Isolated-Parallel UPS Configuration

Transcription

1 Isolated-Parallel UPS Configuration 1

2 Inhalt 1 Introduction IP systems configuration Projects Conclusions

3 Abstract The isolated-parallel UPS configuration has become well established in the marketplace as a system solution for medium-sized and large computer centers. It enables huge savings to be made in UPS units and therefore contributes to an improvement in the efficiency of the overall system. At the same time, the reliability of isolated-parallel systems (IP systems) is comparable to the most reliable Tier 4 system configurations [5]. This white paper deals with the operating principles of IP systems, the integration of Diesel engines, the UPS topologies and the types of energy storage devices that are suitable for this application. Then follows an overview of project-specific designs which illustrates the flexibility and performance of IP systems. 3

4 1 Introduction Power-critical facilities are tending to be larger and have an increasing number of sensitive loads, requiring more and more highly-reliable electrical power. Direct paralleling of UPS systems see Figure 1 is a commonly used method for increasing the available output power and for providing redundancy. A simple arrangement for safeguarding the power supply is an N+X redundant-parallel configuration, with N denoting the number of UPS units needed to supply the load, and X representing the number of redundant units. Figure 1 Parallel UPS configuration Since in this configuration several units are supplying the same paralleling bus, the redundant-parallel scheme is limited to around 5 to 6 MVA at low voltages and is virtually impossible to maintain during operation. Even though the simple N+X redundancy ensures high availability, it is highly probable that a failure of the common bus will cause complete failure of the supply to the loads. To avoid this "single point of failure", a second, isolated bus can be used. In order to achieve full redundancy, the systems need to be realized so that each bus is able to supply the full load. Therefore this so-called N+N redundant system configuration, shown in Figure 2, needs twice the number of UPS units to supply the load. As a result, many of these units are only partially loaded and are not operated at optimum efficiency. Such a system will not therefore be the first choice if a "green" UPS solution is the primary consideration, but nevertheless should still be considered because of its extremely high reliability. Like the parallel-redundant scheme, this also has limited capacity at low voltages. 4

5 Figure 2 System-redundant UPS configuration with static transfer switches (STS) Another way of avoiding the central bus is to abandon paralleling of UPS units. This results in the so-called isolated-redundant configuration, which is shown in Figure 3. In this configuration each UPS supplies its dedicated load while one or more redundant UPS units are running in the standby mode, ready to supply the load if one of the UPS units fails. Figure 3 Isolated-redundant UPS configuration with static transfer switches (STS) Both the isolated-redundant and system-redundant configuration require additional equipment, such as a central load management system and static transfer switches for single-feed loads to switch to the alternate power source in the event of failure of one UPS. A disadvantage of the insulated-redundant configuration is that there is no option for automatic load sharing between modules and the fact that one or more UPS units are running in idle mode, i.e. without supplying a load. When combined with Diesel engines, this results in engines running in idle mode during power system failures, which reduces engine performance if this occurs over several hours. 5

6 In 2005, the idea of a new system configuration was born the so-called isolated-parallel system (IP system). The basic development goal of this new configuration was to avoid the disadvantages of the conventional UPS configurations described above and to combine their advantages in a new highly-reliable system topology. This topology allows repeated paralleling of UPS modules combined with automatic load sharing and high efficiency while ensuring fault insulation between the load buses of the individual units. 2 IP systems configuration In an IP system (see Figure 4) each UPS is connected to a common bus (IP bus) via a choke (IP choke). The loads are supplied directly from the UPS units. The job of the IP choke is to reduce fault current to an acceptable level while ensuring adequate load sharing among the UPS units. Abbildung 4 Basic design of an IP system This load sharing enables the IP system to be designed as a simple N+X redundant configuration and the number of redundant units can be reduced to a minimum. Since in this case each UPS unit is run at a high-efficiency operating point, IP systems represent an energy-saving power supply for "green" data centers and other facilities where high reliability and energy efficiency are extremely important. 6

