06-12-2012, 12:00 PM
Distribution Automation Systems With Advanced Features
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Abstract
This paper examines the use of wide-area
distribution automation (DA) systems in electric power
distribution systems. The number of DA systems installed on an
annual basis is increasing. Many of these projects encompass a
large area of a distribution system. Wide-area DA holds the
promise of further increasing distribution system reliability. This
paper presents wide-area DA strategies and discusses the
potential impact on system operation.
The paper also looks at the effects of communications
outages on the performance of DA systems. Backup solutions are
discussed that allow the DA system to function when a
communications link fails.
INTRODUCTION
Wide-area automatic control systems increase the
reliability of distribution systems but also introduce some
challenges. This paper discusses features of these systems,
their benefits, and challenges in implementing them.
Utility engineers have developed automatic control
strategies and applied them on distribution systems for many
years. In recent years, utilities have deployed more
sophisticated automatic control strategies that act with a larger
scope of responsibility. This has been driven, in part, by the
increasing availability of economical communications
technologies. Automatic network reconfiguration controllers
respond quickly to permanent fault conditions and restore
power to de-energized loads. Some of these systems can be
easily applied to complex feeder arrangements that are not
suitable for simple loop schemes. These control systems can
transfer loads automatically to balance load between feeders
after an outage.
WIDE-AREA CONTROL OVERVIEW
Control Objectives
Wide-area automatic control is a broad concept that
encompasses many system-wide control objectives in the
distribution system, including the following:
• Service restoration
• Miscoordination detection
• Loss reduction
• Dynamic load balancing and load shedding
• Data sharing with adjacent control areas
• Communications link failure detection
• Enhanced situational awareness
These objectives are interdependent. For example,
restoring service to a feeder section may result in overload
conditions, excessive voltage drop, or power factor
degradation. The control system may need to transfer load
from the overloaded feeder to adjacent feeders or, perhaps,
shed noncritical load to alleviate actual or predicted overloads.
In cases where there are multiple alternate feeds that could be
used to restore service, the control system will select the
alternative feeder that has the most capacity.
The control system must be dynamic in determining
overload conditions. As an example, an outage may occur
during the night when the load is very light. The system
reconfigures to isolate the faulted line section and restores
load in the area. When morning arrives, the load increases
dramatically and overloads a section of the line. The system
has to react to these dynamic changes in load and either move
load to another source that has extra capacity or shed load to
keep from damaging equipment.
WIDE-AREA CONTROL EXAMPLE
This section focuses on one example of wide-area control
used in distribution systems. The distribution automation
(DA) system detects permanent faults and open-phase
conditions on the distribution system. The DAC acts to isolate
the affected section of the feeder and restore power to the
unaffected feeder sections from the normal source. Power can
also be restored from an alternate source, if available.
Example Distribution Network
Although the DAC can be applied to a wide variety of
feeder arrangements, it is helpful to describe the system
operation with respect to an example distribution network.
The example network includes four sources, four feeder
breakers, and twelve reclosers. Four reclosers (R3, R6, R9,
and R12) are normally open, and the remaining switching
devices are normally closed. The distribution network consists
of radial feeders only.
Sequence of Operation
The DAC executes a straightforward sequence. Fig. 5
illustrates the sequence of operation of the automation system.
The Initialize step is executed when the controller first
turns on, when an operator disables the sequence, or when an
operator issues a RESET command while the sequence is
unarmed. The Initialize step acts to reset internal variables and
alarm conditions. During this step, the distribution system
configuration is evaluated to determine the normal operating
arrangement. Once the Initialize step has executed, the
sequence transitions to the Unarmed step.
The Unarmed step is executed after the Initialize step is
completed successfully or if a sequence failure occurs during
the Update, Analyze, Isolate, or Restore steps. The Unarmed
step monitors for an operator-issued ENABLE command. If
this command is detected, the sequence transitions to the
Ready step. The Unarmed step also monitors for an operatorissued
RESET command that, if detected, returns the sequence
to the Initialize step.
Feeder Breaker Control Inhibit
In many cases, wide-area automatic control systems are
implemented on parts of the distribution network that
previously were not remotely controlled by the dispatcher.
Dispatchers may not have concerns about automatic operation
of devices outside the substation. However, because the feeder
breaker has been under their exclusive control in the past,
dispatchers may have concerns about allowing an automatic
system to control it.
Many strategies can still be implemented by controlling
only reclosers and motor-operated switches outside the
substation. However, automatic control of the feeder breaker
allows better performance of the control strategy in some
situations. For example, if a loss of voltage occurs on the
substation bus, the control system can automatically open the
feeder breaker. The entire feeder can then be re-energized
from an alternate source. However, if control of the feeder
breaker is not allowed, the first recloser outside the substation
will be automatically opened instead of the feeder breaker.
The feeder beyond the first recloser is then re-energized from
an alternate source. In this case, the feeder section between the
substation and the first recloser is left de-energized
unnecessarily.
Communications System Path Outages
One system was analyzed to determine how the
communications system performed over an 81-day period.
Findings show that one of the paths in the communications
system was down eight times for a total outage time of
approximately 56 minutes. The longest outage time was
10 minutes and 45 seconds. Extrapolating these data to a
period of a year indicates that this same communications
system could be down for as long as 252 minutes a year. It is
this type of communications system outage information that
raises concern about the reliable operation of the DA system.
It also causes us to ask what can be done to allow a
reconfiguration of the distribution system during an outage if,
at the same time, the communications system is not fully
functional. This scenario motivated DA engineers to develop a
backup system that can engage if a power outage occurs when
a link in the communications system is down. The idea is to
keep the DA system as functional as possible, even when
multiple communications links are down. This concept allows
the installed DA system assets to be better utilized, provides
additional benefits to customers by reconfiguring the power
system, and restores power when the communications system
may not be fully functional.