Practical applications of static compensators (III) [Part 3/5: Thyristor switched reactors (TSR) for distributed energy resources]

Thyristor switched / controlled reactors

Thyristor switched reactors (TSR for short) and thyristor controlled reactors (TCR for short) 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 reactors (MSR for short) could not handle.

TSRs and TCRs 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, underground high voltage cable networks, compensation of cables, wires or mains capacitance in industrial installations, and loads with low power factor, to name a few.

Functions

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

Modern TSRs and TCRs 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

TSRs and TCRs 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 TSRs and TCRs 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.

TSRs for distributed energy resources

Electric power systems face the increasing challenge of maintaining efficiency and stability while connecting growing amounts of distributed energy resources (DER for short). Distributed energy resources are usually located close to load centers and can be used individually or in aggregate to provide value to the electric power system. These small or medium-sized power generation and energy storage devices tend to generate more unpredictable and fluctuating power flows than conventional power sources. This may create problems in the system like the increased flow of reactive power due to the varying reactive power of both, generation as well as consumption.

Transmission and distribution system operators need economic solutions for such challenges that would control reactive power flows and thereby stabilise voltages and improve the overall active power transmission and distribution.

Requirements

Background

A distribution system integrates numerous distributed energy resources including combined heat and power systems, rooftop solar photovoltaic systems, small wind power systems, gas microturbines and batteries in electric vehicles used to export power back to the grid.

As part of the overall reactive compensation scheme, the distribution system operator needs to keep the reactive power flow within predefined limits, to maintain steady-state voltage limit conditions and to maintain a desired power factor in response to possible changes in power generation and consumption.

System description

The 30 kV distribution line it is tied with a 220 kV transmission line through a power transformer. Along the distribution line there are several distributed energy resources and load centers.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the system 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 system 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 the distribution system operator’s requirements. There is a need for inductive reactive power compensation for increasing the transmitted active power, reducing the losses in the system and stabilising the voltage. With fluctuating power flows, better fine-tuning of the voltage which leads to a better control of the system can be achieved by using a compensation device formed by several steps, so the power output can be adjusted to actual system needs.

The solution is a thyristor switched reactor rated 30 kV 50 Hz 15 Mvar formed by two steps of 5 Mvar and 10 Mvar. A TSR is the optimal device in this case as it can provide a real-time response for controlling reactive power, maintaining voltage stability and also mitigating possible voltage swells and overvoltages.

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

Conclusions

As the electric power system instability and unpredictability increase, the importance of the reactive power regulation also increases in order to be able to guarantee an economic supply of high-quality power that is reliable and has minimal losses.

Thyristor switched compensation devices offer an instantaneous response to changes in the electric power system that electromechanically switched compensation devices cannot deal with. This makes thyristor switched reactors suitable solutions to improve the overall stability of systems with distributed energy resources that require transient free inductive reactive power compensation as they can dynamically adjust their reactive power output according to the changing conditions of the electric power system.

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

• Cost-efficient local supply of reactive power.
• Reduced reactive power loading of the system.
• Better system voltage control.
• Improved voltage stability at low load conditions.
• Increased active power capacity of the system.
• Reduced 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.