Electrical Equipment Monitoring
Loose connections or worn out contact surfaces are the root causes of electrical system conduction troubles. They cause a local temperature increase, which worsens the contact quality even further as the current increases. This causes thermal runaway, which damages the insulating material at high enough temperatures.
Thermal conditions of electrical equipment and temperature monitoringAdvanced thermal monitoring of electrical equipment is actually the topic of this technical article.
Medium voltage circuit breakers, switchgear, and substations are frequently targets of thermal runaway’s destructive dielectric discharges.
Since temperature rise occurs when the current flows through the system, temperature measurement should be made when the equipment is energized, for example, online. It might be periodic or continuous.
There are many approaches to measuring temperature, some of which will be described in this technical article.
- Temperature Measurement Using Thermography
- Continuous Temperature Measurement:
- Fiber-optic Technology For Temperature Measurement:
- HV Transformer Winding Temperature Monitoring with the Fiber-optic Technique
- Wireless Temperature Monitoring:
- BONUS (PDF) 🔗 Download ‘Infrared thermography with AI for Predictive Maintenance of Power Substation Equipment’
1. Temperature Measurement Using Thermography
To date, the solution most commonly used to protect against conduction faults consists of periodically carrying out maintenance operations, which include temperature measurement.
Typically, at the time of annual servicing, a complete thermal inspection is made of the electrical installation, during which the temperature of the connections is checked. The technique that is usually used for this purpose is infrared thermal analysis.
It consists of periodically inspecting the installation by means of an infrared camera, with the aim of detecting thermal faults likely to reveal a conduction fault.
In addition, the infrared measurement principle is not very accurate since it only allows the emissivity of the radiating bodies to be measured, and not the temperature. To detect a fault by means of infrared thermal analysis, several conditions must be met at the same time: it is necessary to have a substantial fault, a strong current to reveal it, and an expert eye trained to detect differences in emissivity between phases.
The periodic nature of inspection is the main limiting factor with this practice. A fault which appears on the day after the yearly visit will only be detected a year later, but it will continue to degenerate and lead to thermal runaway in the meantime.
Figure 1 – Thermography inspection of substation equipment (click to zoom)
2. Continuous Temperature Measurement
There are several techniques that can be used for continuous temperature monitoring of energized electrical equipment.
2.1 Infrared Non-Contact Temperature Sensors
The technique for continuously monitoring the temperature using infrared emission of heated surface is to measure the temperature with noncontact infrared (IR) thermometers. IR sensors are installed in the close vicinity of the target and send signals to a remote PC.
An optimal distance between the sensor and the target is determined by the size of the target (diameter, D) and the parameters of the sensor. For each particular type of IR sensor, the ratio field of view (FOV) = X/D is a constant value, where X is the distance between the sensor and the target. The smaller the target area (D) is, the closer the sensor should be located to the target. The solution is relatively inexpensive, but there are several disadvantages.
Another downside of using IR noncontact sensors is the need to run the cables from each sensor to receiving units.
Infrared thermometers are used for indicating temperature variations due to dust collection and changes in emissivity resulting from minor surface corrosion, particularly on reflective metallic surfaces like copper bus bars. The recorded temperature may be distorted by reflected infrared radiation from nearby objects, and abrupt fluctuations in ambient temperatures can also lead to measurement inaccuracies.
Figure 2 – Infrared Non-Contact Temperature Sensors
2.2 Electronic Temperature Sensors
Some MV cubicle monitoring systems are based on the use of electronic temperature sensors installed at critical points. This solution offers the advantage of continuous monitoring of the installation and detecting thermal faults very early. The problem is that each sensor must draw its energy from the mains current and transmit digital data via an infrared or optical fiber link.
Such complexity leads to a high cost for each measurement point. Another problem with using electronic temperature sensors is that they require the installation of devices whose level of reliability and service life are incompatible with the associated switchgear and the function to be performed.
A diagnosis system must be much more reliable than the equipment that it is monitoring, especially if, as is the case here, it is not maintainable without a shutdown of the substation.
