I. Introduction
In the previous issue of EE T&D part 1 of the article on Undervotage Load Shedding discussed the investigations of recent blackouts [1,3,7], which indicate that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960 and –‘70s. It explored the nature of recent power system blackouts (2003 east coast, 1996 California and others) and explains why voltage collapse is the leading edge indicator of impending power system problems. This article (part 2) discusses the design and security issues that need to be addressed in the design of an undervoltage load shedding (UVLS) scheme and why relying on underfrequency load shedding (UFLS) maybe “too little, too late.” This article also addresses the current level of UVLS on utility systems as well as current NERC (North American Electric Reliability Council) pronouncements on the subject.
I. STATUS OF UNDERVOLTAGE LOAD SHEDDING (UVLS)
Simply stated, the philosophy of UVLS is that when there is a system disturbance and the voltage drops to a pre-selected level for a pre-determined time, then selected loads are shed. The intention is that when load is shed for a disturbance, the voltage will recover to acceptable levels thereby avoiding a more widespread system voltage collapse. Developing a UVLS program requires coordination between protection engineers and system planners, who together can determine the amount of load and time delay required in the shedding program. System planning engineers conduct numerous studies using P-V (nose curves) as well as other analytical methods to determine the amount of load that needs to be shed to retain voltage stability under credible contingencies. Voltage collapse is most probable under heavy load conditions where large amounts of power are be transported from remote generation sites and the bulk of the system load consists of motors.
Two basic types of UVLS schemes are being applied: decentralized (or distributed) and centralized. A decentralized scheme has relays installed at the loads to be shed. As voltage conditions at these locations begin to collapse, load assigned to that relay is shed. This philosophy is similar to UFLS schemes. A centralized scheme has undervoltage relays installed at key system buses within the area and trip information is transmitted to shed load at various locations. Additional logic is sometimes applied to add security to the system. Many of these schemes are categorized as “special protection“ or “wide area” protection schemes. These schemes require high-speed and reliable communication to properly operate.
If voltage collapse is the major cause of power system blackouts, then what is the current status of UVLS and what are NERC’s pronouncements on the subject? Implementation of UVLS is not mandatory for NERC member systems. However, NERC has recognized it as a cost-effective method to address voltage collapse. Although most systems studies find UVLS very effective in preventing voltage collapse, it may not benefit all systems. For example, systems with fast voltage decay characteristics (less than a second) may find direct load tripping to be a better alternative. However, systems that are at a risk of fast voltage decay may also be at a risk of slower voltage decay under different conditions. Studies should be performed to determine which systems are the potential candidates for a suitable UVLS scheme. Planning engineers within the regional NERC groups in the U.S. generally conduct these studies.
The most aggressive region in the U.S. that has extensively investigated UVLS is the WECC (Western Electricity Coordinating Council), which has established UVLS guidelines for its members. This regional council serves the western part of the U.S. WECC views automatic UVLS as a safety net to protect the system from collapse or cascading for outages beyond the normal NERC and WECC design criteria.
The WECC has established the following design guidelines in developing UVLS schemes [8]:
1. UVLS should be designed to coordinate with protective devices and control schemes for momentary voltage dips, sustained faults, low voltages caused by stalled air conditioners, etc.
2. The time delay to initiate load dropping should be in seconds, not in cycles. A typical time delay is 3 - 10 seconds.
3. UVLS relays must be on VTs that are connected above automatic LTCs (on the source side).
4. Voltage pick-up points for the tripping signal should be set reasonably higher than the “nose point” of the critical P-V nose curve.
5. Voltage pick-up points and the time delays of the local neighboring systems should be checked and coordinated.
6. Redundancy and enough intelligence should be built into the scheme to ensure reliable operation and to prevent false tripping.
7. Enough load should be shed to bring voltages to minimum operating voltage levels or higher and maintain VAR margin according to WECC’s Voltage Stability Criteria.
System blackouts that were caused by voltage collapse are not confined to the U.S. Major blackouts happened around the world. Blackouts in Italy, Japan, eastern Denmark and southern Sweden have occurred within the last few years and are all related to voltage instability. These and the U.S. blackouts have resulted in utilities beginning to implement UVLS schemes. Table I summarizes a few of the existing UVLS applications at various utilities.
Some utilities install centralized controllers to receive information like undervoltage (U/V), high reactive output, or loss of lines from remote substations (S/S’s) or power plants and sending initiation signals for load shedding to substations. In other cases, the UVLS function is applied as part of EMS center program. Others have installed an UVLS program using decentralized undervoltage relays in substations. Sophisticated wide-area controls using signal processing, real-time control computers and phasor measurement are call wide-area protection or special protection schemes.
