Download optimal real-time voltage control with an

OPTIMAL REAL-TIME VOLTAGE CONTROL WITH
AN ADVANCED LTC CONTROL SYSTEM
By Donald L. Hornak, P.E.
Applications Engineer
Beckwith Electric Co., Inc.
ABSTRACT
The implementation of an advanced automatic LTC Transformer Control System for use on the bulk
power system LTC autotransformers is described. The purpose of the Control System is to perform realtime closed-loop control of the bulk power system to achieve the two important results. The first is to
remove any voltage limit violations that can be accomplished by efficiently using all tap positions on
autotransformers. Secondly, the system will help minimize transmission system losses.
When added to new or existing autotransformers, the Control System, with its integrated voltage regulating relay, closes the loop, via SCADA, from an Optimal Power Flow Calculation to the power transformers. The system operator is now relieved from manually adjusting the LTC tap position of each autotransformer to accomplish voltage control. All adjustments can be made automatically by biasing the centerband
setpoint of the automatic voltage regulating relay. This results in a much flatter voltage profile on the
bulk power transmission system. A typical installation and flow diagram is included in the discussion.
INTRODUCTION
The need to control and manage the reactive power supply to meet the increasing use of the bulk power
transmission system and its diminishing reserve capacity has long been a concern for electric utilities.
Numerous techniques to monitor and control reactive supply have been proposed, based on extensive
studies to analyze the problem. Presently, the reactive control techniques are primarily being studied
using Optimal Power Flow (OPF) programs. Automated real-time voltage control is not yet in wide use,
even though there is an acute need for it to achieve secure economic power system operation.
Implementation of an automatic LTC Control System that closes the loop between the autotransformer
and the system energy control center is presented in this paper. The scheme uses the output of a OPF
calculation on the energy management system computer to bias the centerband setpoint via SCADA of
the voltage regulating relay installed on the transformers. Data regarding system parameters at the
transformer are then sent back through SCADA to the energy management system computer for any
further refinements for optimum power flow. The net effect of this control action is to flatten the
transmission network voltage levels and re-balance the reactive flows to meet the existing load conditions and available reactive sources.
PRACTICAL ASPECTS OF REAL-TIME CLOSED-LOOP IMPLEMENTATION
In general, operating practices have permitted devices with local intelligence; such as LTC power transformers, generating unit voltage regulators, automatically switched shunt capacitor banks, shunt reactors
and static var compensators; to adjust voltage levels based on a fixed voltage control setting. However, as
the transmission system becomes more fully utilized, such simple solutions are proving to be inadequate.
It is becoming more frequently necessary for power system operators to intervene and make adjustments
to voltage levels during daily operations in order to match reactive supply to electrical system demands.
Often a delicate balance is achieved, which must then be readjusted an hour or so later as system
conditions change. This manual adjusting process distracts the operator from his normal duties or else
overloads the operator’s resources. Even when OPF programs provide one more step of automation, the
new fixed tap position is only updated about once an hour. Fully automating this process is both
desirable and achievable.
Linking the OPF calculations with real-time voltage control is particularly attractive since much of the
necessary hardware exists as a part of the overall Energy Management System (EMS). For example,
control of load tapchanging transformer taps and capacitor bank switches is already in place within an
existing EMS. If remote control of generator voltage regulators were added, then much of the hardware
to implement a total scheme would be in place. However, adding this hardware needs to be justified on a
cost/benefit basis.
Overall control of the reactive supply for a typical power system might include the following variety of
devices whose reactive capability can be controlled:
•
25 Generators/Synchronous Condensers
•
100 LTC Power Transformers
•
15 Switched Shunt Capacitors/Reactors
•
2 Static Var Compensators
Since LTC transformers are usually the largest compliment of devices and connect the various voltage
levels of the bulk power system, they were selected for actual implementation of the LTC Control
System. An added benefit is that if remote biasing of an automatic voltage regulating relay works
efficiently, then its characteristics can be extended to the other devices.
The implementation strategy for the LTC Control System (M-0557) is as follows:
1.
Test and verify the OPF results on the actual system using manual adjustments to simulate proposed
automatic operation.
2.
Establish software linkages and LTC hardware to permit selected LTC autotransformers to be automatically controlled by changing the LTC control centerband setpoint. Closely monitor the results for
one year.
3.
Increase the number of LTC Control System devices across the network based on actual results.