7 Load sharing In normal operation each load is supplied from the power grid via the associated UPS. Where the UPS units are equally loaded, no power is transferred thru the IP chokes. Each UPS unit independently regulates its output voltage to the specified nominal value. Any voltage differences are absorbed by the IP chokes so that, even in the absence of reactive current control, no reactive current is exchanged between the UPS units. In an uneven load distribution, each UPS still predominantly feeds its dedicated load. Loads which are greater than the average load of the system receive additional active power from the UPS with the lower load via the IP bus (see Figure 5). The power flow and the natural load sharing among the UPS units is determined by the combination of the phase angles between the respective UPS output buses and the impedances of the IP chokes. Figure 5 Example of load sharing in an IP system consisting of 16 UPS modules The load-dependent phase angle at the UPS unit between the supply system and the UPS output is the main factor with regard to natural load sharing. Different phase angles between the UPS outputs due to differing loads, result in active power flow thru the IP chokes located between the units. This active power is directed so that it flows from the unit with the lower load with a small phase angle to the unit with the higher load with the greater phase angle and consequently ensures load equalization between the units. Therefore no active load equalization control is necessary during power grid operation. A suitable UPS topology to achieve this load-dependent phase angle in a natural way is a rotary UPS with an internal coupling choke, as shown in Figure 9. Looking at the mechanisms of natural load sharing in IP systems, it is obvious that a direct connection between any location downstream of the UPS and the power grid would significantly disturb the system. This connection would eliminate the phase angle between the power grid and the load, so that the loads of the whole IP system would be supplied by 7

8 this bypass via the IP bus. Therefore before a UPS unit or its load is switched to bypass, it is imperative that it is first disconnected from the IP bus to avoid this overload situation. However, an IP system would not really be an advantage compared to existing system topologies if there were to be no alternate solutions to this kind of bypass operation. Should a UPS fail suddenly and without warning, its associated load continues, nevertheless, to be connected to the IP bus via the IP choke and power is automatically supplied to it via this connection. As Figure 6 shows, in this operating mode each of the remaining UPS units contributes to the supply to the affected load which, furthermore, also continues to be protected against power failures by the UPS sets.. Figure 6 Example of a redundant supply to a load in an IP system in the event of failure of one UPS Since the affected load is now supplied via the IP chokes, its supply voltage is reduced by the voltage drop across these chokes. Appropriate design of the IP chokes can result in a supply voltage deviation at the load of less than 10% of the nominal voltage and is therefore within the permissible range defined by the ITI Curve [3] for the continuous supply. Up to this point, despite the failure of one UPS, absolutely no switching operations are therefore required to ensure the continuous supply of power to the load. The voltage drop across the IP chokes can be further reduced by connecting the load directly to the IP bus by means of a pair of switches. Figure 7 shows an IP system in which the IP bus is extended to a ring. The additional part is termed an IP return bus and, like other parts of the IP bus, can be used as a bypass for the loads. An additional pair of switches on the load bus enables no-break load transfer between the UPS output and the IP return bus. This enables controlled insulation of an individual UPS from the IP system and also reconnection to this system in order to carry out maintenance operations, for example, without having to interrupt the UPS supply to the load. 8

9 Figure 7 IP-system with the load of UPS #2 directly supplied via the IP return bus Short-circuit response The main reason for using IP topology is that possible short-circuits due to the use of IP chokes have only minor effects on the loads. In spite of this, short-circuits must also be insulated in an IP system as quickly as possible, both to meet the safety requirements and to keep interference on the power supply to the loads to within the limits specified by the ITI Curve. In the event of a short-circuit at the load shown in Figure 8 the effects on the whole system are relatively minor since two IP chokes are always connected in series between the short-circuit and the remaining loads. With suitably designed IP chokes the dynamic voltage sag at the unaffected loads is less than 10% of the nominal voltage. 400 V UPS 1 UPS 2 UPS 3 UPS ka 1166 A 10,5kA 1166 A IP-Bus Figure 8 load side of UPS #2. 46,5 ka 387 V 0 V 387 V 387 V Load 1 Load 3 Load 10 (40ms) 1 Example of fault circuit distribution in an IP system with a short-circuit on the 9

