Increasingly popular over the past several years are the so-called special protection systems, sometimes also referred to as remedial action schemes.
Depending on the power system in question, it is sometimes possible to identify the contingencies or combinations of operating conditions that may lead to transients with extremely disastrous consequences. Such problems include, but are not limited to, transmission line faults, the outages of lines and possible cascading that such an initial contingency may cause, outages of the generators, rapid changes of the load level, problems with HVDC or FACTS equipment, or any combination of those events.
Among the many varieties of special protection schemes, several names have been used to describe the general category: special stability controls, dynamic security controls, contingency arming schemes, remedial action schemes, adaptive protection schemes, corrective action schemes, security enhancement schemes, etc. In the strict sense of protective relaying, we do not consider any control schemes to be SPS, but only those protective relaying systems that possess the following properties:
• SPS can be operational (“armed”), or out of service (“disarmed”), in conjunction with the system conditions.
• SPS are responding to very low probability events; hence they are active rarely more than once a year.
• SPS operate on simple, predetermined control laws, often calculated based on extensive offline studies.
• Oftentimes, SPS involve communication of remotely acquired measurement data (SCADA) from more than one location in order to make a decision and invoke a control law.
The SPS design procedure is based on the following:
• IDENTIFICATION OF CRITICAL CONDITIONS: On the grounds of extensive offline steady state studies on the system under consideration, a variety of operating conditions and contingencies are identified as potentially dangerous, and those among them that are deemed the most harmful are recognized as the critical conditions. The issue of their continuous monitoring, detection, and mitigation is resolved through offline studies.
• RECOGNITION TRIGGERS: These are the measurable signals that can be used for detection of critical conditions. Oftentimes, such detection is accomplished through a complicated heuristic logical reasoning, using the logic circuits to accomplish the task: “If event A and event B occur together, or event C occurs, then…” Inputs for the decision making logic are called recognition triggers, and can be the status of various relays in the system, sometimes combined with a number of (SCADA) measurements.
• OPERATOR CONTROL: In spite of extensive simulations and studies done in the process of SPS design, it is often necessary to include human intervention, i.e., to include human interaction in the feedback loop. This is necessary because SPS are not needed all the time, and the decision to arm, or disarm them remains in the hands of an operator.
Among the SPS schemes reported in the literature, the following are represented:
• Generator Rejection
• Load Rejection
• Under frequency Load Shedding
• System Separation
• Turbine Valve Control
• Stabilizers
• HVDC Controls
• Out-of-step Relaying
• Dynamic Braking
• Generator Runback
• VAR Compensation
• Combination of schemes
Some of them have already been described in the above text. A general trend continues toward more complex schemes, capable of outperforming the present solutions and taking advantage of the most recent technological developments and advances in systems analysis.
Depending on the power system in question, it is sometimes possible to identify the contingencies or combinations of operating conditions that may lead to transients with extremely disastrous consequences. Such problems include, but are not limited to, transmission line faults, the outages of lines and possible cascading that such an initial contingency may cause, outages of the generators, rapid changes of the load level, problems with HVDC or FACTS equipment, or any combination of those events.
Among the many varieties of special protection schemes, several names have been used to describe the general category: special stability controls, dynamic security controls, contingency arming schemes, remedial action schemes, adaptive protection schemes, corrective action schemes, security enhancement schemes, etc. In the strict sense of protective relaying, we do not consider any control schemes to be SPS, but only those protective relaying systems that possess the following properties:
• SPS can be operational (“armed”), or out of service (“disarmed”), in conjunction with the system conditions.
• SPS are responding to very low probability events; hence they are active rarely more than once a year.
• SPS operate on simple, predetermined control laws, often calculated based on extensive offline studies.
• Oftentimes, SPS involve communication of remotely acquired measurement data (SCADA) from more than one location in order to make a decision and invoke a control law.
The SPS design procedure is based on the following:
• IDENTIFICATION OF CRITICAL CONDITIONS: On the grounds of extensive offline steady state studies on the system under consideration, a variety of operating conditions and contingencies are identified as potentially dangerous, and those among them that are deemed the most harmful are recognized as the critical conditions. The issue of their continuous monitoring, detection, and mitigation is resolved through offline studies.
• RECOGNITION TRIGGERS: These are the measurable signals that can be used for detection of critical conditions. Oftentimes, such detection is accomplished through a complicated heuristic logical reasoning, using the logic circuits to accomplish the task: “If event A and event B occur together, or event C occurs, then…” Inputs for the decision making logic are called recognition triggers, and can be the status of various relays in the system, sometimes combined with a number of (SCADA) measurements.
• OPERATOR CONTROL: In spite of extensive simulations and studies done in the process of SPS design, it is often necessary to include human intervention, i.e., to include human interaction in the feedback loop. This is necessary because SPS are not needed all the time, and the decision to arm, or disarm them remains in the hands of an operator.
Among the SPS schemes reported in the literature, the following are represented:
• Generator Rejection
• Load Rejection
• Under frequency Load Shedding
• System Separation
• Turbine Valve Control
• Stabilizers
• HVDC Controls
• Out-of-step Relaying
• Dynamic Braking
• Generator Runback
• VAR Compensation
• Combination of schemes
Some of them have already been described in the above text. A general trend continues toward more complex schemes, capable of outperforming the present solutions and taking advantage of the most recent technological developments and advances in systems analysis.