Redundant multiphase power supplies for common load device

An apparatus and a method for using at least two multiphase AC power sources to provide seamless uninterruptible power to a common load by isolating a faulty phase or source upon detection of a fault in one of the sources or in one of the phases thereof. To combine at least two three-phase power sources in parallel, for example, the apparatus includes a power paralleling circuit comprising two three-phase silicon controlled rectifier (SCR) bridges and a controller. Each bridge receives power from one of two separate and/or independent three-phase power sources. The controller monitors the status or condition of respective phases of power and selectively gates an associated SCR in each bridge so as to simultaneously power the common load from the two sources. Isolation switches are located in series with each phase circuit of each power supply. In the event of a fault, e.g., a loss of phase synchronization between corresponding phases of the respective power sources, an over-voltage condition of a phase, an under-voltage condition of a phase, etc., the controller issues gating signals to the appropriate SCR(s) or isolation switch(es) to isolate the defective phase(s) or power source from the load. In an alternative embodiment, an alarm is issued to alert maintenance personnel of a fault condition. A corresponding method for achieving the aforestated fault tolerance is also disclosed.

FIELD OF THE INVENTION
 The present invention relates to a method and apparatus for improving
 reliability in a multiphase power supply arrangement. More specifically,
 the invention relates to SCR-controlled, multi-phase power sources that
 supply a common load in a manner that reduces or eliminates a "single
 point of failure."
 BACKGROUND OF THE INVENTION
 Faults in an AC power supply may exist as a phase loss, an under-voltage
 condition, an over-voltage condition, or other condition. Such faults may
 result from a variety of reasons including breaker trips, fuse loss,
 inverter faults, shorted turns in a transformer, and ground faults.
 Frequency or phase errors may also occur when the AC power frequency falls
 outside of specified ranges. Power supply faults have deleterious impact
 on electrical equipment, and in many applications cannot be tolerated.
 Achieving reliability by using multiple power sources to supply a
 single-phase or DC load device is relatively simple. Multiple sources, for
 example, can be connected in parallel with the load device so that a
 failure of one source will not shut down the load. For single-phase AC
 supplies, this works fine, provided the phases of the respective sources
 are, maintained in substantial synchronism. Other precautions, however,
 must be taken if the difference between the respective phases become
 significant. Connecting a single-phase AC load device to multiple
 single-phase AC sources does not require precise phase alignment because
 most load devices can tolerate minor phase differences. Using two or more
 multiphase AC sources to power a multiphase load, however, presents
 certain challenges. While it is possible to connect multiphase sources in
 parallel, a failure of any phase usually results in a failure in another
 phase or in the other source. Furthermore, prior systems cannot handle
 multiphase AC power sources in parallel to supply high performance loads
 where high reliability is essential.
 SUMMARY OF THE INVENTION
 To address the above and other problems, an embodiment of the present
 invention comprises respective silicon controlled rectifiers (SCRs)
 connected in parallel to respective phases of at least two multiphase AC
 power sources for supplying power to a common load. A first group of SCRs
 connects a first power source to the load and a second group of SCRs
 connects a second power source to the load. During normal operation, the
 SCRs are switched at or near zero crossings of the respective phases in a
 bridge rectifier fashion to allow source current from each AC power source
 to supply the load. Upon detection of a fault, a controller issues gating
 signals to selected SCR(s) to effectively remove the "defective" phase
 from the load, thereby permitting the corresponding phase of the other
 source to seamlessly continue powering the common load without
 interruption. The controller may optionally issue an alarm upon detection
 of a fault.
 According to another embodiment of the invention, the topology of a
 paralleling SCR circuit comprises an input for a first multi-phase power
 source, a first 3-pole in-line input disconnect device, a first SCR group
 and a second in-line output disconnect device. A first multiphase power
 source connects to a first 3-pole in-line disconnect device which, in
 turn, connects a first group of silicon controlled rectifiers to second
 group of in-line output disconnect devices. The paralleling circuit also
 includes an input for second multiphase power source, a similarly
 connected third disconnect devices, and a similarly connected fourth
 disconnect devices. This scenario may be repeated for additional
 multiphase power sources. The topological structure of the paralleling
 circuit still further includes a paralleling output that combines the
 outputs of the first an second plurality of silicon controlled rectifiers
 so that they may power a common source.