Figure 3 – Common thermal spots in medium-voltage switchgear
3. Fiber-optic Technology For Temperature Measurement
Various combinations of fiber-optic technology are in use for temperature measurement and offer solutions for a variety of tasks. In the environment of high field strengths, the usage of optical sensors or fiber-optic technologies offers the possibility to measure important values without any influence of EMI.
Also the galvanic separation (isolation) is an advantage. Alternative temperature measuring systems for temperature measurements inside the equipment use thermal sensors based on fiber-optic technologies. This complex method consists of fiber-optic temperature sensors installed in the apparatus.
Different types of sensors and functional principle are in use in the electrical industry.
3.1 Optical Fiber Sensing Probe
One possibility is to measure the temperature at one point of the fiber. For example, the temperature measurement principle is based on an optical fiber sensing probe. In such a probe, a small portion of the fiber cladding is removed and replaced by a suitable “reference” liquid whose refractive index versus temperature characteristic is known.
Another technique involves an optical fiber with a temperature-sensitive phosphor tip at the end. After excitation with blue-violet light the phosphor fluoresces with red light. The intensity of this light decays exponentially with time. The decay time is measured (the time constant of decay is inversely proportional to the sensor temperature) and correlated to the tip temperature.
Various combinations of fiber-optic technology come in many forms and offer solutions for a variety of applications. A fiber-optic sensing device is generally made up of two separate components, namely the amplifier unit and a sensing head assembly. The sensing head assembly comprises a pair of optical fibers, a transmitter and a receiver, terminating in the optical fiber-based head.
A fiber-optic wire may be used as a temperature data carrier or as an active sensing element.
Figure 4 – Fiber Optic Temperature Sensors Applied to Power Transformers
The critical parameter in a transformer’s life cycle depends on the reliability of the winding’s hot spot value throughout its operational lifespan.
Fiber optic sensors are a very effective and progressively economical method for assessing capacity and obtaining data on the unit’s condition via direct temperature measurement of the hot spot. Fiber optic temperature sensors were formerly reserved for big power transformers mostly due to the expense of the fiber optic systems in relation to the cost of the transformer itself. The application of a direct technique facilitates precise measurement of the transformers’ hot spot, enhancing understanding of operational condition assessment, load planning, asset management, and end-of-life evaluation.
In an active sensing element, temperature-dependent modification of the optical transmission properties is used in distributed fiber-optic temperature-sensing systems. In other systems, a fiber-optic cable is used as a temperature data carrier, which delivers light to the sensing element installed at the measurement point.
Fiber-optic temperature transducers are compact, immune to EMI and radio-frequency interference (RFI), resistant to corrosive environments, and provide high accuracy and reliability in temperature measurements. The various combinations come in many forms and offer solutions for many applications. Sensitive elements at the fiber tip provide rapid and accurate temperature measurement.
Mounting the sensing element at the end of a small optical fiber allows placing the sensor in difficult-to-access locations.
Figure 5 – Fibre optic temperature sensor setup in the transformer winding
3.2 Distributed Fiber-Optic Temperature Sensing
Another possibility is to measure the temperature distributions along the length of the fiber-optic cable with the use of thermo-optical effects. The light impulse injected into the fiber is subjected to scattering as it travels and the backscattered light impulse is returned to the detector. Among the returned light pulses, the intensity of Raman scattering is closely related to the temperature of the position of scattering.
So the temperature along the fiber can be measured from the intensity of Raman scattering.
In the case of the draw-out MV circuit breaker, for example, it would necessitate designing a special automatic disconnect to comply with the requirements of the ANSI Standards. So the application of such a system, while very reliable and relatively inexpensive for the new equipment, becomes prohibitive for the existing equipment.
Figure 6 – Working principle of Distributed Fiber-Optic Temperature Sensing
The DTS unit transmits laser light pulses into an optical fiber and measures the backscattered light. The temperature along the fiber can be ascertained from the interval between the emission and detection of light, as well as the ratio of the amplitudes of backscattered light from designated sites along the fiber.
