Optimal Real Time Voltage Control
Content
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. 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. Increase the number of LTC Control System devices across the network based on actual results.
2.
3.
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
Interconnection to Adjoining Utility 230 kV Network
To Remainder of Transmission System
Static Var Compensator
230/115 kV 90 MVA Sub 115 kV Network
M-0557
Shunt Capacitors 230/115 kV 135 MVA Sub 115/69 kV Sub G
M-0557
230/115 kV 90 MVA Sub M-0557 115/69 kV Sub
G
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, T ap 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 VOLTAGE CENTERBAND VOLTAGE LOWER BANDEDGE VOLTAGE OUT OF BAND 121 VOLTS 120 VOLTS 119 VOLTS
TD
ACTUAL BUS VOLTAGE
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) 119 V ACTUAL BUS VOLTAGE
TD
TD
TAPCHANGE INITIATED TAPCHANGE INITIATED BIAS INITIATED
ORIGINAL VOLTAGE BAND WITHOUT BIAS NEW VOLTAGE BAND AFTER 5 STEPS OF BIAS TD CB TIME DELAY 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 1. David I. Sun, Tsang-I Hu, Gen-Sheng Lin, Chia-Jen Lin, Chun-Ming Chen, “Experiences with 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, “VAR Management-Problem Recognition and Control”, IEEE Transaction on Power Apparatus and S y s t e m s , Vol. PAS-103, No. 8, pp. 2108-2116, Aug. 1984. 3. K.C. Lai, W.J. Lee, M.S. Chen, “Design of a Microcomputer Based Operator Assistance System for Real Time Voltage and Reactive Power Correction”, IEEE Transactions on Power Systems, Vol. 6, No. 2 , pp. 723-728, May 1991. 4. Walter A. Johnson, J.F. Aldrich, R.A. Fernandes, H.H. Happ, K.A. Wirgau, R.P. Schulte, W.R. Bosshard, J.D. Willson, R.E. Reed, “EHV Operating Problems associated with Reactive Control”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 3, pp. 1376-1381, March 1981. 5. Show-Kang Chang, Farrokh Albuyeh, Michel L. Gilles, George E. Marks, Ken Kato, “Optimal Real-Time Voltage Control”, IEEE Transactions on Power Systems, Vol. 5, No. 3, pp. 750-758, Aug. 1990. 6. “Optimize Bulk Power Flow with SCADA Controlled LTC Control System”, Application Note, Beckwith Electric, Aug. 1990.
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. 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. Increase the number of LTC Control System devices across the network based on actual results.
2.
3.
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
Interconnection to Adjoining Utility 230 kV Network
To Remainder of Transmission System
Static Var Compensator
230/115 kV 90 MVA Sub 115 kV Network
M-0557
Shunt Capacitors 230/115 kV 135 MVA Sub 115/69 kV Sub G
M-0557
230/115 kV 90 MVA Sub M-0557 115/69 kV Sub
G
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, T ap 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 VOLTAGE CENTERBAND VOLTAGE LOWER BANDEDGE VOLTAGE OUT OF BAND 121 VOLTS 120 VOLTS 119 VOLTS
TD
ACTUAL BUS VOLTAGE
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) 119 V ACTUAL BUS VOLTAGE
TD
TD
TAPCHANGE INITIATED TAPCHANGE INITIATED BIAS INITIATED
ORIGINAL VOLTAGE BAND WITHOUT BIAS NEW VOLTAGE BAND AFTER 5 STEPS OF BIAS TD CB TIME DELAY 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 1. David I. Sun, Tsang-I Hu, Gen-Sheng Lin, Chia-Jen Lin, Chun-Ming Chen, “Experiences with 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, “VAR Management-Problem Recognition and Control”, IEEE Transaction on Power Apparatus and S y s t e m s , Vol. PAS-103, No. 8, pp. 2108-2116, Aug. 1984. 3. K.C. Lai, W.J. Lee, M.S. Chen, “Design of a Microcomputer Based Operator Assistance System for Real Time Voltage and Reactive Power Correction”, IEEE Transactions on Power Systems, Vol. 6, No. 2 , pp. 723-728, May 1991. 4. Walter A. Johnson, J.F. Aldrich, R.A. Fernandes, H.H. Happ, K.A. Wirgau, R.P. Schulte, W.R. Bosshard, J.D. Willson, R.E. Reed, “EHV Operating Problems associated with Reactive Control”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 3, pp. 1376-1381, March 1981. 5. Show-Kang Chang, Farrokh Albuyeh, Michel L. Gilles, George E. Marks, Ken Kato, “Optimal Real-Time Voltage Control”, IEEE Transactions on Power Systems, Vol. 5, No. 3, pp. 750-758, Aug. 1990. 6. “Optimize Bulk Power Flow with SCADA Controlled LTC Control System”, Application Note, Beckwith Electric, Aug. 1990.
