Common Ancillary Protective and Control Functions

Working Group K5 Report

The IEEE Power System Relaying Committee (PSRC), Substation Protection Subcommittee, Working Group K5 on Ancillary Protective and Control Functions Common to Multiple Protective Relays have produced a document that addresses the considerations in applying the ancillary protection, control and monitoring functions that are common in multiple relays and the integration of these functions into the overall protection system. Today's modern protection schemes come with fully integrated protection, control and monitoring functions to accommodate the implementation of the different design requirements to achieve a reliable protection and control solution.

This freedom of implementation can be an exhilarating challenge to the engineer’s imagination. This document addresses subjects related to specific protection and control topics with application examples. Topics on breaker failure, automatic reclosing, synchronism check, voltage status monitoring, breaker controls, event and fault recording are discussed. Testing and maintenance considerations are also covered in the document.

The applications of duplicate protective schemes in modern protection is of big interest to users as almost all numerical protective relays now give the user the ability to either modify the existing protection and control logic inside the relay, or add specific logic tailored to the user’s requirements. This advancement in the state of the art has enabled the user to implement a whole host of tripping, transfer, monitoring and control schemes as part of a custom made logic inside the main protective relay, thus allowing the elimination of external relays, auxiliary relays, timers, and wiring. Whether they are installed in new substations or as retrofits in old substations, multifunction relays can be successfully applied to satisfy both the protection and control requirements of the power system equipment. The choice of implementing protection and control functions depends largely on the equipment to be protected, the power system operating requirements, and the owner’s comfort level with multifunction relays. There are virtually no limits on the variety of new protection schemes that can be designed to satisfy specific application requirements. A major challenge for the engineer is to balance the ability to provide redundancy of functions against the requirement to “keep the system simple.”

Application examples of improved protection and control schemes have been documented in this report in the areas of breaker failure scheme logic; line reclosing scheme logic; sync check, or interlocking. In the area of monitoring, programmability gives the multifunction relay powerful monitoring and alarming capabilities. Breaker trip coil and loss of voltage monitoring are good examples of these capabilities. Monitoring the status of terminal components for the purposes of modifying protection schemes, such as for open terminal conditions or to provide stub bus protection when the line motor operated disconnect is open, are also excellent examples of the enhanced monitoring features of multifunction relays. Event data recorded in microprocessor based relays, both analogs and status, is an important tool for the post analysis of the event.

In breaker failure (BF) schemes, maintenance practice and relaying philosophy impact significantly the selection of a BF scheme. Factors to consider are the preferred degree of security and reliance on remote versus local backup; degree of integration of the fault detection and BF functions on a single multifunction relay and the existing maintenance/ testing practice; willingness and capacity to adjust and preferences with respect to simplicity and cost targets.

In automatic reclosing schemes, present technology multifunction devices offer the 79 function at no extra cost. Whether automatic reclosing (AR) is implemented as a dedicated reclosing relay or as a programmable function within a multifunction protection relay, several external input signals may be required for successful implementation, depending on the design requirements of the reclosing scheme. Typical input signals required by reclosing relay schemes include but are not limited to reclose initiation, breaker status (open or closed,) drive to lockout, pause, and voltage or synchronism check supervision.

In a case when redundancy of the AR function is required, both primary and backup tripping relays are equipped with an AR function. One of the ARs is typically selected as the normal (master) device, and the other is enabled only if the master device is not operational.

This report is supported by many application examples in the annex section and gives thorough discussions on intelligent electronic device (IED) control function schemes used in modern protection numerical relays where point to point wiring associated with the cascading device outputs of a traditional scheme might be reduced or eliminated. Control system architecture, will depend upon the required redundancy, and the choice of which hardware platforms are to contain the line, bank, bus, or breaker failure protection. The degree of integration practiced by the utility may range from fully integrated, where relays provide not only remote and local breaker control but also status, alarm, and metering information, to partial integration, where relays provide only for local automation such as in automatic transfer or isolation schemes.

Schemes that most effectively measure the conditions from only a single power station can be migrated from remote manual to local automated control. The resulting simplified schemes have fewer components that can fail (no telecommunication channel.) The potential for human error is also reduced. Remote arming or manual backup to this automated control may be included. One example is the automation of switching of a shunt capacitor bank to control local voltage. A relay that is applied to protect the capacitor bank can also measure the bus voltage, make a control determination, and initiate switching. This automatic control scheme can be removed from service by a remote SCADA operator.

If redundancy has already been provided to satisfy protection requirements, the dependability, security, availability and simplicity of control schemes may be improved with functions that are inherent in the numerical relays. New techniques may require changes to process and thinking. However, it is important that the addition of control functions in multifunction relays does not degrade protection performance. Inappropriate or failed control can cause equipment damage or unsafe conditions.

In this report, maintenance is also considered. Physical switches for isolation (make-before-break), injection testing, and cutting the relay out of service may be provided at the option of the utility. Connectorized cables might be applied to the control circuit outputs for possible disconnection. Virtual switches that reside as logic elements within the IED can prevent unwanted transmission of alarms during maintenance.

Event and fault recording are helpful tools when analyzing faults on the electric system. Most microprocessor relays provide these tools in some form. This document describes situations where multiple relays are used for protection, control and monitoring to retrieve event or fault data from multiple sources. The ability to compare records from several sources, however, may prove useful. Different relays handle several issues such as frequency response, record length, triggering, record storage, setting files, software, off-nominal frequency, and other issues in different methods. One may be able to gather more information by gathering information from multiple relays. To obtain the full benefit of these comparisons it helps to have the relays time synchronized. The usage of an IRIG-B signal from a global positioning system (GPS) time source can provide the time synchronization. Another useful method to ensure more data is collected during events is cross-triggering or cross-initiation. The benefit is that all relays provide data so that analysis can be more complete. It can be accomplished by hard wiring an output of one relay to the input of other relays or by peer to peer communications.

Testing and maintenance are also covered and although the overall maintenance frequency is reduced for microprocessor relays due to the self-monitoring, the use of many functions in one microprocessor relay can create issues with maintenance and testing. The maintenance of one microprocessor relay could remove needed protection for a circuit and therefore, the circuit may have to be cleared.

Clear documentation is also needed to ensure proper protection is available during maintenance. The report discusses these important issues in the documentation section.

Multifunction relays have protective functions that interact with each other, making testing more complicated. They can also be programmed to do control logic, which is usually verified along with the protection logic during commissioning. Digital relays also can have multiple setting groups that may be switched to address varying system conditions. This flexibility increases the commissioning complexity. Due to the additional complexity it is important for the user to fully understand the schemes and how they interact. The simple testing of one integrated protection element in an IED could trigger an undesirable operation of a breaker failure scheme. The documentation of the schemes is very important and those who test the relays have to completely understand the behavior of a system.

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