 Another embodiment of the invention includes a method of supplying
 continuous uninterruptible power to a common load using a power
 paralleling circuit that effectively combines separate and independent
 power sources. SCRs of first and second groups are connected to at least
 first power and second power sources, respectively. The method comprises
 providing respective gating signals to the first group of SCRs at or near
 zero crossing points of each phase of the multi-phase power source, and
 providing respective gating signals to the second group of SCRs at or near
 zero crossing points of the multi-phase power source, thereby alternately
 firing the respective SCRs at appropriate times during their AC cycles.
 The method additionally includes monitoring fault conditions in the
 respective phases of the power sources and controlling the SCR gating
 signals to isolate a phase or source from the load upon detection of a
 fault, thereby seamlessly supplying the load with uninterrupted power. The
 method optionally includes issuing an alarm upon detection of a fault.
 Other objects, features, and advantages of the present invention will
 become apparent when considered in conjunction with the accompanying
 drawings. The invention, though, is pointed out with particularity by the
 appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
 FIG. 1 shows a power paralleling circuit for supplying power to a load 10
 in accordance with one embodiment of the invention. A first three-phase
 power source 2 connects to a first power bridge 6. A second three-phase
 power source 4 connects to a second power bridge 8. In the United States,
 each power source 2 and 4 typically supplies three-phase alternating
 current ("AC") power at 208/120 volts and 60 hertz, although other
 electrical services are available. Power specifications, however, may vary
 in other countries according to standards adopted.
 To power a common load, the respective phases of each power supply should
 be synchronized. Both power bridges 6 and 8 are under the control of a
 controller 7, which sends gating signals to SCRs contained in the bridges
 6 and 8 via signal lines 3 and 5, respectively. The output of power bridge
 6 connects in parallel to the output of power bridge 8 via an output line
 9. The voltage on output fine 9 is a three-phase combined source that
 supplies a common load 10.
 FIG. 2 depicts the topology of power bridges 6 and 8 of FIG. 1 in greater
 detail. Specifically, FIG. 2 shows two three-phase voltage or power
 sources (.phi..sub.A1, .phi..sub.B1, .phi..sub.C1 and .phi..sub.A2,
 .phi..sub.B2, .phi..sub.C2) connected in parallel by way of a plurality of
 silicon controlled rectifiers (SCRs). A fist-phase parallel connection of
 the first voltage source includes a first SCR 20 having its anode
 connected to the .phi..sub.A1 line and the cathode connected to a first
 parallel output line 40. The first-phase parallel connection of the first
 voltage source also includes a second SCR 21 having its cathode connected
 to the .phi..sub.A1 line and its anode connected to the first parallel
 output line 40. A second-phase parallel connection for the first voltage
 source includes a third SCR 22 having its anode connected to the
 .phi..sub.B1 line and the cathode connected to a second parallel output
 line 41. The second-phase parallel connection for the first voltage source
 also includes a fourth SCR 23 having its cathode connected to the
 .phi..sub.B1 and the anode connected to the second parallel output line
 41. A third-phase parallel connection for the first voltage source
 includes a fifth SCR 24 having its anode connected to the .phi..sub.C1
 line and the cathode connected to a third parallel output line 42. The
 third-phase parallel connection for the first voltage source also includes
 a sixth SCR 25 having its cathode connected to the .phi..sub.C1 line and
 the anode connected to the third parallel output line 42.
 A first-phase parallel connection for the second voltage source includes a
 first SCR 30 having its anode connected to the .phi..sub.A2 line and the
 cathode connected to the first parallel output line 40. The first-phase
 parallel connection for the second voltage source also includes a second
 SCR 31 having its cathode connected to the .phi..sub.A2 line ad the anode
 connected to the first parallel output fine 40. A second-phase parallel
 connection for the second voltage source includes a third SCR 32 having
 its anode connected to the .phi..sub.B2 line and the cathode connected to
 the second parallel output line 41. The second-phase parallel connection
 for the second voltage source also includes a fourth SCR 33 having its
 cathode connected to the .phi..sub.B2 fine and the anode connected to the
 second parallel output fine 41. A third-phase parallel connection for the
 second voltage source includes a fifth SCR 34 having its anode connected
 to the .phi..sub.C2 line and the cathode connected to a third parallel
 output line 42. The third-phase parallel connection for the second voltage
 source also includes a sixth SCR 35 having its cathode connected to the
 .phi..sub.C2 line and the anode connected to the third parallel output
 line 42. Parallel output lines 40, 41, and 42, and a common neutral line
 43 are supplied to common load 10.