10 The IP bus is the central element of an IP system. A short-circuit on the IP bus has repercussions for all UPS units and their loads connected to them and is therefore a particularly critical event for the entire system. Compared to a short-circuit in the load distribution system, in the event of a short-circuit on the IP bus, only one IP choke is located in each case between the short-circuit and the UPS output or the load. As a result, in this case the voltage sag at the loads is much larger than the voltage sag in the case of a short-circuit in a load distribution system. With a low subtransient UPS reactance, the initial voltage sag at the load is limited to a value below 30%. For sensitive computer power supplies, according to the ITIC curve such a voltage sag is permissible for a maximum period of 500 ms. A protection concept which is specially tailored to the requirements of the IP system enables a short-circuit on the IP bus to be selectively insulated within 70 ms and at the same time enables the section of the IP system that is not directly affected to remain fully operational. Although this involves a less serious short-circuit for the loads, these numbers make it clear that it is advisable to take measures by which a possible short-circuit on the IP bus can be largely eliminated. The use of busbar systems, for example, which because of their compact construction are well-suited to isolating the IP bus from external effects, can contribute to a short-circuit-proof design. An essentially simple design using components with the lowest possible interference susceptibility, such as outgoing circuits and switch connections, likewise contributes to the prevention of possible short-circuits. UPS system An IP system consists of a number of UPS systems. Such a UPS system comprises the following main components: a UPS with an energy storage device, an IP choke for linking the UPS unit to the IP bus and the breakers necessary for the safe operation of the system. Figure 9 shows a system utilizing a rotary UPS. 10

11 Figure 9 Example of a UPS system utilizing a Piller UNIBLOCK UBT rotary UPS and a separate Diesel generator. 1 Public utility power grid 2 IP bus 3 IP return bus 4 Rotary UPS flywheel energy storage device 5 Diesel generator for long-term power supply interruptions (optional) 6 IP choke (with choke circuit-breaker) 7 Breaker pair for bypass transfer 8 IP bus disconnecting switches 9 Load An IP system is not limited to one special type of UPS installation. If, however, it is desirable to make optimum use of the advantages of such a system, then a rotary UPS with internal coupling choke is mandatory. Its high short-circuit capacity and its natural load sharing makes this type of UPS the best choice for an IP system. Rotary Diesel UPS systems are a suitable solution for bridging long-term power supply interruptions. Alternately, rotary UPS units combined with external Diesel generators connected to the UPS output can also be employed. 11

12 Even though a UPS in an IP system can be equipped with any kind of energy storage device, for stability reasons a bidirectional-operating energy storage device, such as an electricallycoupled flywheel is to be preferred. Otherwise, if batteries or electromagnetic clutches are used, the UPS system should be able to supply power and take up power equally in order to ensure the stability of the entire system, for example in the event of sudden changes in load. Control The control of voltage, current and frequency, plus all synchronization operations are realized by the controllers in the respective UPS unit. The UPS also controls its associated breakers and ideally, if required, should also be able to synchronize itself to other voltage systems, such as the IP bus, for example. A so-called system control is assigned to each UPS system. This is the central unit for operating and monitoring the UPS system. A mimic diagram which visualizes the status of the UPS system and also enables safe operation of the system is located at the front end of each system control. At the same time, the system control monitors all essential voltages, currents, phase angles and performance data of the associated UPS system. Signal lines between the system control and the UPS enable the functions of the UPS needed for system integration to be controlled, but without impairing the basic functionality of the individual UPS units. If required, the control of a separate Diesel generator integrated in the system, as well as its synchronization, can be realized by the system control. A digital communications interface allows easy integration of the system control in a building management system (BMS). Another PLC, the so-called master control, serves as primary control and monitoring for the entire IP system. Because of its central function, it consists of two redundant controls which communicate with the system control via a redundant digital communications bus. The master control collects the measurement data from the system controls and continuously compares the system utilization with the available UPS capacity. The data resulting from this comparison are sent back to the system control which, on the basis of these data, is able to react with the correct measures to a possible overload situation or to the failure of one UPS. The master control also controls the restoration of the power supply for the individual UPS systems and therefore enables a controlled transfer of the load of the IP system to the utility power grid. Figure 10 shows the arrangement of controls of an IP system. 12

