Power Plant Training in Protective Relays

A protective relay activates in the electrical distribution system, sending a trip signal. Switchgear circuit breakers trip. An entire bus is lost. Equipment shuts down. The plant trips off line. A routine operations shift has turned to confusion and anxiety. How will your operators respond?

I have a client who recently had this very scenario. Disappointingly, those operators were stumped. They did not know how to diagnose what had happened and therefore were unsure how to respond, leading to a lengthier than necessary shutdown.

Over 99% of the time a plant’s protective relays sit there quietly monitoring the electrical distribution system. Perhaps a green LED gives the operator comfort that everything is normal. However, it is the fractional percentage of the time that the operators need to be prepared for; that time when a relay does its job and responds to protect the system from a harmful event.

There was once a time when protective relays were not too hard to decipher. A wall of panels loaded with electromechanical relays stood guard over the electrical distribution system. An operator could look at the nameplate for each relay and find overcurrent relays, differential relays, etc. If there was a trip, the operator knew to look for the trip target. Upon finding the target, the operator immediately knew which relay had caused the trip and therefore the basic cause of the trip.

With current technology, relays are now all solid state. The protective relays that once took up an entire relay panel are now contained within a single Schweitzer, Multilin, Basler Electric or other solid-state device. Mechanical targets are replaced with LED indicators. While each LED is labeled with a device function number (51, 51, 87, etc.), all of the programming and integration of the various protective schemes occur within the relay’s software. Understanding how the protective system works and what is involved is now more complicated. A well-designed power plant training program is essential to ensure operators have the necessary knowledge to understand and operate this complex system.

The science and skill of protective relaying is fundamental to ensuring high reliability in the operation of the electrical distribution, transmission and generation systems. To ensure your operators are up to the task, an operations training curriculum should include training on relay theory and basic system protection, specific to your site. The objectives of such a power plant training course should include:

• Understand the basic philosophy of system protection
• Understand the function and operation of Current Transformers (CTs) and Potential Transformers (PTs)
• Understand Phasor Notation and Polarity, Fault Current Basics, Fuse Characteristics, and Non-directional Time Overcurrent Relays
• Understand Directional Overcurrent Relays, Impedance Relaying, and Blocking Schemes
• Understand Blocking Schemes, Bus Differential Relaying Concepts, Restraint Windings, and Electromechanical and Microprocessor Relays
• Understand different relay systems and how they respond to a fault, including:
  • Generator Protection
  • Transformer Protection
  • Bus Protection
  • Feeder Protection
  • Motor Protection
  • Transmission Line Protection
• Use relay LED indications and event recordings to analyze relay operations

With power plant training in protective relays, operators will become confident that when (not if) the next relay trip occurs, they will be able to quickly and effectively respond to the event.