Применение технологии СВИ для задач мониторинга, оперативно-диспетчерского и автоматического управления,
а также задач мониторинга за состоянием силового оборудования и ЛЭП
Report "Real-Time Application of Synchrophasors for Improving Reliability"
В отчете рассматривается множество способов применения технологии СВИ в режиме реального времени и в офлайн режиме для повышения надежности управления энергосистемой. Доклад подготовлен по заказу Североамериканской корпорации по обеспечению надежности (NERC).
(Оценивание состояния электрического режима)
Для управления ЭЭС требуется полная и точная информация о параметрах режима, которая характеризует текущее состояние ЭЭС. В ОИК ДЦ информация поступает с помощью средств телемеханики в виде ТС и ТИ. Как правило, объемы и качество телеметрической информации недостаточны для полной наблюдаемости схемы ЭЭС, что негативно отражается на ОС.
ОС – одна из основных задач подсистемы оперативного управления режимами. Она выполняет функцию формирования моделей текущих режимов работы ЭЭС для выполнения дальнейших расчетов. ОС является обязательной составной частью всех комплексов централизованной противоаварийной автоматики и системы мониторинга запаса устойчивости.
Благодаря возможности получения СВИ задача ОС ЭЭС приобретает новое качество. Требуется развитие методики ОС, ранее базирующейся на несинхронизированных SCADA-измерениях, для улучшения свойств решения задачи ОС ЭЭС. Применение данных СВИ позволит избежать основных проблем, возникающих при решении задачи ОС, связанных с низким качеством ТИ и ТС. В полученных SCADA измерениях проявляется эффект «замершего измерения», что может привести к ненаблюдаемости в расчетной схеме и некачественному ОС. Получение высокоточных данных СВИ с минимальной задержкой открывает новые перспективы для решения задачи ОС:
наличие дополнительных измерений увеличивает надежность системы при отказах отдельных измерительных каналов;
возрастает обоснованность решений в процессе отбраковки измерений, содержащих грубые ошибки;
повышается вероятность выработки правильных рекомендаций при проверке состояния топологии сети;
наличие прямых измерений независимых переменных, к которым относятся модули и фазы напряжений узлов, повышает устойчивость вычислительного процесса (за счет улучшения свойств матриц Якоби):
более высокая точность дополнительных измерений способствует повышению точности оценки режима в целом.
Low frequency oscillation monitoring
Мониторинг низкочастотных колебаний
Fundamentals of Power System Oscillations
Oscillations are always present in the BPS due to the electromechanical nature of the electric grid. Under no significant external influence to the system, the grid oscillates at its natural frequencies for small perturbations such as constant changes in load. These oscillations are usually well-damped and contained; however, growing or high-energy oscillations can present system instability, potential equipment damage, safety concerns, and power quality issues. In addition to these natural oscillations, forced or “rogue” inputs to the system can also cause oscillations and should be detected and mitigated to the extent possible. It is important to clearly differentiate between the types of electromechanical oscillations that are present. From a practical standpoint, power system oscillations can be categorized as follows:
• System (Natural): low-frequency rotor angle oscillations caused by instantaneous power imbalances. These are often differentiated further as follows:
Local: oscillations where one power plant or generating unit oscillates with the rest of the system, generally caused by heavy loading and generator controls
Intra-plant: oscillations where generating units within a power plant oscillate with each other at the same location5 , generally caused by poor tuning, unit control interactions, and unit operating modes
Inter-area: oscillations characterized by several coherent units or parts of the system oscillating against other groups of machines, often predominant in power systems with relatively weaker interarea connections
Torsional: high (subsynchronous) frequency oscillations caused by resonance conditions between highly compensated transmission lines and the mechanical modes of a steam-turbine generator (typically referred to as subsynchronous resonance6 ).
• Forced: sustained oscillations driven by external inputs to the power system that can occur at any frequency, such as unexpected equipment failures, control interactions, or abnormal operating conditions.
Situational Awareness, Wide Area Visualization
Визуализация динамики изменения параметров электрического режима в энергосистеме
Situational awareness is defined as, “the perception of elements in the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future.” (Endsley, 1995) Within the electric industry we want grid operators to have good situational awareness; we know that good visualization tools can help operators understand grid conditions and react to them. Advanced visualization software using synchrophasor data allows control room operators to see what is happening on the bulk power system within fractions of a second, rather than the industry standard practice of using data updated every four seconds. This technology can give operators precise snapshots of real-time conditions, provides clear information on unfolding events, and helps operators analyze the situation and take informed mitigation actions to protect and enhance grid reliability.
