Transmission Operations -
Wide-Area Control System Advanced Auto-Restoration
Contents
Overview
The purpose of advanced auto-restoration is to automatically restore power to
un-faulted sections of a line or feeder, after a fault is
isolated, in networks having complex topologies and multiple
organizational boundaries.
Currently, automatic restoration of service
is performed only within a restricted set of conditions and
network topologies, as described in the WACS Automated Controls
Baseline use case. In the near future, it is expected that these
restrictions will be removed and the automation system will be
able to restore power in systems which:
·
There are multiple sources from which to
restore power
·
The multiple sources may belong to different
organizations
·
There are multiple possible connection points
between the sources
·
It is necessary to split the de-energized load
into sections because any one source cannot re-energize the
whole load
The remainder of this
narrative describes an example scenario illustrating these
capabilities.
Initial State
As shown in Figure 1, two neighboring substations are connected in a
manner to make traditional auto-restoration possible, in other words:
·
Per typical utility operation, there is
breaker located in the substation connected to each feeder,
provided with an automatic reclosing function. These are
labeled 1A1, 1B1, 2A1, and 2B1 in the figure, following the
naming convention <substation1/2><feederA/B><switch/breaker#>.
·
Normally-closed switches are located at
intervals along each feeder to permit auto-sectionalizing
around a fault. (e.g. 1B2, 1B3, 1B4). These switches are
typically of the “no-load break” variety, for economic
reasons. They can open only when there is no load on the
line. Some may be of the “load break” variety, which can
open under normal current. Usually only those devices at
the head of the feeder (such as 1A1, 1B1 etc.) will be
“fault interrupting” breakers capable of opening under fault
current.
·
A normally-open switch is located at the end
of adjacent feeders. (e.g. 1C or 2C). This switch can be
closed to share load or restore power from one feeder to the
other.
·
Each breaker and switch is monitored and
controlled by an Intelligent Electronic Device (IED).
·
A Substation Computer (SC) in each substation
gathers information and controls the IEDs connected to its
feeders. It reports to the Operator for that utility by way
of a Graphical User Interface (GUI).
In this example, the two adjacent sets of feeders
can also be connected to each other (if necessary) using a number of
normally-open switches (1X, 1Y and 1Z). This interconnection is
rarely performed because the two substations belong to different
utilities. In this example, the interconnection switches are owned by
the utility that controls Substation 1. However, Operator 1 must have
approval from Operator 2 before closing any of these switches.
The scenario begins with each IED reporting its
downstream load and switch status to the Substation Computer. For the
purposes of this example, we assume that all this information
is reported to both Substation Computers. There are two ways
to do this:
·
Each IED reports its data separately to each
Substation Computer
·
Each IED only reports to its “own” Substation
Computer, and the Substation Computers exchange
information.
The latter case is most likely to be implemented
because:
·
It reduces the number of communications
connections between the two utilities, which is desirable
for security reasons.
·
It reduces the bandwidth and processing power
required by each IED.
Figure 1
Initial System State
From the current data reported by each IED (shown
in italics with arrows), the Substation Computers can calculate the
load on each individual section of the feeders. This example assumes
that the maximum capacity limit on each feeder is 100A. Feeder 1B, in
that case, is operating near capacity, while the other feeders are
about 50% loaded.
Fault Detection
As shown in Figure 2, a fault occurs on feeder 1B between switches 1B2
and 1B3. Breaker 1B1 trips and de-energizes 90A of load, including
60A that is downstream from the fault.
All IEDs on feeder 1B report to Substation
Computer 1 the fault and the loss of current. Those IEDs that saw the
fault current (1B1 and 1B2) may send an estimated distance to the
fault. IED 1B1 reports that it has tripped and has started reclosure
timers. Substation Computer 1 forwards the information to Substation
Computer 2, but SC2 takes no action because the fault is not in its
territory.
Figure 2
Fault Detection
Auto-Sectionalization
As shown in Figure 4, the IEDs on feeder 1B take action to isolate, or
auto-sectionalize, the fault. There are two possible methods for
doing so, with different communications requirements.
·
High-Speed Communication. One possible
method is that Substation Computer 1 determines which two
switches (1B2 and 1B3) to open using fault direction and
distance information provided by the IEDs. This method
would require fast communication between the 1B IEDs and
SC1, in order to open the switches between reclosings of the
breaker (measured in seconds). It would likely also require
the IEDs to provide a specialized communications service,
i.e. “open the next time you see zero current”.
