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by Reza Tajali, PESchneider Electric, Power Systems Engineering

Designing, commissioning, and maintaining the electrical networks of commercial facilities such as telecommunication server farms and Internet data centers are technically complex projects. Continuing

deployment of e-commerce activity is taxing capacity of existing facilities. That means assuring the reliability of critical power is increasingly impor-tant to designers, contractors, and technicians who plan new facilities.

For more than 100 years, the world has been generating, transmitting, and using electrical power. Technology has evolved, but the principles of electricity have not. From the pages of the National Electrical Code to the manuals of high-voltage engineering, the value and necessity of power system grounding is a common theme.

There are three ways to ground low-voltage power systems. They can be solidly-grounded, ungrounded, or impedance-grounded. When it comes to mission-critical power applications, there is no one best method – each involves tradeoffs in power quality versus power availability.

Solidly-Grounded Systems and the Power Availability Problem

With a solidly-grounded sys-tem, an intentional connection to ground provides stable voltages between the phase conductors and ground (Figure 2). The gen-erator generates power which is delivered to the loads through the cable system. The intentional con-nection to ground plays no part in transferring three-phase power from the generator to the load. If this connection is removed, power will continue to flow.

Solidly-grounded systems are the most stable from a power quality point of view. The neu-tral-to-ground bond provides sta-bility, and transient voltage surge suppression (TVSS) devices can be applied with great advantage.

However, these systems are de-signed to trip and isolate ground faults efficiently. Making an inten-tional connection to ground creates a return path for ground fault cur-rents. When a fault occurs, the cur-rent flows back to the source along this path causing a circuit breaker to trip, thereby interrupting power. Note, however, by definition, mis-sion-critical systems cannot toler-ate an interruption in power.Figure 1 — Multiple Source System

Feature

System Groundingfor Mission-Critical Power Systems

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Data centers below five MVA are typically sup-plied from 480-volt commercial power. Larger facili-ties are supplied at distribution voltage and utilize facility-owned power transformers to step down the voltage to 480 volts. These utility services typically have many interruptions. In order to make the bulk source of power continuously available, multiple sources of power are tied together. Figure 1 depicts one such system, which may include one or more tie circuit breakers. Power can be supplied from two or more alternate sources through a transfer scheme. The generator could be standby, or it could be run in parallel with the utility sources.

While multiple power sources can solve power availability problems, they can greatly complicate the ground fault system. Common practice in the United States is to use three-pole circuit breakers, and the neutral conductor is not switched. Depend-ing on the preference of the designer, the neutrals of the two systems could be tied together inside the main switchboard – or they can remain untied.

Figure 3 illustrates a ground fault on side A of a distribution bus where the neutrals are tied together. Let us assume that the tie circuit breakers are normally kept open. Ideally, for a fault on side A only circuit breaker A should trip – assuming the tie circuit break-ers are already open. However, the ground fault cur-rent can return through the neutral-to-ground bond of side B or the neutral-to-ground bond of the generator.

This current would then flow over the neutral bus of the switchboard, which can cause circuit breaker B and/or the generator circuit breaker to trip.

This problem can be solved by appropriate design of the ground-fault protection system. Unfortunately, dif-ferent manufacturers have taken different approaches to solving this problem, and no standardization ex-ists to guide the consulting engineers who ultimately specify this equipment.

Ungrounded Systems and the Power Quality Problem

Unlike solidly-grounded systems, ungrounded power systems provide excellent power availabil-ity. Because there is no return path for ground fault current, overcurrent protective devices will not trip. The first ground fault essentially provides a corner-grounded system. An alarm is initiated so that the maintenance crew can locate and repair the fault.

However, the phase-to-ground voltages in un-grounded systems are unstable, creating power quality problems. Phase-to-ground voltage cannot be ignored because the “major insulation” in all power systems and utilization equipment is sandwiched between the phase voltage and ground. (Major insulation refers to the insulation between the phase conductors and

Figure 2 — Simple Power System

Figure 3 — Ground Fault Current Causing Incorrect Tripping

Figure 4a — Stable System Neutral of the System is at Ground Potential

Figure 4b — Solid Line to Ground Fault in Ungrounded System

Figure 4c — Unstable SystemVLG>VLL

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ground. This is in comparison with minor insulation which in power transformers refers to the turn-to-turn insulation.)

