment, module  120  includes electrical terminals  202 ,  204 , which are electrically couplable between output bus  118  in MSTS  100  and either first input bus  114  or second input bus  116  depending on whether module  120  is part of first plurality of modules  124  or second plurality of modules  126 . Module  120  in this embodiment further includes a controller  206 , a communication interface  208 , a sensing circuit  210 , a coil driver circuit  212 , a gate driver circuit  214 , a mechanical disconnect  216 , and a snubber circuit  218 . Controller  206 , communication interface  208 , sensing circuit  210 , coil driver circuit  212 , gate driver circuit  214 , mechanical disconnect  216 , and snubber circuit  218  include any component, system, or device which implements their respective functionality as described herein. 
     Controller  206  controls the operation of module  120  and interacts with communication interface  208  to send and/or receive information over communication bus  128 . Controller  206  utilizes gate driver circuit  214  to control whether solid-state switch  122  is open or closed. Controller  206  also utilizes coil driver circuit  212  to control whether mechanical disconnect  216  is open or closed. Generally, mechanical disconnect  216  provides galvanic isolation for module  120  when mechanical disconnect  216  is open. 
     Sensing circuit  210  measures information for module  120 , including a temperature of module  120 , a humidity, a current flowing between terminals  202 ,  204  (including a current flowing through solid-state switch  122 ), a voltage at terminals  202 ,  204  and/or a voltage at solid-state switch  122 , a power factor at terminals  202 ,  204 , a harmonic distortion at terminals  202 ,  204 , etc. 
     Module  120  utilizes snubber circuit  218  during on-off transitions of solid-state switch  122  to clamp voltages transients across solid-state switch  122 . The components depicted for snubber circuit  218  are illustrative only, and snubber circuit  218  has different configurations in other embodiments. In this embodiment, solid-state switch  122  is depicted as a pair of anti-series MOSFETs  220 . In some embodiments, MOSFETs are arranged in serial and parallel combinations depending on the current capability of module  120 . 
     Although module  120  in this embodiment is depicted as a single-phase device, module  120  is a 3-phase device in other embodiments. In 3-phase embodiments, module  120  includes additional instances of terminals  202 ,  204 , solid-state switch  122 , mechanical disconnect  216 , and snubber circuits  218  for each phase. 
     In some embodiments, controller  206  modifies the operation of module  120  based on information measured by sensing circuit  210  by opening and closing mechanical disconnect  216  and/or solid-state switch  122  based on the temperature of module  120 , a humidity, the current flowing between terminals  202 ,  204  (including a current flowing through solid-state switch  122 ), the voltage at terminals  202 ,  204  and/or a voltage at solid-state switch  122 , the power factor at terminals  202 ,  204 , the harmonic distortion at terminals  202 ,  204 , etc. For example, during current faults, currents higher than a threshold current may flow through solid-state switches  122 , which are sensed by controller  206  and cause controller  206  to open solid-state switches  122  and/or mechanical disconnect  216 . Such fault currents may further be detected by comparing the measured currents to a pre-determined time-current curve. 
     In another example, a temperature of module  120  higher than a threshold temperature may cause controller  206  to open solid-state switches  122  and/or mechanical disconnect  216 . In another embodiment, the voltage at terminals  202 ,  204  and/or a voltage at solid-state switch  122 , the power factor at terminals  202 ,  204 , the harmonic distortion at terminals  202 ,  204 , a humidity, etc., may cause controller  206  to initiate a transfer between first power source  104  and second power source  106 . In other embodiments, any of the prior factors may trigger controller  206  to communicate this information to management controller  132 , which may take any further action deemed appropriate in order to rectify non-standard operating conditions at MSTS  100 . 
       FIG.  3    is a flow chart of a method  300  of operating a modular static transfer switch in an example embodiment. Method  300  will be discussed with respect to MSTS  100 , although method  300  may be performed by other systems or devices, not shown. The methods described herein are not all inclusive and may include other steps not shown. Further, the steps of the methods described herein may be performed in a different order. 
