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• Switchgear Temperature Monitoring Technology Overview

Switchgear Temperature Monitoring
Technology Overview and Selection Criteria

The Need to Continuously Monitor Switchgear Temperature

Overheating caused by overloaded circuits, unbalanced loads, or loose or damaged connections will shorten equipment life, and potentially lead to catastrophic failure. Temperature rises in medium-voltage switchgear and switchboard components can be sudden, often causing thermal run-away, resulting in burning, melting and destruction of components. Periodic visual inspections are costly, require special safety considerations and are unlikely to detect these conditions in time.  Best practices now demand 24x7 thermal monitoring of switchgear assets, catching up to 70% more failures than periodic inspection, protecting valuable equipment from failure and increasing personnel safety.

What Switchgear Components should be Thermally Monitored?

Hot spots can arise in busbars, circuit breaker contacts, cables, or anywhere where a joint or connection point exists. Critical monitoring points may include:

• Line and load side of Air Circuit Breakers (ACB)
• Bus bar joints
• Inside the lower and upper contacts of Vacuum Circuit Breakers (VCB) connected to main bus bar and feeder bus bar circuits
• Stabs / disconnect switches
• Connections on Current Transformers (CT)
• Incoming cable / feeder joints
• Joints on the main bus bar
• Voltage Transformer (VT) electrical joints

Medium-Voltage Switchgear Thermal Monitoring Selection Criteria

The following criteria should be carefully considered when selecting the best technology to monitor the temperature of switchgear components:

• The temperature sensors should be inherently safe, without risk of causing shorts, partial discharge, arc-flashes or any other electrical event.
• The temperature sensors should reliable and free of electromagnetic or radio-frequency interference.
• The temperature sensors should be highly-durable, free of long-term deterioration, and last the life-time of the equipment (20+ years).
• The temperature sensors should be accurate, easy to monitor, and plug-and-play with existing monitoring systems such as SCADA.
• The temperature sensors should be easy to install in new or existing deployments and free of calibration.
• The temperature sensors should be cost-effective, in terms of capital purchase, installation and operating costs.

Switchgear Temperature Monitoring Choices

The following technology options exist to continuously monitor switchgear temperature and hot spots:

Infrared Red (IR) Temperature Sensors

Infrared temperature sensors can be used as an alternative to periodic temperature monitoring with an infrared thermometer. The IR temperature sensors are contactless, in that a sensor is not directly mounted to the measuring point of interest. Although this solution is inherently safe and free of electromagnetic and radio-frequency interference, there are some key limitations:

Insulting boot / bus barrier covering busbar joints

• Obstacles are often in the way to provide direct line-of-site measurement.
• Insulting boots often cover the connection point, rendering IR measurement useless.
• IR measurement is inaccurate and not always repeatable. Accuracy is typically ±2°C and ±4°C at higher temperatures.
• Limited sensing range and can’t measure temperatures below 0°C.
• Lens mounting brackets and routing of multiple conductive wires around the HV cabinet is required.
• Often need to mount special black targets with known emissivity at sensor locations to improve reliability.
• Installation is expensive and tedious given the need for accurate alignment and the need for different lenses for different sensing distances.

Wireless Surface Acoustic Wave (SAW) Temperature Sensors

Another choice is Antenna SAW sensors, which consists of a component mounted directly to the measurement point and wireless antennas mounted close by to read the temperature.   Such solutions eliminate the line of sight requirement introduced by IR temperature sensors, but some other deficiencies are introduced.

• Wireless sensors are prone to interference and slow to respond.
• Commissioning is difficult and expensive due to the allocation of frequencies and the setup of multiple antennas.
• Sensors are bulky and often require custom boots or cut-out holes in existing boots to fit.
• Challenging to mount antennas close enough to sensors for reliable communication.
• Typical installation must also be done on-site to manage real-world interference issues.
• Wireless (SAW) technology has a limited reach (often as short as 30 cm).
• Accuracy is typically ±2°C and ±4°C above 80°C.

Fiber Optic Temperature Sensors

Fiber optic temperature sensors provide direct contact with the measurement point of interest, typically via a ring lug attached to the end of the fiber. The fiber is made from high durability plastic or glass, and is inherently safe from electrical events or noise.   The fiber is run back to a temperature transmitter located in the LV cabinet for connection to a monitoring system.   Advantages of fiber optic temperature sensors for continuous monitoring of switchgear assets are:

Fiber optic temperature sensor installed under busbar boot cover

• The temperature sensors are inherently safe, without risk of causing shorts, partial discharge, arc-flashes or any other electric event.
• Highly reliable due to direct, wired connection to the measurement points.
• Fiber optic cabling ensure sensors are free from electromagnetic or RF interference.
• Sensors can be mounted underneath insulating boots for accurate and real-time measurement.
• Accurate and repeatable measurements.
• 20+ years of calibration and maintenance-free installation.

OSENSA Innovations Fiber Optic Temperature Sensing Solutions

OSENSA Innovations develops and manufactures fiber optic temperature sensor products for industrial applications including switchgear, power transformers, switchgear, generators, semiconductor, and MRI equipment. OSENSA’s fiber optic temperature sensors are solutions consisting of one or more fiber optic probes attached to a temperature transmitter (also known as signal conditioner) electronic device. OSENSA’s fiber optic sensing technology enables industrial-grade fiber optic solutions that are price-competitive with conventional wired thermocouples and RTDs (resistive thermal devices). Furthermore, OSENSA’s fiber optic temperature sensors are more durable and easier to install than competitive optical technologies. OSENSA’s fiber optic sensors are immune to electromagnetic radiation and compatible with high-voltage, high-RF (radio frequency), and high-magnetic-field environments, making them ideal for applications where thermocouples and RTDs fail.

OSENSA’s fiber optic temperature sensors do not suffer any of the technical challenges associated with wireless and infrared sensors, as the sensors are mounted directly on the critical switchgear monitoring points. OSENSA’s low-cost optical temperature sensors are rigidly attached to the hotspot locations and are completely immune to electromagnetic interference and noise bursts caused by high-voltage switching. OSENSA’s fiber optic sensors are robust, can be manufactured to various lengths, and work like a conventional thermocouple. In addition, each OSENSA optical temperature sensor transmitter can monitor three phases providing both analog output and digital RS-485 Modbus RTU communication. OSENSA’s fiber optic temperature probes are perfectly suited for smart grid switchgear temperature monitoring.

In addition to the advantages of fiber optic temperatures sensors stated above, OSENSA’s switchgear fiber optic temperature sensors provide the following:

• Wide temperature range up to 200°C, with accuracy better than ±1°C.
• Low noise, high-accuracy, and repeatability temperature measurements.
• Scalable, DIN-rail mounted temperature transmitters (signal conditioners).
• Simple installation at time of manufacturing or in retrofit applications.
• Highly cost-effective.

Visit www.osensa.com or email info@osensa.com to learn more.

What Components do OSENSA’s Switchgear Solutions Include?

Temperature Transmitter – FTX-910/610/310-PWR+R

Temperature Probes with Fasteners – PRB-110, PRB-910

Display and Control Module – HMI-001, HMI-001-RELAY

Summary

Numerous technology choices exist for continuous temperature measurement of switchgear components, included IR, Wireless (SAW) and direct-contact fiber optic temperature sensors.

Fiber optic temperature sensors from OSENSA Innovations provide direct-contact, continuous monitoring of switchgear assets. With the lowest-cost and highest-accuracy, combined with easy and maintenance-free installation, OSENSA Innovations is the technology of choice.