 Having the parallel connection as described with reference to FIG. 2, load
 10 is supplied with continuous power with reduced points of failure
 between the utilities and equipment rack. As earlier indicated,
 communication and host server equipment require seamless, uninterrupted
 power supply in the event of a failure. Through the configuration as shown
 in FIG. 2, load 10 draws power from each of the power sources in a shared
 manner, although certain tolerable imbalances may occur due to differences
 in mean voltage levels of the respective AC sources. To compensate for
 such variation, an impedance may be inserted in series with corresponding
 pairs of silicon controlled rectifiers to obtain a balance or equal supply
 of source power to the load. Diode conduction properties of the SCRs
 prevent power of the first source from flowing back towards the second
 source, and vice versa. By using the two AC power sources and the two gate
 firing circuits, the SCR bridges are in what is called a "2N
 configuration." The gating trigger signals for the above SCRs will be
 explained hereafter with reference to FIGS. 4A and 4B.
 FIG. 3 shows an electrical schematic diagram of a power paralleling circuit
 according to a second embodiment of the present invention. The illustrated
 parallel power connection includes two three-phase UPS voltage sources 2
 (.phi..sub.A1, .phi..sub.B1, .phi..sub.C1) and 4 (.phi..sub.A2,
 .phi..sub.B2, .phi..sub.C2) connected in parallel by way of a plurality of
 silicon controlled rectifiers (SCRs), a plurality of manual disconnect
 switches, and a pair of three-phase three-pole electrically operated
 breakers. The term plurality is also referred to as group. A first phase
 parallel connection of a first voltage UPS source 2 includes a series
 connection of a first cable termination 52, a first phase pole manual
 disconnect switch contact 60, a second cable termination 70, a first pair
 of reverse-connected parallel SCRs 20' and 21', and a third cable
 termination 80. A second phase parallel connection of the first voltage
 UPS source 2 includes a series connection of a fourth manual disconnect
 switch 54, a second phase pole manual disconnect switch contact 60', a
 fifth cable termination 72, a second pair of reverse-connected parallel
 SCRs 22' and 23', and a sixth cable termination 82. A third phase parallel
 connection of the first voltage UPS source 2 includes a series connection
 of a seventh cable termination 56, a third phase manual disconnect switch
 contact 60", an eighth cable termination 74, a third pair of
 reverse-connected parallel SCRs 24' and 25', and a ninth cable termination
 84. A first set of power lines 81, 83, and 85 are connected to the cable
 terminations 80, 82, and 84, respectively. Power lines 81, 83, and 85
 supply a first half of a parallel power source to a 100 amp breaker 86
 power panel 100.
 Still referring to FIG. 3, a first phase parallel connection of a second
 voltage UPS source 4 includes a series connection of a first cable
 termination 52', a first phase manual disconnect switch contact 62, a
 second cable termination 70', a first pair of reverse-connected parallel
 SCRs 30' and 31', and a third cable termination 80'. A second phase
 parallel connection of the second voltage UPS source 4 includes a series
 connection of a fourth cable termination 54', a second phase manual
 disconnect switch contact 62', a fifth cable termination 72', a second
 pair of reverse-connected parallel SCRs 32' and 33', and a sixth cable
 termination 82'. A third phase parallel connection for the second voltage
 UPS source 4 includes a series connection of a seventh 56', a third phase
 pole manual disconnect switch contact 62", an eighth cable termination
 74', a third pair of reverse-connected parallel SCRs 34' and 35', and a
 ninth cable termination 84'. A second set of power lines 81', 83', and 85'
 are connected to the cable terminations 80', 82', and 84', respectively.
 Power lines 81', 83', and 85' supply the second half of a parallel power
 source to the power cage 100. A cable termination 90 electrically connects
 the neutral lines from both UPS sources (2, 4) to a neutral line 91 for
 the power panel 100.
 Still referring to FIG. 3, manual disconnect switch contacts (60, 60', 60")
 and (62, 62', 62") are manually opened or closed by disconnect handles 61
 and 63, respectively. Having the parallel connection as described with
 reference to FIG. 3, power is delivered to panel 100 from two sources.
 Uninterrupted power to panel 100 will continue with a failure of one of
 the power sources. Disconnect switches 61, 63 and breakers 86, 86' isolate
 source power from the SCRs and power panel for maintenance repairs.
