Problem: Continuously and Cost Effectively Monitor Switchgear Temperature

Switchgear, which serve as important points of control within power distribution systems, transformers and other transmission and distribution (T&D) components, are susceptible to failure if not closely monitored and controlled. Increased loads can greatly stress switchgear. The resulting increases in temperatures of critical switchgear components can cause significant degradation of metal contacts and insulation. In turn, this increases the probability of internal short-circuits or increases in contact resistance, which, if unchecked can lead to system failure and possibly cause personal harm to system operators. The problem is especially relevant in emerging economies where the burgeoning need for power greatly stresses outdated legacy electric power infrastructure. Conventional methods of monitoring switchgear temperature are expensive and not entirely effective. Surface Acoustic Wave (SAW) technology can provide a passive (no battery or energy harvesting) wirelessly-interrogated temperature measurement solution that is ideally suited for switchgear.

Situation: Conventional Methods are Expensive and Provide Sporadic Coverage

"Hot Spots" within the switchgear are readily identified by taking thermal images of cable connections, insulators and bus bar connections. The non-contact testing afforded by Infrared (IR) cameras address a variety of safety concerns with respect to operating in a high voltage/high current environment. However, thermal images have distinct disadvantages associated with them:

• Spot Assessments
• Cost
• Reference Images in Winter and Spring
• Images Distorted by Dust
• Absence of Real-Time Data

Other traditional temperature measurement methods including battery powered sensors also have distinct disadvantages:

• Burden of Battery Replacement
• Restricted Access to Equipment Compounding Battery Replacement Burden
• Environmental Impact
• Time Consuming Installation

Solution: SAW Based Switchgear Temperature Sensing

A Surface Acoustic Wave (SAW) based temperature measurement solution for switchgear includes SAW Temperature sensors mounted at critical contact points within the switchgear box, as shown in the Figure below, and a reader capable of interrogating multiple SAW temperature sensors in rapid sequence. The reader antenna is mounted within the box, offering good radio frequency shielding of the interrogation process by the box itself. The locally powered reader sends short RF pulses into the switchgear and, if the pulses are at the frequency of the sensor, the sensor receives, modifies and passively returns the pulses. Returned pulses contain information related to sensor temperature.

SAW based temperature sensors take advantage of the controlled change in material properties of a crystal, converted to an electrical signal automatically via the piezoelectric effect. The sensing mechanism involves electrically inducing a surface acoustic wave into a piezoelectric material and then reconverting the energy of the wave (influenced by the temperature to which the sensing element is exposed) back into an electrical signal for temperature measurement.

Figure: SAW Based Switchgear Temperature Sensing Solution

One significant advantage of SAW devices is their passive operation, which makes them very amenable to operation in harsh environments via wireless interrogation and inherently have low maintenance requirements. A wireless SAW based temperature sensing solution consists of a reader (RF Transceiver) electromagnetically linked to a SAW sensing element as shown in the Figure below.

Figure: Wireless SAW Temperature Sensing System

Solution Differentiators
A SAW based temperature measurement system addresses many of the disadvantages associated with other means of temperature monitoring:

• Continuous Monitoring: A SAW based temperature measurement solution allows for the continuous monitoring of temperature and thereby provides for the ability to continually monitor the switchgear for adverse events or the prelude adverse events.
• Cost: The cost of a SAW based temperature monitoring system is much lower than traditional methods of temperature monitoring including IR Thermography.
• No Seasonal Effects: Calibration consists of a one-time adjustment to a software parameter that accounts for manufacturing variations of the sensing elements. The sensors can be calibrated at any known temperature within their stated operating range and so seasonal effects do not affect a SAW based temperature monitoring solution. Calibration to a reference temperature allows good accuracy over the entire operating range. Typically the sensor is calibrated once, at installation, and remains calibrated for years of reliable operation.
• Dust: SAW based temperature measurement solutions are immune to the effects of dust accumulation.
• Environmentally Friendly: SAW wireless sensors are passive (do not require batteries) and thereby provide for an environmentally friendly temperature measurement solution.

The key solution differentiator of the TempTrackr™ system is that it measures temperature at the source of expected failure: the bus bar connection, the cable connection and the circuit breaker connection. Unlike other solutions that measure the ambient temperature of the air within the switchgear box, the TempTrackr™ system measures the temperature at the most vulnerable points within the switch. The core value of the system lies in its ability to provide early warning signs of expected failure by continuously monitoring the temperature of critical switchgear components.

TempTrackr™ SAW Sensor System
SenGenuity has developed a SAW based temperature sensing solution that fully addresses the unique needs of T&D infrastructure temperature monitoring. The TempTrackr™ SAW Sensor System is capable of wirelessly monitoring multiple (up to eighteen) passive SAW temperature sensors per breaker box. The Figure below shows the different mounting schemes available to install the sensor on bus-bar and conductors.

Figure: Temperature Sensor Mounting Options

Temperature data is obtained from the sensor via a wireless reader (connected to an antenna) that is appropriately located within the switchgear cabinet as shown in the Figure below. The wireless reader can generate temperature data on demand or on a continuous basis. For Switchgear OEM's who are interested in integrating the TempTrackr™ system into the switchgear control architecture, SenGenuity can provide wireless readers capable of communication via RS232.

Figure: Wireless Temperature Sensor Reader

For end-users of legacy switchgear or end-users who are interested in monitoring switchgear temperature remotely, SenGenuity can provide control room software, shown in the Figure below, with alarm capabilities.

Figure: Control Room Software with Alarm Capability

The various components of the TempTrackr™ system have undergone certification tests and are in compliance with CE and FCC requirements.

Key Features:

• Wireless Passive Temperature Sensors
• -20°C to 120°C Operating Temperature Range
• 428 MHz to 439 MHz RF Operating Range
• Scalable System Configurations
• 6 Sensor System (basic configuration)
• 12 Sensor System (with sensors partitioned into two isolated sub-compartments)
• 18 Sensor System (with sensors partitioned into three isolated sub- compartments)

Click here for Appnote in pdf format

Temperature Monitoring System Using Passive Wireless Sensors for Switchgear and Power Grid Asset Management (313Kb)

Applications of Wireless Temperature Measurement Using SAW Resonators

Contact us at sensors@sengenuity.com for more information.