Low-voltage (LV) motor control centers (MCCs) are numerous in industrial power distribution systems. MCC are commonly a safety concern because operator and maintenance personnel have close interactions with the MCC. Also, the recognition of arc flash as a unique electrical hazard has led to a new expectation for circuit protection devices: the safeguarding of personnel from the hazards of thermal burns and explosive blasts.
Traditional arc flash mitigation is dependent on single-function circuit protection devices (like molded case circuit breakers (MCCB)) to operate as designed in order to limit arc flash energy. Unfortunately MCCB and other components in power system short circuit protection schemes can fail with no indication that functionality is compromised until an arc flash event occurs. In such events, the arc flash energy release can be orders of magnitude greater than expected. Work methods, tools, and personal protective equipment selected based on predicted energy levels may not be sufficient to protect workers from injury. Protective relay technology has evolved to enable self-diagnostics and communication to personnel so the failure does not manifest as a hidden failure.
Arc-flash mitigation schemes built into microprocessor equipment must have ultra reliability, extensive on board diagnostics, self reporting upon failure, and remote monitoring of current, voltage, contactors, and overload devices. These technologies help reduce exposure to electrical shock hazards by decreasing the need to troubleshoot and perform other maintenance tasks that place workers in close proximity to potentially hazardous voltages.
This paper describes methods of smart MCC controls using a new low voltage motor overload relay with advanced protection and integrated arc-flash detection. These protection and control systems provide improved safety, advanced protection, time-synchronized event diagnostics, reduced cost, and higher reliability than previous technologies.
Next-generation low voltage motor relay
Simultaneous to the evolution of legacy intelligent MCC systems, a vastly more sophisticated set of electronics, software tool sets, diagnostics, reporting, and communications methods were developed for the high voltage (HV) electric power protection industry throughout the world. These more sophisticated protection devices have been used since the 1980s at voltages up to 765,000 V. These HV relays are designed for severe environmental testing and reliability requirements, such as temperature, shock, and electromagnetic interference. The mean time between failures of these HV relays exceeds 300 years .
A new generation of low voltage motor relay (LVMR) has come on the market which has features brought from the high voltage (HV) transmission industry. These new motor relays bring reliability, safety, and reduced cost to the LV MCC which has never before been possible.
This section explains the unique feature set of this new generation of microprocessor-based low-voltage motor relay (LVMR).
Advanced automation and control features
Many features differentiate the modern microprocessor-based multifunction LVMRs from older legacy motor overload technologies. These features include the following:
- A built in arc-flash detection light sensor.
- Three phase Rogowski Current Transformers built into the LVMR, eliminating the need for auxiliary CTs.
- An integrated power supply, eliminating auxiliary power supplies.
- A small form factor.
- Sequence of events (SOE) recording.
- Oscillography capture.
- Motor operating statistics report.
- Total harmonic distortion (THD) measurement.
- Network time synchronization of all LVMR.
- Multiple Ethernet and serial ports.
- IEC 61850 and Modbus communication protocols.
- Trip-rated digital outputs constructed with dry contacts to avoid use of interposing ice cube relays.
- Continuous self-assessment of the health of the power supply, processor, analog-to-digital converters, memory, and other components.
- Conformal-coated boards for dirty and corrosive environments.
- A simplified setup from a web-based human-machine interface (HMI).
- Built-in metering with fundamental and harmonic data.
- Programmability similar to a miniature programmable logic controller (PLC).
- Multilevel password login to assist in Cybersecurity policy compliance.
Advanced protection features
As shown in Figure 1, this new generation of LVMR include the following protection functions:
- Arc-flash detection (AFD) element.
- Undervoltage and overvoltage (27 and 59) elements.
- Underfrequency and overfrequency (81U and 81O) elements.
- Load loss detection (37CP) element.
- Instantaneous and time-overcurrent (50 and 51) elements.
- Thermal (49T and 49P) elements.
- Locked rotor detection (50PLR) element.
- Load jam detection (50PLJ) element.
- Current unbalance detection (46) element.
- Breaker failure protection.
- Motor starting and running (14 and 66) elements.
- Power factor (55) element.
- Phase reversal (47) element.
- Loss-of-potential (60) element.
- Variable frequency drive (VFD) protection.
- Negative-sequence overcurrent (50Q and 51Q) elements.