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T&D technology

This paper summarizes all basic technology you can find in transmission and distribution systems worldwide, starting from overhead lines and underground cables through substation equipment like switchgears, transformers, shunt reactors, HVDC, etc.

Transmission and distribution technology overview
Transmission and distribution technology overview

Static VAR compensators (SVC)

The SVC is designed to preserve voltage stability by rapidly supplying reactive power to support the voltage during the transient period. It is part of the flexible AC transmission system (FACTS) genre of equipment.

Essentially, SVCs and static compensators (STATCOMs) deliver a similar function using different power electronic technologies and methods SVCs also provide power oscillation damping where instabilities could arise between different parts of a power system.

The SVC function provides variable inductive and capacitive reactive power using a combination of thyristor controlled reactors (TCR), thyristor switched reactors (TSR) and thyristor switched capacitors (TSC).

These are connected to the AC network using a compensator transformer or via the transformer tertiary winding.


An SVC can provide a fast, continuously variable reactive power response using the TCRs, with the coarser reactive control provided by the TSRs and TSCs.

The reactive power MVAr output of the SVC may be controlled directly or be configured to automatically control the voltage by changing its MVAr output accordingly.

Typical SVC configuration
Figure 1 – Typical SVC configuration

As the SVC uses AC components to provide reactive power, the MVAr production reduces in proportion to the square of the voltage. A suitably rated SVC can provide fault ride through capability at the interface point of the offshore transmission network and the onshore transmission system, as required by the System Operator/Transmission Owner Code (STC).

SVCs can be used for AC connections or with a current source converter (CSC) HVDC-based solution. Voltage source converter (VSC) HVDC solutions can inherently control MVAr output, so do not necessarily need an SVC.

SVCs are made up to 500kV and 720MVAr and they have been in operation for many years at higher ratings and voltages than STATCOMs SVCs tend to be cheaper than STATCOMs on a like-for-like basis, but have a larger footprint.

Dependencies and impacts

The TCRs produce harmonics that normally require fifth and seventh harmonic filters, and star-delta winding transformers to block third and ninth harmonics.

Six-pulse SVCs are typical, but where there’s space and concern about harmonic performance, twelve pulse SVCs can be considered.

A step-up transformer is usually required to couple the SVC to the required bus section. These are specialised transformers with low voltage secondary windings (e.g. 10kV) and the capability to handle the reactive power flow and block triplen harmonics.

If a transformer fails, the SVC will be out of service until the transformer is repaired or replaced The fast dynamic response of the SVC is provided by thyristor valves that are water cooled, air insulated and designed for indoor use.

The reactors and capacitors are usually housed outdoors unless there are noise considerations.

Title:Transmission and distribution technology overview – National Grid ESO
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Transmission and distribution technology overview
Transmission and distribution technology overview

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More Information


  1. Jason Colman
    Jul 11, 2019

    Ampage n voltage?

  2. Linus Abraham
    Mar 27, 2019

    Am into power operations and generator

  3. ibrahima DIOP
    Mar 14, 2019

    c’est tres instructif mais le niveau est assez élevé pour un simple technicien par contre j’ai bien aimé l’article sur la mise à la terre et la définition de certains termes.Dakar -SENEGAL-WESTERN AFRICA

  4. innocent zvekare
    Mar 13, 2019

    Resistance of a cable depend on what..

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