Module 6 : Preventive, Emergency and Restorative Control
Lecture 31 : Power System Restoration
Objectives
In this lecture you will learn the following
Restoration of a power system after a blackout has taken place.
Problems associated with restoration.
An Illustrative Example
After a blackout
If a blackout (a near total loss of generation and load) takes place, efforts have to be taken to bring back the
system to a normal state at the earliest. It may surprise you to know that this (black starting!) is not an easy
task. We shall see why in this lecture.
Once a generator is tripped, restarting it requires a significant amount of power. Power is required for 2 types of
activities:
Survival Power: For emergency lighting, battery chargers etc. Usually the requirement is 0.3% of the
a) generator capacity.
Startup Power: For starting power plant auxiliaries (pumps etc.) Interestingly, nuclear and thermal units
b) require approximately 8 % of the unit capacity for auxiliaries alone! Therefore, a 500 MW generator
requires approximately 40 MW for running its auxiliaries.
Hydro and Gas units, on the other hand, require only about 0.5-2% of unit capacity for auxiliaries and can be
started usually from in-house DG sets.
The major steps required for restoration are:
Islands which have survived need to be stabilised for frequency and need to be used for starting other
a) units
b) Hydro/Gas units which require less startup power need to be started using in-house DG sets.
Larger thermal units need to be fed "startup power" from: 1) Islands which have survived 2)
c) Blackstarted generators 3) Other synchronous grids (temporarily)
d) Started units are synchronised with one another.
Loads and Generation have to be matched as much as possible to avoid large frequency variations.
e) Governors have a major role in stabilizing frequency in an island.
Problems in Restoration
a) Securing Islands
After a blackout a few islands may survive due to separation of the system in time. A few hydro or gas
generators could be blackstarted using in-house D-G sets. Therefore some small pockets will be there in the
otherwise blacked out grid wherein generators are supplying some loads. However, the situation in these
islands is usually precarious due to the small number of generators within the island (having very little
cumulative inertia).
Recall that the initial rate of change of frequency is determined by cumulative machine inertia and the initial
load-generation imbalance, while the final settling frequency is determined by the governor and load
, frequency characteristics (see Module 3).
Therefore if the load in the island is fluctuating (for instance, traction loads), the rate of change of frequency
within the island due to fluctuating loads may be quite large -- large enough for the island to collapse due to
excessive frequency variations - causing generators to trip. Therefore control of generated power (by
governors) and frequency based tripping or energisation of load is important.
Black-starting of large generators is done by availing startup power from other started generators or islands.
Startup power may also be availed from neighbouring synchronous grids if an AC transmission link exists
(normally disabled). Unfortunately, startup power cannot be availed via DC links (which use AC line voltages
for commutating thyristors), because AC voltages are not available in the system which is blacked out.
Therefore a generator at Vindyachal (near the border of the western region and northern region grid of India,
which are not synchronised but exchange power through DC asynchronous links during normal conditions) can
avail startup power through an AC line from the northern grid.
Problems in Restoration
b) Extending Power to Loads from Generators which are black-started
Lecture 31 : Power System Restoration
Objectives
In this lecture you will learn the following
Restoration of a power system after a blackout has taken place.
Problems associated with restoration.
An Illustrative Example
After a blackout
If a blackout (a near total loss of generation and load) takes place, efforts have to be taken to bring back the
system to a normal state at the earliest. It may surprise you to know that this (black starting!) is not an easy
task. We shall see why in this lecture.
Once a generator is tripped, restarting it requires a significant amount of power. Power is required for 2 types of
activities:
Survival Power: For emergency lighting, battery chargers etc. Usually the requirement is 0.3% of the
a) generator capacity.
Startup Power: For starting power plant auxiliaries (pumps etc.) Interestingly, nuclear and thermal units
b) require approximately 8 % of the unit capacity for auxiliaries alone! Therefore, a 500 MW generator
requires approximately 40 MW for running its auxiliaries.
Hydro and Gas units, on the other hand, require only about 0.5-2% of unit capacity for auxiliaries and can be
started usually from in-house DG sets.
The major steps required for restoration are:
Islands which have survived need to be stabilised for frequency and need to be used for starting other
a) units
b) Hydro/Gas units which require less startup power need to be started using in-house DG sets.
Larger thermal units need to be fed "startup power" from: 1) Islands which have survived 2)
c) Blackstarted generators 3) Other synchronous grids (temporarily)
d) Started units are synchronised with one another.
Loads and Generation have to be matched as much as possible to avoid large frequency variations.
e) Governors have a major role in stabilizing frequency in an island.
Problems in Restoration
a) Securing Islands
After a blackout a few islands may survive due to separation of the system in time. A few hydro or gas
generators could be blackstarted using in-house D-G sets. Therefore some small pockets will be there in the
otherwise blacked out grid wherein generators are supplying some loads. However, the situation in these
islands is usually precarious due to the small number of generators within the island (having very little
cumulative inertia).
Recall that the initial rate of change of frequency is determined by cumulative machine inertia and the initial
load-generation imbalance, while the final settling frequency is determined by the governor and load
, frequency characteristics (see Module 3).
Therefore if the load in the island is fluctuating (for instance, traction loads), the rate of change of frequency
within the island due to fluctuating loads may be quite large -- large enough for the island to collapse due to
excessive frequency variations - causing generators to trip. Therefore control of generated power (by
governors) and frequency based tripping or energisation of load is important.
Black-starting of large generators is done by availing startup power from other started generators or islands.
Startup power may also be availed from neighbouring synchronous grids if an AC transmission link exists
(normally disabled). Unfortunately, startup power cannot be availed via DC links (which use AC line voltages
for commutating thyristors), because AC voltages are not available in the system which is blacked out.
Therefore a generator at Vindyachal (near the border of the western region and northern region grid of India,
which are not synchronised but exchange power through DC asynchronous links during normal conditions) can
avail startup power through an AC line from the northern grid.
Problems in Restoration
b) Extending Power to Loads from Generators which are black-started