II. DESIGNING A SECURE UVLS SCHEME
A. UVLS vs. UFLS Schemes
As discussed above, UVLS programs are designed into utility electrical systems to operate as a last resort, under the theory that it is wise to shed some load in a controlled fashion if it can forestall the loss of a great deal of load to an uncontrolled cascading event. There are two kinds of automatic load shedding installed in North America: undervoltage load shedding—which sheds load to prevent local area voltage collapse, and under-frequency load shedding—which is designed to rebalance load and generation within an electrical island once it has been created by a system disturbance.
Typically, automatic UVLS responds directly to voltage conditions in a local area. UVLS drops several hundred megawatts of load in pre-selected blocks within load centers, triggered in stages when local voltage drops to a designated level—likely 89 to 94%—with a several second delay. The goal of a UVLS scheme is to shed load to restore reactive power relative to demand, to prevent voltage collapse and to contain a voltage problem within a local area rather than allowing it to spread in geography and magnitude. If the first load-shed step does not allow the system to rebalance, and voltage continues to deteriorate, then the next block of load is dropped.
In contrast, automatic under-frequency load shedding (UFLS) is designed for use in extreme conditions to stabilize the balance between generation and load after an electrical island has been formed, dropping enough load to allow frequency to stabilize within the island. By dropping load to match available generation within the island, UFLS is a safety net that helps to prevent the complete blackout of the island, and allows faster system restoration afterward. UFLS is not effective if there is electrical instability or voltage collapse within the island.
Today, UFLS installation is a NERC requirement, designed to shed at least 25-30% of the load in steps within each reliability coordination region. These systems are designed to drop predesignated customer loads automatically if frequency gets too low (since low frequency indicates too little generation relative to load), starting generally when frequency drops to 59.3 Hz. More load is progressively dropped as frequency levels fall farther. The last step of load shedding is set at the frequency level just above the setpoint for generation under-frequency protection relays (typically 57.5 Hz), to prevent frequency from falling so low that generators could be damaged
B. Selection of Voltage Relays for UVLS
Voltage relays will sense all voltage depressions regardless of cause. Some techniques can be used to improve the ability of undervoltage relays to discriminate between conditions—those which require load shedding, and those that do not [7].
• The relay may measure all three-phase voltages or positive sequence voltage. With this technique the relay is less likely to respond to unbalanced short circuits.
• The relay may initiate timing only if the measured voltages are within a window, below a maximum level, and above a minimum level to ensure that load will not be shed for accidental loss of signal to the undervoltage relay or for slowly-cleared, three-phase faults which depress the system voltage to less than the minimum level.
• The relay applied must have a high reset ratio. This is necessary so that only a small recovery in voltage level is required to stop the shedding sequence. High accuracy relays are required, with low setpoint drift. The accuracy of the voltage transformers supplying the relay must also be considered in assessing the overall accuracy of a scheme.
Modern digital relays are an ideal relay to use in undervoltage load shedding application since they have the characteristics cited above.
C. Secure UVLS Schemes
There are two basic types of automatic UVLS schemes that utilities have installed. Both types involve the installation of undervoltage relays at key utility substations. These relays must measure the transmission system voltage and are typically installed at the primary of distribution substations that are located close to key transmission substations. Fig. 1 (SEE PDF) shows a typical utility installation of both undervoltage (27) and under frequency (81) relays.
Because of VT availability, underfrequency relays are usually connected on the secondary of the distribution station because frequency is the same on both the high and low side of the transformer. The voltage measurement for UVLS must be on the transformer primary since transformer losses and load tap changing (LTC) controls will distort the true transmission system voltage level. Fig. 1 (SEE PDF) illustrates a direct tripping type of UVLS. To add security, some UVLS schemes are only enabled if system conditions have occurred that indicate that the power system is in a “stress condition.” Conditions such as net power import versus local generation or undervoltage measurements at key transmission substation buses are used to arm these UVLS schemes. Some utilities call such schemes “special protection schemes.” These schemes add an additional level of complexity and generally rely on communications to arm the scheme. Also, they may not be armed quickly enough to be activated for undervoltage events caused by slow-clearing, multi-phase transmission system faults that occur during heat storm conditions.
Design of a secure undervoltage separation scheme that avoids false operations for such events as slow clearing system faults requires some logic as well as a relay that can accurately measure voltage within acceptable limits. The undervoltage relay needs to be highly accurate. A measurement accuracy of +/-0.5 V on a 120 V basis is required. Also, the undervoltage relay that is used needs to have a high pickup/dropout ratio. This ratio needs to be near 100% so that when voltage recovers after a system fault, the relay will quickly reset to the non-trip condition. To meet these requirements, as well as the logic described below, digital relays are almost exclusively being used for UVLS.