·
Fault-Interruption Counting. A more
robust and distributed method would be for each IED to be
programmed to open its switch after a pre-configured
reclosure attempt. Each IED would open its switch under the
following conditions:
o
The IED has observed fault current
o
The IED has seen the fault current drop to zero,
indicating the breaker has tripped
o
These two conditions have occurred a pre-configured
number of times. The number is different for each IED on the feeder.
Figure 3 illustrates how this occurs in the example. No IED
is permitted to open its switch between the initial fault and the
first reclosure attempt, in case the fault is transient. 1B4 is
permitted to open its switch between the first and second reclosure
attempts, but does not do so. Because 1B4 is downstream from the
fault and has no other source of current, it does not observe the
fault current and its opening conditions are therefore not met.
Similarly, 1B3 does not observe fault current and so does not open in
its time window.
IED 1B2, however, has seen the same current as
1B1, and has been counting the fault interruptions. After the third
reclosure attempt, 1B2 opens its switch, isolating the fault from any
source of current. This is auto-sectionalization, and is shown
as step (1) in Figure 4. When 1B1 recloses the fourth time, it is
successful, and 10A of load is restored to that section of feeder 1B.
This is step (2) in Figure 4.
Figure 3
Fault-Interruption Counting for Auto-Sectionalization
Figure 4
Auto-Sectionalization and Load
Splitting
Isolating the Fault
The final step in auto-sectionalization, shown in
step (3) of Figure 4, is to isolate the fault. Substation Computer 1
observes that 1B3 and 1B4 have reported zero current and voltage
without having reported fault current. It therefore determines
(possibly with the assistance of distance-to-fault data from 1B1 and
1B2) that the fault is between 1B2 and 1B3. Substation Computer 1
recommends to Operator 1 that switch 1B3 be opened in order to isolate
the fault. Operator 1 confirms this operation, and SC1 sends the
message to 1B3 causing it to open.
Load Splitting
Whichever auto-sectionalizing method is used, the
fault is now isolated and auto-restoration can begin. Substation
Computer 1 reviews the data provided prior to the fault. It
calculates the loading on each segment of each feeder, as shown in
Figure 4. It determines that there is 60A of load that can
be restored.
However, the “traditional” solution, to close
switch 1C, will not solve the whole problem. Feeder 1A is already
loaded at 60A. If it accepts the whole downstream load of 60A, it
will be overloaded, since the example began with the assumption of
100A maximum limit per feeder.
The Substation Computer determines that it will
be necessary to “split” the downstream load and re-energize it from
multiple sources. Substation Computer 1 recommends to Operator 1 that
switch 1B4 be opened, receives confirmation from Operator 1, and opens
the switch by sending a message to 1B4. This is step (4) in Figure 4.
Auto-Restoration
The final steps in auto-restoration are shown in
Figure 5. Utility 1 has a policy in place that load is to
be restored from Utility 1 sources whenever possible. Therefore
Substation Computer 1 recommends that switch 1C be closed, rather
than, for instance, switch 1Z. Operator 1 confirms this operation and
SC1 sends the message to IED 1C, restoring 30A of service.
Substation Computer 1 recommends that switch 1Y
be closed to restore the remaining un-faulted section of feeder
between 1B3 and 1B4. Operator 1 contacts Operator 2 at Utility 2,
requesting permission to close switch 1Y.
Before making this decision, Operator 2 does the
following:
·
Reviews the sequence of events logs generated
by SC2 showing the auto-sectionalizing sequence.
·
Confirms that Utility 1 has isolated the fault
between 1B2 and 1B3.
·
Confirms from records generated by SC2 that
the de-energized section between 1B3 and 1B4 previously was
loaded at 30A.
·
Checks on the SC2 GUI that feeder 2A can
handle the additional 30A load.
Finally, Operator 2 contacts Operator 1, giving
permission to close switch 1Y. Operator 1 confirms the operation with
SC1, which sends the message to 1Y and restores the remaining 30A of
service.
Figure 5
Auto-Restoration
Load Balancing
Following auto-restoration, feeder 1A is loaded
at 90A and 2A is loaded at 80A, while 2B is only loaded at 50A.
Operator 2 may choose to close switch 2C in order to lighten the load
on feeder 2A.
In theory, the whole system could be more
efficiently loaded by also closing switch 1Z. However, neither
Substation Computer would make this recommendation because:
·
The power on the two feeders is likely
incompatible due to differences in frequency, voltage, and
phase angle. Therefore, it would be necessary to open 1C
before closing 1Z.
·
Opening 1C would cause a momentary outage
downstream of 1B4. Furthermore, if 1C was not a “load
break” switch, it would be necessary to first break the load
at 1A1, meaning that the outage would occur for all of
feeder 1A.