4. Winding Temperature Monitoring of HV Transformers
With the Fiber-optic Technique
The loading capability of power transformers is limited mainly by the temperature of the winding. As part of acceptance tests on new units, the temperature rise test is intended to demonstrate that at full load and rated ambient temperature, the average winding temperature will not exceed the limits set by industry standards.
However, the temperature of the winding is not uniform and the real limiting factor is actually the hottest section of the winding commonly called the Winding Hot Spot (WHS). The temperature of solid insulation is the main factor in transformer aging. When insulation is exposed to high temperature for extended periods of time, the cellulose insulation undergoes a depolymerization process during which the cellulose chains get shorter.
Therefore, winding temperature is a prime concern for transformer operators. This variable needs to be known under any and all loading conditions, especially in the unusual condition involving rapid dynamic load changes. Accurate knowledge of the temperature of the Winding Hot Spot is a critical input for the calculation of the insulation aging, assessment of the risk of bubble evolution, and short-term forecasting of the overload capability.
It is also critical for efficient control of the cooling banks to ensure that they can be set in motion quickly when needed.
Watch Video – Five Fast Facts About Fiber Optics Temperature Sensors – Transformers
The Winding Hot Spot area is located somewhere toward the top of the transformer, and is not accessible for direct measurement with usual methods. For about 40 years, fiber-optic temperature sensors have been available for temperature measurement in HV transformers. The first units were fragile and needed delicate handling during manufacturing and installation. Over the past 20 years, significant development has taken place to improve ruggedness and facilitate connection of the five cables through the tank wall.
New fiber-optic probes are protected with a permeable protective PTFE Teflon sheath to withstand manufacturing conditions including long-term immersion in transformer oil.
Online monitoring of winding temperature can provide a dynamic evaluation of insulation degradation.
There are several temperature-monitoring systems available on the market that are developed specifically for monitoring Winding Hot Spot in HV transformers.
Figure 7 – Transformer winding temperature fibre optical sensors
5. Wireless Temperature Monitoring
5.1 How it Works, Benefits, and Problems
The most troublesome issue in building a reliable condition monitoring system is wires that are used to get temperature readouts from the sensors. Elimination of wires required for constant communication with the sensor unit provides a great opportunity to build a temperature measurement system that:
- is easily installable, even on the already assembled switchgears, and
- provides online temperature measurements.
The most important advantage of using wireless technique to monitor thermal condition of the energized equipment is eliminating any cables and wires from the online system. Another important benefit of wireless technology is much lower installation costs than that of any other type of online monitoring equipment. Wireless systems should work well in difficult or dangerous-to-reach locations or in moving applications.
An ideal wireless temperature-monitoring system (WTMS) would consist of the specific components of hardware and software. Hardware consists of sensors and receivers or interrogators. To achieve the goal of wireless continuous temperature monitoring of HV and MV energized electrical equipment, there are specific requirements to be met by the sensors. The sensors should be wireless units equipped with unique identification and should be made of miniature and dielectric components.
Signal transmissions from multiple sensors should not interfere with each other.
Receivers or interrogators should be installed at a significant distance from the sensors in the central location; they should collect the data from all sensors and transfer the data to a PC. They should work independently in series with other receivers. In case a temporary EMI exists, they should easily recover.
Technology progress on wireless systems and devices introduces every year at least a few new wireless solutions so that there are several mature technologies that can be used to implement wireless sensor solutions. Unfortunately, because of the fact that often the wireless sensors are battery-powered, the lifetime of the sensor is limited to at most a few years.
For critical applications, the average switchgear lifetime is about 30 years; thus the condition monitoring system should last for the same time and require little maintenance.
However, a major maintenance interval of MV and HV electrical equipment of 5–7 years is not uncommon. There are also solutions based on custom protocol optimized for wireless temperature monitoring, but similarly to the standard sensors, the lifetime of the solution is limited by battery capacity.
Figure 8 – Wireless temperature sensors installed on circuit breaker’s finger clusters
5.2 Thermal Diagnostics
Thermal diagnostics has two separate goals. One is an ability to detect an early conduction fault (warning), another would be a thermal runaway detection, which results in forced interruption of electric current (protection). Conduction downgrading phenomena are generally slow and progressive. Early detection of conduction faults is based on an analysis of small changes in temperature.
