Increasing demand for power supply requires the construction of new power plants and additional transmission lines. The demand for equipment such as transformers and reactors is expected to grow further driven by the twin phenomena of growth in HVDC lines and renewable energy-based installation.
Transformer and reactor testing is thus becoming a key priority area for utilities. Testing the equipment prior to installation helps transcos to expedite the installation and minimise right-of-way issues as it helps them determine their reliability in extreme conditions. It also helps transcos to avoid expenses on maintenance issues on a regular basis. Therefore, regulators, equipment procurers, transcos and discoms are increasingly putting emphasis on processes concerning transformer and reactor testing. In addition, testing transformers and reactors is crucial to determine their reliability in adverse weather conditions so as to ensure public safety and compliance with various standards/regulations.
Transformer and reactor testing facilities
Transformer/reactor testing facilities measure and evaluate quality, effectiveness and compatibility of transformers and reactors. These facilities calibrate and assess the transformer and reactor on parameters such as oil leakage, vacuum and pressure. Some tests specific to transmission reactors, that is, shunt reactors, include tests measuring zero sequence reactance, mutual reactance, acoustic sound level, tan delta of winding, and gas analysis of insulating oil. Furthermore, the Central Electricity Authority (CEA) requires transmission companies to routinely assess reactors post commissioning in order to ensure that they are maintained throughout their lifetime.
As of now, there are 10 transformer testing facilities in India. Most of these testing centres are also transformer manufacturing companies which provide these entities a sharp edge in delivering on their transformers with limited delay. Emerging technologies also incorporate 3D modelling to simulate and stress-test the strength and resilience of transformers and reactors.
CEA procedures for testing
In April 2021, the CEA issued a revised notification delineating parameters and specifications for transformers and reactors along with procedures for testing them.
The CEA document mandates testing the transformers, reactors and auxiliary equipment as per the manufacturing quality plan provided by the purchaser. The purchaser is empowered to conduct tests on equipment as well as the raw materials used in the equipment.
The notification requires the manufacturer to design, manufacture and test the equipment in accordance with the best international engineering practices pertaining to transformers and reactors. The equipment needs to be specifically tested for conformance to safety standards and hence it is important to test it for mechanical, thermal, dielectric, and electrical stress, among others.
These new regulations also empower the purchaser/consultant appointed by the purchaser to visit the manufacturing unit in order to inspect design as well as manufacturing processes. The new regulations also state that the manufacturer will be penalised if losses during routine tests are in the range of 2 per cent on maximum specified values with further possibility of rejection if the values exceed 2 per cent.
The procurer of a transformer and reactor is also allowed to conduct stage inspection during the manufacturing stage. The stage inspection will be focused on testing the precision and compatibility of the core, winding, core-coil assembly and tank. Further, the manufacturer is mandated to provide a list of materials along with requisite documentation about the raw materials used in the production of the transformer as well as reactor. Additionally, the transco or a consulting entity representing the transco can test for interchangeability of components prior to their shipment. The inspector also needs to authenticate whether the manufacturer is supplying accessories such as conservators and control cubicles. Over the past couple of months, transcos have conducted virtual inspections of equipment owing to the Covid-19 pandemic. In line with this, the CEA has introduced rules relating to virtual inspection.
Types of test
Oil leakage test: The tanks and oil-filled compartments are filled with air or oil of high temperature at substantial pressure to determine if there is any oil leakage from these compartments. Oil testing consists of measuring the breakdown voltage, and other chemical and physical properties of the oil, either through portable test equipment or in a laboratory. Through proper testing, a transformer’s lifespan is increased, reducing the need to pay for replacements. Generally, moisture and oxygen can enter a transformer through leaking gaskets causing accelerated ageing of insulation and insulation failure. Leaking oil can get into the ground and possibly into groundwater, which can be expensive to treat; leaking oil can carry lead from transformer paint (if the paint is lead-based) into the ground, causing further contamination. Therefore, it is important to test oil leakage.
Vacuum test: A tank designed for full vacuum (760 mm of mercury) is mandated to be tested at an internal pressure of 25 torr for one hour. After subjecting it to such high pressure vaccum, the inspector then checks for damage to the flat plate. It ensures that the transformer is able to bear a vacuum under intense pressure.
Pressure test: All transformer/reactor tanks, radiators, and other fittings together or separately are subjected to a pressure corresponding to twice the normal head of oil or normal oil head pressure plus 35 km per sq. m, whichever is lower and maintained for eight hours. The inspector will then check if the deflection in the flat plate exceeds the specified figure or not. The permanent deflection of flat plates after the excess pressure has been released shall not exceed the figure specified above for the vacuum test.
Issues and challenges
The CEA aims to improve the quality of transformers and reactors provided by manufacturers by standardising equipment and equipment testing processes by formalising parameters related to the testing process. Additionally, these regulations outline details regarding the maintenance and repair of reactors. Several years down the line, transcos are expected to gain enormous benefits from the application of these regulations that ensure reliability, robustness and consistency of standardised, low-maintenance and interchangeable equipment. These norms will also empower transmission utilities to reduce their T&D losses and expedite project execution with minimal cost and time overruns.