Arc Flash / Electrical Safety / Electrical Systems

How to improve the safety of your motor control centers

Implement a smart solution by integrating a low voltage motor overload relay with advanced protection and integrated arc-flash detection.

By Scott Manson, IEEE; and H. Landis Floyd, Life Fellow, IEEE

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 [1].

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.


Figure 1. Modern LVMR Functionality

User programmable logic

Programmable logic in the modern LVMR enables the user to create sophisticated protection and control schemes. When used in concert with the high-speed relay-to-relay communication and sensitive protection elements, users can construct schemes to prevent human electrocution [2], automatic load re-acceleration schemes, monitoring wear on molded case circuit breakers and contactors, breaker failure schemes, fast bus tripping schemes, earth fault monitoring, and more.

Arc-flash protection

The modern LVMR includes a light sensor for detecting arc flash events. The light produced by an arc flash provides a large-magnitude signal that is used in conjunction with overcurrent sensing to securely and reliably detect an arc fault. Upon detection of the arc-fault condition, the relay initiates the high-speed tripping of an upstream breaker to minimize the arc-fault duration and resultant incident energy.

When a light flash is detected in an MCC bucket, high-speed IEC 61850 GOOSE messaging is sent from the LV protective relay to an upstream relay associated with the motor bus circuit breaker. If the upstream relay detects an overcurrent condition coincident with the MCC bucket light flash, a high-speed trip is initiated on the motor bus circuit breaker to minimize the arc-fault duration. A typical scheme for such a system is shown in Figure 2.

Significant testing of the relays was done in real arc-flash environments to ensure survival. Field tests have proven that even in a catastrophic arc-flash test event, at least four GOOSE messages indicating the arc-flash event are sent within 16 milliseconds. The total time from the start of fault conditions to an upstream relay having trip-rated contacts fully closed and conducting was between 4 to 13 milliseconds, with the variance caused by the asynchronous processing cycles of the microprocessor-based relays.

By quickly tripping the upstream circuit breaker, the LVMR-based arc flash mitigation scheme reduced the associated incident energy from 12 to 1.2 cal/cm2. (see Figure 3).

Hardened equipment specifications

The modern, microprocessor-based LVMR must survive an arc-flash event long enough to trip upstream breakers. The LVMRs is therefore designed and tested to survive in the harsh environment of an arc-flash plasma cloud. This environment includes very high temperatures, bright light, ionized air, strong magnetic fields, flying molten metal, and mechanical shock. The LVMR was therefore designed to meet several IEEE, IEC, and other standards.

Condition monitoring

The LVMR monitors temperature, current, and counts the number of contactor and MCCB operations. The LVMR also captures the current waveforms during every MCCB operation. This functionality provides a simple method to determine the health of contactors, MCCB, and loads.

Internal self-testing

Modern, microprocessor-based LVMRs must continuously monitor, self-detect, and report internal failures with internal memory, power supply, input/output (I/O) board, current transformer (CT) or voltage transformer (VT) board, clock inaccuracies, or processing errors. These tests run simultaneously with the active protection and automation logic and do not degrade the device performance.

The LVMR reports out-of-tolerance conditions as a status warning or a status failure. For conditions that do not compromise functionality yet are beyond expected limits, the LVMR declares a status warning and continues to function normally. A severe out-of-tolerance condition causes the LVMR to declare a status failure and automatically switch the device into a device-disabled state. During a device-disabled state, the LVMR suspends protection elements and trip/close logic processing and de energizes all control outputs.


Figure 2. Use of Relay-to-Relay GOOSE Messaging for Arc-Flash Protection


Figure 3. MCC arc-flash test (480V MCC and 7kA fault current) without LMVR protection at left, and with LMVR protection at right.

Integrated smart motor control system

It is recommended to use the new LVMR as part of an integrated smart motor control system (ISMCS). Because these systems are factory configured and tested, they simplify the configuration, commissioning, and testing of large numbers of LVMRs. The ISMCS also replaces PLCs, remote terminal units (RTUs), and significant cabling with fiber optic Ethernet cables. Figure 4 shows the communications architecture of the ISMCS.

The ISMCS also acts as a complete protection, control, and monitoring solution for an MCC. It provides process diagnostics that simplify maintenance by allowing users to detect and correct problems before they become critical, preventing damage and minimizing process downtime.

The ISMCS provides users with immediate real-time information on motor performance, centralized touchscreen HMI access, and historical reporting and analysis. This networked ISMCS solution integrates the latest LVMR and incoming feeder relay for advanced motor protection, control, metering, and process automation. Valuable motor and system process data are automatically gathered, consolidated, and made available simultaneously to both process control systems and power management systems.


Figure 4. Example ISMCS Architecture

The ISMCS centralized HMI include the following features:

  • Centralized HMI control and monitoring of individual loads.
  • Multi-level security access
  • Aggregated power metering from every relay to give real-time feedback about process operations.
  • No third-party operating systems
  • Methods based on decades of utility and industrial electric power protection experience
  • Standard integration and communications techniques
  • Factory preconfigured and programmed relays, controllers, HMIs, and managed switches.
  • Engineering access to every smart device on the Ethernet network.
  • Centralized event diagnostic software.
  • Standardized Equipment Condition Monitoring.

Critical for the long-term maintenance of an MCC are multiple levels of system annunciation. Should the central ISMCS HMI fail, a local HMI on the front of the bucket is available. For improved reliability, the preferred solution is for the LVMR to communicate to dual switches in a dual-star arrangement, as shown in Figure 5.


Figure 5. Dual-Star Network

Conclusion

The following points capture the essential takeaways about a comprehensive LV MCC protection and control system:

  • Hidden failures that can compromise arc flash mitigation are reduced.
  • Light detectors built directly into the LVMR are used to reduce the incident energy of arc-flash events.
  • End users save money and time with a preconfigured, standardized ISMCS solution.
  • Data from the LVMR supports a preventive and predictive maintenance program.
  • Comprehensive feature sets in the LVMR increase reliability, improve safety, and reduce the operating costs of LV MCC systems.
  • The system reduces motor failures with advanced protection elements.
  • Ruggedized designs and thorough type-testing of LVMRs improve the reliability of an LV MCC system and reduce process downtime.
  • An LVMR with internal testing and onboard diagnostics immediately identifies if the protection,  control, and arc-flash system is functioning.
  • Motor starting reports, fault oscillography, and binary sequence of events aid in the diagnosis of equipment problems.

References

[1]    R. D. Kirby and R. A. Schwartz, “Microprocessor-Based Protective Relays Deliver More Information and Superior Reliability With Lower Maintenance Costs,” proceedings of the IEEE Industrial and Commercial Power Systems Technical Conference, Detroit, MI, August 2006.
[2]    P. S. Hamer, “The Three-Phase Ground-Fault Circuit-Interrupter System—A Novel Approach to Prevent Electrocution,” proceedings of the 55th Annual IEEE Petroleum and Chemical Industry Conference, Cincinnati, OH, September 2008.