_RP_550-1-11-1981_scan.pdf

Manual on Installation of Refinery Instruments and Control Systems Part 1-Process Instrumentation and Control Section 11-Eiectrical Power Supply Refining Department API RECOMMENDED PRACTICE 550 THIRD EDITION,JUNE 1981 OFFICIAL PUBLICATION REG.U.S.PATENT OFFICE CONTENTS SECTION II-ELECTRICAL POWER SUPPLY PAGE 11.1 Scope.11.2 General.11.2.1 Plant Power Reliability.I 11.2.2 Power Outage.2 11.3 Instrument Load Characteristics.2 11.3.1 Pneumatic Systems.2 11.3.2 Electronic Analog Systems.2 11.3.3 Digital Systems.2 11.4 Instrument Load Requirements.3 11.4.1 General.3 11.4.2 Reliability Requirements.3 11.4.3 Power Quality Requirements.4 11.4.4 Emergency Power Source Capacity.4 11.5 Electrical Power Supply.4 11.5.1 General.4 11.5.2 Conditions.4 11.5.3 Instrument Power Supply Source.5 11.5.4 Power Supply Regulation.5 11.5.5 Pneumatic System Power Supplies.5 11.5.6 Electronic System Power Supplies.5 11.5.7 Typical Designs.6 11.6 Automatic Transfer and Parallel Power Sources.7 11.6.1 General.7 11.6.2 Design Problems.7 11.6.3 Problem Areas.7 11.7 Distribution System.8 11.7.1 General.8 11.7.2 Requirements.8 11.7.3 Criteria for System Design.8 11.8 Wiring Methods.9 11.8.1 General.9 11.8.2 Power Wiring.9 11.8.3 Special Procedures.9 11.9 System and Equipment Grounding.9 11.9.1 Instrument Signal Grounding.9 11.9.2 Instrument Power Supply Grounding.9 11.9.3 Equipment Grounding.9 vii LIST OF ILLUSTRATIONS Figures Il-l-Secondary Transfer Using Circuit Breakers.I 11-2-Fuel Supply Shutdown Circuit for Momentary Power Failure Security.2 11-3-Power Supply for Critical Instrument Load.3 11-4-Typical Automatic Transfer Switching Methods.4 11-5-Semi-Critical Supply.6 11-6-Critical Power Supply.7 11-7-Critical Power Supply with Redundancy.8 viii Part!-Process Instrumentation and Control SECTION 11-ELECTRICAL POWER SUPPLY 11.1 Scope This section is intended as a guide for the design and se-lection of equipment for a highly reliable electrical power distribution system for plant process instrumentation and controls.For additional information on the installation of electrical systems,refer to API RP 540,Recommended Practice for Electrical Installations in Petroleum Process-ing Plants.11.2 General A supply of continuous electrical power to a plant process instrumentation and control system is essential ror safe operation and the manufacture of on-grade products.When power interruptions occur,the plant must be de-signed either to continue to operate on standby steam-driven turbines or to shut down in a safe and orderly man-ner.Control systems and monitoring devices must continue to provide information and control during this period;therefore,the electrical power system reliability re-quirement is much greater for instrument system supplies.11.2.1 PLANT POWER RELIABILITY The reliability of the service to a plant depends on many factors.Utility companies invest considerable time and ef-fort in improving the reliability of their systems.This in-cludes providing redundant equipment,alternate sources of power such as neighboring electrical utilities,and various contingency plans to permit operation during emergency conditions.The reliability of electric power varies from region to re-ELECTRIC UTILITY SYSTEM/-INTERLOCK(_ J _)gion,and in most areas seasonal outages are predictable.Unpredicted events,such as earthquakes or man-caused disasters,also cause long,unexpected outages or curtail-ments.The configuration of the transmission and distribution system can be arranged to provide maximum reliability.For an all-electric-drive plant where service is critical,at least one redundant feeder line is required together with transformers,main breakers,and a tie-breaker for fast au-tomatic transfer.Typical utility substation arrangements are shown in Figure 11-1.If the plant has sufficient steam and electric operation is only a matter of economics,then a single utility feed could be used with steam-driven turbines to back up electric mo-tors.Most utility companies will provide whatever config-uration the customer requires.In return for this service,a facilities charge may be added to the monthly billing for any electrical equipment which exeeds what the utility company considers necessary for standard service.The trend in the processing industry is to rely more heavily on the utility companys ability to deliver power with minimal,if any,interruption.New plants are relying Jess on uneconomical standby steam drivers,especially in locations where the utility has a reliable service record.As a result,only the critical services,such as essential digital and analog control systems and shutdown circuits,are being served with uninterruptible power supplies in new installations.The processing portion of the plant is left to circulate down as much as possible by using critical-standby steam drivers(generally on circulating pumps)when power failures occur.ELECTRIC UTILITY SYSTEM/INTERLOCKS(-)I I I Ill 11111111 BUS:TIE BUS 1-.-.-I-BUS 2 LOADS LOADS LOADS Figure 11-1-