How to choose the right static compensator for your application [Part 3/8: Main functions]

After the overview of static compensators in the first two articles, this third article will discuss the main functions of these devices.

In principle, a shunt static compensator can correct a wide variety of power quality problems. They are able to provide ancillary services and they can also support the development of clean energy.

In transmission level applications (up to 800 kV), shunt static compensators are usually applied for power flow control, stability enhancement, voltage regulation and power oscillation damping. In distribution level applications (up to 38.5 kV), they are usually applied for power factor correction, reduction of voltage variations, mitigation of voltage fluctuations, elimination of harmonics, load balancing and for providing fault right through capability.

Some of the most common functions that shunt static compensators can provide are:

• Power factor correction (PFC).
• Elimination of harmonics and interharmonics.
• Reduction of voltage variations (sags and swells).
• Mitigation of voltage fluctuations (flicker).
• Load balancing.
• Voltage control.
• Reactive power control (RPC).
• Fast reactive current injection (FRCI).
• Fault ride through (FRT) capability.

Power factor correction (PFC)

Low power factor is typically caused when inductive or capacitive loads like motors, transformers, cables or furnaces are present in the electric power system. Other contributors to low power factor are harmonic currents produced by nonlinear loads or renewable generators, or the change of load in the electric power system.

Shunt static compensators detect the phase angle difference caused by inductive or capacitive equipment and they generate and inject in real time leading or lagging current into the electric power system, making the phase angle of the current the same as that of the voltage, which brings fundamental power factor to unity.

Elimination of harmonics and interharmonics

Most electric power systems are not designed to support the nonlinear loads and renewable generators that nowadays make up a large percentage of the installed equipment. One of the main power quality problems that these devices bring are harmonic and interharmonic currents and voltages.

Shunt static compensators can eliminate current or voltage harmonics (odd and even) and interharmonics by injecting the harmonic and interharmonic current signal measured into the electric power system. The injected current signal is of same magnitude but opposite in phase of the measured signal.

Reduction of voltage variations (sags and swells)

Some power quality phenomena occur extremely fast requiring the mitigation to be even faster. If the installation is affected by voltage sags or voltage swells created by the loads, a shunt static compensator with fast response time could be the solution to reduce those voltage variations and protect sensitive loads from those disturbances.

Shunt static compensators can support the voltage of the installation via reactive power injection. They can monitor the voltage of the electric power system on which they are connected and determine the proper amount of reactive power needed to either raise the system voltage or lower it. They can inject continuously leading reactive power to raise the voltage and lagging reactive power to lower the voltage.

The amount of reduction of voltage variations that shunt static compensators can provide to keep the electric power system within its stated voltage tolerance level depends on system parameters like impedances and cannot be specified alone. The device can only be specified for the rated capacity of current and reactive power that can be used for the reduction of voltage variations.

Mitigation of voltage fluctuations (flicker)

Shunt static compensators are a very cost-effective solution to deliver instantly voltage fluctuation mitigation functionality. They can mitigate voltage fluctuation caused by the load by providing a rapid detection and injection of the needed reactive current. This prevents the reactive current of the system from over loading the network and causing rapid voltage deviations, also known as voltage fluctuations or flicker.

The amount of mitigation that shunt static compensators can provide depends on system parameters like impedances and cannot be specified alone. The device can only be specified for the rated capacity of current and reactive power that can be used for the mitigation of voltage fluctuations.

Load balancing

Using single-phase loads or single-phase generators on a three-phase electric power system results in unbalanced conditions in the system. The unbalanced currents result in unbalanced voltages and affect other loads and generators connected to the electric power system resulting for example in overheating transformers, power losses and overall lower system efficiencies. Therefore, load balancing is helping to improve the power quality and efficiency of the system.

Shunt static compensators are using current control to deliver load balancing functionality in three-phase systems. They can provide load balancing with its full rated capacity. How this relates to power depends on variables like load power factor. They can also balance system voltages if the unbalance is caused by unbalanced active or reactive power of the load.?

Voltage control

Voltage control refers to controlling voltage node profile to a target value or within a target range. This control is commonly achieved by shunt static compensators injecting or absorbing reactive power at a voltage controlled node. The transmission or distribution system operator can dispatch manually or automatically the reactive power from the shunt static compensators located near the generation or in the transmission and distribution system using optimal power flow methods.

Reactive power control (RPC)

The need for reactive power varies as demand varies and as the sources of generation vary. As reactive power is not viable to be transmitted over long distances in the electrical grid, its production is distributed across the electric power system, usually closer to the locations where it is needed.

Shunt static compensators can provide reactive power control in real time by injecting or absorbing reactive power as demanded by the application.

Fast reactive current injection (FRCI)

The main purpose of FRCI is to support the electric power system during a fault-induced voltage collapse. The injection in real time of reactive current could support short term voltage stability of the system. FRCI during grid faults by wind farms and solar power plants is nowadays a mandatory requirement in most national grid codes.

Oriented towards system dynamic security and voltage quality, FRCI capability can be easily provided by shunt static compensators.

Fault ride through (FRT) capability

Generating plants are expected to remain connected at system voltages within a defined voltage range and not have a detrimental effect on the ability of the TSO or DSO to maintain system voltage within that range. They should remain in service under normal operating conditions or system disturbances, and during voltage and frequency excursions associated with system short circuit faults.

The inability for a generating plant to ride through a fault could result in the disconnection of large amounts of generation and possibly system collapse. The FRT capability of generation to remain connected through disturbances on the power system is critical to power system security, system stability, and it is one of the most challenging requirements in grid codes. The FRT capability of generating plants depends on generator technology, design and control characteristics. In order to comply with FRT requirements, certain generating technologies may require additional dynamic voltage control capabilities that can be provided by shunt static compensators.

Low voltage ride through (LVRT) capability refers to the ability of a generating plant to remain connected to the electric power system and operate for a certain amount of time through periods of low voltage at the connection point (voltage drops, undervoltages or interruptions). Injecting enough reactive power can guarantee an uninterrupted operation.

High voltage ride through (HVRT) capability refers to the ability of a generating plant to remain connected to the electric power system and operate for a certain amount of time through periods of high voltage at the connection point (voltage swells or overvoltages). Absorbing enough reactive power can guarantee an uninterrupted operation.

This article finalizes with the introduction of static compensators, and shunt static compensators in particular. The next article of this series will discuss features and applications of static synchronous compensators (STATCOM).

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.