On Wednesday 11th December, 2019, the power system collapsed, this time for a different but equally avoidable reason. The umbrella association for workers in the Nigerian Electricity Supply Industry (NESI), after the expiration of a 21-day ultimatum given to the Federal Government (FG) on some outstanding issues of payments and conditions of service for its members, decided to shutdown the power system, embarking on a nationwide strike. A system collapse of this sort is different from one arising from the failure of the transmission network for which we are used to a few hours of restoration of supplies. This is a Black Start situation as it involves the shutdown of generators and rotating equipment with high system inertia that cannot be started immediately once they have been shut down.
What is Black Start?
A black start is the process of restoring an electric power station or a part of an electric grid to operation without relying on the external electric power transmission network to recover from a total or partial shutdown. It is the procedure to recover from a total shutdown of the power system which has caused an extensive loss of power supplies. Here, there is no longer synchronism between power generating stations which have hitherto been operating in parallel. This entails isolated power stations being started individually and gradually being reconnected to each other in order to form an interconnected system again. Power generators are required to start up their main generators one after the other in a black start situation. Most of the main generators have minimum downtime period which are in several hours before which they can be switched on after being switched off. Knowing this, power engineers will do all they can to avoid what can cause a black- start situation; not a strike action that resulted from the insensitivity of non-power Engineers in power! Two Ministers (Labour and Power) were summoned by the National Assembly at the eleventh hour while trying to avert the strike. The workers union gave a 21-day ultimatum to ministers but nothing was done in terms of round table discussions with the workers according to union executives. Why wait till the last minute, and when generators which are meant to sustain critical loads, have been shut down?
Why a Black Start Generator is needed?
Once there is a total shutdown, efforts have to be made to get the power system up and running as soon as possible. The restoration of power system after a complete blackout is a very important problem. Black-start generators or units are used for this purpose. The decision support system for black-start can help the system operator to establish a black- start procedure where the power system is restored section-by-section as soon as it possibly can. This will gradually restore power to transmission and distribution substations within the power system.
The Restoration Stages
Following the total blackout of a power system, restoration of power occurs in three critical stages – preparation, system restoration and load restoration.
The first thing is to update the Generator start-up sequence, cranking path and critical load pick-up sequence. Next, there is need to update the transmission network path, dispatchable load pick-up sequence and re-synchronisation of islands if there are any. Finally, an update of load pick-up sequence helps to provide electricity to other customers connected to the distribution and transmission networks.
The impact of loss of supplies and disruption to consumers can be minimised if the power system is decentralized and operated as regional grids. In this case, the transmission system will still be used for switching in terms of critical power transfer. More importantly, a stitch in time saves nine. Ministers and employers of labour have to respond to staff demands as quickly as possible to avoid strike actions that can cripple businesses and processes.
Source: Engineer Idowu Oyebanjo
