Application of Four-Pole Circuit Breakers within Data Centers

Transcription

1 Application of Four-Pole Circuit Breakers within Data Centers December 2013/AT324 by Frank Waterer, Fellow Engineer, Schneider Electric Engineering Services Make the most of your energy SM Revision #1 12/13

2 Summary Abstract... p 3 IEC lexicon (vs) North American Nomenclature... p 4 Brief Introduction of the Four Wire Electrical System... p 5 Origins of Four-Pole Circuit Breakers... p 6 Radial Circuit Breaker Applications in North America... p 7 The use of MDGF Protection Systems versus Four-Pole Circuit Breakers... p 9 The use of four-pole circuit breakers as UPS and generator circuit breakers with HRG systems... p 10 White paper on data centers 2

3 Abstract Since the introduction of the four pole circuit breaker in the 1950s, there has been ambiguity as to the appropriate applications for its utilization. Of particular interest is the use of four pole circuit breakers in multiple source power distribution systems such as service entrance rated equipment, optional stand-by generators, and uninterruptable power system (UPS) units where some of the three phase power systems are resistively grounded. This paper provides a brief history on the origins of three phase, four wire systems and the use of four pole circuit breakers within data centers today. Also discussed are the applications for using four pole circuit breakers with power distribution systems in lieu of the installation of Modified Differential Ground Fault (MDGF) Protection Systems. Get connected to power White paper on data centers 3

4 IEC lexicon (vs) North American Nomenclature Below is a short list of notable abbreviations and their definitions that are commonly used in countries that follow the mandates of the International Electrotechnical Commission (IEC) and the equivalent term in North America. Closed Coupled: The connection of two or more electrical power sources together in a synchronized manner. (North America: closed transition) Earth: Connected directly to the planet Earth or through a selected resistance where soil resistivity is measured and maintained. (North America: ground) Line: The energized conductor relative to the earth or another line. (North America: line of phase) Non - Coupled: The prevention and restriction of connecting two or more electrical power sources together. (North America: open transition or make before break) PEN: Protected Earth Neutral (USA within NPFA documents: the grounded conductor or neutral) RCD: Residual Current Device [USA: ground fault circuit interrupter (GFCI) or a ground fault relay (GFR)] RCCB: Residual Current Circuit Breaker. Also known in other IEC countries in Eastern Europe and India as an Earth Leakage Circuit Breaker or ELCB. (USA: circuit breaker with an integral ground fault protective function) RE: Resistively Earthed. [USA: low resistive grounded (LRG) system or a high resistive grounded (HRG) system. A resistively earthed or ground system is one in which a resistor with a specific resistance is intentionally installed in series between the Xo terminal of the supplying transformer and earth.] SCPD: Short-Circuit Protection Devices (circuit breakers or fuses) STD: Short Time Delay protection (protection against short-circuit over currents by circuit breaker with rapid trip release) White paper on data centers 4

5 Brief Introduction of the Four Wire Electrical System Beginning in the late 1950 s and continuing through the 1960 s, in response to a growing energy and raw materials shortages and energy supply issues, the manufacturing and industrial base within the United States began a slow and steady conversion from 3Ø, 3W, ungrounded DELTA, systems to 3Ø, 4W, solidly grounded, WYE systems. The use of three separate single phase transformers were replaced with a design in which all of the three phase windings are enclosed within a single transformer tank. This was a dramatic savings in materials and real estate. Commercial and industrial lighting systems in the United States rapidly changed from 120V to 277V. The operating voltage for industrial motors changed from 220/230V to 440/460V. The use of higher utilization voltages resulted in the use of less copper. Smaller conductors and smaller conduits equated to significant savings in material and labor cost. Eventually all commercial and industrial utilization equipment within the United States were converted to a power distribution systems supplied with a 3Ø, 4W, solidly grounded, WYE, 480Y/277V, 60Hz. The reconfigured transformers and systems provided power for both motors and facility lighting. In Canada, the voltage for the 3Ø, 4W configured systems were converted to 600Y/347V, 60Hz. Similar voltage system conversions had already begun in Europe and some other IEC countries years before for the same economic and raw material reasons. In the United Kingdom, the voltage for the 3Ø, 4W configured systems were 415Y/240V, 50Hz. In Europe, North Africa, and some Middle Eastern countries, the 3Ø, 4W configured systems were 380Y/220V, 50Hz. See Figure 1 below. Presently in the European Union, the utilization voltages are being converted to a 3Ø, 4W, 400Y/230V, 50Hz for standardization purposes. Figure 1 Three Phase, Four Wire, Solidly Earthed/Grounded WYE White paper on data centers 5