13 Figure 10 Steuerung eines IP-Systems: Anordnung der Systemsteuerungen (S), der Mastersteuerungen (M) und der redundanten Kommunikationsleitungen Additional control cables which interconnect the individual system controls, enable the exchange of all the most important signals, even in the unlikely event of simultaneous failure of both master controls. An IP system can therefore be safely operated without a master control and with little restriction of its functionality. Operating modes In the power grid mode each UPS is connected to the power grid and supplies active and reactive power to its dedicated load. At the output side the UPS is connected via the IP choke to the IP bus, which enables independent control of the output voltage while equalizing active power with the other UPS units. The energy storage devices are fully charged and the Diesel engine is switched off. In the event of a power failure, each UPS is automatically isolates itself from the power grid and for the time being the load is supplied by the energy storage device of the UPS. From this moment on, the load sharing between the UPS units is controlled by a droop function, which is based on a power/frequency characteristic implemented in each UPS. Because the same characteristic is present in each UPS, no communication regarding the load sharing is 13

14 required between the individual systems. After the Diesel engines are started, the supply to the load is automatically transferred from the energy storage device of the UPS to the Diesel engine, so that the energy storage device can be recharged and, if required, again becomes fully available. To ensure load sharing also in the Diesel mode, each Diesel engine is controlled by the associated UPS via its internal droop function, irrespective of whether the engine is mechanically connected to the generator of the UPS (Diesel UPS) or an external Diesel generator is used. From a control point of view, due to this integration an external Diesel generator is also an integral part of the UPS system and the combination of UPS and Diesel generator can be considered as one unit. A special control structure in the UPS, in combination with the bidirectional energy storage device, enables active frequency and phase stabilization, which is also effective in the Diesel mode. The restoration of the power supply to the individual UPS systems is verified by the master control. Following a predefined power grid stabilization period and after all energy storage devices have been recharged, the master control ensures that the UPS systems are restored in sequence to the power grid mode. This procedure also enables high-power IP systems to be switched back to the supply system, without causing sudden, excessive load fluctuations in the power grid. Synchronization to the power grid uses a common synchronization voltage, which is obtained centrally from the main incoming supply. The shared use of this synchronization voltage as the preferred source, as well as the line voltage at each individual UPS as redundancy, ensures uniform synchronization of all systems, even if the line voltage may not be available to some UPS systems. As soon as the first UPS system is reconnected to the grid, the load sharing between the UPS systems which are still partially in the Diesel mode, can no longer be realized via the regular droop function because the system is now limited to the constant frequency of the power grid. Therefore, a new control method for load sharing had to be developed when operating at a fixed frequency. This new control, which was developed and patented by Piller Group GmbH, is called Delta Droop Control (DD control) and enables load sharing in mixed grid and Diesel mode, without the UPS systems having to communicate with each other regarding the load sharing. DD control is based on the physical characteristics of the individual IP system and enables each UPS unit in Diesel mode to adjust its power output at the units in grid mode, without the need for a central control unit. 14

15 Figure 11 Example of the load flow in an IP system: UPS #1 is already in grid mode, while units #2 to #16 are still in Diesel mode and their load sharing is controlled by the DD control. Stability An electrical network whose elements are interconnected by chokes has a slight tendency to oscillations which can be caused by sudden changes in the load distribution resulting from load fluctuations, Diesel engine starts and power outages. In this case, each load fluctuation at a UPS initially causes a change in the phase angle between the power grid and the UPS output. This change delayed by the IP chokes - is then transmitted to the other UPS units until, resulting from the new load distribution, a new operating point of the entire system has been established. To prevent this process from producing oscillations, transient frequency changes which inevitably occur as a result of changes in phase angles, must be effectively attenuated. So that this can happen, irrespective of the polarity of the load change, the energy storage device of the UPS should be able to absorb power of the same order of magnitude as it can also supply. Maintainability Assuming a redundant system configuration, the option to transfer the load from a UPS system to the IP return bus allows maintenance of a UPS system while its load is safely supplied by the remaining UPS units via the IP bus. 15