Detection of local overheating right from the start allows scheduling of maintenance operations at a convenient time, when it does not create a significant disturbance of the process. Timely scheduled maintenance preserves the installation, mainly the contacts and insulating materials, since the thermal phenomena are still weak.
A few principles may be used when developing the algorithms for early detection. For example, when monitoring the temperature in electrical switchgear, knowledge of cubicle thermal models is important. Correlation of contact temperature data with the current values and ambient temperature measurements, and temperature measured on all three phases may provide very important information on the thermal processes within the installation.
In the event of a failure, it is necessary to set up a temperature-driven function which, when it observes a measurement greater than a set point value, sets an output signal that controls the tripping of a circuit breaker. Such systems should be very reliable to not give false alarms or, even worse, cause nuisance tripping of the CB.
Wireless temperature-monitoring sensors may be installed on electrical equipment at the points that are not accessible with the power on. To provide continuous uninterrupted service, the sensors must have a very high service life and high mean time to failure (MTTF), which is the sensor’s reliability calculated over its designed lifetime.
Long service life of temperature-monitoring system is defined by a reliable and independent power source for the sensors.
Figure 9 – Wireless temperature monitoring system installed in MV switchgear
A centralized receiving device is capable of receiving wireless data from numerous switchgear cabinets and a multitude of temperature measuring sites. The wireless temperature measuring sensor can be affixed at the designated measurement location. The complete refurbishment project is significantly smaller than the current items.
The advancement of temperature measurement systems holds substantial practical importance. The centralized receiving apparatus possesses robust capabilities and a user-friendly interface. It can be linked to the main station via 485 communication, which is more appropriate for unmanned stations.
5.3 Wireless Temperature Sensors: Power Source
In wireless temperature sensing, the search for reliable power supply is very important. A traditional way of supplying energy to a wireless sensor is by using primary (nonrechargeable) batteries, whose main weakness is the limited amount of available energy. The life of the battery depends on ambient temperature.
Considering the application of sensors in electrical installation, ambient temperature will be always elevated, thus significantly shortening the battery’s useful life. To keep the sensor functioning, the battery should be replaced when depleted.
By definition, wireless autonomous sensors must not depend on an external power supply. Energy harvesting is a means of extending the lifetime of the autonomous sensor beyond the life of a battery. Several dominant energy-harvesting technologies are known. Choosing one or another energy-harvesting technology application depends on the type of equipment where the wireless sensors are used.
One of these technologies uses photovoltaic elements, which produce electricity from ambient light—either indoors or outdoors. Photovoltaic cells produce electricity from photons by means of a semiconductor p−n junction. This technology is at a relatively advanced stage of development.
Vibration harvesters use electromagnetic, piezoelectric, or electrostatic phenomena and may produce electricity from vibrations of the surface the sensor is deployed on.
Electromagnetic generators use a resonant magnet and coil arrangement to generate electricity, whereas piezoelectric-based generators use a piezoelectric resonant beam which generates electricity when subjected to strain. Electrostatic generators exploit capacitive effects but, due to machining and practical issues, have not become widespread.
Vibration energy harvesters are sensitive to the frequency of vibrations of a surface, and their deployment is generally limited to machinery that vibrates at a constrained range of frequencies and amplitudes.
Thermoelectric elements produce electricity from a temperature gradient. Thermoelectric energy sources exploit the Seebeck effect, in which electricity is generated from a temperature difference across a thermocouple. In general, thermoelectric devices require a large and sustained temperature gradient between two surfaces in order to provide useful power.
The sensors use interdigital transducers (IDTs) that convert the electric field energy to mechanical wave energy and then back to an electric field. The transducers are connected to the antenna to convert the received signal into an acoustic wave, which propagates along the sensor.
Depending on the construction of the device, the IDTs can retransmit the signal to the receiver. The received signal is amplified and then converted to a baseband frequency in the RF module and then analyzed by a signal processor.