POWER SYSTEM DESCRIPTION
A portion of the power system was chosen that would best test the OPF program and demonstrate the
effectiveness of the LTC Control System. The configuration of the four transformers chosen to include the
enhanced voltage regulating relays is shown in Figure 1. All four transformers were 230/115 kV; two
were rated at 90 MVA and two were 135 MVA. The network contains all the necessary elements for a full
test of the system, such as generators both on the high and low voltage system, a weak interconnection to
an adjoining utility, and static var compensators. Switched shunt capacitor banks provide additional
To Remainder
of Transmission
System
Interconnection to
Adjoining Utility
230 kV Network
Static
Var
Compensator
230/115 kV
90 MVA
Sub
M-0557
115 kV Network
Shunt
Capacitors
230/115 kV
135 MVA
Sub
M-0557
115/69 kV
Sub
G
230/115 kV
90 MVA
Sub
M-0557
G
115/69 kV
Sub
230/115 kV
135 MVA
Sub
M-0557
FIGURE 1 Application of SCADA-Adjustable LTC Control System for Optimal Power Flow
reactive supply to this portion of the grid. Controlling four LTC transformers would demonstrate how
the technique would work when applied to the whole system.
Initial testing of the OPF calculations and manual adjustment of the LTC taps several times a day to
simulate what would happen when the Control Systems were installed was very effective, but also labor
intensive. However, the results of this effort lead to the decision to specify, build and install four
demonstrator units for the transformers shown in this diagram.
Figure 2 is a block diagram showing how the EMS software is linked to the LTC autotransformers in the
field. Starting with the EMS computer, the optimal power flow calculations determine the new centerband
setpoint that will be used to improve the voltage profile. The SCADA master sends the request for
adjustment of the centerband through a communication link to the SCADA remote terminal unit (RTU);
in this case by microwave, although telephone lines or other means could be used. The LTC Control
System senses an input from the SCADA RTU and adjusts its centerband accordingly. A raise or lower
signal is then sent to the autotransformer. Closing the loop entails sending data about local conditions
from the autotransformer back to the SCADA RTU, and ultimately to the OPF so that calculations can be
made on information that is constantly being updated. For example, if there are no more tap positions
available, the system operator can immediately know that he must choose another option, such as
changing generator control limits or interchange power transfer.
LTC CONTROL SYSTEM DESCRIPTION
The M-0557 LTC Control System includes a voltage regulating relay with provisions for circulating
current paralleling and a SCADA interface. The M-0557 is designed to allow the Energy Management
System to remotely change the voltage centerband setpoint of the voltage regulating relay in 20 discrete
steps via SCADA. Typically, one might choose ±2.5% as a total bias adjustment range, or ±3 V on a 120 V
base. With ±10 steps of bias available, the centerband can be adjusted over a total range of 6 V in a series
of 0.3 V steps.
Figure 3 depicts a comparison of the performance characteristics of a conventional voltage regulating
relay and the LTC Control System. In the conventional relay shown, the centerband is set for 120 V with a
two volt bandwidth. When the bus voltage decreases below the lower bandedge, the time delay timer
will start, and a tapchange will be initiated when the timer has timed out.
The bottom figure shows how the bus voltage will be affected by the LTC Control System. The centerband
on the Control System is set to 120 V, the bandwidth is set at 2 V, and the bias range selected for the
Control System is ±2.5%. Starting from the left in the drawing, the bus voltage decreases until it is below
the lower voltage limit of 119 V, which starts the time delay. When the timer is timed out, a tapchange is
initiated to raise the voltage back within band. The vertical line in the center of the figure indicates that a
bias is initiated a short time later via SCADA; in this case, five steps for a total increase in centerband of
+1.5 V. Since the bus voltage is still below the new biased lower limit of 120.5 V, the relay will start to
time out for a tapchange operation to bring the voltage within the new biased range of 120.5 V to 122.5 V.
Similarly, the bias could be accomplished in the other direction to a maximum lower limit at these
settings of 116 V.
