Connection moment control system for improved electricity quality
In order to ensure high power quality on the transmission grid, the electricity must be at the right voltage, but voltage disturbances do occur from time to time. “Litgrid” plans to solve the problem of voltage drops caused by magnetizing currents when connecting high-power transformers by applying an innovation: a moment of connection control system.
“Earlier, during the first partial test of isolated operation, we noticed that the magnitude of the magnetisation current when the transformers are connected to the system causes problems. Voltage drops in the system are most often observed when switching on the transformer of the combined cycle unit of the Lithuanian Power Plant, while connection of any device whose power exceeds 200 MVA can have an impact on the quality of electricity,” says Tadas Kivilis, the System Technical Requirements Engineer in the System Reliability Department at “Litgrid”.
Such voltage drops are caused by magnetising currents appearing during the connection of a transformer. The scope of negative impact depends on the location of a device and the voltage angle point during its connection. Calculations of “Litgrid” show that the combined-cycle block transformer of the Lithuanian Power Plant is connected about 20 times per year on average, while the negative impact on the voltage is noticed in about 10 per cent of cases.
“As little as one-tenth of cases is still too much: in public hearings of the National Energy Regulatory Council, users expressed concerns about the quality of electricity more than once, while complaints from network users regarding incidents related to the quality of electricity are received more and more often. As consumption of electricity increases and we begin managing our energy system independently following synchronisation with European networks, the importance of this aspect will continue to grow,” says the engineer of “Litgrid”.
Up until now, negative impact was attempted to be reduced by shunting the main contacts with resistance during the point of connection of the device to the network and only then switching it to regular contacts. However, this method is extremely expensive, as it requires the use of circuit breakers with a complex design.
Therefore, “Litgrid” has started developing an innovative idea, the implementation of which would help to ensure the quality of electricity and would reduce the impact of the connection of high-power transformers to the electricity network. That idea is a connection point control system, which has not been applied in Lithuania up until now.
This principle has been used all over the world ever since the end of the 20th century. New methods, which analyse the state of a circuit breaker, perform corrections over the course of connection and record data that can be analysed later on, have been introduced relatively recently.
“The connection point control system is used abroad and applied to other equipment, for example, when connecting or disconnecting shunts, long high-voltage transmission system lines. As every system is unique, we will first test it during the connection of transformers, aiming at the reduction of a negative impact and easier process of ensuring the quality of electricity. We expect good results”, says T. Kivilis