Single-Phase UVLS Logic -- Logic can be used to enhance the security of an undervoltage separation scheme to prevent false operation due to slow-clearing system faults. Fig. 2 (SEE PDF) illustrates a scheme using single-phase line to ground voltage measurements.
The voltage collapse is generally a balanced voltage event with voltage on all three phases being approximately equal. Fault conditions (with the exception of three-phase faults) result in unbalanced phase voltages. This fundamental difference between low voltages caused by faults versus voltage collapse can be used to add security to a separation scheme.
The logic shown in Fig. 2 (SEE PDF) requires that all three line-to-neutral voltages must drop below setpoint #1. Additional security can be added using undervoltage (27B) blocking. Since the magnitude of undervoltage due to impending voltage collapse is 89-94%, blocking operation for low voltages that are fault-induced adds more security. Fig. 2 indicates that any line-to-neutral phase voltage that drops below setpoint #2 will block the operation of the scheme. The last security measure in the logic scheme in Fig. 2 (SEE PDF) is the use of negative sequence voltage (47B) to block operation of the separation scheme.
During unbalanced fault conditions (all fault except three phase faults), negative sequence voltage will be present. Since voltage collapse events are balanced voltage conditions, only a very small level of negative sequence voltage is present. The equation that defines negative sequence voltage is shown below.
To account for the 120O phase angle displacement between phases, unit phasors (a and a2) are used in symmetrical component terminology. For completely balanced three-phase voltages, the negative sequence voltage is zero. Negative sequence voltage blocking is used to detect unbalanced fault conditions and block the undervoltage scheme from improper operation.
Positive Sequence UVLS Logic -- Another logic scheme to enhance security for voltage separation is show in Fig. 3 (SEE PDF)
The scheme is similar to that shown in Fig. 2 (SEE PDF). The blocking elements are the same. But this logic scheme uses positive sequence rather than individual phase-to-neutral voltages to detect an undervoltage condition. Positive sequence voltage is a symmetrical component term and is defined by the following equation:
For completely balanced three-phase voltages, the positive sequence voltage is equal to the value of the normal phase to neutral voltages –that is, V1=Va=Vb=Vc. Positive sequence voltage provides a single quantity as the actuating voltage for undervoltage separation and does not require that all three voltages be below a given setpoint as required in the logic scheme discussed in Fig. 2 (SEE PDF). Both schemes discussed in Fig. 2 and 3 (SEE PDF) are easily programmed into modern digital relays. One of the benefits of digital relay logic is that the blocking logic can be modified to suit the user. If undervoltage and/or negative sequence blocking is not desired by the user, it can be easily eliminated in the logic.
Additional security can be provided at critical facilities using a “voting logic” scheme. The “voting logic” means that multiple protective relays are applied with identical settings and logic at the same measuring point on the system. A majority of the devices must agree before action is taken. The purpose of voting logic is to get confirmation of the system conditions from more than one protective relay, thus avoiding potential false tripping based upon a malfunctioning protective relay. If two relays are installed at each location, two out-of-two logic is used. This logic requires both relays to operate before tripping is initiated. If three relays are used, two-out-of-three logic is used requiring any two relays to confirm the trip condition. Two-out-of-three logic is common in nuclear plant voltage separation schemes.
III. UVLS SETTING CONSIDERATIONS
As previously discussed, setting and design of an UVLS requires close cooperation between the relay engineers and system planners. System planning engineers conduct numerous studies using P-V curves and other analytical methods to determine the amount of load that needs to be shed to retain voltage stability under various contingency conditions. Voltage collapse is most probable under heavy load conditions, so the amount of load to be shed depends on system peak load and generation sources. When considering the type of load to be shed, constant KVA loads such as motors are good candidates for shedding since they draw more current as voltage is decaying. The following is an example that discusses the consideration in setting UVLS relays. The first step is to determine the P-V curves for creditable voltage collapse scenarios.
Fig. 4 (SEE PDF) show an example P-V curve for a creditable contingency. The knee of the curve at which the voltage will collapse is identified as Vcollapse. A setting margin or safety factor is desired and then the accuracy band of the relay and VT is shown. The setting (Vsetting) must be set above these margins. As with all relay settings, dependability and security need to be balanced. If too small a margin is chosen, there is a risk of the scheme operating during allowable emergency conditions that do not yet require load shedding. If too small a margin is chosen, then load shedding could occur after the system passes below the nose curve voltage collapse point (Vcollapse) shown in Fig. 4 (SEE PDF).
