EP1327292A2

EP1327292A2 - Rotary dynamic system power distribution unit

Info

Publication number: EP1327292A2
Application number: EP01979538A
Authority: EP
Inventors: Thomas De Vries
Original assignee: GE EDG Inc
Current assignee: GE EDG Inc
Priority date: 2000-10-04
Filing date: 2001-10-04
Publication date: 2003-07-16
Also published as: JP3927907B2; AU2002211486A1; WO2002030174A2; JP2004511199A; WO2002030174A3

Abstract

A power distribution unit (PDU) provides power to end loads, such as critical computer systems. The PDU receives a primary (101) and a secondary source (102) of power and is able to switch to the secondary source of power when there is a problem with the primary source of power. The PDU stores energy, in a device such as a flywheel (122), so that if one of its power sources fail, the PDU can continue to supply high quality power to its loads while the alternate power source is brought on-line. A pair of three-pole mechanical switches (111-112) control whether the primary or alternate power supplies are used. A standard motor generator (115) is connected to the output of the switches, and is used to transform the input power and to use the stored energy during a switch from the primary to secondary source of power.

Description

ROTARY DYNAMIC SYSTEM POWER DISTRIBUTION UNIT

BACKGROUND

1. Field of the Invention

The present invention relates generally to power switching systems and, more particularly, to highly reliable sub-cycle power switching and distribution systems capable of supporting loads sensitive to power interruptions.

2. Description of Related Art

The electrical power requirements of buildings housing large computer centers can be very high. The difficulty in efficiently supplying such buildings with adequate power is compounded when the computers are mission critical computers that must have a constant source of uninterrupted power.

Conventionally, mission critical computers are powered using uninterruptable power source (UPS) power circuits. The UPS circuits monitor power flowing to the computers (i.e., the circuit loads), and in the event of a power failure, switch power to the circuit loads from a secondary power source such as a battery. From the point of view of the computers, power was never lost and operation continues as normal.

In very critical data centers, power for the computer equipment is supplied by two UPS systems and two UPS power distribution systems. The systems are redundant, which allows for maintenance of the systems or for a failure or mis-operation in one of the power pathways while maintaining power to the data processing equipment via the second UPS system and power route. This system architecture is called the "System-Plus-System" design. As close as possible, referenced electrically, to the critical loads, switching apparatus, typically a static transfer switch (STS), is installed that allows the selection of the primary, or normal power source, and the secondary, or alternate power source, to the critical loads. This is usually placed in the Power Distribution Unit (PDU) that serves the computer equipment. A typical PDU contains at least one step-down transformer and distribution panelboards. The transformer converts the voltage from the 480-Volts UPS power output to 208/120-Volts required for the computer equipment. If the static switch is installed ahead of the transformation, only one transformer is required; if the static switch is installed after transformation, two transformers are required.

As discussed above, conventional critical power distribution circuits switch power between the primary and secondary power sources using power switching circuits. At the heart of these conventional power switching circuits is a static transfer switch (STS). The STS automatically transfers the system loads from the primary to the secondary source upon a failure in the primary source. STS circuits are typically designed using silicon-controlled rectifier (SCR) power-switching elements. In other words, solid-state semiconductor elements are used to quickly reroute power from the primary to the secondary power source. Although circuits based on STSs are generally reliable in continuing to supply power to mission critical applications when the primary power source fails, a vulnerability point still exists in that the STS circuit itself may fail.

A typical mean-time-between-failure ( TBF) for a STS is 400,000 hours. Although seemingly a long time, as the number of STSs installed at a site increases, the overall reliability of the site decreases proportionally. For example, a site with twenty STSs will have a system wide MTBF of approximately 20,000 hours (slightly less than two and one-half years). A larger data center containing 50 or more will have a MTBF of less than one year. For modem data centers, which are designed to operate 24 hours a day, every day of the year, this is a meaningful failure rate.

Accordingly, there is a need in the art to provide a more reliable switching circuit for routing power from primary and backup power sources to an end load.

SUMMARY

Systems and methods consistent with the principles of the present invention address the need identified above by providing an improved power distribution unit for supplying electrical power to mission critical devices that demand continuous power.

