Practical applications of static compensators (II) [Part 4/5: Thyristor switched capacitor banks (TSC) for overhead distribution lines]

Thyristor switched capacitor / filter banks

Thyristor switched capacitor banks (TSC for short) and thyristor switched filter banks (TSF for short), also called thyristor switched harmonic filters, have been around since the 1970s. Description of their topology and operating principle can be found as far back as 1974.

They were developed thanks to the technological evolution of thyristor valves in the 1970s to take care of the problems in the electric power system created by fast changing reactive power demand or by highly dynamic loads that conventional solutions like mechanically switched capacitor banks (MSC for short) or mechanically switched capacitors with damping network (MSCDN for short) could not handle.

TSCs and TSFs can be applied to medium or large installations in a range of segments. They have several high voltage potential applications where their use offers many benefits including equipment or facilities where reactive power and/or power factor fluctuate rapidly or in big steps, installations with solar inverters or wind turbine generators, transmission and distribution lines, railway electrification systems, starting and impact loading of large motors and other dynamic loads, and loads with low power factor, to name a few.

Functions

TSCs and TSFs are suitable solutions for applications that require real time transient free capacitive reactive power compensation. They can provide an stable and accurate capacitive reactive current flow without the drawbacks of conventional solutions, reducing system losses and stabilising the system voltage.

Modern TSCs and TSFs can take care of several power quality problems, provide ancillary services and support the development of clean energy by combining different control functions in a single device.

Connection

TSCs and TSFs are power electronics-based shunt compensation devices connected in 3-wire electric power systems in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. They behave as controlled impedances providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 5 milliseconds and typical overall response time is under 10 milliseconds).?

The most common operating voltage range for TSCs and TSFs is 3 kV up to 36 kV as they are built using high voltage thyristor valves. It is possible to connect them to higher voltages using a suitable step-up transformer.

TSCs for overhead distribution lines

In the electric power system generators produce energy, which is then delivered to end users through the conductors of transmission and distribution lines. Due to the nature of alternating current, the system produces reactive power in its transverse reactance, better known as capacitive reactance, and consumes it in its longitudinal reactance, known as inductive reactance.

The reactive power demand in the electric power system varies throughout the day. During lightly loaded conditions there will be excess of capacitive reactive power and during peak load conditions there will be excess of inductive reactive power. Both situations need to be compensated for maintaining system voltage stability. Voltage fluctuations or voltage collapse are associated for example with reactive power demands not being met because of limitations on the production and distribution of reactive power.

Reactive power compensation is an essential function in a power system required to minimise power distribution losses, to maximise power distribution capability, to stabilise the power system and to maintain the supply voltage within required limits.

Requirements

Background

The higher demand for electrical power supply at the load center at the end of the distribution line caused by increased use of electricity at both domestic and industrial level is a threat to power system stability.?

The main requirements from the system operator are to control the voltage of the line between 0.95 pu and 1.05 pu, avoiding any possible voltage sags and undervoltages, maximise power distribution capability and to decrease the distribution losses.

The loads at the end of the distribution line are time varying, so it is needed a dynamic solution that can adapt fast to the daily and seasonal varying needs of the system to be able to provide the needed voltage stability.

System description

The 30 kV overhead distribution line has a length of 200 kilometres. It is tied with a 220 kV transmission line through a power transformer. At the end of the distribution line there is a load center.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the distribution line over a period of time by using a power quality analyser. The collected measurement data need to be studied to determine the power quality problems and energy efficiency requirements.

A series of comprehensive analytical power studies including load flow analysis are needed to find out the magnitude of the problems and the ratings of the possible solutions.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution that would comply with customer’s requirements. The solution is a thyristor switched capacitor bank rated 30 kV 50 Hz 20 Mvar formed by two steps of 10 Mvar each.

Thyristor switched capacitor banks can be used to compensate for inductive reactive power generated by long and heavily loaded distribution lines, thus allowing the flow of more active power through the electric power system and mitigating voltage sags and undervoltages.

The TSC located in the distribution line provides a real-time response, maintaining voltage stability during seasonal or daily load variations. The TSC can effectively control the voltage and also reduce the distribution system losses as demanded by the distribution system operator.

Based on the values monitored, the following functions are proposed for the TSC.

Conclusions

Thyristor switched compensation devices offer near-instantaneous response to changes in the electric power system that electromechanically switched compensation devices cannot deal with. This makes thyristor switched capacitor banks suitable solutions for networks that require transient free capacitive reactive power compensation.

The use of TSCs in overhead distribution lines help to improve the voltage profile of the lines and prevent system collapse due to overloading, line faults or sudden high inductive reactive power demand.

The benefits of using TSCs for this kind of applications can be summarised in:

• Increase the efficiency and reliability of the electric power system.
• Limit the voltage level decrease on the system (increasing the voltage during high load periods).
• Limit switching transients during energisation or reclosing operations.
• Increase distribution system capability.
• Reduce system losses.

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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.