Fig. 5 (SEE PDF) illustrates this point. The choice of time delay and the number of setpoints are also critical settings, especially for distributive or de-centralized schemes which trip load directly. Again, planning studies can provide help in selecting the time and setpoints. Typically, there are fewer setpoints in UVLS schemes then are used for UFLS. Some utilities have chosen one voltage pickup point with different time delays for each block of load shed. Time delays are generally set at 2 - 10 seconds— not in the cycle range common for UFLS.
IV. CONCLUSIONS
Investigations of recent blackouts confirm that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960’s and –‘70s. The operation of today’s power system with load centers remote from the generation source makes today’s power system very dependent on the transmission systems that interconnect load and generation. Loss of transmission lines result in high VAR losses that cause voltage collapse at the load center. UVLS is a viable method of providing protection to avoid system voltage collapse. Implementation of UVLS is not mandatory for NERC member systems. However, NERC has recognized it as an important method to address voltage collapse. Although most systems studies find UVLS very effective in preventing voltage collapse, it may not benefit all types of voltage collapse scenarios. For example, systems with fast voltage decay characteristics (less than a second) may find UVSL to be too slow to prevent collapse. UVLS provides a “system safety” net and is an economical method of addressing voltage collapse situations using the philosophy that it is better to shed some load if shedding that load can prevent a much larger outage. Ultimately, however, transmission lines need to be built to address creditable undervoltage conditions.
UVLS schemes are more difficult to design and to set than UFLS and require close cooperation between utility relay engineers and utility system planners. There are two types of UVLS schemes --- decentralized or distributed and centralized. Both types of schemes are being applied, as individual utilities are beginning to apply UVLS on their own without being mandated to do so by NERC.
This article discussed the considerations in developing a secure UVLS scheme and it is the hope of the author that it has focused on the key design and setting questions that need to be addressed.
VII. REFERENCES
[1] C. J. Mozina, Power Plant Protection and Control Strategies for Blackout Avoidance, Georgia Tech Protective Relay Conference, April 2005.
[2] B.R. Williams, W.R. Schmus, D.C. Dawson, Transmission Voltage Recovery Delayed by Stalled Air Conditioner Compressors, IEEE PES Transactions on Power Systems, Vol. 7, No.3 August 1992.
[3] North American Electric Reliability Council (NERC), 1987 System Disturbance Report, p19, July 1998.
[4] U.S. – Canada Power System Outage Task Force, Final Report on the August 14, 2003 Blackout in the United States and Canada: Causes and Recommendations” April 5, 2004.
[5] G.C. Bullock, Cascading Voltage Collapse in West Tennessee, August 22,1987, Georgia Tech Relay Conference, May 1990.
[6] S. Imai, Undervoltage Load Shedding Improving Security as Reasonable Measure for Extreme Contingencies. IEEE PES Transactions on Power Delivery.
[7] IEEE Power System Relaying Committee Report, Summary of System Protection and Voltage Stability, Transactions on Power Delivery, Vol. 10. No. 2, April 1995.
[8] Undervoltage Load Shedding Task Force (UVLSTF), Technical Studies Subcommittee of the WECC, Undervoltage Load Shedding Guidelines, July 1999.
About the Author
Chuck Mozina is a consultant for Beckwith Electric. He is an active 25-year member of the IEEE Power System Relay Committee (PSRC) and is the past chairman of the Rotating Machinery Subcommittee. He is active in the IEEE IAS I&CPS, PCIC and PPIC committees, which address industrial system protection. He is a former U.S. representative to the CIGRE Study Committee 34 on System Protection and has chaired a CIGRE working group on generator protection. He also chaired the IEEE task force that produced the tutorial “The Protection of Synchronous Generators,” which won the PSRC’s 1997 Outstanding Working Group Award. Chuck is the 1993 recipient of the Power System Relay Committee’s Career Service Award and he recently received the 2002 IAS I&CPS Ralph Lee Prize Paper Award. His papers have been republished in the IAS Industrial Applications Magazine.
Chuck has a Bachelor of Science in Electrical Engineering from Purdue University and is a graduate of the eight-month GE Power System Engineering Course. He has authored a number of papers and magazine articles on protective relaying. He has over 25 years of experience as a protection engineer at Centerior Energy, a major investor-owned utility in Cleveland, Ohio where he was the manager of the system protection section. In that capacity, he was responsible for the electrical protection of the company’s generating plants as well as the transmission and distribution system that served over 1.2 million customers. For ten years, he was employed by Beckwith Electric, a manufacture of protective relays, as Application Manager for Protection Products. He is also a former instructor in the Graduate School of Electrical Engineering at Cleveland State University as well as a registered Professional Engineer in Ohio.
