1. Thermal monitoring of low voltage switchgear using thermal ionization detector

Insulation and arcing faults are common faults in the power components. Insulation deterioration based PD faults mostly appear in the high voltage (HV) and medium voltage (MV) components while due to very nature its inception, arcing faults can occur at all levels (HV, MV, and LV) of voltage. A great interest has been shown to explore better condition monitoring solutions for predictive maintenance of power networks. However for efficient monitoring, dedicated sensors should be used for PD and arcing faults. The authors have been extensively working in PD and arcing monitoring in MV components such as cables, joints, terminals, and switchgear using high frequency Rogowski coil, radio frequency (RF) antenna, high frequency current transformer (HFCT), and D-dot sensors [3]–[4][5][6][7]. The present work is focused on arcing faults monitoring in LV switchgear based on heat and smoke particle sensing using thermal ionization detector (TID).

In this research work, major objective was to study the use of thermal ionization detector for possible online condition monitoring of low voltage (LV) switchgear. A closed thermal chamber was considered for the study. Bus bars having three joints were installed inside the chamber. Thermal ionization detector was installed in the ceiling of the chamber and its output was compared with the ambient and joint temperatures measured by thermocouples.

Section II discusses various thermal sensors and their shortcomings for online condition monitoring of switchgear. Section III explains the basic principle of thermal ionization detector, followed by the test setup used in this research, in section IV. In the last part of this paper, results and discussions are presented with notable conclusions.

Infrared thermal imaging cameras are used widely in the commercial power installations in order to identify over-heated connections before they blow away. These sensors measure the heat radiations emitted from a hot object in the infrared range of the radiation spectrum. There are various types of sensors available commercially. Sizes of the sensors range from small eye type (1 inch diameter) to conventional big thermo-graphic cameras [13]. Thermo-graphic cameras can only be used offline through a glass window of the switchgear or with open door. Moreover, typically all the connections are not visible from outside the switchboard, through the window.

Small eye type IR sensors are also not useful because these sensors are directional sensors. Online IR thermal monitoring using this sensor requires complicated mechanical assembly to help 180° rotary movement of the sensor in both horizontal and vertical directions. Unfortunately switchgears don't have enough space to install such a complicated and reasonably large assembly inside them. It also incurs excessive wiring inside the switchgear. Such limitations make the use of infrared eye sensors unfavorable too [12].

The busbars were fed high current (about 150 A) at 26 V, through step down heating transformer (maximum current of 2000 A and minimum voltage of 2.74 V). The input current was continuously measured by a current transformer (CT). Joints' resistances were measured by using micrometer. Measured values were 10μΩ for the installed bus bar joints shown in Fig. 2 and Fig. 3. Temperatures of the joints were continuously monitored by thermocouples installed at each joint. The time variant output of the sensor was amplified by a dedicated amplifier and then fed to a digitizer (computer), along with the time variant outputs of thermocouples. Following parameters were varied during the measurement.

• The temperature of the compartment (ambient temperature)

• Input current and consequent variation in temperature of joints

• Cooling of joint temperature by opening the door of compartment

Two types of measurements have been carried out in this work. During the first measurement, the compartment was kept at room temperature and the power supply was switched on. Also the compartment/ambient temperature and TID output were recorded. TID actually measured potential (in Volts) but it was calibrated to read temperature for comparison purpose.

In the second measurement, the compartment was first heated up to a certain temperature and then bus bar supply was switched ON. And then after a certain time elapse, the power was turned OFF. In order to record fast cooling phenomenon, the doors were opened after a certain time of turning the power OFF.

The sensor's sensitivity and output is compared with the thermocouples installed at various locations inside the switchgear compartment containing energized bus bar joints. TID is found to have a satisfactory performance in detecting abnormal heating phenomenon at bus bar joints. The sensor's output follows the actual hotspot temperature curves, as seen from the results. Whereas, the thermocouple installed at the ceiling of the compartment doesn't react to changes in hot spot temperatures. Referring to the operating principle of the TID sensor, the conduction between the electrodes of this sensor is directly proportional to the number of smoke particles entering the sensor. Hence, it should behave approximately linear at higher temperatures because thermal ionization is a linear phenomenon. TID sensor can easily detect the abnormal temperatures or hot spots inside the switchgear but another technique is required to be followed after detection of excessive heating in the switchgear. A thermo-graphic camera can be a suitable option for following fault localization.