·
Utility 1 would lose the 30A of load
downstream of 1B4 to Utility 2 until the fault could be
repaired. This would be unacceptable from a business point
of view.
Summary
Performing advanced auto-restoration will require
the following measures beyond those required for existing
auto-restoration mechanisms:
·
Real-time sharing of data between Substation
Computers
·
Calculation of loads on each feeder or line
section, and storing these recent historical values in the
Substation Computer.
·
More advanced logic in each Substation
Computer to evaluate each possible switching action, perhaps
on the order of the Contingency Analysis programs currently
used by EMS stations.
·
Reliable communications between neighboring
operators, either by voice or by data
·
One of the following features:
o
Full breakers and protection
relays on each section, or “load break” or
“fault-interrupting” switches. Utilities are
unlikely to do this because of the significantly
higher cost.
o
Fault-Interruption counting,
as discussed in this example. Fault-interruption
counting has one major drawback: Ideally, it
requires the same number of reclosures as there
are switches on the feeder. Typically, utilities
do not use a high number of reclosures because:
-
It causes excessive wear on the breaker
-
It annoys the customers, who see multiple small outages within a short
period of time.
Therefore it is rare to see
more than two or three reclosures. This example, with four
reclosures, would be extremely rare. This limits the granularity with
which load can be restored, and increases the number of subscribers
affected by an fault.
o
High-speed communications between remote IEDs and
the substation computer. In this example, it would permit the
Substation Computer to immediately determine that 1B2 and 1B3 switches
should open, and do so quickly, between the first and second
reclosings of breaker1B1. This is shown as an alternate scenario in
the use case below.
|
# |
Event |
Name of Process/Activity
|
Description of
Process/Activity |
Information Producer
|
Information
Receiver |
Name of Info Exchanged |
IntelliGrid Architecture Environments |
|
1A |
Fault |
Report
Fault |
IEDs
upstream from the fault report seeing
fault current |
IED 1B1,
IED 1B2 |
Substation Computer
Device 1 |
Fault
Detected |
Deterministic Rapid Response Intra-Sub |
|
1B |
Loss of
current and voltage |
Report
Loss of Service |
IEDs
downstream from the fault report loss of
current and voltage |
IED 1B3,
IED 1B4 |
Substation Computer
Device 1 |
No
Current Detected,
No
Voltage Detected |
Deterministic Rapid Response Intra-Sub |
|
2.1 |
|
Initial
Trip |
First
feeder IED (relay) trips breaker and
reports the action. Starts reclosure
timer. |
IED 1B1 |
Substation Computer
Device 1 |
Trip |
Deterministic Rapid Response Intra-Sub |
|
2.2 |
Recloser
timer expires |
First
Reclose Attempt |
Upon
expiry of reclosure timer, first IED
recloses the breaker. Reports the
action. |
IED 1B1 |
Substation Computer
Device 1 |
Switch
State (close) |
Deterministic Rapid Response Intra-Sub |
|
2.3 |
Fault |
Report
Fault |
IEDs
upstream from the fault report seeing
fault current again. This message
indicates that the fault was not
intermittent and that the SC should
attempt to auto-sectionalize. |
IED 1B1,
IED 1B2 |
Substation Computer
Device 1 |
Fault
Detected |
Deterministic Rapid Response Intra-Sub |
|
2.4 |
|
2nd
Trip |
First
IED trips breaker and reports the
action. Starts reclosure timer. This
message indicates that the SC can now
attempt to open a switch for auto-sectionalization. |
IED 1B1 |
Substation Computer
Device 1 |
Trip |
Deterministic Rapid Response Intra-Sub |
|
2.5 |
|
Auto-sectionalize |
Computer
determines the correct switch to open
based on the fact that the upstream
switches reported fault current, while
the downstream switches reported no
current or voltage.
Directs
the correct switch to open between
reclosures of the breaker. |
Substation Computer
Device 1 |
IED 1B2 |
Switch
Control (open) |
Deterministic Rapid Response Intra-Sub |
|
2.6 |
Reclosure timer expires |
Report
Upstream Power Restored |
Upon
expiry of the reclosure timer, first
feeder IED recloses the breaker.
