Practical applications of static compensators (I) [Part 4/5: Thyristor switched filter banks (TSF) for natural gas compressor stations]

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.

TSFs for natural gas compressor stations

Nowadays, natural gas is transported in two major ways, by LNG carriers, which is transportation done by tanker trucks, railway tankers and purpose built ships in liquid state, or by pipelines in gas state. Compressor stations are an integral part of the natural gas pipeline network that moves natural gas from individual producing well sites to end users. As natural gas moves through a pipeline, distance, friction, and elevation differences slow the movement of the gas, and reduce pressure. Compressor stations are placed strategically within the pipeline network to help maintain the pressure and adjust and monitor the flow of the gas.

Centrifugal compressors are typically used in natural gas compressor stations. They have relatively lower compression ratio limits and higher capacities compared with reciprocating compressors but their energy efficiency is less than that of reciprocating compressors especially at partial loads. Centrifugal compressors are built to withstand the high temperatures and rugged conditions found in demanding operating environments. Their interrupted and stable operation are critical to ensure safe, efficient and reliable performance of processes in compressor stations.

Requirements

Background

The starting torque of motors varies by the square of the voltage, so the starting of large motors on weak networks is often a problematic operation.

The compressor station experiences failed motor starts due to voltage drop during the start-up of the centrifugal compressors. Loss of capacity because of compressor or motor failure has a significant financial impact on the operations of the compressor station.

Customer’s requirements for this application include reactive power compensation support, voltage stabilization and inrush current control during the start-up of the centrifugal compressors.

System description

The compressor station has four centrifugal compressors having synchronous motors rated 11 kV 50 Hz 3.7 MW each. They operate in parallel and are fed by two independent power transformers rated 30 MVA each.

Natural gas enters the compressor station through station yard piping and is passed through scrubbers and filters to extract any liquids and remove solids or other particulate matter that may be in the gas stream. Once the natural gas stream has been cleaned, it is directed through additional yard piping to individual centrifugal compressors.

The compressor station control system regulates the flow and number of compressors that are needed to handle the scheduled system flow requirements. Each of the individual compressors provides the needed additional pressure before directing the gas back into the pipeline with full operational pressure restored.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the compressor station 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.

Comprehensive power studies including flicker and motor starting studies are needed to find out the magnitude of the problems and the ratings of the possible solutions.

Proposed solution

Thyristor switched filter banks are designed for applications requiring instantaneous reactive power support to mitigate voltage sags, flicker, and high inrush currents associated with large dynamic loads. They can also improve installations’ power factor and reduce harmonic distortion.

The study results indicate that two TSFs rated 11 kV 50 Hz 6 Mvar each would be required to keep the system voltage stable during start-up operation.

The TSF is connected in parallel with the compressors to limit and control the inrush current produced during the starting operation. They temporarily reduce the load and torque in the power train of the motor during start-up. This reduces the mechanical stress on the motor and the shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network.

The most important parameters to take into account during the design of the system are:

• Duty cycle of the compressors (for example, if one unit is running per incomer and one unit is on standby).
• Power factor required during start-up and power factor required after acceleration.
• No load current and full load current of the motor.
• Time required for the start-up.

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

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 TSFs suitable solutions for networks that require transient free capacitive reactive power compensation.

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

• Real time reactive power generation for the start-up.
• Installation’s voltage improvement.
• Harmonics mitigation.
• Reduced maintenance frequency and costs.
• Extension of the lifespan of the compressors and the rest of the components of the electric power system.

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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.