As the fifth in a series of tutorials on the power system, Leonardo ENERGY introduces its minute lecture on voltage and frequency control, using the analogy of a metal/rubber plate to demonstrate the centralised nature of frequency control, whereas voltage control is more a local matter.

1. Voltage and frequency control of the grid
Minute Lectures
A complex system
In Europe, all electrical grids are interconnected
This large international grid is highly complex to control
It differs largely from water, gas, or data networks
because electrical energy in alternating current is not
storable
The stability of the electrical grid is maintained by
frequency and voltage control

2. Voltage and frequency control of the grid
Minute Lectures
The only possible ways of storage:
• Transform the alternating current in direct current by power
electronics, and then store the energy in the electric field of a
capacitor or in the magnetic field of a coil
• Transform the electric energy into other forms of energy
None of these options are conceivable on large scale in the
near future
In the long run, storage in hydrogen could contribute to the
management of the electric system
Alternating current is not storable

3. Voltage and frequency control of the grid
Minute Lectures
Since storage is difficult, two separate equilibriums should
be kept on the grid. Both are the responsibility of the
Transmission System Operator (TSO)
A double equilibrium (1/2)

4. Voltage and frequency control of the grid
Minute Lectures
A. The active power generated should at each moment equal the
active power consumed. A deviation from this equilibrium results
in a deviation from the 50 Hz frequency. So keeping this
equilibrium between active power consumption and generation
means maintaining frequency
B. The reactive power on the grid should be kept in equilibrium as
well. Reactive power is an extra load for the grid, leaving less
capacity for active power, resulting in a local voltage drop. So
keeping reactive power in equilibrium means maintaining voltage
A double equilibrium (2/2)

5. Voltage and frequency control of the grid
Minute Lectures
Consumption of active power varies strongly according
to the time of the day, the season, or weather
conditions
The TSO keeps reserve capacity in power plants at hand
to be able to react quickly and deliver extra power
when necessary (= spinning reserve)
The increasing number of wind turbines make control
even more complex, since their generation output is
difficult to predict
A. Frequency control = active power control

6. Voltage and frequency control of the grid
Minute Lectures
A. Frequency control = active power control
If necessary, the TSO can ask for extra power from other
countries on the European interconnected grid
Indeed, for this active power equilibrium, the complete
interconnected grid behaves as a whole
The larger the grid, the easier to keep the equilibrium, since
variations in consumption and generation will level out

7. Voltage and frequency control of the grid
Minute Lectures
A. Frequency control = active power control
The active power equilibrium of the grid can be represented by an
inflexible sheet with loads (L) pulling it down, and generators (G)
compensating this force pulling the sheet up to keep it in place:

8. Voltage and frequency control of the grid
Minute Lectures
B. Voltage control = reactive power control
Reactive power is inextricably related with the active
power, and oscillates between generators, inductive
elements (motors, transformers, electric ovens…) and
capacitive elements (capacitor batteries) on the grid
It doesn’t participate in the energy transmission, but is
nevertheless an extra charge for the grid, resulting in:
• Extra losses on the line
• Less capacity left for the transmission of active energy,
becoming evident by a voltage drop on the line

9. Voltage and frequency control of the grid
Minute Lectures
B. Voltage control = reactive power control
Energy users will be stimulated to compensate their
inductive elements by capacitor batteries
The remaining reactive power on the grid will be
compensated by the synchronous generators in power
plants
This compensation should be done as close as possible to
the inductive loads, since reactive power
• Is difficult to transport on the grid
• Leads to extra losses on the lines
• Leads to voltage drops on the lines, limiting their capacity

10. Voltage and frequency control of the grid
Minute Lectures
B. Voltage control = reactive power control
The reactive power equilibrium of the grid can be represented by a
flexible sheet (e.g. a sheet of plastic) with reactive loads (QL) pulling
it down, and power stations compensating this force in the
immediate vicinity of the load by generating reactive power (QG),
limiting in this way the tension on the sheet and the risk of
breaking:

11. Voltage and frequency control of the grid
Minute Lectures
Round-up
Since electrical energy is not storable on an AC network,
generation should at each moment equal consumption
This equilibrium has to be kept for active power (= actual
energy transmission) as well as for reactive power (= an
oscillation between inductive and capacitive elements)
The active power equilibrium is related with the frequency,
and should be kept for the whole of the interconnected grid
The reactive power equilibrium is related with the voltage on a
line, and should be kept at local level

12. Voltage and frequency control of the grid
Minute Lectures
Links and references
• Michel Crappé, Evolution du système électrique
européen. Les défis pour les ingénieurs, Symposium
“Avenir de l’énergie et énergies de l’avenir”, Brussels,
December 8, 2005
• Leonardo ENERGY Minute Lecture on
Spinning Reserve
• Leonardo ENERGY Minute Lecture on
Reactive Power