Reports the action. Power is now
restored from the substation to switch
1B1. |
IED 1B1 |
Substation Computer
Device 1 |
Switch
State (close),
Current,
Voltage |
Deterministic Rapid Response Intra-Sub |
|
3.1 |
Logic
timer expires |
Request
Isolation |
Computer
detects (using a timer) that no fault
has occurred since 1B1 reclosed the
breaker. Determines that switch 1B3 is
the first switch downstream from the
fault and should be opened. Requests
confirmation from System Operator . |
Substation Computer
Device 1 |
System Operator 1 |
Request
(open
1B1) |
User
Interface |
|
3.2 |
|
Confirm
Isolation |
Tells
the Substation Computer Device it is
permitted to open the first downstream
switch (1B3). |
System Operator 1 |
Substation Computer Device 1 |
Confirm |
User
Interface |
|
3.3 |
|
Isolate
Fault |
Requests
that the first downstream switch (1B3)
open. |
Substation Computer Device 1 |
IED 1B3 |
Switch
Control (open) |
Deterministic Rapid Response Intra-Sub |
|
3.4 |
|
Report
Isolation Complete |
The IED
controlling the first downstream switch
reports that the switch is open. |
IED 1B3 |
Substation Computer Device 1 |
Switch
State (open) |
Deterministic Rapid Response Intra-Sub |
|
4.1 |
|
Request
Load Split |
Computer
determines from the Current and Voltage
information stored prior to the fault
that service cannot be restored from a
single source. Determines which switch
to operate (1B4) and requests
confirmation from the System Operator . |
Substation Computer Device 1 |
System Operator 1 |
Request
(open
1B4) |
User
Interface |
|
4.2 |
|
Confirm
Load Split |
System Operator confirms that the
Computer may open the switch to split
the load (1B4) |
System Operator 1 |
Substation Computer Device 1 |
Confirm |
User
Interface |
|
4.3 |
|
Split
Load |
Computer
opens the switch to split the load (1B4) |
Substation Computer Device 1 |
IED 1B4 |
Switch
Control (open) |
Deterministic Rapid Response Intra-Sub |
|
4.4 |
|
Report
Load Split Complete |
IED
(1B4) reports that the switch is open
and the load is split. |
IED 1B4 |
Substation Computer Device 1,
Substation Computer Device 2 |
Switch
State |
Deterministic Rapid Response Intra-Sub |
|
5.1 |
|
Request
Local Restoration |
Computer
determines that half the load can be
restored by closing the local normally
open switch (1C), and requests
permission from operator. |
Substation Computer Device 1 |
System Operator 1 |
Request
(close
1C) |
User
Interface |
|
5.2 |
|
Confirm
Local Restoration |
System Operator confirms that the
Computer may close the normally open
switch (1C) |
System Operator 1 |
Substation Computer Device 1 |
Confirm |
User
Interface |
|
5.3 |
|
Restore
from Local Source |
Computer
closes the switch to restore power from
the local source (1C). |
Substation Computer Device 1 |
IED 1C |
Switch
Control (close) |
Deterministic Rapid Response Intra-Sub |
|
5.4 |
|
Local
Restoration Complete |
IED (1C)
reports that the switch is closed and
current is restored to half the load. |
IED 1B4 |
Substation Computer Device 1,
Substation Computer Device 2 |
Switch
State (close),
Current |
Deterministic Rapid Response Intra-Sub |
|
6.1 |
|
Request
Inter-Utility Restoration |
Computer
determines that the other half of the
load can be restored by closing the
inter-utility switch (1Y), and requests
permission from operator. |
Substation Computer Device 1 |
System Operator 1 |
Request
(close
1Y) |
User
Interface |
|
6.2 |
|
Request
Linking Utilities |
System Operator 1 verifies Computer 1’s
request and forwards it to
System Operator 2 at the other utility. |
System Operator 1 |
System Operator 2 |
Request
(close
1Y) |
User
Interface |
|
6.3 |
|
Confirm
Linking Utilities |
System Operator 2 verifies the request
and gives System Operator 1 permission to
proceed. |
System Operator 2 |
System Operator 1 |
Confirm |
User
Interface |
|
6.4 |
|
Confirm
Inter-Utility Restoration |
System Operator confirms that the
Computer may close the normally open
inter-utility switch (1Y) |
System Operator 1 |
Substation Computer Device 1 |
Confirm |
User
Interface |
|
6.5 |
|
Restore
from Inter-Utility Source |
Computer
closes the switch to restore power from
the other utility source (1Y). |
Substation Computer Device 1 |
IED 1Y |
Switch
Control (close) |
Deterministic Rapid Response Intra-Sub |
|
6.6 |
|
Inter-Utility Restoration Complete |
IED (1Y)
reports that the switch is closed and
current is restored to the remaining
load. |
IED 1Y
|
Substation Computer Device 1,
Substation Computer Device 2 |
Switch
State (close),
Current |
Deterministic Rapid Response Intra-Sub |
