Maintaining the reliability of power supply systems is becoming increasingly important as electrical installations get more and more complex. Power failures and even blackouts of short durations can have a significant impact on consumers, especially in the manufacturing and industrial segments. Faults in insulators are common occurrences that disrupt operations in transmission and distribution networks, and they must be fixed through swift detection, localisation and elimination.
Testing the integrity of a system’s insulation requires measuring its resistance to the current flow across it. A high level of resistance means very little current is escaping through the insulation, while a low resistance level indicates a significant amount of current could be leaking through and along the insulation.
There are several methods of detecting insulation faults. Measuring insulation resistance is a common one as it can indicate the availability and operating safety of an electrical system. It is particularly important for preventing damage and maintaining the reliability of systems and equipment. The factors that affect insulation resistance are the nature of electrical installation or equipment, operating conditions and usage.
These days, there are several online insulation fault and residual current location systems that offer programmed solutions. They provide increased system security and safety within reasonable cost and time limits as well as enhanced accuracy in detecting exact fault locations without interrupting operations. Flexibility in use, extension and retrofitting, portable size, and universal applicability are their other benefits.
Causes
Insulator faults in power systems can occur due to various reasons: climatic conditions, defective material, inadequate glazing, flashovers, mechanical stresses, pollution, etc. Extreme tem-
peratures can cause the expansion and contraction of different materials in porcelain insulators and lead to cracks. The use of defective materials and the manufacturing of insulators at low temperatures can increase their porosity, making them absorb moisture and
causing failures and current leakage. In addition, flashovers often result in overheating and insulator faults, and insulators on transmission systems in heavily industrialised zones are affected by pollution and corrosion.
To ensure their proper functioning, it is important to conduct flashover tests, routine tests and performance tests. The different kinds of flashover tests include dry, wet, voltage and impulse frequency tests, while the routine tests include corrosion testing and proof load testing. The latter are conducted by applying direct current (DC) voltage through the de-energised circuit using an insulation tester; if no failures occur during measurement, the test is a success.
In performance testing, different variables like the insulator’s temperature, puncture voltage, porosity and mechanical strength are examined. For instance, during temperature cycle testing, the insulator is first heated in water at 70 degrees Celsius for an hour, and then cooled immediately for another hour. This cycle is repeated thrice. On completion, the insulator is dried and its glazing thoroughly examined for irregularities. In a puncture voltage test, the insulator is suspended in an insulating oil and 1.3 times the flashover voltage is applied to check the puncture resistance. To test an insulator’s mechanical strength, a maximum working strength of 2.5 times is applied for about one minute, which should not cause any damage to the insulator.
Resistance measurement
Depending on the life cycle of a system or item of equipment, its insulation resistance should be tested, measured and monitored. An electrical system’s life cycle can be divided into eight phases: installation, commissioning, operation, maintenance, repair, major modifications, upgradation, and decommissioning. Depending on the phase, high voltage (HV) testing, insulation measurement or insulation monitoring are required. Insulation monitoring devices or fault current monitoring systems can also be used for inspection, depending on earthing. These methodologies help in the timely planning of power supply system maintenance through the early detection of impending insulation faults.
Before an electrical system is commissioned, insulation resistance should be measured between the active conductors and protective earth conductor. During this exercise, the active conductors are electrically connected. The DC measuring voltage and the magnitude of insulation resistance must comply with the specified requirements. If each circuit reaches the required value without electrical load, the insulation resistance is considered apt.
Insulator inspection in HV lines
HV transmission lines and systems play a critical role in the bulk transfer of energy, especially in light of the increasing grid integration of renewables. The pre-emptive inspection of lines thus assumes great importance in the interests of reducing downtime. However, it has significant cost, time and labour implications. Of late, mechanised methods of insulator inspection that analyse the images of transmission lines have begun to be developed.
Faults in power transformers
Insulation faults are a common occurrence in power transformers as they are subjected to various electrical, mechanical and thermal stresses. Insulation can start deteriorating due to moisture, overheating, vibration, voltage surges, or mechanical strain and lead to transformer winding failure. Apart from transformer failure, this can result in outages in the distribution network and revenue losses for utilities. Thus, early fault detection in the insulating systems of HV transformers is crucial.
Faults in ungrounded systems
Insulation faults in ungrounded systems can be detected and indicated through an insulation monitoring device. The ground fault location is activated by alarm contact with the insulation monitor or manually.
Then, the test device generates a signal of limited amplitude and duration for a defined period of time. This signal then flows via the location of the insulation fault through all the measuring current transformers within the insulation fault path. It is finally detected by the current transformers and evaluated by the electronics of the monitoring device, which provides fault location path information through light-emitting diodes (LEDs).
Faults in grounded systems
In grounded power supply systems, residual current measurement systems (RCMS) are used for monitoring insulation. All conductors of the circuit to be monitored are led through a current transformer. In a fault-free system, the resulting sum of all currents is equal to zero so that no voltage is induced in the secondary winding of the measuring current transformer. In the event of a fault current, the imbalance is sensed by the current transformer and evaluated by the electronic circuitry.
All RCMS units in a system work in parallel to minimise system evaluation. They scan all measuring current transformers and indicate the faulty circuit via LEDs. They also provide a central control and display of the measuring results and a pre/main alarm warning for each circuit. The use of current transformers allows the RCMS to be independent of the load current and nominal voltage of the installation.
Faults in disconnected loads
In the case of loads that are disconnected for different periods of time (such as valve drives, lift motors, fire extinguishing pumps and emergency generators), insulation faults are often ignored while the systems are turned off. This can lead to unexpected failures when the power is switched back on. To avoid these instances, offline monitors can be used to monitor the insulation resistance between all active conductors and the earth. If there is an insulation fault, the operator receives an alert before it reaches a critical state.
Summing up
Insulation monitoring and insulation measurement are separate activities that are performed according to the phase of an electrical system’s life cycle. Net, net, it is important to avoid insulation failures through preventive maintenance and the use of monitoring devices and systems for fault location.