     During operation, first plurality of modules  124  operate to selectively connect first power source  104  to load  102  (see step  302 ) and second plurality  126  of modules  120  operate to selectively connect second power source  106  to load  102  (see step  304 ). 
     First, consider that MSTS  100  is initially supplying electrical power to load  102  from first power source  104 . If an issue is detected with first power source  104 , MSTS  100  ensures the proper operation of load  102  by transferring load  102  from first power source  104  to second power source  106  (see step  306 ). To do so, solid-state switches  122  in first plurality of modules  124  open to start the transfer, and after a time delay to ensure that first power source  104  is disconnected from load  102 , solid-state switches  122  in second plurality of modules  126  are closed, which completes the transfer. This activity can occur in a number of different ways. In one embodiment, management controller  132  generates instructions that direct controllers  206  of modules  120  to perform this activity. In another embodiment, controllers  206  of modules  120  communicate with each other to perform this activity. For instance, controllers  206  of first plurality of modules  124  coordinate with each other to open their solid-state switches  122  to start the transfer, and controllers  206  of second plurality of modules  126  coordinate with each other to close their solid-state switches  122  to complete the transfer. In addition, controllers  206  of modules  120  communicate with each other to implement the time delay to ensure that first power source  104  is disconnect from load  102  prior to connecting second power source  106  to load  102 . 
     Next, consider that MSTS  100  is initially supplying electrical power to load  102  from second power source  106 . If an issue is detected with second power source  106 , MSTS  100  ensures the proper operation of load  102  by transferring load  102  from second power source  106  to first power source  104  (see step  306 ). To do so, solid-state switches  122  in second plurality of modules  126  open to start the transfer, and after a time delay to ensure that second power source  106  is disconnected from load  102 , solid-state switches  122  in first plurality of modules  124  are closed, which completes the transfer. This activity can occur in a number of different ways. In one embodiment, management controller  132  generates instructions that direct controllers  206  of modules  120  to perform this activity. In another embodiment, controllers  206  of modules  120  communicate with each other to perform this activity. For instance, controllers  206  of second plurality of modules  126  coordinate with each other to open their solid-state switches  122 , and controllers  206  of first plurality of modules  124  coordinate with each other to close their solid-state switches  122 . In addition, controllers  206  of modules  120  communicate with each other to implement the time delay to ensure that second power source  106  is disconnect from load  102  prior to connecting first power source  104  to load  102 . 
       FIG.  4    is a flow chart of a method  400  of servicing a modular static transfer switch in an example embodiment, and  FIG.  5    depicts additional details of method  400 . Method  400  will be discussed with respect to MSTS  100 , although method  400  may be performed by other systems or devices, not shown. 
     During operation, first plurality of modules  124  operate to selectively connect first power source  104  to load  102  (see step  302  of  FIG.  4   ) and second plurality of modules  126  operate to selectively connect second power source  106  to load  102  (see step  304  of  FIG.  4   ). First, consider that MSTS  100  is currently supplying electrical power to load  102  from first power source  104 , and an issue is detected with module  134  (see  FIG.  1   ). For example, module  134  may require service, repair, or may have failed. When module  134  fails, the remaining modules in first plurality of modules  124  continue to supply electrical power to load  102  from first power source  104 , by, for example, each supplying a larger portion of the current for load  102  due to the loss of module  134 . Module  134  is hot-swapped in MSTS  100  without disconnecting first power source  104  from load  102  (see step  402 ). To do so, first power source  104  remains connected to load  102  (see step  502  of  FIG.  5   ). In order to remove module  134  in MSTS  100  (see  FIG.  2   ), solid-state switch  122  of module  134  is opened (see step  504  of  FIG.  5   ), followed by opening mechanical disconnect  216  (see step  506  of  FIG.  5    and  FIG.  2   ) of module  134 , which isolates module  134  from MSTS  100 . Module  134  is removed, and the remaining modules of first plurality of modules  124  continue to supply the electrical power to load  102  from first power source  104  (see step  508 ). If a spare/backup module  135  (see  FIG.  1   ) is available, spare/backup module  135  is hot plugged into MSTS  100  (see step  510 ). Mechanical disconnect  216  of spare/backup module  135  is closed (see step  512 ), and solid-state switch  122  of spare/backup module  135  is closed (see step  514 ), which electrically couples spare/backup module  135  to MSTS  100 . Spare/backup module  135  begins to supply load  102  with a portion of the electrical current from first power source  104 . In another embodiment, module  134  is serviced/repaired after removal (see step  516 ) and is hot-plugged into MSTS  100  (see step  518 ). Mechanical disconnect  216  of module  134  is closed (see step  512 ), and solid-state switch  122  of module  134  is closed (see step  514 ), which electrically couples module  134  to MSTS  100 . Module  134  begins to supply load  102  with a portion of the electrical current from first power source  104 . 