16 Additional breakers in the IP bus ideally on both sides of the IP choke connections as shown in Figure 9 allow sections of the IP bus to be insulated so that maintenance work can be carried out even on the IP choke and the IP bus itself. The IP system remains fully functional during maintenance, since the resulting sections of the IP bus are always connected by the IP return bus and the IP bus therefore continues to be available as an alternate supply for the load. By utilizing DD control it is also possible to carry out maintenance on the incoming feeder of a UPS system, with only the affected UPS then running in Diesel mode and the remaining UPS units able to remain in grid mode. Summarizing the above-mentioned functions, the IP system is probably the simplest system topology which, combined with maximum reliability, can be maintained during operation. All other solutions with similar maintainability (system redundancy, isolated redundancy and distributed redundancy) have a far more complex infrastructure requiring more maintenance, which in this case is possibly also subject to greater risks. Efficiency The high efficiency of an IP system is obtained because its very high reliability is achieved using a small number of redundant UPS units. Combined with load sharing, all UPS units can therefore be operated in a working point with high efficiency. In an IP system with a total power of 20 MW, for example, UPS units can be operated at 94 % capacity utilization. In a system redundant arrangement with similar high reliability the UPS units have only a maximum utilization of 45%. Consequently, there is a difference in the electrical efficiency of the individual UPS units of 3 to 4 %. This translates as a potential energy loss of around 5 to 7 GWh per year and therefore additional operating costs of up to (based on a price of 10 ct/kwh), not including additional costs of the increased cooling requirement. In addition to the advantages described above, an IP system therefore also constitutes a "green" solution and meets the economic requirements of a modern data center. 16

17 3 Projects The first isolated-parallel system was developed in 2007 for a data center in Ashburn, Virginia, [2]. It supplies a total load of 36 MW. The installation consists of two separate IP systems, each equipped with 16 Piller UNIBLOCK UBT 1670 kva UPS units with flywheel energy storage devices in a 14+2 redundant configuration. Each UPS is backed up by a separate 2810 kva Diesel generator, which is connected to the load bus downstream of the UPS and is able to supply critical as well as general loads. Since the success of the initial installation, further data centers with IP systems have been commissioned worldwide, many of which are in the order of 10 to 20 MW. 4 Conclusions The isolated-parallel system uniquely enables the advantages of combining isolatedredundant and parallel-redundant UPS configurations. It enables: distributed redundancy between the UPS units with simultaneous decoupling of the respective load buses and therefore the simple design of large, parallel UPS systems with several outputs. What also distinguishes the IP system from other systems are the outstanding maintainability and the unique fault tolerance, which ensure an extremely reliable and robust power supply for highavailability data centers, for example. Due to the use of flywheel energy storage devices and the possibility of realizing large UPS systems of up to 20 MW in low-voltage technology, the IP system is particularly suited to applications in which minimizing space requirement and maximizing return on capital are of the highest priority. By reducing the redundant UPS units to a minimum and avoiding systems which run in the standby mode, in addition to an extremely "green" solution, the IP system is an excellent choice when a safe and ecologically acceptable power supply is demanded. 17

18 References [1] Mike Mosman, Iso-Parallel Rotary UPS Configuration, 7x24exchange, 2005 Fall Conf., [2] Mike Mosman, An Isolated-Parallel UPS System for a large Data Center, 7x24exchange, 2007 Fall Conf., [3] Published by Information Technology Industry Council, revised 2000 [4] Ralf Briest, Kevin Collins, Hilmar Darrelmann, Mike Mosman Load Sharing and Stability Analyses of Iso-Parallel UPS Systems PCIM Europe 2007 [5] Frank Herbener, Reliability of UPS Piller White Paper No. 054 Frank Herbener, Piller Group GmbH, Frank.Herbener@piller.com, Germany White Paper No / Mrz