Transmission & Distribution

The Electricity Grid



They are present along the electricity grid to step down (or, less often, up) the voltage within the transmission grid, the distribution grid and the final distribution network. They are also present at all connections between these various networks. There are various types of transformers, the main ones used in the grid are power transformers and distribution transformers.



Switchgear is equipment that isolates, protects and controls electrical networks or installations. The term “switchgear” is a generic one and include:
Disconnectors ensure, in the open position, an isolating distance to provide "dielectric independance" between the two circuits.
Switches are used to reconfigure the electrical system or in combinations with fuses to protect an MV/LV transformer.
Fuses are used for protection of MV/LV transformers and MV motors in case of short circuit.
Circuit breakers are used for fast interruption of short circuit current to minimise installation damage.


Instruments transformers

Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents.


A substation is a connecting point in the electricity grid and an important point for information on the power flow travelling along the lines. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.
Substations may be owned and operated by an electrical utility, or may be owned by a large industrial or commercial customer. Generally, substations are unattended, relying on remote supervision and control.



HVDC refers to an electric power system for the large-scale conversion of electrical power between direct current (DC) and the alternating current (AC) that is used for the electricity grid more generally. HVDC has several applications:

  • It can be used for the large-scale transmission of bulk electric power over long distances, when HVDC systems can have lower lifetime cost and suffer lower electrical losses than an AC system

  • It can be used for transmission of electric power through marine or underground cables where the capacitance of the cables would create high losses for transmission by AC

  • It can be used in “back-to-back” mode for connecting adjacent AC electrical grids that are either at different frequencies or are out-of-phase, thereby allowing power exchange between these host AC grids whilst helping to stabilise both.


FACTS is the acronym for “Flexible Alternating Current Transmission Systems” and refers to a group of network elements incorporating power electronics based circuits used to overcome certain limitations in the transmission capacity of electrical grids.

The main purpose of these systems is to supply the network as quickly as possible with power that is adapted to its particular requirements, while also improving transmission quality and the efficiency of the power transmission system.

Network Management

The electricity grid needs a continuous control system that manages both generation and load on time. Network Management is a must to maintain the stability and value of all relevant system parameters of the transmission and distribution networks (such as voltage, power, phase balance, reactive power and frequency) within predetermined limits. Network Management includes an expert system able to balance phases, manage reactive power and reconfigure circuits to achieve the best performance. Predictive and forecasting methods are used to anticipate the conditions and requests that the network is going to support. That allows to get the network prepared in advance. That includes a huge amount of information and data such as: weather forecast to anticipate the capacity of the renewable sources, historical data of controllable and uncontrollable load, prediction of exceptional consumptions, availability of demand side management, etc. The Network Management system must digest and match all data predicted and measured in order to ensure the correct functioning of the whole electricity system.

Protection & Control

Faults occurring in power systems must be isolated quickly and selectively in order to avoid serious consequences for the equipment and the customers to be supplied. The shutdowns must be selective so that only faulted lines are isolated. Main important devices of the protection chain are protection relays. They control the tripping of circuit breakers to disconnect the fault from the grid. Modern relays are digital based on IED-technology (Intelligent Electronic Device). Digital control technology is applied in order to ensure safe operation of the networks at transmission, regional as well as local level: Energy Management Systems (EMS) are used for balancing between generation and load. SCADA systems (Supervisory Control and Data Acquisition) are applied for tasks such as network management through operational planning as well as operational monitoring, control and information processing.

Renewable energy integration

As solar- and wind-based generation capacities in electrical power networks soar, maintaining grid stability and reliability becomes a challenge for grid operators. Reasons for this are the short-term variability and low predictability inherent to renewable sources.

With renewable integration distribution grid’s power lines also need to cope with two-way power flows from and to the consumer side. The line voltage can vary in a wider ranges also exceeding the permitted limits. Transformers with flexible transformation ratio or Line Voltage Regulator (LVR) devices can help to avoid overvoltages.

Charging infrastructure

To decarbonise the transport sector countries have large ambitions to grow the use of electric vehicles, using decarbonised electricity supplies. Meeting the power new demand for electricity will mean expanding the electricity infrastructure. Vehicles will require charging points at their homes, on street, at parking lots and charging stations. There is also a strong demand for very high charging rates to allow vehicles to be re-charged in a few tens of minutes during long journeys.

Meeting this demand will require innovation at a number of levels. Smart grid technologies will be required to maximise the capacity of the existing grids to deliver power. There will also be a need to reinforce the networks at both the distribution level and transmission level to carry greater power flows. There is also the possibility to use metering and digital technology to control when cars are recharged so as to avoid very high loads in the evening when people return home and to maximise the use of low carbon power from renewable generators.


An energy consumer who also produces and consumes his own energy, for instance with solar panels. This consumer may sell his ability to increase or decrease his energy consumption during short period of time through an aggregator.