In the previous issue of EE T&D part 1 of the article on Undervotage Load Shedding discussed the investigations of recent blackouts [1,3,7], which indicate that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960 and –‘70s. It explored the nature of recent power system blackouts (2003 east coast, 1996 California and others) and explains why voltage collapse is the leading edge indicator of impending power system problems. This article (part 2) discusses the design and security issues that need to be addressed in the design of an undervoltage load shedding (UVLS) scheme and why relying on underfrequency load shedding (UFLS) maybe “too little, too late.” This article also addresses the current level of UVLS on utility systems as well as current NERC (North American Electric Reliability Council) pronouncements on the subject.
I. STATUS OF UNDERVOLTAGE LOAD SHEDDING (UVLS)
Simply stated, the philosophy of UVLS is that when there is a system disturbance and the voltage drops to a pre-selected level for a pre-determined time, then selected loads are shed. The intention is that when load is shed for a disturbance, the voltage will recover to acceptable levels thereby avoiding a more widespread system voltage collapse. Developing a UVLS program requires coordination between protection engineers and system planners, who together can determine the amount of load and time delay required in the shedding program. System planning engineers conduct numerous studies using P-V (nose curves) as well as other analytical methods to determine the amount of load that needs to be shed to retain voltage stability under credible contingencies. Voltage collapse is most probable under heavy load conditions where large amounts of power are be transported from remote generation sites and the bulk of the system load consists of motors.
Two basic types of UVLS schemes are being applied: decentralized (or distributed) and centralized. A decentralized scheme has relays installed at the loads to be shed. As voltage conditions at these locations begin to collapse, load assigned to that relay is shed. This philosophy is similar to UFLS schemes. A centralized scheme has undervoltage relays installed at key system buses within the area and trip information is transmitted to shed load at various locations. Additional logic is sometimes applied to add security to the system. Many of these schemes are categorized as “special protection“ or “wide area” protection schemes. These schemes require high-speed and reliable communication to properly operate.
If voltage collapse is the major cause of power system blackouts, then what is the current status of UVLS and what are NERC’s pronouncements on the subject? Implementation of UVLS is not mandatory for NERC member systems. However, NERC has recognized it as a cost-effective method to address voltage collapse. Although most systems studies find UVLS very effective in preventing voltage collapse, it may not benefit all systems. For example, systems with fast voltage decay characteristics (less than a second) may find direct load tripping to be a better alternative. However, systems that are at a risk of fast voltage decay may also be at a risk of slower voltage decay under different conditions. Studies should be performed to determine which systems are the potential candidates for a suitable UVLS scheme. Planning engineers within the regional NERC groups in the U.S. generally conduct these studies.
The most aggressive region in the U.S. that has extensively investigated UVLS is the WECC (Western Electricity Coordinating Council), which has established UVLS guidelines for its members. This regional council serves the western part of the U.S. WECC views automatic UVLS as a safety net to protect the system from collapse or cascading for outages beyond the normal NERC and WECC design criteria.
The WECC has established the following design guidelines in developing UVLS schemes [8]:
1. UVLS should be designed to coordinate with protective devices and control schemes for momentary voltage dips, sustained faults, low voltages caused by stalled air conditioners, etc.
2. The time delay to initiate load dropping should be in seconds, not in cycles. A typical time delay is 3 - 10 seconds.
3. UVLS relays must be on VTs that are connected above automatic LTCs (on the source side).
4. Voltage pick-up points for the tripping signal should be set reasonably higher than the “nose point” of the critical P-V nose curve.
5. Voltage pick-up points and the time delays of the local neighboring systems should be checked and coordinated.
6. Redundancy and enough intelligence should be built into the scheme to ensure reliable operation and to prevent false tripping.
7. Enough load should be shed to bring voltages to minimum operating voltage levels or higher and maintain VAR margin according to WECC’s Voltage Stability Criteria.
System blackouts that were caused by voltage collapse are not confined to the U.S. Major blackouts happened around the world. Blackouts in Italy, Japan, eastern Denmark and southern Sweden have occurred within the last few years and are all related to voltage instability. These and the U.S. blackouts have resulted in utilities beginning to implement UVLS schemes. Table I summarizes a few of the existing UVLS applications at various utilities.
Some utilities install centralized controllers to receive information like undervoltage (U/V), high reactive output, or loss of lines from remote substations (S/S’s) or power plants and sending initiation signals for load shedding to substations. In other cases, the UVLS function is applied as part of EMS center program. Others have installed an UVLS program using decentralized undervoltage relays in substations. Sophisticated wide-area controls using signal processing, real-time control computers and phasor measurement are call wide-area protection or special protection schemes.