EQUIPMENT CONFIGURATION
The LTC Control System is installed in a NEMA 4X enclosure. It consists of a front control panel that is
swing-mounted and an interior panel that contains the interface equipment for the LTC mechanism and
the SCADA RTU. The M-0557 incorporates the following units:
Autotransformer
230/115 kV or 115/69 kV
with Load Tapchanging
Equipment
M-0557 LTC Control System
with SCADA-Adjustable Centerband
Setpoint Bias
Data
MW, Mvar, Tap Position,
Voltage, Amps
SCADA RTU
Communication Link:
Telephone or
Microwave
SCADA Master
Energy Management System
Computer
Calculates Optimal Power Flow and
Sends New Bandcenter Setpoint to
Autotransformer via SCADA
System Energy Control Center
FIGURE 2
Closed-Loop Voltage Control with Remote Automatic Operation of the
LTC Autotransformer by Biasing of the Control’s Setpoint
TIME
VOLTAGE UPPER BANDEDGE
121 VOLTS
VOLTAGE CENTERBAND
120 VOLTS
VOLTAGE LOWER BANDEDGE
119 VOLTS
VOLTAGE OUT OF BAND
ACTUAL BUS VOLTAGE
TD
TAPCHANGE INITIATED
Conventional Voltage Regulating Relay Performance Characteristics
122.5 V (BIASED UPPER LIMIT)
121.5 V (BIASED CB IN 5 STEPS)
121 V
120.5 V (BIASED LOWER LIMIT)
120 V (CB SETPOINT)
ACTUAL BUS VOLTAGE
119 V
TD
TD
TAPCHANGE INITIATED
TAPCHANGE INITIATED
BIAS INITIATED
ORIGINAL VOLTAGE BAND WITHOUT BIAS
NEW VOLTAGE BAND AFTER 5 STEPS OF BIAS
TD
TIME DELAY
CB
CENTERBAND
LTC Control System Performance Characteristics with 120 V Centerband Setting
FIGURE 3
Comparison of Conventional and Biased Relay Performance Characteristics
•
An automatic voltage regulating relay provides selectable voltage centerband, bandwidth, time
delay, and resistive and reactive line drop compensation. The control is modified to accept signals
from the centerband setpoint bias module.
•
The centerband setpoint bias module provides all functions necessary to bias the centerband setpoint.
•
A parallel balancing module provides LTC transformer paralleling capability using the circulating
current monitoring technique. The module accepts commands from SCADA or locally from the front
panel controls for parallel or independent operation.
•
An ac current relay guards against excessive circulating current. The trip current is adjustable from
0.01 A to 0.1 A. Local or remote automatic operation is locked out on excessive circulating current.
CONTROL SYSTEM FEATURES
The LTC Control System has the following control mode capabilities:
•
Remote or local control is selectable with a two-position switch.
•
Automatic or manual control selectable with a two-position switch.
The following features can be selected either locally or via SCADA remote input command:
•
Centerband setpoint control on/off
•
Supervisory raise/lower control on/off
The length of time that the supervisory raise and lower circuit is powered after the remote command
is removed can be adjusted from 0.1 to 1.0 sec., effectively stretching the SCADA pulse to ensure that
the required tapchange has been initiated.
•
Paralleling on/off
Visual indication of the status of these functions is provided both locally and remotely.
A ±1.0 mA analog signal is also provided, which may be used as input to SCADA to feed back the
setpoint information for monitoring purposes. The signal is linearly scaled so that +1.0 mA represents
+(percent bias selected) centerband adjustment, zero current is zero adjustment, and –1.0 mA represents
–(percent bias selected) adjustment.
Installation of the LTC Control System took place for a one year trial basis. Results from the installations
have been beyond expectations, and savings in reduced transmission losses are funding the system-wide
implementation of additional LTC Control Systems. The overall objective is to expand to 50 units over a
five-year time frame. The system will then have automatic LTC control with remote bias of the voltage
centerband setpoints on all of its 230/115 kV transformers. A review of the project results will then be
done and a decision made on whether or not to expand the installation of the LTC Control System to the
115/69 kV transformers.
CONCLUSIONS
Since algorithms exist that can optimize controlling the voltage on the bulk power supply system, then
closing the loop between the OPF calculation and the operation of the voltage control device has several
advantages. Using the LTC Control System permits the OPF program to use real-time information in its
calculations, and allows finer adjustments of the LTC autotransformers.
Results have shown that this approach is very cost effective, including decreased man hours spent
manually adjusting taps, and has improved overall system response to voltage swings caused by fluctuating loads, large intersystem transfers, and disturbances. System response to disturbances is much
improved because the system reactive supply is being dispatched more closely to system reactive load
requirements in a balanced fashion. In addition, the LTC transformers are operated with automatic
voltage control, allowing for proper voltage levels at all times.
In general, by regulating the autotransformers in this manner, the system voltages are maintained at
levels closer to optimum throughout the day. The additional benefit is that var flow throughout the loop
is minimized, thereby maximizing real power flow and thermal capability, and minimizing system
losses.
REFERENCES
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Implementing Optimal Power Flow for Reactive Scheduling in the Taiwan Power System”, IEEE
Transactions on Power Systems, Vol. 3, No. 3, pp. 1193-1200, Aug. 1988.
2. Walter L. Snyder Jr., John G. Raine, Richard D. Christie Jr., Fred Ritter, Robert Reed,
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