• Electromechanical relay
  These mechanically driven electrical (disk or cylinder) relays are single phase only and are dedicated to a single protection function. As these units approach the end of their useful life (20 years or more), the probability of failure increases dramatically. Moreover, they are no longer manufactured and spare parts are not available. In some cases equivalent electromechanical relays are still available.
• Solid state relays
  Solid state (static) relays are in most cases single phase and significantly smaller and lighter than electromechanical relays. They are less sensitive to dusty conditions and are not subject to wear. However, electronic component drift can occur with age or in extreme ambient temperatures and voltage surges may be a problem.
• Integrated relay
  Integrated relays are similar in design and function to solid state relays but use mostly integrated circuits rather than discrete components. Integrated circuitry allows these relays to combine three phase ground and current protection, metering, communication, control and monitoring capabilities. Although these relays do not perform as well as digital ones, they are comparably priced.
• Digital relays
  These microprocessor- and software-based relays feature advanced programmable functions which maximize flexibility and monitoring capabilities, and offer a wide choice of trip curves and self-test capabilities. The microprocessor replaces most of the electronic circuitry, thereby maximizing integration of advanced protection functions, control/monitoring, alarms and annunciation, metering and communication into a single device. Use of digital technology significantly reduces the risk of drift. The unit has a high immunity to electromagnetic fields and transients, and is designed for harsh environments and a wide range of operating temperatures.

Relay reliability
The reliability of the protection system is crucial to the dependable operation of the electrical distribution system. When a relay fails, the circuit breaker may not trip under a fault condition or it may false trip. The consequences of such a failure in a medium voltage system can be catastrophic (personnel safety, equipment damage, lost production, lost data etc.). Traditional reliability (electromechanical and static relays) depends on the frequency of routine maintenance. This generation of relays does not have self testing or status monitoring as do most of the new digital units. A problem may go undetected until routine maintenance is performed, or until the relay fails to operate. To avoid this problem, digital relays feature built-in self-diagnostics.

Maintenance
Electromechanical and static relays require annual maintenance as recommended by the manufacturer. On site, the owner continues to maintain the relays following the original manufacturer's maintenance procedures. This includes cleaning, calibration, testing and replacement of defective or worn parts based, if possible, on deviation from original specifications.

Most of the newer digital relays do not need the yearly maintenance required for electromechanical and static relays. Due to its continuous self-test feature, Schneider Electric's SEPAM digital relay only requires a simple check-up on a yearly basis to ensure correct circuit breaker operation via the relay and to check alarm indicators. The secondary injection test need only be performed every five years. This benefit offers a considerable reduction in maintenance costs and downtime.

Required protection
With increased pressure on productivity, more complex processes and improved production asset management, facilities cannot afford unscheduled downtime or costly equipment damage.

Protection systems installed 20 years ago use basic, generic overcurrent and ground fault protection regardless of the type of load. Twenty-year old equipment is oversized and was installed to strike a compromise (balance) between the reliability of the system and the capacity of the load to sustain a fault or improper applications.

Because the new generation of electrical equipment and its loads must be able to operate at full and sometimes above capacity, the old overcurrent and ground fault protection system may no longer meet the current needs of the existing system. The protection system's "security margin" must be significantly reduced to meet the electrical system operating requirements without damaging the equipment being protected.

Because of the flexibility and large choice of protection settings and curves, digital relays can provide application-specific protection (including feeders, motors, transformers, generators, and capacitors) with accurate settings. This enables them to meet specific operational requirements and better protect the distribution equipment. When considering relay upgrades, users should conduct application engineering and coordination studies to establish protection requirements based on loads and electrical system operating requirements - thereby maximizing the effectiveness of digital relays.

For example, a transformer with electromechanical overcurrent protection could be converted to digital protection using -- in addition to the overcurrent protection -- thermal overload protection and temperature monitoring. Depending on the size and cost of the transformer, differential protection could also be considered.

This alternative takes into consideration the thermal inertia and real temperature of the transformer during operation, and therefore allows more power through the same transformer without jeopardizing equipment safety. Overload can be sustained for a limited time (based on the thermal inertia of the transformer and temperature), and the digital relay can annunciate the trip and delay. The information can also be sent remotely if required.

Coordination
Electromechanical relays provide only a single curve choice and feature very limited adjustment settings. Also, different types of relays are required to meet the coordination requirements.

Digital relays, on the other hand, provide a wide choice of curves and in most cases allow tripping curves to be combined. This improves the coordination scheme, which in turn improves reliability and equipment safety. Also, the choice of relay is simplified and spare parts inventory is minimized.

Most modern digital relays provide zone protection coordination. The choice of settings depends only on the type of load being protected (the upstream protection setting no longer needs to be considered) and does not reduce system reliability.

Circuit Breaker Control and Predictive Maintenance
Some digital relays provide features allowing the control and monitor of circuit breakers. This information can be used for maintenance purposes in order to plan the required maintenance for individual circuit breakers depending on their actual performance and operation. This information can be collected remotely, and can be used to detect variations of circuit breaker parameters.

System Analysis
Many digital relays provide programmable event and waveform capture. When a fault occurs, the relay can record trip curve, date and time, and the fault amplitude and phase. Waveform capture also allows remote data collection and in-depth event analysis. A time stamping feature allows waveforms captured from digital relays at different locations in the electrical system to be combined to analyze the fault propagation in the system.

Integrated Input/Output
Most digital relays offer additional input/output modules (digital or analog) for integrated control and monitoring, thereby eliminating the need for an external input/output system. Dedicated RTD input modules are also available for transformer and motor applications, which eliminates the need for dedicated temperature monitoring modules.

Communication
One of the main advantages of digital relays is their capacity to communicate to a SCADA system. This allows, in most digital relays, remote operation (opening and closing circuit breakers), remote settings (password protected) and remote annunciation of alarm, temperature, amperes voltage as well as power metering, events and waveform capture.

Remote control and monitoring provides cost savings, increased reliability, and improved management of the electrical system. Maintenance personnel now have the ability to optimize the distribution system by tracing faults, detecting power imbalances in the system and preventing tripping and faults before they occur. Cost allocation and power factor optimization is also possible.

Conversion Considerations
The conversion to digital technology requires a comprehensive plan to ensure a fast, reliable conversion from the old units to the new digital relays with minimum downtime. To achieve this, a new cell door, complete with the new digital relay and cable harness, is designed and prefabricated in the factory. The actual downtime will, therefore, be limited to the removal of the existing doors and wiring, the installation of the new door, the connection of the new wiring and the commissioning of the new system.

The protection settings can be downloaded to the relay from a laptop computer. Most of the digital relay manufacturers offer software which runs in Windows to program the settings defined by the coordination study. The downloading time is almost immediate and the settings are kept in a master file.

Conclusion
Modern digital relay technology is a major breakthrough for protecting electrical systems. This technology is now mature, very cost effective and should be considered as part of any switchgear modernization program.

Don Bartlett is Manager, Marketing, Schneider Canada Services. He can be reached at (905) 678-5325 or bartletd@squared.com ET