 Operation of the SCRs as shown in FIGS. 2 and 3 will now be explained with
 reference to FIGS. 4A, 4B, and 5. Fault detection and gating controller
 120 of FIG. 5, hereafter "the controller", supplies gate trigger signals
 to the SCRS. As shown in FIGS. 4A and 4B, periodic trigger signals occur
 at or near zero crossing points of the three-phase AC power signals. With
 reference to the first-phase .phi..sub.A1 (FIG. 2 and 3) of the first
 voltage source, SCR (20, 20') is triggered at or near the zero crossing
 point for the positive going wave cycle and SCR (21, 21') is triggered at
 or near the zero crossing point for the negative going wave cycle.
 Triggering of the other SCRs is done in a similar manner. Accordingly, SCR
 (22, 22') is triggered at or near the zero crossing point for the
 .phi..sub.B1 positive going wave cycle and SCR (23, 23') is triggered at
 or near the zero crossing point for the .phi..sub.B1 negative going wave
 cycle. With reference to the third-phase, SCR (2424') is triggered at or
 near the zero crossing point for the .phi..sub.C1 positive going wave
 cycle and SCR (25, 25') is triggered at or near the zero crossing point
 for the .phi..sub.C1 negative going wave cycle. By the time the
 first-phase .phi..sub.A1 completes a full cycle in 0.01666 sec., all of
 the triggering pulses have been issued except for SCR (25, 25'). In the
 preferred embodiment, the width of each triggering pulse is approximately
 0.4 ms or less. The triggering pulses for the SCRs in line with the second
 voltage source operate in the similar manner. Accordingly, the triggering
 pulses for SCRs 30, 30, 31, 31, 32, 32', 33, 33', 34, 34, 35, and 35 can
 be similarly described.
 Returning to FIG. 5, controller 120 senses the voltage sources AC.sub.1 and
 AC.sub.2 waveforms and accordingly supplies the triggering pulses as shown
 in FIGS. 4A and 4B. In sensing the AC.sub.1 and AC.sub.2 waveforms, the
 controller also checks for any faults that ray occur in the sources. If a
 fault is detected, the controller 120 controls the triggering pulses to
 effectively remove a source from the parallel circuit by shuting down the
 SCRs for that source. For example, if controller 120 senses that AC.sub.2
 has a fault, the controller 120 discontinues the triggering pulses to SCRs
 30, 30, 31, 31, 32, 32', 33, 33', 34, 34', 35, and 35'. In addition, when
 a fault is detected, an alarm 130 can be triggered to alert personnel that
 a fault has occurred. When it is determined that AC.sub.2 has returned to
 normal, the controller then resumes the triggering pulses for the SCRs in
 the AC.sub.2 parallel circuit. Controller 120 may also respond to a fault
 by opening an in-line switch (solid state or breaker) between a phase or
 source, on one hand, and the load device, on the other hand.
 Controller 120 further senses the voltage levels across the various SCRs
 via sensing lines, as indicated FIG. 5. Again, if the controller
 determines that an SCR is malfunctioning or does not perform according to
 specification, then the controller 120 issues trip signals via lines 121
 or 123. For example, if it is determined that SCR 34' in the phase line
 .phi..sub.C2 for the source AC.sub.2 is faulty, e.g., out of sync or
 higher or lower than a prescribed voltage, then the controller 120 will
 issue a trip signal, via line 123, to the breaker 86'. The tripping of
 breaker 86' directs source power into or out of panel 100. Once again, the
 controller 120 activates the alarm 130. Once the breaker 86' is opened,
 maintenance personnel may then remove the defective SCR 34' by opening the
 manual disconnect switches 63. For economic reasons, the SCR pair 34' and
 35' may be removed together as a single unit. The parallel circuit of FIG.
 3 includes a plurality of manual disconnect switches, as indicated above,
 to facilitate the isolation of defective components.
 Controller 120 may also provide self-diagnostics by monitoring the status
 of the SCRs themselves in order to issue an alarm or to isolate a
 defective power phase or a defective SCR where there is redundancy in SCR
 devices or switches. Similarly, provision for monitoring the sensors
 themselves may be provided.
 Additional advantages and modifications will readily occur to those skilled
 in the art. The invention in its broader aspects is therefore not limited
 to the specific details, any representative apparatus, or the illustrative
 examples shown and described. Accordingly, departures may be made from
 such details without departing from the spirit or the scope of the general
 inventive concept. The invention is defined by the following claims.