One aspect of the present invention is a power distribution unit comprising a plurality of elements. The elements include: a synchronous motor generator, first and second mechanical switches, and a transfer logic circuit configured to monitor normal and alternate input power sources and to reverse switching states of the first and second mechanical switches when the transfer logic circuit detects a defect in the normal power source such that the first mechanical switch is opened and the second mechanical switch is closed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this Specification, illustrate an embodiment of the invention and, together with the description, explain the objects, advantages, and principles of the invention. The same or like parts in multiple drawings are indicated with the same reference numbers. In the drawings:

Fig. 1 is a circuit diagram illustrating the components of a power distribution unit consistent with the present invention; and

Fig. 2 is a diagram illustrating a physical layout of the power distribution unit.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings that illustrate the embodiments of the present invention. Other embodiments are possible and modifications may be made to the embodiments without departing from the spirit and scope of the invention. Therefore, the following detailed description is not meant to limit the invention. Rather the scope of the invention is defined by the appended claims. As described herein, a power distribution unit (PDU) provides power to end loads, such as critical computer systems. The PDU receives a primary and a secondary source of power and is able to switch to the secondary source of power when there is a problem with the primary source of power. The power transfer is done transparently to the end loads so that the end loads continue to operate as if the power was never interrupted. The PDU uses rotary technology in the form of a synchronous motor generator set, including a flywheel, to provide "ride-through" during power transfers.

Fig. 1 is a single line diagram illustrating the components of the power distribution unit (PDU) consistent with the present invention.

In normal operation, PDU 100 receives input power through two independent power sources, labeled as normal power source "A" 101 and alternate power source "B" 102. The input power is transmitted through one of the two 400-Amp circuit breakers 105 and 106. Power from sources 101 and 102 is normally 480-Volt power.

Contactors 111 and 112 are standard three-pole mechanical switches that are operated by an electric signal under the control of transfer logic 110. The line sides of contactors 111 and 112 are connected to the power supply and the load side to the motor generator 115. In case of a failure in normal power source 101, transfer logic 110 controls contactors 111 and 112 to switch to alternate power source 102.

Transfer logic 110 bases its control on information input from sensing devices. In particular, current transformers (CT) 140 and potential transformers (PT) 141 are located ahead of contactors 111 and 112 and at the output of generator 121 to: (a) sense a power failure at normal input 101 or alternate input 102, and (b) locate an undervoltage or over current anomaly ahead of the inputs 101 and/or 102 and/or after the output of generator 121.

Synchronous motor generator 115 is indicated in Fig. 1 by the dashed lines surrounding components 116-122. Components 116-122 include: contactors 116 and 117, start/run logic 118, pony start motor 119, synchronous motor 120, generator 121 , and flywheel 122. The motor generator 115 performs two functions in PDU 100: (1) it transforms the 480-Volt input power to the 280/120- Volt power typically required by the loads, and (2) it provides transparent ride-through during transfer from normal power source 101 to alternate power source 102 (or vice-versa).

Occasionally, it may be necessary to take synchronous motor generator 115 off-line for repairs or maintenance. In this situation, power from the active power source is routed through step-down transformer 125 using bypass control circuitry 126, which actuates circuit breakers 127 and 128 to pass power from either motor generator 115 or transformer 25. Transformer 125, like synchronous motor generator 115, converts the input power to the 208/120-Volt power used at the output. Transformer 125, however, is not capable of continuing to supply stable power during a power failure.

Power from either motor generator 115 or transformer 120 is eventually passed through circuit breakers 130, which leads to circuit distribution panels through which the load devices are connected. The circuit distribution panels may be, for example, 225-Amp, three-pole, four-wire, 42 circuit distribution panels. Normal Operation Normal operation of PDU 100 will now be described. In normal operation, power from normal power source 101 flows through circuit breaker 105 and to contactor switch 111. Contactor switch 111 is closed, which allows the power to pass to synchronous motor generator 115. Contactor 112, conversely, is open, thus shutting off any power from alternate power source 102 to motor generator 115. Circuit breakers 107 and 108 are preferably set open at this time, as bypass transformer 125 is not being used.

Within synchronous motor generator 115, contactor 116 is open and contactor 117 is closed, allowing the input power to flow to motor 120 which, in turn, spins generator 121. Motor 120 and generator 121, when connected as shown in Fig. 1, transform the 480-Volt input power to 280/120-Volt output power, which is supplied, through circuit breaker 128 and circuit breakers 130, to the output circuit distribution panels and the end loads. Since the connection between motor 120 and generator 121 is mechanical, complete electrical isolation is present between the electrical power input to the motor and the power output of the generator to the critical computer equipment loads.

Motor 120 additionally powers flywheel 122. Flywheel 122 provides short term backup power to generator 121 in the event of an interruption of the generator's input power. Flywheels are well known in the art. In general, flywheels store energy as mechanical kinetic energy through the rotation of the flywheel. If power to generator 121 is cut-off, the kinetic energy of the rotating flywheel is converted into electrical energy by generator 121 until a stable supply of power is brought on-line.