II. DESIGNING A SECURE UVLS SCHEME
A. UVLS vs. UFLS Schemes
As discussed above, UVLS programs are designed into utility electrical systems to operate as a last resort, under the theory that it is wise to shed some load in a controlled fashion if it can forestall the loss of a great deal of load to an uncontrolled cascading event. There are two kinds of automatic load shedding installed in North America: undervoltage load shedding—which sheds load to prevent local area voltage collapse, and under-frequency load shedding—which is designed to rebalance load and generation within an electrical island once it has been created by a system disturbance.
Typically, automatic UVLS responds directly to voltage conditions in a local area. UVLS drops several hundred megawatts of load in pre-selected blocks within load centers, triggered in stages when local voltage drops to a designated level—likely 89 to 94%—with a several second delay. The goal of a UVLS scheme is to shed load to restore reactive power relative to demand, to prevent voltage collapse and to contain a voltage problem within a local area rather than allowing it to spread in geography and magnitude. If the first load-shed step does not allow the system to rebalance, and voltage continues to deteriorate, then the next block of load is dropped.
In contrast, automatic under-frequency load shedding (UFLS) is designed for use in extreme conditions to stabilize the balance between generation and load after an electrical island has been formed, dropping enough load to allow frequency to stabilize within the island. By dropping load to match available generation within the island, UFLS is a safety net that helps to prevent the complete blackout of the island, and allows faster system restoration afterward. UFLS is not effective if there is electrical instability or voltage collapse within the island.
Today, UFLS installation is a NERC requirement, designed to shed at least 25-30% of the load in steps within each reliability coordination region. These systems are designed to drop predesignated customer loads automatically if frequency gets too low (since low frequency indicates too little generation relative to load), starting generally when frequency drops to 59.3 Hz. More load is progressively dropped as frequency levels fall farther. The last step of load shedding is set at the frequency level just above the setpoint for generation under-frequency protection relays (typically 57.5 Hz), to prevent frequency from falling so low that generators could be damaged
B. Selection of Voltage Relays for UVLS
Voltage relays will sense all voltage depressions regardless of cause. Some techniques can be used to improve the ability of undervoltage relays to discriminate between conditions—those which require load shedding, and those that do not [7].
• The relay may measure all three-phase voltages or positive sequence voltage. With this technique the relay is less likely to respond to unbalanced short circuits.
• The relay may initiate timing only if the measured voltages are within a window, below a maximum level, and above a minimum level to ensure that load will not be shed for accidental loss of signal to the undervoltage relay or for slowly-cleared, three-phase faults which depress the system voltage to less than the minimum level.
• The relay applied must have a high reset ratio. This is necessary so that only a small recovery in voltage level is required to stop the shedding sequence. High accuracy relays are required, with low setpoint drift. The accuracy of the voltage transformers supplying the relay must also be considered in assessing the overall accuracy of a scheme.
Modern digital relays are an ideal relay to use in undervoltage load shedding application since they have the characteristics cited above.
C. Secure UVLS Schemes
There are two basic types of automatic UVLS schemes that utilities have installed. Both types involve the installation of undervoltage relays at key utility substations. These relays must measure the transmission system voltage and are typically installed at the primary of distribution substations that are located close to key transmission substations. Fig. 1 (SEE PDF) shows a typical utility installation of both undervoltage (27) and under frequency (81) relays.
Because of VT availability, underfrequency relays are usually connected on the secondary of the distribution station because frequency is the same on both the high and low side of the transformer. The voltage measurement for UVLS must be on the transformer primary since transformer losses and load tap changing (LTC) controls will distort the true transmission system voltage level. Fig. 1 (SEE PDF) illustrates a direct tripping type of UVLS. To add security, some UVLS schemes are only enabled if system conditions have occurred that indicate that the power system is in a “stress condition.” Conditions such as net power import versus local generation or undervoltage measurements at key transmission substation buses are used to arm these UVLS schemes. Some utilities call such schemes “special protection schemes.” These schemes add an additional level of complexity and generally rely on communications to arm the scheme. Also, they may not be armed quickly enough to be activated for undervoltage events caused by slow-clearing, multi-phase transmission system faults that occur during heat storm conditions.
Design of a secure undervoltage separation scheme that avoids false operations for such events as slow clearing system faults requires some logic as well as a relay that can accurately measure voltage within acceptable limits. The undervoltage relay needs to be highly accurate. A measurement accuracy of +/-0.5 V on a 120 V basis is required. Also, the undervoltage relay that is used needs to have a high pickup/dropout ratio. This ratio needs to be near 100% so that when voltage recovers after a system fault, the relay will quickly reset to the non-trip condition. To meet these requirements, as well as the logic described below, digital relays are almost exclusively being used for UVLS.
