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

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

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)

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)

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

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

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:

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

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

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:

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

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 onSpinning Reserve

• Leonardo ENERGY Minute Lecture onReactive Power