Islanding, Blackstart, Early warning of Instability
Мониторинг выделения части энергосистемы на изолированную работу
Islanding involves the separation of a part of the power system from the larger power grid. Electrical islands occur when relays, circuit breakers, switches, and other topology control equipment operate in such a way as to isolate a part of the grid from the larger interconnection. The isolation could be a disturbance response to protect either the equipment or system. On rare occasions, multiple protection and control actions can create an electrical island. If there is sufficient generation in the electrical island to supply the load, service may be sustained and operators may not detect the situation for some time. The more probable outcome, however, is that the islanded region blacks out because the balance between generation and load on an island is difficult to sustain. PMUs monitor frequency, voltages, and phase angles which can be indicators of an island. For example, the frequency in the island can diverge from the rest of the power system and decrease from normal values. The sooner an operator knows that an electrical island has formed, the greater the options available for avoiding a blackout. As soon as system operators are aware that they are effectively operating two separate electric grids – the islanded region and the rest of their interconnection – they immediately take steps to keep the generation and load in balance. Once the electrical island is stable, operators then adjust the generating units on the island so that voltage magnitudes, frequencies, and phase angles on and off the island match. Now the island can be reconnected to the rest of the interconnection.
Synchrophasors in Power System Protection
There is a growing body of research into the ways that synchrophasor technology can be used for system protection. Figure provides a topical breakdown of the current research under way over the last five years, based on reports in IEEE and NASPI publications.
The future of synchrophasors in power system protection systems starts with analyzing their potential for use in standard protection schemes. A simple explanation of these schemes and the current research and vendor solutions available offers a platform for future progress.
Мониторинг угла вектора напряжения
The timing or angular (phase angle) differences in voltages between locations in a power system provide information on power system stress. The differences in phase angles between ends of a transmission line grow with loading of the line. The phase angle difference is not only useful for determining the stress on the transmission line, but also is a decisive factor for reclosing transmission lines. Information on voltage phase angles assists in operating the power system in a reliable fashion without impacting stability. Traditionally, phase angles are calculated off-line with simulations and state estimation based on line flows. Now PMUs can measure phase angles directly, making them immediately available to system operators and enabling them to monitor and remediate stressed power system conditions as they develop.
Определение типа и вида возмущения
Event detection and classification
Voltage sage detection and analysis
Low- and high-impedance faults
Equipment health diagnostics
When an event takes place on the power system, operators must take action to mitigate its effects. The type and extent of the event that occurred are not always obvious to system operators; in some cases, the event is not even discernable to operators. Good event detection, management, and restoration allow system operators to understand the event, minimize its impact, and restore service as quickly as possible. PMU data provide early indications of grid stress, including abnormal voltages, phase angles, frequencies, and power flows. Before the event occurs, PMU data can provide clues to operators that parts of the system are stressed. With this information operators may have time to take mitigation action before the event worsens. As the event occurs, PMU data provide a view of the system’s response to the event in the form of high-resolution graphs of the anomalous power measurement waveforms. This provides insights into the type of event taking place and its extent. The sooner an operator understands the event, the more options are available for mitigating the event.
After a system disturbance or blackout, utilities study the sequence of events that led to the problem in order to prevent it from reoccurring. Analyzing system events such as oscillations, islanding, cascading outages, or other unfavorable behaviors is an important component of maintaining power system operational reliability. The insights gained by these analyses allow operators and planners to learn what initiated the system event, how the system responded to the incident, and most importantly, what can be done to prevent another. To understand how the system responded to the incident, the analysts develop a sequence of events that organizes every action due to the event, such as precise time at which protection equipment (i.e., relays) and circuit breakers involved in the occurrence operated. Using non-time-synchronized technology to determine the sequence of events, as during the analysis of the August 14, 2003 blackout in the United States and Canada, is time consuming and tedious. Significant manual effort was needed to time-align the thousands of various data items from the companies involved in that blackout. It took a large group of engineers over six months to compile the sequence of events because none of the recordings were time-stamped with a common time. As a result, one of the major recommendations of the U.S.-Canada Power System Outage Task Force was to “Require use of time-synchronized data recorders in order to establish time-synchronized disturbance monitoring to help evaluate the performance of the interconnected system under stress….” PMUs provide high resolution, time-stamped data so that an accurate aligned record of events can be constructed quickly, often within hours. Such a record of events is needed to be able to perform a root cause analysis of the event. Synchrophasor measurements taken from geographically dispersed parts of the interconnection are time-aligned with precision, and a picture of the dynamic behavior of the entire interconnection can be created quickly along with the determination of the sequence and source of an event.
Synchrophasor technology significantly reduces the labor and cost of post-event analysis. PMU-enabled automated tools have allowed engineers to investigate many events that they previously would not have had time to investigate. The more events that are analyzed, the greater the insights gained by engineers and operators. NERC can now initiate a technical investigation of an event within hours of obtaining the PMU data. PMU data also provide deeper insight into the dynamic behavior of the grid, enabling faster modeling and simulation of a disturbance and faster diagnosis of an event to develop recommendations for corrective and mitigation measures.
Voltage Stability Monitoring
Мониторинг устойчивости по напряжению
Maintaining the voltage at levels that remain stable is of paramount importance to a power system operator. Voltage stability is the ability of a power system to maintain adequate voltage levels through changes in generation, load, and topology. Loss of voltage stability almost always results in a voltage collapse: the voltage drops to zero and the load blacks out at that location. Depending on system conditions at the time, a local voltage collapse can propagate to adjacent areas and can ultimately lead to a widespread outage. Because voltages tend to decline when the electric grid is most stressed, the loss of a generator, transmission line, or other key asset during those times becomes much more significant than losing that same asset at other times. As the bulk electric system is challenged to transport increasing amounts of electric power, the likelihood of voltage collapse increases.