     Next, consider that MSTS  100  is currently supplying electrical power to load  102  from second power source  106 , and an issue is detected with module  136  (see  FIG.  1   ). For example, module  136  may require service, repair, or may have failed. When module  136  fails, the remaining modules in second plurality of modules  126  continue to supply electrical power to load  102  from second power source  106 , by, for example, each supplying a larger portion of the current for load  102  due to the loss of module  136 . Module  136  is hot-swapped in MSTS  100  without disconnecting second power source  106  from load  102  (see step  402 ). To do so, second power source  106  remains connected to load  102  (see step  502  of  FIG.  5   ). In order to remove module  136  in MSTS  100  (see  FIG.  1   ), solid-state switch  122  of module  136  is opened (see step  504  of  FIG.  5   ), followed by opening mechanical disconnect  216  (see step  506  of  FIG.  5    and  FIG.  2   ) of module  136 , which isolates module  136  from MSTS  100 . Module  136  is removed, and the remaining modules of second plurality of modules  126  continue to supply the electrical power to load  102  from second power source  106  (see step  508 ). If a spare/backup module  137  (see  FIG.  1   ) is available, spare/backup module  137  is hot plugged into MSTS  100  (see step  510 ). Mechanical disconnect  216  of spare/backup module  137  is closed (see step  512 ), and solid-state switch  122  of spare/backup module  137  is closed (see step  512 ), which electrically couples spare/backup module  137  to MSTS  100 . Spare/backup module  137  begins to supply load  102  with a portion of the electrical current from second power source  106 . In another embodiment, module  136  is serviced/repaired after removal (see step  516 ) and is hot-plugged into MSTS  100  (see step  518 ). Mechanical disconnect  216  of module  136  is closed (see step  512 ), and solid-state switch  122  of module  136  is closed (see step  514 ), which electrically couples module  136  to MSTS  100 . Module  136  begins to supply load  102  with a portion of the electrical current from second power source  106 . 
     In other embodiments, module  134  or module  136  are serviced by transferring load  102  between first power source  104  and second power source  106 .  FIG.  6    is a flow chart of another method  600  of servicing a modular static transfer switch in an example embodiment, and  FIG.  7    depicts additional details of method  600 . Method  600  will be discussed with respect to MSTS  100 , although method  600  may be performed by other systems or devices, not shown. 
     During operation, first plurality of modules  124  operate to selectively connect first power source  104  to load  102  (see step  302  of  FIG.  6   ) and second plurality of modules  126  operate to selectively connect second power source  106  to load  102  (see step  304  of  FIG.  6   ). First, consider that MSTS  100  is currently supplying electrical power to load  102  from first power source  104 , and an issue is detected with module  134  (see  FIG.  1   ). For example, module  134  may require service, repair, or may have failed. When module  134  fails, module  134  is cold-swapped in MSTS  100  after transferring load  102  to second power source  106  (see step  602 ). To do so, load  102  is transferred to second power source  106  (see step  702  of  FIG.  7   ), which de-energizes module  134 . In order to remove module  134  in MSTS  100  (see  FIG.  2   ), solid-state switch  122  of module  134  is opened (see step  704  of  FIG.  7   ), followed by opening mechanical disconnect  216  (see step  706  of  FIG.  7    and  FIG.  2   ) of module  134 , which isolates module  134  from MSTS  100 . Module  134  is removed, and second plurality of modules  126  continue to supply the electrical power to load  102  from second power source  106  (see step  708 ). If a spare/backup module  135  (see  FIG.  1   ) is available, spare/backup module  135  is cold-plugged into MSTS  100  (see step  710 ). Mechanical disconnect  216  of spare/backup module  135  is closed (see step  712 ), and solid-state switch  122  of spare/backup module  135  is closed (see step  714 ), which electrically couples spare/backup module  135  to MSTS  100 . Load  102  is transferred back to first power source  104  (see step  716 ), and spare/backup module  135  begins to supply load  102  with a portion of the electrical current from first power source  104 . In another embodiment, module  134  is serviced/repaired after removal (see step  716 ) and is cold-plugged into MSTS  100  (see step  718 ). Mechanical disconnect  216  of module  134  is closed (see step  712 ), and solid-state switch  122  of module  134  is closed (see step  714 ), which electrically couples module  134  to MSTS  100 . Load  102  is transferred back to first power source  104  (see step  716 ), and module  134  begins to supply load  102  with a portion of the electrical current from first power source  104 . 