Single-Phase UVLS Logic -- Logic can be used to enhance the security of an undervoltage separation scheme to prevent false operation due to slow-clearing system faults. Fig. 2 (SEE PDF) illustrates a scheme using single-phase line to ground voltage measurements.
The voltage collapse is generally a balanced voltage event with voltage on all three phases being approximately equal. Fault conditions (with the exception of three-phase faults) result in unbalanced phase voltages. This fundamental difference between low voltages caused by faults versus voltage collapse can be used to add security to a separation scheme.
The logic shown in Fig. 2 (SEE PDF) requires that all three line-to-neutral voltages must drop below setpoint #1. Additional security can be added using undervoltage (27B) blocking. Since the magnitude of undervoltage due to impending voltage collapse is 89-94%, blocking operation for low voltages that are fault-induced adds more security. Fig. 2 indicates that any line-to-neutral phase voltage that drops below setpoint #2 will block the operation of the scheme. The last security measure in the logic scheme in Fig. 2 (SEE PDF) is the use of negative sequence voltage (47B) to block operation of the separation scheme.
During unbalanced fault conditions (all fault except three phase faults), negative sequence voltage will be present. Since voltage collapse events are balanced voltage conditions, only a very small level of negative sequence voltage is present. The equation that defines negative sequence voltage is shown below.
To account for the 120O phase angle displacement between phases, unit phasors (a and a2) are used in symmetrical component terminology. For completely balanced three-phase voltages, the negative sequence voltage is zero. Negative sequence voltage blocking is used to detect unbalanced fault conditions and block the undervoltage scheme from improper operation.
Positive Sequence UVLS Logic -- Another logic scheme to enhance security for voltage separation is show in Fig. 3 (SEE PDF)
The scheme is similar to that shown in Fig. 2 (SEE PDF). The blocking elements are the same. But this logic scheme uses positive sequence rather than individual phase-to-neutral voltages to detect an undervoltage condition. Positive sequence voltage is a symmetrical component term and is defined by the following equation:
For completely balanced three-phase voltages, the positive sequence voltage is equal to the value of the normal phase to neutral voltages –that is, V1=Va=Vb=Vc. Positive sequence voltage provides a single quantity as the actuating voltage for undervoltage separation and does not require that all three voltages be below a given setpoint as required in the logic scheme discussed in Fig. 2 (SEE PDF). Both schemes discussed in Fig. 2 and 3 (SEE PDF) are easily programmed into modern digital relays. One of the benefits of digital relay logic is that the blocking logic can be modified to suit the user. If undervoltage and/or negative sequence blocking is not desired by the user, it can be easily eliminated in the logic.
Additional security can be provided at critical facilities using a “voting logic” scheme. The “voting logic” means that multiple protective relays are applied with identical settings and logic at the same measuring point on the system. A majority of the devices must agree before action is taken. The purpose of voting logic is to get confirmation of the system conditions from more than one protective relay, thus avoiding potential false tripping based upon a malfunctioning protective relay. If two relays are installed at each location, two out-of-two logic is used. This logic requires both relays to operate before tripping is initiated. If three relays are used, two-out-of-three logic is used requiring any two relays to confirm the trip condition. Two-out-of-three logic is common in nuclear plant voltage separation schemes.
III. UVLS SETTING CONSIDERATIONS
As previously discussed, setting and design of an UVLS requires close cooperation between the relay engineers and system planners. System planning engineers conduct numerous studies using P-V curves and other analytical methods to determine the amount of load that needs to be shed to retain voltage stability under various contingency conditions. Voltage collapse is most probable under heavy load conditions, so the amount of load to be shed depends on system peak load and generation sources. When considering the type of load to be shed, constant KVA loads such as motors are good candidates for shedding since they draw more current as voltage is decaying. The following is an example that discusses the consideration in setting UVLS relays. The first step is to determine the P-V curves for creditable voltage collapse scenarios.
Fig. 4 (SEE PDF) show an example P-V curve for a creditable contingency. The knee of the curve at which the voltage will collapse is identified as Vcollapse. A setting margin or safety factor is desired and then the accuracy band of the relay and VT is shown. The setting (Vsetting) must be set above these margins. As with all relay settings, dependability and security need to be balanced. If too small a margin is chosen, there is a risk of the scheme operating during allowable emergency conditions that do not yet require load shedding. If too small a margin is chosen, then load shedding could occur after the system passes below the nose curve voltage collapse point (Vcollapse) shown in Fig. 4 (SEE PDF).
