The energy emergency test was organised by the state-owned Slovak Electricity Transmission System (SEPS) in cooperation with Slovenské elektrárne, the state-owned Vodohospodárská výstavba and the West Slovak Distribution Company. The test took place in a separate part of the electricity system, the so-called electricity island, separated from the rest of the electricity network. The provider of the start-from-blackout service was the Gabčíkovo hydroelectric power plant and the system power plant, which, if necessary, will be provided with voltage for starting from blackout, was for the first time in Slovakia in a nuclear power plant, namely the Bohunice V2 nuclear power plant (EBO V2).
The test could have taken place during the shutdown of the fourth Bohunice unit to the general outage. It also started on Saturday 14 May with the shutdown of both turbine-generators of the nuclear unit (TG41 and TG42) and the subsequent cooling of the unit, which was subjected to a special emergency test of the power system at a temperature of 170°C.
The aim of the test was to supply voltage from a source providing a support service, i.e. from the Gabčíkovo Hydroelectric Power Plant. Through 110 and 400 kV lines and dedicated distribution and transmission system equipment, the voltage was fed to selected appliances of the nuclear power plant's own consumption as it would be provided for the start-up of the power plant in the event of an actual total blackout of the country's electricity system.
Own consumption appliances - a set of technological equipment of the power plant itself necessary for the start-up and operation of the power plant. These are mainly transformers, control and control elements, drives, controls, various types of pumps, etc.
The weekend test was preceded by preparatory work, when the various entities released from normal operation the equipment necessary for its execution. Subsequently, the substations along the route were put into an emergency state.
After creating the whole route for the test, the loss of power supply of the self-consumption of the dedicated generator for the test in Gabčíkovo NPP was simulated, thus inducing the conditions of local blackout. After the detection of the no-voltage condition, the generator dedicated to the test switched to the blackout mode. After the generator was started up and brought up to nominal values, it was connected to the already established route and the voltage thus applied was transmitted through the island lines to the nuclear power plant building.
In the next phase, selected EBO V2 self-consumption appliances were connected, which ensure the start of the power plant when the system is restored after a blackout. Throughout the test, it was necessary to monitor and regulate the compliance with the operational safety limits with the available control options of the equipment.
After all selected equipment was connected, stabilization of a separate part of the power system - the power island - was carried out. After its stabilisation, the connection of the self-consumption facilities of EBO V2 was assessed as successful and the disconnection of the self-consumption of EBO V2 and the disconnection of the island followed. All the island equipment used in the test was returned to its original state and reconnected back to the power system.
Trials of this type are very important for everyone involved. With the continuous growth in the volumes of electricity transmitted and the mix of the resource base, the demands on the reliability and security of transmissions and the risk of failure are increasing. Interconnection with the pan-European power system brings not only benefits but also the risk of cascading an extraordinary fault into the electricity system of the Slovak Republic, which would lead to overloading of the system elements. This could lead to a collapse of the system and a complete loss of electricity supply.
Therefore, it is necessary to regularly check how the individual companies operating or managing the transmission and distribution system or energy sources are able to cope not only with common accidents, but also with a possible blackout type failure. The obtained knowledge serves not only the dispatchers of the transmission system operator - Slovak Power System Dispatching, but also for the handlers and dispatchers of all the parties concerned, starting from the power plant providing the voltage (provider of the "Start from dark" support service) through the distribution system operators up to the system power plant, as well as for the needs of further development of the simulation environment.
