Consulting-Specifying Engineer - January 2009 - (Page 39) tion. Sensitive electronic equipment may require a connection through power conditioning and energy storage and/or devices such as battery or UPS systems. • Consequences and /or economic impact of normal supply interruption. • Economic impact of complete power outage and investment available to address the issues. • In-house capability to operate and maintain an emergency power supply system. Figure 1 shows the emergency power system as a radial, single power supply source with an emergency generator as an alternate source. This is the simplest system to operate and is easy for O&M staff to understand. It can be fully automated with a minimum level of control. This configuration has a low initial investment but is vulnerable to outages for faults and maintenance. If the normal utility source is lost, the entire system is lost for approximately 10 seconds until generators can be brought online. If the generator unit fails to start during an outage, then the emergency loads will be lost. The key to a reliable emergency power system is to minimize single point of failure by having multiple sources, multiple paths, and multiple components configured in the simplest way possible. A more elaborate emergency power system is illustrated in Figure 2. Level of redundancy In Figure 2, the system has multiple levels of redundancy. It consists of multiple power supply sources connected to a split bus, primary switchgear with bus tie capability. Each side of the switchgear bus serves a facility service transformer for added capacity redundancy. The secondary service switchgear also is arranged in a split bus configuration with bus tie capability. The emergency power supply system has multiple generating units to emergency backup power if both the normal and the alternate power supply sources fail. Both the utility switchgear and the emergency generator switchgear have paralleling capability. The system has the ability to switch between two utility sources via a source transfer scheme implemented by the medium voltage relay for the main and the tie circuit breakers. If both utility sources are lost, the essential electrical loads are sup- Emergency source G Normal switchgear bus Emergency switchgear bus Normal loads To other critical/essential transfer switches ATS Critical/essential loads Figure 1 This emergency power system has a radial, single power supply source with an emergency generator as an alternate source. Source: Stanley Consultants Source A Source B (alternate) MV switchgear MV switchgear Emergency source G LV emergency switchgear bus G LV switchgear LV switchgear Normal loads Normal loads ATS ATS Critical/essential loads Critical/essential loads Figure 2 This more complex system has multiple levels of redundancy. Source: Stanley Consultants Consulting-Specifying Engineer • JANUARY 2009 39
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.