As the operating frequencies are high, SAW sensors are well protected from EMI that often occurs in the vicinity of industrial equipment, such as motors and HV lines.
Further Study – A case study of maintenance and condition monitoring of the power transformers
A case study of maintenance and condition monitoring of the power transformers
5.4 Wireless Temperature-Monitoring Techniques
Traditional methods of measuring temperature have relied on the temperature dependence of resistance (thermistors or RTDs), a variety of different types of thermometers, the temperature dependence of a diode junction (silicon), or the emission of infrared radiation from heated objects (IR sensors).
Passive devices such as thermocouples and RTD require battery-powered transmitters to communicate information. This complicates measuring the temperature of contacts and connections in HV switchboxes and transmission. A standard requirement for these systems is that there be no metallic or fiber-optic cabling from the contact or connection of interest to the supporting structure or frame, as this can cause a dangerous and potentially explosive path to the ground.
Typically, the infrared measurement systems used for this type of monitoring are cost prohibitive. Battery-powered temperature-transmitting systems have drawbacks related to the typical physical size and the need for inconvenient periodic replacement of the battery.
With a few exceptions, batteries are not well suited for high-temperature operation, especially above 150°C.
Figure 10 – Wireless temperature measurement of LV/MV busbars inside switchgear
5.5 Wireless Temperature Monitoring with SAW Sensors (WTMS)
SAW technology uses the effect of changing the parameters of an acoustic wave, such as velocity and amplitude, when it propagates through or on the surface of the piezoelectric material depending on the physical condition of the material. Any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the acoustic wave.
Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity being measured. SAW sensors are very sensitive in measuring mechanical properties such as stress or strain. Special cuts of piezoelectric substrate create SAW devices with a very linear SAW frequency versus temperature
dependence. The result is a very high-resolution temperature sensor.
In WTMS based on SAW technology, the number of sensors is limited to no more than a few. In this technology, the measured quantity (temperature) is calculated from the frequency range; thus every sensor must occupy its own frequency range.
Switchgear interiors contain a number of conducting metal plates and other components that reflect and degrade radio signals, which cause difficulties in using this technology.
To apply a set of SAW sensors to measure temperature in all 12 locations in each cubicle of switchgear, a special layout of the sensors is required.
Figure 11 – Wireless temperature monitoring sensor, type surface acoustic wave (SAW)
The solution was found by antenna tuning, so that shielding capabilities of the switchgear cubicle boxes divided the switchgear into sections that contain sensors operating at different frequencies, while frequencies in different sections were duplicated. It allowed installation of 72 sensors on the switchgear to monitor all expected hot spots.
Utilization of miniature SAW sensors enables monitoring of breaker connectors and noninvasive installation inside the switchgear. The small size of sensors allows the system to be extremely flexible in choosing measurement locations. Passive power supply enables indefinite lifetime of the system regardless of the external condition, which is a significant benefit of self-powering devices.
Application of wireless temperature monitoring allows preventing expensive outages and improving the safety of switchgear technicians and maintenance personnel. It may be considered as a valuable replacement for existing traditional temperature-sensing solutions within the switchgear application.
Suggested Guide – Condition monitoring and diagnostic techniques for MV vacuum switchgear
Condition monitoring and diagnostic techniques for MV vacuum switchgear
6. BONUS (PDF): Infrared thermography with AI for Predictive Maintenance of Power Substation Equipment
Download: Infrared thermography with AI for Predictive Maintenance of Power Substation Equipment (for premium members only):
Reference: Transmission, Distribution, and Renewable Energy Generation Power Equipment Aging and Life Extension Techniques by Bella H. Chudnovsky
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Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
Profile: Edvard Csanyi
Have you looked into RFID-based temperature sensors? These sensors are also passive/wireless, they receive energy via RFID technology and also transmits data back via RFID, ensuring digital communication between sensors and receiver.
Compared to SAW sensors, which rely on analog transfers that are easily disrupted by electromagnetic waves of similar frequencies, RFID sensors are also passive, wireless with digital communication that are no longer afraid of interference.