1. Minute Lectures
Spinning Reserve
Balancing the net
Electricity cannot be stored in an economic way (in large
quantities)
Therefore, at each moment the electrical power consumed
should equal the electrical power generated.
Companies that sell electricity estimate the amount of
energy their clients will consume. They buy this
“Scheduled Power” from generation companies, who
put it on the transmission grid.

2. Minute Lectures
Spinning Reserve
The Load Diagram
But what if the clients (or power stations) consume (or
produce) more, or less, than scheduled?
24 hour load
diagram for
a power system

3. Minute Lectures
Spinning Reserve
The Transmission System Operator
The difference between the scheduled consumption and the real
load is counteracted by the transmission system operator,
who is responsible for keeping the network balanced
The difference between the scheduled production and the actual
production should be also taken into account
An imbalance results in a frequency that is lower or higher than
the target frequency (generally 50Hz)
The system operator maintains the balance by making use of
the “spinning reserve”

4. Minute Lectures
Spinning Reserve
Definition
Spinning reserve = the unused capacity which can be
activated on decision of the system operator
Spinning reserve is provided by devices that are
synchronized to the network and able to affect its
active power
Negative spinning reserve = capacity that can be
switched off quickly to compensate a dip in energy
demand.

5. Minute Lectures
Spinning Reserve
Who provides it? (1/2)
The system operator pays a generation company to
provide spinning reserve
This makes the system operator a client of generation
companies
Though less common, a consumer can provide spinning
reserve as well, if he agrees to reduce its load upon
request of the system operator

6. Minute Lectures
Spinning Reserve
Who provides it? (2/2)
• Thermal power plants at less than full
output
• (Large) consumers
• Pumped storage stations

7. Minute Lectures
Spinning Reserve
Secondary frequency control
A centralized automatic control that delivers reserve
power on short notice in order to bring the
frequency back to the target value.

8. Minute Lectures
Spinning Reserve
Tertiary frequency control
A manual change in the dispatching in order to
 restore the secondary reserve and provide a more
permanent solution if the imbalance between
consumed power and scheduled power persists
 react if the secondary reserve does not suffice
 manage congestions (which the secondary control
does not do)

9. Minute Lectures
Spinning Reserve
The complete picture (1/2)
Schematically, frequency regulation can be represented
as follows:
A tie-line is a circuit (e.g. a
transmission line) connecting two
or more control areas or systems
of an electric system
A transmission system operator
(TSO) is a company that is
responsible for operating,
maintaining and developing the
transmission system for a control
area and its interconnections
[UCTE Handbook]

10. Minute Lectures
Spinning Reserve
Spinning reserve and frequency control
Link between the spinning reserve and the frequency
control reserves
The spinning reserve consists of
 the secondary frequency control reserve
 the synchronized tertiary frequency control reserve
The primary frequency control is not part of the spinning
reserve, because it is not controlled by the
Transmission System Operator.

11. Minute Lectures
Spinning Reserve
The complete picture (2/2)
Allocation of the
capacity of a
generation unit
(in this example
spinning is
reserve is
positive, but it
can also be
negative)

12. Minute Lectures
Spinning Reserve
What about the primary control?
Just like the secondary control, the primary frequency
control automatically delivers reserve power to
counteract frequency changes although it also helps
balance consumption and production
However, the primary frequency control is not part of
the spinning reserve, but a local control with network
stability as its primary focus (although it can also
help balance consumption and production).

13. Minute Lectures
Spinning Reserve
Round-up
The spinning reserve is a synchronised unused capacity
that can be activated quickly by the TSO to offset the
differences between the scheduled load/production and
the real load/production. It controls the frequency on the
net.
The spinning reserve is the responsibility of the
transmission system operator, who will “rent” the
required capacity from generation companies.

14. Minute Lectures
Spinning Reserve
Further information
• Y Rebours & D Kirschen (University of Manchester)
What is Spinning Reserve?
A survey of Definitions and Specifications of Reserve
Services
• UCTE Operations Handbook
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