If the power system does not have a stable reference to ground, the magnitude of line-to-ground voltages are completely undefined. As shown in Figures 4a, 4b, and 4c, the line-to-ground voltages can be lower, equal to, or greater than the phase-to-phase voltages.

Figure 5 depicts an arcing ground fault in an ungrounded system. Chaotic by nature, the arc strikes, goes through extinction, and restrikes, which can create very high line-to-ground over-voltages. These overvoltages can wreak havoc with equipment insu-lation and sensitive electronics.

The mechanics by which these overvoltages are generated is rather simple to illustrate. Power systems have a certain amount of stray capacitance. The arc ex-tinction and restrike process is similar to opening and closing a switch. Every time an inductive/capacitive circuit is switched, an oscillatory transient condition is created. Overvoltages with mag-nitudes higher than three per unit can appear in an ungrounded system.

In any system, arcing faults be-come more common as the power system ages. As illustrated previ-ously, ungrounded systems are unstable and, therefore, unsuitable for mission-critical applications.

High-Resistance Grounded Systems

High-resistance grounded pow-er systems (Figure 6) have been touted as providing the best com-promise between solidly-grounded designs and ungrounded systems. These systems are simple to design and implement, especially on mul-tiple-source power systems.

Inserting a resistor in the grounding circuit caps the mag-nitude of ground fault currents to below 10 amperes, limiting the

Figure 5 — Arcing Ground Fault Generates High Line to Ground Overvoltages. The Capacitor Symbol Serves to Illustrate System Stray (Distributed) Capacitance.

Figure 6 — High-Resistance Grounded System and the Arcing Ground Fault. The Capacitor Symbol Serves to Illustrate System Stray (Distributed) Capacitance.

damage at the point of fault. Similar to the ungrounded system, the first fault will not cause an interruption. While the resistor significantly limits the magnitude of transient overvoltages, they are not eliminated. An arc-ing ground fault will not trigger the alarm circuit, so an arcing fault can exist and remain undetected for extended periods of time.

Application of TVSS devices to high-resistance grounded systems en-counters some limitations. In these systems the line-to-ground voltage can be as high as the phase-to-phase voltage (as in the phase-to-ground fault depicted in Figure 6). Thus, any TVSS device used in these systems must have a rating equal to or exceeding the phase-to-phase voltage. The higher voltage rating means higher clipping voltage, which limits the level of protection offered by the TVSS.

However, the design of data center power systems typically provides complete isolation between the 480-volt high resistance-grounded power and the 208/120-volt solidly-grounded critical power, which is delivered to sensitive electronics. Transformers in power distribution units provide this isolation. Therefore, the line-to-ground transient activity in the 480-volt power will not transfer to the critical power of the computers.

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Seek professional advice on which system best meets your power availability and power quality needs, keeping in mind the tradeoff between com-plexity and maintainability. High-resistance grounded power systems are extremely simple and provide high power availability, but they require diligent mainte-nance personnel who look for and repair ground faults. The biggest problem with these systems is that the first ground fault usually is ignored by the maintenance personnel because the power continues to flow.

Solidly-grounded systems provide excellent power quality, but they trip each time a ground fault is en-countered, creating a power availability problem. Cre-ating an optimized system – and avoiding costly errors – requires an understanding of these tradeoffs.

Reza Tajali, a registered electrical engineer in California and Tennessee, is a staff engineer for Square D’s Power Systems Engi-neering group in Nashville, Tennessee. He has more than 20 years experience with electrical power distribution and control. Tajali holds two United States patents on switchgear products. He is responsible for performing power quality audits on industrial and commercial facilities. That work includes measurement, analysis, and simulation of power systems.