Programmable scheme logic

Modern state-of-the-art numerical relays incorporate many protection and control elements. These elements can be used as building blocks to configure the device to a customer's specific application.

The Programmable Scheme Logic is the means by which the internal elements, in reality numerical algorithms, are interlinked in accordance with conventional gate logic rules. The PSL also assigns how opto inputs assert control over the enabled elements, and which outputs should change state, should a particular combination of elements operate. The PSL is used to supplement built-in protection and control schemes, such as breaker-failure protection, which are already implemented within the standard building blocks of elements. Some of the benefits offered by multifunctional devices configured in PSL include:

• Easy configuration of customer-specific schemes
• Protection reconfiguration, when required
• Removing the need for a programmable logic controller and circuit breaker bay controller.

The user can design a logic scheme based upon three main types of logic gates: logical AND, OR, and MAJORITY gates. Any gate input or output can be inverted, allowing NOT, NAND, and NOR configurations. Auxiliary timers may be assigned at any stage of the logic for sequence of events coordination, also within feedback loops if required.

Event driven PSL only writes to output contacts or front plate LEDs once all PSL re-evaluation is complete. The principle of event driven PSL is to reduce the quiescent overhead on the processor to a very low level, and to keep it low even during fault and autoreclose conditions on the power system. This ensures that fast and repeatable trip times are obtained from relays, and that non-real time tasks such as fault location and disturbance record management take place immediately afterwards.

Monitoring

Predictive maintenance is becoming extremely important in the efforts of utilities to deal with reduced personnel and increasing customer requirements for improved power quality and reliable supply of electric power. Modern distribution protection IEDs measure and calculate numerous analog parameters and provide additional monitoring functions that allow the transition from scheduled to event driven maintenance, without the need for specialized equipment.

A variety of tools, available in microprocessor based relays can help the user to determine the need for primary or secondary substation equipment maintenance based on user defined alarm signals from the protective device.

The data is divided in several categories:

• Breaker maintenance related data, including breaker interrupted current, breaker operation counters, fault counters and breaker opening and close time monitors
• Breaker auxiliary contacts and trip coil monitoring
• Voltage transformer supervision schemes
• Current transformer supervision logic
• Broken conductor detection

All the above information is available from the relays based on built in analysis tools that process the fault data and convert it to information, thus reducing the need for protection, control and maintenance personnel in the decision process.

Recording

The relays need to record and time tag different events and store them in non-volatile memory. This enables the system operator to establish the sequence of events that occurred within the relay following a particular power system condition, switching sequence etc. The real time clock provides the time tag to each event, with a resolution of 1ms.

The event records consist of fault flags, pre-fault and fault measurements etc. Fault location is part of the data included in the fault record.

Internal failures detected by the self-monitoring circuitry, such as watchdog failure, field voltage failure etc. are logged into a maintenance report.

Recording of the waveforms during a distribution system event can help with the analysis of the operation of the protection relays. The number of records that may be stored by the relay is dependent on the selected recording duration. The duration of the recording may need to be extended to several seconds in order to capture the operation of time-overcurrent protection elements.