Design of System Integrity Protection Schemes (SIPS)

Authors: Jonathan Sykes, and Yi Hu, USA

Considering the importance of SIPS to the stability of the power system, the IEEE PES Power Systems Relaying Committee WG C15 published a report “Design and Testing of Selected System Integrity Protection Schemes (SIPS) .“
The design of SIPS varies from scheme to scheme and is very different from the design of conventional protection systems. It even varies from application to application for the same type of scheme. The proper design, documentation, and testing of SIPS is the basis for reliable and accurate operation of such schemes, which often are the last line of defense for preventing the protected system from cascading outages.

Generic SIPS Description Model
A SIPS is designed to detect abnormal system conditions, to decide on a course of actions, and to take the appropriate corrective actions. This generalized concept is shown in Figure 1.

Monitor and Detect: The Monitor function generally involves the measurement of power flows on lines, voltages, frequency, rate of change of frequency, and/or other factors pertinent to the specific system condition. Detection is the determination that a line, generator, transformer, or other power system component is out of service. Security of detection can be accomplished through the measurement of two quantities. For example, on a line, a line outage can be securely detected by requiring that the circuit breaker status is monitored to be “Open” and the current on the line is less than a minimum pickup value. Most SIPS are critical to system performance. As such, these schemes are generally designed (and often mandated) to be composed of fully redundant “A” and “B” sets of equipment.

Communicate: The information from the various monitoring and detection sites must be communicated to a decision function. In a physical implementation, the decision function can either be centralized in one location or distributed throughout multiple locations. The size of the scheme and the number of locations involved are usually factors in determining the physical architecture. The performance capabilities of available communications technology generally removes communication latency as a factor in the centralized vs. distributed decision. Measurements on actual implementations have demonstrated that device-to-device latencies in the order of 3 to 7 ms are readily achievable.

Similar to the implementation architecture of the monitor and detection function, the communications system for the SIPS must generally be designed for redundant operation such that no single communications system element failure can prevent the SIPS from performing its functions. Although high-speed communications technology is available, economics may dictate that existing communications infrastructure with higher latency is used. In this scenario, communications latency must be factored into the overall system performance criteria.

Decide: Once all relevant data has been communicated to the decision function, the data is typically analyzed for conformance to pre-determined critical operating conditions. A SIPS will typically consist of multiple contingencies as described above. As most SIPS are required to operate “as fast as possible”, there is typically very little operator interaction with the real time decision. Operator interfaces are usually provided to monitor the status and health of the system, to facilitate in the testing of scenarios, and sometimes to “arm” the scheme when warranted by system conditions.

Mitigate: Once a decision to take action is made, the desired action must be communicated to actionable devices in the field. The actionable devices are typically known as mitigation devices. Depending on the required action, mitigation devices can span all the way from high-voltage substations down to distribution substations. In the cases where dynamic stability is being addressed, mitigation will most likely take place in high voltage substations. When overload conditions are being addressed, mitigation will usually take place at sub-transmission to distribution levels. Again, typical installations will include a redundant “A” and a “B” system.

Communications Requirements
Communications are used to acquire data from distant locations for the central controller where the decision for any action is made and to send commands to the field to execute the action. The communications paths may not be the same or of the same criticality.

Mitigation action signals are sent from the controller to the field. These signals typically cause tripping or closing of breakers. Status signals, representing the condition of the power system are sent from the field to the controller. Status signals communicate breaker or switch positions to the controller. Monitoring signals, also sent from the field to the controllers for evaluation, are analog value signals such as MW and MVar representing system loading. SIPS signals are carried over various communication systems including but not limited to: SONET, microwave, leased line, fiber, Ethernet, satellite, radio and power line carrier.

To assure system stability, SIPS signals transmission requires speed, security and dependability from the communications system that it uses. Security is for the communications system to not produce inappropriate action as a result of a component failure or a communications circuit media issue and dependability is to pass the control signal in spite of a single system component failure. SIPS event detection signals are sent to the controller to trigger SIPS decision makings and SIPS control action signals to the field for mitigation actions such as opening or closing breakers.
SIPS monitoring signals are measured and/or calculated signals representing analog values of power delivery. The signals may be in a digital format such as DNP, proprietary protocol, IEC 61850, or analog frequency shift key (FSK). SIPS monitoring signals from the field that are transmitted to the controller may not require the same speed of delivery as event detection and control action signals if these signals are not quantities that trigger the decision making and mitigation actions of the SIPS but are quantities that influence what action should be taken when certain events occur. These SIPS monitoring signals from the field may be dependent on existing low speed communications architecture at the utility.

Direct fiber, digital microwave, and SONET provide low latency for a given transmitted signal. Satellite signals have the greatest delay. Ethernet delays are variable based on the equipment, the number of nodes, and configuration used.
Communication delays vary depending on the communications system used. Direct fiber is different than digital multiplexing schemes, and packet based communications (e.g. Ethernet) differ from time division multiplexing (T1/E1 and SONET/SDH). “Through node delay” is the time it takes for a signal to travel through a node. “Re-frame delay” is the amount of time a multiplexer will require to resume communications after an interruption of service. A list of typical delays is presented in Table 1:
Manufacturers have different methodologies to reduce the impact of delays, reframing, and outage detection.



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