Feature
System Grounding
for Mission-Critical Power Systems
by Reza Tajali, PE
Schneider Electric, Power Systems Engineering
D
esigning, 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 important 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.
Figure 1 — Multiple Source System
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Solidly-Grounded Systems
and the Power Availability
Problem
With a solidly-grounded system, an intentional connection to
ground provides stable voltages
between the phase conductors
and ground (Figure 2). The generator generates power which is
delivered to the loads through the
cable system. The intentional connection 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 neutral-to-ground bond provides stability, and transient voltage surge
suppression (TVSS) devices can be
applied with great advantage.
However, these systems are designed to trip and isolate ground
faults efficiently. Making an intentional connection to ground creates
a return path for ground fault currents. When a fault occurs, the current flows back to the source along
this path causing a circuit breaker
to trip, thereby interrupting power.
Note, however, by definition, mission-critical systems cannot tolerate an interruption in power.
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Figure 2 — Simple Power System
Data centers below five MVA are typically supplied from 480-volt commercial power. Larger facilities 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. Depending 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 breakers are already open. However, the ground fault current can return through the neutral-to-ground bond of
side B or the neutral-to-ground bond of the generator.
Figure 4a — Stable System Neutral of
the System is at Ground Potential
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Figure 3 — Ground Fault Current Causing Incorrect Tripping
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, different manufacturers have taken different approaches
to solving this problem, and no standardization exists 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 availability. Because there is no return path for ground fault
current, overcurrent protective devices will not trip.
The first ground fault essentially provides a cornergrounded system. An alarm is initiated so that the
maintenance crew can locate and repair the fault.
However, the phase-to-ground voltages in ungrounded 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 4b — Solid Line to Ground Fault
in Ungrounded System
Figure 4c — Unstable System
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ground. This is in comparison with
minor insulation which in power
transformers refers to the turn-toturn 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 overvoltages. These overvoltages can
wreak havoc with equipment insulation 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 extinction 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 magnitudes higher than three per unit
can appear in an ungrounded
system.
In any system, arcing faults become more common as the power
system ages. As illustrated previously, ungrounded systems are
unstable and, therefore, unsuitable
for mission-critical applications.
High-Resistance Grounded
Systems
High-resistance grounded power systems (Figure 6) have been
touted as providing the best compromise between solidly-grounded
designs and ungrounded systems.
These systems are simple to design
and implement, especially on multiple-source power systems.
Inserting a resistor in the
grounding circuit caps the magnitude of ground fault currents
to below 10 amperes, limiting the
Winter 2004-2005
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 arcing 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 encounters 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 480volt 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 complexity and maintainability. High-resistance grounded
power systems are extremely simple and provide high
power availability, but they require diligent maintenance 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 encountered, creating a power availability problem. Creating 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 Engineering 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.
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