Column: Transformer Lifecycle
Transformer energisation after network blackout
According to ENTSO-E Network policy 5, responsibility for system restoration after a blackout is on the shoulders of Transmission System Operators (TSOs). They are advised to have a firm plan in place for regular testing of system restoration functionality. Part of the network restoration includes energising transformers to supply various underlying grids. However, transformer energisation after a blackout is complex and there is a significant risk for network restoration to fail because the network is quite weak in the first phase of the process and due to specific transformer behaviour.
This paper will focus on relevant items in regard to energising transformers and an alternative robust ‘soft-energisation’ methodology to supply transformers simultaneously after a network blackout. A qualitative assessment is made for the ‘classical’ and ‘soft-energisation’ methodology for network restoration purposes. Also, feasibility studies are elaborated.
A robust and fast network restoration process after a blackout is a key element for TSOs, in order to restore power as quickly, reliably and safely as possible, while minimising the stress on network components. This last condition is vital as too much electrical stress, e.g. transient voltages or currents, could trigger component failure or protection relays. Trips of network components during the restoration process are undesirable and might cause another blackout, extending the damage for consumers. Dealing with voltage oscillations is one of the technical challenges that a TSO has to deal with for network restoration purposes.
For network restoration after a blackout, there are two main methodologies that can be applied to energise network components: the ‘classical’ and the ‘soft-energisation’ methodology. In this contribution, the focus is on these methodologies and not on the start of the involved black start generator that has to act as a voltage source.
- Network restoration: ‘Classical’ and ‘soft-energisation’ methodology
2.1 Classical methodology
For the ‘classical’ network restoration methodology, a dedicated black start generator must be started and accelerated via a black start facility. In Western Europe, a gas turbine is typically used as a black start generator. The black start facility usually comprises (emergency) diesel generators and/or a small gas turbine to accelerate the black start generator and to supply its auxiliaries. As soon as the black start generator is running at full speed at no load and its voltage regulator is excited, the unit can supply a transmission substation, overhead lines, cables and transformers.