6 Origins of Four-Pole Circuit Breakers In IEC countries, the use of 3Ø, 4W configured voltage systems are not limited to commercial and industrial power equipment. The respective voltage and current between a line-to-protected earth neutral is utilized to supply residential housing, shops, small stores, and clinics with a 240V or 220V single line (or phase) electrical service. In IEC countries, two-pole circuit breakers have been utilized for decades in single line or phase applications and with RCDs to open the line conductor as well as the PEN or protected earth neutral conductor during a faulted condition or as a result of the detection of leakage currents in order to completely isolate the supply from the load for safety concerns. The use of four pole circuit breakers originated in Europe after the installation and use of 3Ø, 4W, solidly grounded, WYE configured power systems became widely common. The use of four pole circuit breakers in Europe was deemed a logical technological evolution with the two-pole circuit breakers utilized in single phase application and RCDs to open the line conductor as well as the PEN for same safety concerns. The initial use of four pole circuit breakers in IEC countries were on low ampacity control and power circuits and in applications requiring a RCCB or ELCB. RCCB or ELCB that interrupted all energized phase conductors and the PEN were employed in IEC countries where the traditional molded case circuit breakers or fuses (SCPD and STD) did not detect leakage currents dangerous to humans and livestock or that could result in fire hazards if not detected and interrupted. It has been historically common in IEC countries to provide complete mechanical isolation of the PEN from the electrical current source after any faulted condition or when isolation from the PEN is desirable for maintenance or repairs to existing electrical power circuits. As the power distribution system grew in ampacity in IEC countries due to power demands, the size of four pole circuit breakers also increased in ampacity and application. When two source power systems in commercial applications began to be installed in IEC countries to provide an alternate source of electrical power during abnormal or emergency conditions, the use of four pole circuit breakers were the applicable technological means of choice to avoid any circulating current between two separate electrical power source and a common load. The use of four pole circuit breakers in a non-coupled or open transition application also provided a means to avoid nuisance tripping of RCCB or ELCB (ground fault circuit breakers or ground fault relays) due to inadequate sensing and relaying of current imbalances caused by single phase loads. White paper on data centers 6

7 Radial Circuit Breaker Applications in North America The 3Ø, 4W, solidly grounded, WYE, 480Y/277V and 600Y/347V power distribution system used in North America are not utilized to supply electrical power to residential housing, shops, small stores, and clinics as are the 3Ø, 4W systems in IEC countries. Also, two-pole circuit breakers and two pole RCDs are not installed in single phase 277V and 347V applications to open the neutral conductor associated with a feeder or branch circuit in North America. In North America a three-pole circuit breaker has been the standard for an over current protective device in three phase power circuits since molded case circuit breakers were first developed and introduced into the electrical market in the USA in See Figure 2 below. In North America, the neutral conductor within electrical power equipment has traditionally been installed as a solidly connected conductor and is not automatically opened to mechanically separate the grounded conductor or neutral in the service rated equipment or any distribution equipment from the source of electrical power as part of any electrical protective function. Instead a mechanical means is provided in the electrical service rated equipment, pursuant Underwriter s Laboratory (UL) requirements, to physically separate the grounded conductor or neutral associated with all loads from the source of electrical power if necessary. This mechanical means is identified as the neutral disconnect link. See Figure 2 below. The intended purpose of a neutral disconnect link is to provide a mechanical means to completely separate an electrical service from its electrical supply provided by a local electrical utility, electrical co-operative, or electrical energy provider. If a neutral circuit connection still remained between Xo terminal of the supplying transformer and the neutral terminal within the service entrance equipment after the opening of the associated main circuit breaker, then connected loads would still remain actually connected to the supplying system via the respective neutral or associated grounded conductor(s). By operating and opening the main circuit breaker or fused disconnect within the service entrance equipment, as well as mechanically disconnecting and removing the neutral disconnect link, all electrical loads can be completely isolated and electrically separated from its electrical supply. Also, when an integral ground fault protective function is required or specified on a three phase, four wire, solidly grounded, WYE power circuit, then a separate neutral sensor is installed to monitor imbalanced current flow in the neutral conductor in order to provide effective ground fault protection along with a Main Bonding Jumper. See Figure 2 below. White paper on data centers 7

