19-10-2012, 11:05 AM
Auxiliary DC Control Power System Design for Substations
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Abstract
The most critical component of a protection, control,
and monitoring system is the auxiliary dc control power
system. Failure of the dc control power can render fault detection
devices unable to detect faults, breakers unable to trip for faults,
local and remote indication to become inoperable, etc. The auxiliary
dc control power system consists of the battery, battery
charger, distribution system, switching and protective devices,
and any monitoring equipment. Proper sizing, design, and maintenance
of the components that make up the auxiliary dc control
system are required. Many references for stationary battery system
design address only a specific battery technology, making it
difficult to compare different types of batteries for their overall
suitability to substation application. Also, most references do not
address the particular requirements of the electrical substation
environment and duty cycle. This paper provides an overall review
of things to consider in designing the auxiliary dc control
power system for an electrical substation.
INTRODUCTION
The most critical component of a protection, control and
monitoring (PCM) system is the auxiliary dc control power
system. Failure of the dc control power can render fault detection
devices unable to detect faults, breakers unable to trip for
faults, local and remote indication to become inoperable, etc.
In many cases, the dc system is not redundant, which makes
reliability an extremely important consideration in the overall
design. The auxiliary dc control power system consists of the
battery, battery charger, distribution system, switching and
protective devices, and any monitoring equipment. Proper
design, sizing, and maintenance of the components that make
up the dc control power system are required.
PCM systems that do not include an auxiliary dc control
power system can be used if properly designed. These
schemes typically use devices that do not require a station
battery source to function. Examples include fuses, selfcontained
reclosers, CT/VT-powered relays, capacitor trip
devices, integral battery trip devices, etc. These same devices
might also be used in backup systems for unmanned facilities
without remote monitoring and without redundant battery systems.
The design of these types of PCM systems is beyond the
scope of this document.
BATTERY SYSTEMS
Battery Sizing Requirements
Under normal operation, the battery charger supplies dc
power to recover the battery voltage after a discharge and to
maintain the float voltage while supporting any self-discharge
losses in the battery system. The charger also supplies the continuous
loads on the auxiliary dc system, while the battery
supports intermittent medium-rate and momentary high-rate
loads, such as trip coils and dc motors. Upon failure of the
battery charger or loss of its ac supply, the battery has to support
the continuous loads along with the intermittent and momentary
loads that may occur before the battery charger is
repaired or the ac supply is restored. Battery sizing calculations
are based upon assumptions of a worst-case scenario
load profile of continuous, intermittent, and momentary loads
during outage of the battery charger and/or loss of ac supply.
The total battery charger outage duration is a critical factor
that must be based upon realistic operational criteria. For example,
upon failure of a battery charger, if the design criteria
for sizing the battery uses an eight-hour load profile, then
available spare equipment, operating, monitoring, and inspection
practices must ensure that maintenance personnel can
respond to and resolve the problem in less than eight hours.
When evaluating a battery’s ability to meet the design criteria.
Circuit Layout
The arrangement of the dc circuits is important to the overall
reliability of the PCM system. Careful analysis of which
PCM circuits provide backup or redundancy to other PCM
circuits protecting each zone of the power system is required.
An effort should be made to ensure that there are no single
points of failure where loss of a single branch circuit will
cause loss of protection and control for any protective zone.
This analysis is easier when dual protection systems are applied.
It is more difficult, but no less important, when backup
is provided by overlapping relays tripping different devices.
Fig. 3 shows arranging the redundant systems on separate
auxiliary dc system feeders. If dual auxiliary dc systems are
available, the PCM and breaker systems that are redundant to
each other would be connected to the separate supplies instead
of merely separate feeders. If the PCM system is not based
upon dual PCM devices and instead uses overlapping protective
devices tripping separate breakers, we would carefully
arrange the overlapping relays and their associated circuit
breakers on the separate auxiliary dc system feeders. In the
arrangement shown in Fig. 3, the breaker trip circuits are
common to the PCM circuits.
BATTERY ROOM VENTILATION
The area or enclosure where the battery system is located
should have an adequate level of air changes to prevent
buildup of hydrogen gas to an explosive level. The minimum
explosive level is 4% by volume. A design value of 2% should
typically be used. The amount of hydrogen evolution during
operation varies based upon the battery technology in use and
the operating conditions. VLA batteries will evolve very little
gas during normal float and equalize operation. Nonrecombinant
N-C batteries only evolve hydrogen during the final stage
of charging.
Charger malfunction can cause high current to be driven
through a fully charged battery, which will cause the worstcase
gas evolution rate in both VLA and N-C type batteries.
Most chargers have a failure mode of low or no voltage output;
however, ferroresonant-type chargers do have a failure
mode that can result in high output voltage. When adequate
monitoring of the auxiliary dc control power system is in
place, high battery voltage associated with a malfunction can
be alarmed and addressed before excessive gas accumulation
can occur.