Operation During a Power Transfer Transfer logic circuit 110, through current transfer 140 and potential transformer 141, monitors the input power sources for defects. When a power defect is detected, such as a break or drop-off in the supplied power, transfer logic 110 begins the process of switching to alternate power supply 102 by opening contactor switch 111 and simultaneously closing contactor switch 112. A feature of three-pole mechanical contactors is that it takes longer to close a contactor, and thus create an electrical circuit, then it does to open a contactor, and thus break the electrical circuit. Accordingly, when the electrical signal is issued from transfer logic 110 to open contactor 111 and close contactor 112, contactor 111 will open before contactor 112, thus ensuring electrical isolation between power supplies 101 and 102.

One possible implementation of contactors 111 and 112 is as a three- pole, 400-Amp contactor. Such contactors are available commercially from a number of companies, such as General Electric Corporation. The contactors have a "pick-up" (PU) time (i.e., elapsed time to close) of 110 to 115 milliseconds and a "drop-out" (DO) time (i.e., elapsed time to open) of 70 to 80 milli-seconds. The PU and DO are measured from the energizing or de- energizing respectively, of the operating coil to the completed operation of the contactor armature. When transfer logic circuit 110 de-energizes the coil of one contactor while simultaneously energizing the coil of the second contactor, at the minimum, 30 milli-seconds will elapse between opening and closing of the contactors, thus ensuring power source isolation.

Transfer logic 110 is constructed so that it is not sensitive to undervoltage or over current conditions downstream of the system; only for an undervoltage, or single-phase condition, upstream of the contactor switches 111 and 112.

The break in power caused by the time delay before alternate power source 102 is switched over to motor generator 115 may cause synchronous motor 120 to lose synchronization with its input power. Pony motor 119 is used to re-synchronize motor 120. Start/run logic 118 closes contactor 116 when it senses that motor 119 has lost synchronization, thus supplying power to pony motor 119, which in turn re-synchronizes motor 120 with the input power. At this point, start/run logic 118 opens contactor 116, removing the power supply to pony motor 119.

While the alternate power source is being brought on-line and motor 120 is being re-synchronized, flywheel 122 supplies generator 121 with energy so that generator 121 can continue to supply the end loads with useable power. From the viewpoint of the end loads, power was never interrupted.

During the above-discussed transfer process, the frequency of the generator 121 , which normally produces power at 60 Hertz, may decay to approximately 59.5 Hertz. This decay lasts for about 6 Hertz (one tenth of a second), until the alternate power source is brought on-line and the motor 120 is re-synchronized. The frequency is restored back to normal within an additional one quarter second, and should never drop below the minimum input requirements for the loads (e.g., computer equipment).

Unlike power distribution units based on static-transfer-switches, which cause a complete power interruption of between 4 and 8 milliseconds during operation, PDU 100 causes no power interruption. Generator 121 continues to produce power during the power outage, with typical voltage fluctuations of approximately only 1 volt.

Operation During Maintenance

Motor generator 115 may be manually removed from the electrical circuit of Fig. 1 for maintenance or inspection. In particular, circuit breakers 105, 106, and 128, when opened, will electrically isolate motor generator 115 from the system. Before isolating motor generator 115, power is routed via circuit breaker 107 or 108 to transformer 125. Transformer 125 may be a delta-wye type step-down transformer, that transforms the input 480-volt power to the output 208/120 volt power. Bypass control circuitry 126 controls the switching from motor generator 115 to transformer 127.

Delta-wye transformers insert a phase shift between the input and output voltage (e.g., 30 degree phase shift). When transformer 125 is a delta-wye transformer, a phase shift should also be inserted in the output of motor generator 115 to match the output of the transformer.

Physical Implementation Fig. 2 is a diagram illustrating the physical layout of PDU 100. As shown, the proposed PDU package is 82 inches high and 210 inches long. The PDU may be 38" deep. These sizes are exemplary; one of ordinary skill will recognize that other equivalent PDUs could be constructed in other dimensions. Generally, PDU 100 is installed internally in a building and proximate to the end loads.

Synchronous motor generators, such as synchronous motor generator 115, are known in the art. One potential manufacturer of such motor generators is Kato Manufacturing, of Mankato, Minnesota. The Kato motor generator may be modified with a non-standard winding to introduce a phase shift, if such a shift is required to match a phase shift introduced by the transformer 125.