     Next, consider that MSTS  100  is currently supplying electrical power to load  102  from second power source  106 , and an issue is detected with module  136  (see  FIG.  1   ). For example, module  136  may require service, repair, or may have failed. When module  136  fails, module  136  is cold-swapped in MSTS  100  after transferring load  102  to first power source  104  (see step  602 ). To do so, load  102  is transferred to first power source  104  (see step  702  of  FIG.  7   ), which de-energizes module  136 . In order to remove module  136  in MSTS  100  (see  FIG.  2   ), solid-state switch  122  of module  136  is opened (see step  704  of  FIG.  7   ), followed by opening mechanical disconnect  216  (see step  706  of  FIG.  7    and  FIG.  2   ) of module  136 , which isolates module  136  from MSTS  100 . Module  136  is removed, and first plurality of modules  124  continue to supply the electrical power to load  102  from first power source  104  (see step  708 ). If a spare/backup module  137  (see  FIG.  1   ) is available, spare/backup module  137  is cold-plugged into MSTS  100  (see step  710 ). Mechanical disconnect  216  of spare/backup module  137  is closed (see step  712 ), and solid-state switch  122  of spare/backup module  137  is closed (see step  714 ), which electrically couples spare/backup module  137  to MSTS  100 . Load  102  is transferred back to second power source  106  (see step  716 ), and spare/backup module  137  begins to supply load  102  with a portion of the electrical current from second power source  106 . In another embodiment, module  136  is serviced/repaired after removal (see step  718 ) and is cold-plugged into MSTS  100  (see step  720 ). Mechanical disconnect  216  of module  136  is closed (see step  712 ), and solid-state switch  122  of module  136  is closed (see step  714 ), which electrically couples module  136  to MSTS  100 . Load  102  is transferred back to second power source  106  (see step  716 ), and module  136  begins to supply load  102  with a portion of the electrical current from second power source  106 . 
     Any of the prior load  102  transfers between first power source  104  and second power source  106  may be performed intelligently in order to minimize the disruption to load  102 , such as by minimizing the phase angle between first power source  104  and second power source  106  during the transfer, by, for example, modifying the delay time between opening and closing solid-state switches  122  during the transfer. In one embodiment, in order to minimize the phase angle difference between first power source  104  and second power source  106  when transferring load  102  from first power source  104  to second power source  106 , the delay time after opening solid-state switches  122  in first plurality of modules  124  is modified, which modifies when solid-state switches  122  in second plurality of modules  126  are closed, which minimizes the disruption to load  102 . In another embodiment, in order to minimize the phase angle difference between first power source  104  and second power source  106  when transferring load  102  from second power source  106  to first power source  104 , the delay time after opening solid-state switches  122  in second plurality of modules  126  is modified, which modifies when solid-state switches  122  in first plurality of modules  124  are closed, which minimizes the disruption to load  102 . 
     An example technical effect of the apparatus and methods described herein includes one or more of: (a) minimizing disruptions to a load even with single-point module failures; (b) providing hot-swap capability for repair or replacing modules; and (c) improving the transfer time between different power sources using solid-state switches that can switch in a non-zero current scenario. 
     Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.