Fig. 5 (SEE PDF) illustrates this point. The choice of time delay and the number of setpoints are also critical settings, especially for distributive or de-centralized schemes which trip load directly. Again, planning studies can provide help in selecting the time and setpoints. Typically, there are fewer setpoints in UVLS schemes then are used for UFLS. Some utilities have chosen one voltage pickup point with different time delays for each block of load shed. Time delays are generally set at 2 - 10 seconds— not in the cycle range common for UFLS.
IV. CONCLUSIONS
Investigations of recent blackouts confirm that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960’s and –‘70s. The operation of today’s power system with load centers remote from the generation source makes today’s power system very dependent on the transmission systems that interconnect load and generation. Loss of transmission lines result in high VAR losses that cause voltage collapse at the load center. UVLS is a viable method of providing protection to avoid system voltage collapse. Implementation of UVLS is not mandatory for NERC member systems. However, NERC has recognized it as an important method to address voltage collapse. Although most systems studies find UVLS very effective in preventing voltage collapse, it may not benefit all types of voltage collapse scenarios. For example, systems with fast voltage decay characteristics (less than a second) may find UVSL to be too slow to prevent collapse. UVLS provides a “system safety” net and is an economical method of addressing voltage collapse situations using the philosophy that it is better to shed some load if shedding that load can prevent a much larger outage. Ultimately, however, transmission lines need to be built to address creditable undervoltage conditions.
UVLS schemes are more difficult to design and to set than UFLS and require close cooperation between utility relay engineers and utility system planners. There are two types of UVLS schemes --- decentralized or distributed and centralized. Both types of schemes are being applied, as individual utilities are beginning to apply UVLS on their own without being mandated to do so by NERC.
This article discussed the considerations in developing a secure UVLS scheme and it is the hope of the author that it has focused on the key design and setting questions that need to be addressed.
VII. REFERENCES
[1] C. J. Mozina, Power Plant Protection and Control Strategies for Blackout Avoidance, Georgia Tech Protective Relay Conference, April 2005.
[2] B.R. Williams, W.R. Schmus, D.C. Dawson, Transmission Voltage Recovery Delayed by Stalled Air Conditioner Compressors, IEEE PES Transactions on Power Systems, Vol. 7, No.3 August 1992.
[3] North American Electric Reliability Council (NERC), 1987 System Disturbance Report, p19, July 1998.
[4] U.S. – Canada Power System Outage Task Force, Final Report on the August 14, 2003 Blackout in the United States and Canada: Causes and Recommendations” April 5, 2004.
[5] G.C. Bullock, Cascading Voltage Collapse in West Tennessee, August 22,1987, Georgia Tech Relay Conference, May 1990.
[6] S. Imai, Undervoltage Load Shedding Improving Security as Reasonable Measure for Extreme Contingencies. IEEE PES Transactions on Power Delivery.
[7] IEEE Power System Relaying Committee Report, Summary of System Protection and Voltage Stability, Transactions on Power Delivery, Vol. 10. No. 2, April 1995.
[8] Undervoltage Load Shedding Task Force (UVLSTF), Technical Studies Subcommittee of the WECC, Undervoltage Load Shedding Guidelines, July 1999.
About the Author
Chuck Mozina is a consultant for Beckwith Electric. He is an active 25-year member of the IEEE Power System Relay Committee (PSRC) and is the past chairman of the Rotating Machinery Subcommittee. He is active in the IEEE IAS I&CPS, PCIC and PPIC committees, which address industrial system protection. He is a former U.S. representative to the CIGRE Study Committee 34 on System Protection and has chaired a CIGRE working group on generator protection. He also chaired the IEEE task force that produced the tutorial “The Protection of Synchronous Generators,” which won the PSRC’s 1997 Outstanding Working Group Award. Chuck is the 1993 recipient of the Power System Relay Committee’s Career Service Award and he recently received the 2002 IAS I&CPS Ralph Lee Prize Paper Award. His papers have been republished in the IAS Industrial Applications Magazine.
Chuck has a Bachelor of Science in Electrical Engineering from Purdue University and is a graduate of the eight-month GE Power System Engineering Course. He has authored a number of papers and magazine articles on protective relaying. He has over 25 years of experience as a protection engineer at Centerior Energy, a major investor-owned utility in Cleveland, Ohio where he was the manager of the system protection section. In that capacity, he was responsible for the electrical protection of the company’s generating plants as well as the transmission and distribution system that served over 1.2 million customers. For ten years, he was employed by Beckwith Electric, a manufacture of protective relays, as Application Manager for Protection Products. He is also a former instructor in the Graduate School of Electrical Engineering at Cleveland State University as well as a registered Professional Engineer in Ohio.