8 Figure 2 Typical Example of a Three-Pole Circuit Breaker with Integral Ground Fault Protection White paper on data centers 8

9 The use of MDGF Protection Systems versus Four-Pole Circuit Breakers There are specific benefits of installing four pole circuit breakers in multiple ended power distribution systems. During the past decades there have been numerous attempts to avoid the challenges, complexities, and cost associated with the design and installation of complex MDGF Protection Systems by the use of four-pole circuit breakers in switchboard and switchgear construction where there are two or more sources of electrical power. In some applications the use of four-pole circuit breakers can be a direct substitute for a well engineered and effectively commissioned MDGF Protection System. However, there are specific benefits of installing and utilizing four-pole circuit breakers in lieu of designing and installing a MDGF Protection System. The benefits for installing four pole circuit breakers in multiple ended power distribution systems are listed below. No MDGF Protection System is required if the Service Entrance Equipment is constructed in a Main-Main or in a Main-Tie-Main style configuration in which all four-pole circuit breakers are only operated in an open transition operation. No two electrical power sources can ever be interconnected in any closed transition operation at any time. In effect, the power distribution system will always be operating as a radial power distribution system with only one electrical power source at any given time for any and all connected electrical loads. The use of four-pole circuit breakers removes the necessity and cost to a project for a complex MDGF Protection System along with complex interconnection wiring between individual sections or between remote line ups of switchboards or switchgear. The use of four-pole circuit breakers in an open transition will prevent circulating current between the neutral bus and the ground bus within the interior of the switchboard or switchgear Service Entrance Equipment. The use of four-pole circuit breakers enhances the switching operations when employed with power systems with multiple generators by limiting the dielectric stresses on the insulation of components and equipment connected to power distribution systems that can result from switching transient. However, the installation of four-pole circuit breakers cannot not be utilized in every configured multiple ended power distribution systems to avoid the necessary use of a MDGF Protection System. Below is list of common issues and limitations. Four-pole circuit breakers can NOT be employed as a solution to avoid the installation of a MDGF Protection System if these circuit breakers are ever employed in continuous paralleled or closed transition operations. Inherent selectivity between Mains and Ties can no longer exist without allowing a space in the TCC curves for the insertion of one or more tie circuit breakers. Consequently, selective coordination can be compromised resulting in circuit breakers being subjected to the forces of greater I2T energy under a phaseto-ground fault. White paper on data centers 9

10 The use of four-pole circuit breakers as UPS and generator circuit breakers with HRG systems HRG systems can be installed with the intended purpose of dramatically reducing the current flow to ground on the three phase, four wire output windings of a UPS Unit, paralleled UPS Units, a single generator, or paralleled generators for a limited period of time during a phase-to-ground fault condition. However, where any UPS Units or generators contain a HRG System there must not be any interconnection of the service entrance neutral conductor with the neutral associated with the Xo output windings of any UPS Unit or generators containing a HRG System. This would result in the unintended consequences of the HRG for the UPS Unit(s) and generators functioning as a HRG for the service entrance system. Also, if the output windings of the multiple UPS Units or generators are paralleled together, they must not have a respective interconnected common neutral bus if each UPS Unit or generator has an individual HRG connected in series between its respective Xo terminal and ground. The use of four pole circuit breakers for UPS Unit(s) or generators with HRG Systems can be beneficial to completely isolate the outputs of UPS Unit(s) or generators from upstream service entrance equipment supplied by a utility or customer provided power class transformer with secondary winding configured as 3Ø, 4W, solidly grounded, WYE. White paper on data centers 10

11 Schneider Electric USA, Inc S. Roselle Road Palatine, IL Tel: Fax: Document Number AT324 This document has been printed on recycled paper January 2013 sf 2013 Schneider Electric Industries SAS, All Rights Reserved.