Renewable Resource Integration
Large-scale renewable generation plants are relatively new to the bulk electric system, so their plant characteristics and control algorithms need to be better understood in the context of the operation of the whole grid. Renewables can be challenging to manage in the grid due to the intermittent nature of their production and the peculiarities of their control systems. This is especially true as they become a greater portion of the generation. Conventional generation is used to balance the variability of wind generation, but is often located far from the wind generation. Synchrophasor systems are particularly useful for monitoring, managing, and integrating distributed generation and renewable energy into the bulk power system.
Functionality provided by synchrophasor technology includes:
• PMU data provide better visibility into how renewable generation affects conventional generation, voltages, frequency and oscillations on the power system both locally and across wide areas.
• Dispatchable fossil-fueled generation plants traditionally provide reactive power to the grid. As the amount of renewable energy resources increases, it becomes more difficult for system operators to provide necessary reactive power. PMU-based applications allow operators to more effectively manage reactive power sources to maintain voltage stability with renewable integration present.
• PMU data provide more accurate information for congestion management tools; this can increase capacity for renewable generation that would otherwise be curtailed due to fixed reliability limits.
Model Validation & Calibration
- System Model Validation
- Load models
- Renewable Generation Modeling
- Generator models
- HVDC Modeling
- Phase (ABC) identification
- Line segment impedances
- Transformer and other device models
Models are used in all aspects of power system planning and operations. Owners and operators of electric power assets use models to predict asset and system behavior under a variety of conditions. These models become more complex as these assets become more complex – the modeling and simulation of the integrated power system is driven by the accuracy of these models. The performance of power system simulations and operating forecasts directly correlates with the accuracy of system models. Improved models also increase the accuracy of dynamic predictions of asset response to events. Thus, the accuracy of these models drives the efficiency and effectiveness of long-term capital investments and real-time system operations. The quality of grid operation is completely dependent on the quality of these models, tools, and input data. Synchrophasor data provide a high-resolution view of power system and asset behaviors under a wide range of conditions across the interconnection. This capability enables engineers to evaluate and improve their models. PMU data also provide the ability to observe dynamic performance of a generation facility under conditions that may not be achievable under test conditions and to more accurately model its performance or validate the existing model. Truing up generator and system models reduces operational uncertainty, so system operators can better
accommodate congestion and other system limitations and reduce the operating margin. With better models, analysis tools provide more accurate insights to decision makers.
Line Parameter Estimation
Overhead transmission lines’ parameters are of great importance to the normal operation of the power system. The inaccuracy of the parameters of transmission lines will result in negative impact on the analysis of the power system, such as the power flow calculation, short circuit calculation, fault analysis, relay settings, fault location and so on. It will also affect the operating mode selection. It can be seen that acquiring accurate parameters of transmission lines is crucial. Traditionally the line parameters are measured off-line. However there might have large difference between the offline and on-line parameters due to operating temperature, skin effect of the current and other factors (weather, environment and geography). Moreover, off-line measuring approach needs several kinds of meters and usually complex wiring. Therefore, on-line parameter identification gains great interests, especially after the advent of WAMS The synchrophasor technology opens a new path to on-line parameter identification. There is a quickly increase in the number of PMU commissioned in power grid. Deployment of WAMS makes PMU data based on-line parameter identification of overhead transmission lines becomes possible. It should be pointed out that accurate on-line parameter identification is good not only for the operating calculation (power flow, fault analysis and so on), but also for early warning of abnormal operating condition. Conductor sag of overhead transmission lines is one of the root reasons of faults. If the conductor sag can not be identified in early stage, then the transmission line might be lower enough to touch trees and get grounding fault. Conductor sag might happen due to overload, wind, ice-coating of the transmission line, etc.
Equipment Problem Detection
Detecting equipment problems early can prevent unplanned outages due to equipment failures. Equipment failures can cause customer outages, costly equipment replacement, and can even damage other equipment. Inspections, diagnostic tests, time-based maintenance, condition-based maintenance, repairs, minor overhauls, major overhauls, and even long-term lay-up are planned and scheduled at times that are least costly to the overall grid. Transmission owners strive to anticipate and prevent equipment failures and schedule equipment replacement or upgrades on a wellmanaged, non-emergency, safe, and cost-effective basis. Synchrophasor technology enhances the capability to detect and diagnose failing and malfunctioning equipment, allowing for replacement during planned outages. Traditional SCADA, monitoring at four second sampling rates, cannot detect many of the data variations that indicate problems. PMUs’ faster reporting rates make it possible to observe and analyze grid events and the condition of grid equipment in unprecedented detail. A number of project participants have identified incipient failures using synchrophasor data and been able to schedule orderly replacement of the problem equipment, avoiding or minimizing customer outages.