As described above, a fail-safe power distribution module was described. The PDU uses an internal backup energy source, such as a flywheel, to ensure nearly transparent transfer from the primary power supply to the secondary power supply during a power failure. Moreover, the PDU does not rely on solid- state static transfer switches, thus providing more reliable operation, and can be constructed primarily from readily available off-the-shelf parts.

While the return to mechanical switching devices, in lieu of static switching devices, may seem to be a step backward, the reliability of the mechanical devices are greater than that of their electronic counterparts. It is only necessary to provide a power "ride-through" during the operation of the mechanical switches. This is supplied by the synchronous motor generator, inserted in the power circuit between the switching devices and the critical loads. Because step-down of the voltage is performed, the motor operates on 480- Volts while the generator produces power at 208/120-Volts. This replaces the transformer in standard PDUs.

The foregoing description of preferred embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible consistent with the above teachings or may be acquired from practice of the invention. For example, although described as converting 480-volt power to 120-volt power, one of ordinary skill in the art will recognize that the given levels could be easily changed based on the power requirements of a particular site. It is also evident that the number of power sources is not limited to two. Three or more power sources may be used within the scope of the invention. The scope of the invention is defined by the claims and their equivalents.

Claims

WHAT IS CLAIMED:

1. A power distribution unit comprising: a synchronous motor generator having a power input connection for receiving power and a power output connection for transmitting power, the synchronous motor generator storing energy received from the power input connection and transmitting the stored energy to the power output connection when the received input power connection fails; a first mechanical switch connected to a normal power source and to the power input connection of the synchronous motor generator; a second mechanical switch connected to an alternate power source and to the power input connection of the synchronous motor generator; and a transfer logic circuit configured to monitor the normal and alternate power sources and to reverse switching states of the first and second mechanical switches when the transfer logic circuit detects a defect in the normal power source such that the first mechanical switch is opened and the second mechanical switch is closed.

2. The power distribution unit of claim 1 , further comprising: electrical distribution panels coupled to the output connection of the synchronous motor generator; and a step-down transformer connected, via circuit breakers, to the normal and the alternate power sources at one input and connected to the electrical distribution panels at an output of the step-down transformer.

3. The power distribution unit of claim 2, further including a bypass control circuit configured to route power through one of the step-down transformer and the synchronous motor generator.

4. The power distribution unit of claim 1 , wherein the power received by the synchronous motor generator is of 480 volts and the synchronous motor generator transforms the received power to 120 volts at the output connection of the synchronous motor generator.

5. The power distribution unit of claim 1 , wherein the energy stored by the synchronous motor generator is stored as kinetic energy in a flywheel.

6. The power distribution unit of claim 1 , wherein the transfer logic circuit simultaneously signals the first and second mechanical switches when reversing the switching states of the first and second mechanical switches, the first mechanical switch opening before the second mechanical switch closes, whereby isolation between the normal and alternate power sources is ensured.

7. A power distribution unit comprising: a first power supply; a second power supply; a first three-pole mechanical switch connected at line side to the first power supply; a second three-pole mechanical switch connected at line side to the second power supply; a synchronous motor generator system including a motor, a generator, and a flywheel, the synchronous motor generator connected at an input to the load sides of the first and second mechanical switches, the synchronous motor generator receiving power from the first and second power supplies through the first and second mechanical switches, respectively, and outputting a transformed version of the received power; and transfer logic configured to monitor the first power source and connected to potential and current transformers positioned ahead of the first and second mechanical switches and at the output of the motor generator, the transfer logic simultaneously issues commands to reverse switching states to the first and second mechanical switches when the transfer logic detects a defect in the first power source; wherein the simultaneous issuance of the commands causes a temporary break in the power received by the synchronous motor generator, the generator of the synchronous motor generator compensating for the temporary break in the power by generating power from energy stored in the flywheel such that the power supplied by the synchronous motor generator continues to output useable power.

8. The power distribution unit of claim 7, further comprising: electrical distribution panels coupled to the out of the synchronous motor generator; and a step-down transformer connected, via circuit breakers, to the first and the second power sources at one input and connected to the electrical distribution panels at an output of the step-down transformer.

9. The power distribution unit of claim 8, further including a bypass control circuit configured to route power from the normal power source A, or the alternate power source B, through the step-down transformer and the synchronous motor generator.

10. The power distribution unit of claim 7, wherein the power received by the synchronous motor generator is of 480 volts and the synchronous motor generator transforms the received power to 120 volt output power.

11. The power distribution unit of claim 7, wherein when the first and second mechanical switches reverse switching states, the first mechanical switch opens before the second mechanical switch closes, whereby isolation between the first and second power sources is ensured.