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Delta
Versus Wye Connected
Capacitor Banks
Introduction
Delta
connected capacitor banks are often specified for medium
voltage (2.4kV to 34.5kV) systems. This is contrary
to IEEE Std 1036-1992, "IEEE Guide for Application
of Shunt Power Capacitors". This standard states
that delta connected capacitors are generally only used
at low voltages, e.g., 2400 V, where a standard capacitor
rating is not available for a wye-connection. The standard
further states that wye-connected capacitor installations
are less complicated to construct, and are more economical.
The
purpose of this bulletin is to discuss the basic aspects
of the delta and ungrounded-wye connected capacitor
bank. A related bulletin, titled, "Why
Your Capacitor Bank Should be Left Ungrounded",
is available from NEPSI, and it discusses the aspect
of grounded versus ungrounded-wye connected capacitor
banks.
Delta
Versus Ungrounded-Wye
Figure 1 shows the capacitor
bank connections that are the topic of this bulletin.
The only other popular connection that is not shown,
is the grounded-wye and split wye-connected capacitor
bank. The following key points can be made in regard
to bank connection under normal and abnormal system
conditions.
Bus
Bar Connections: From
looking at figure one, it should be evident that the
ungrounded-wye connection is much simpler in design
than either of the two delta connected banks. The cross-over
connection that connects phase "A" to phase
"C" to close the delta is complicated at the
medium voltage level due to clearance requirements.
Figure
1 -Connection
and fusing arrangements for ungrounded-wye and
delta connected capacitor banks
Fusing:
Figure one also shows common fusing practices for each
of the bank arrangements. The figure shows that the
delta connected bank can be protected by placing the
fuses inside or outside of the delta. Two fuses per
single phase capacitor are required when fusing inside
of the delta, but their rating is decreased to 57% of
the outside fuse rating. The fuses outside of the delta
are sized in the same way as the fuses for the ungrounded-wye
connected capacitor bank.
Capacitor:
Except for voltage rating, the capacitors in both ungrounded-wye
and delta-connected banks are the same and will have
the same kvar rating. They consists of a double bushing
design, meaning both terminals are fully insulated from
their case (ground). On delta connected banks, the capacitors
have a line-to-line voltage rating, and on a wye-connected
banks, they have a line-to-neutral voltage rating.
Fault
Conditions: A capacitor
typically fails in two ways: 1) A bushing to case fault
occurs. 2) The internal sections fail (commonly known
as a dielectric fault), which basically shorts the capacitor
terminals. Whether the capacitors are delta or wye-connected,
a bushing fault will have the same impact on the power
system.
Internal
section faults, or dielectric faults appear differently
to a power system. On a delta connected bank, an internal
section fault subjects the power system to a phase-to-phase
bolted fault. This fault will cause a major voltage
sag on the facilities power system (until the capacitor
fuse(s) blow) and may cause capacitor case rupture if
not properly protected. It also subjects the power system
to high magnitude fault currents which can impose mechanical
and thermal stress on components in the fault path.
On a wye-ungrounded capacitor bank, internal section
faults subject the power system to a fault current that
is three times the banks rating (until the capacitor
fuse blows). Therefore, the voltage sag, mechanical
and thermal stressing associated with the fault, and
case rupture concerns are reduced.
Common
Misconceptions
The following misconceptions are common reasons why
medium voltage capacitor banks are specified as delta
connected:
•
The system that the bank is being applied to is delta
connected or high resistance grounded. So therefore
the bank should be delta connected.
(Statement Not True)
•
It is a typical practice for low voltage (600 volts
and below) systems, and therefore it must be appropriate
for medium voltage systems. (Not technically appropriate
for medium voltage systems)
•
A faulted wye-connected capacitor bank will go undetected.
(Not true when a blown
fuse detection system is installed).
•
Bank kvar rating will not be correct. (Not True)
All
of the above reasons do not justify the use of a delta
connection over an ungrounded-wye connection.
Conclusion
Medium Voltage Capacitor Banks
for commercial, industrial, and utility power system
applications should be specified as wye-connected unless
a standard voltage rating does not exist. This typically
occurs for 2.4kV systems, because 1.38kV capacitors
do not exist. At other medium voltage levels through
34.5kV, line-to-neutral rated capacitors are available
and should normally be connected in an ungrounded-wye,
or grounded-wye connection.
Northeast
Power Systems, Inc.
66
Carey Road
Queensbury, New York 12804
Phone: 518-792-4776
Fax: 518-792-5767
E-mail:
sales@nepsi.com
Website:
www.nepsi.com
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