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The Main Reactor concept breathes new life into SVCs

The Main Reactor concept breathes new life into SVCs

06/08/2015 - 3.53 pm 
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  • FACTS
  • Renewables
  • SVC – STATCOM

The more recent FACTS products such as STATCOMs are enjoying increasing market acceptance. However, traditional FACTS systems such as SVCs, often considered as mature, still have plenty of room for development, thanks to their reliability and attractive investment and operational costs. The Main Reactor concept, applied in a utility SVC topology, is an innovative enhancement that delivers benefits such as cost savings, improved power quality and increased system availability.

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Beauly Substation, owned by SHE Transmission
 

As part of the FACTS (Flexible AC Transmission Systems) family of devices, utility Static VAr Compensators (SVC) have the job of regulating grid voltage or compensating reactive power. An SVC is connected to a selected point in a transmission grid using a step-down transformer, which typically has a secondary (MV) voltage of between 10 kV and 35 kV to accommodate the voltage level suitable for SVC equipment. An SVC consists generally of thyristor controlled reactors (TCR), thyristor switched capacitor banks (TSC) and mechanically switched filter capacitor banks. The TCR is the controlled reactive element. The task of the filter banks, as well as providing capacitive reactive power, is to filter harmonics.

 
The source of the harmonics could be the background network. “However,” explains Alstom Grid R&D Engineer Jarmo Aho, “the TCR itself generates variable order of harmonics depending on the thyristor firing angle.” The filter banks cannot fully filter these TCR-originated harmonics, meaning that the harmonic level would then increase at the Point of Common Coupling (PCC). An effective method of reducing these harmonics at the PCC is to increase the impedance between it and the SVC busbar. Adding reactance in series with the step-down transformer and its built-in impedance increases overall impedance. “Integrating additional reactance into the step-down transformer”, adds Aho, “would mean designing and manufacturing a special transformer, which would be costly and time-consuming. It occurred to us that it was much more practical to insert an external blocking reactor at the transformer secondary. We have named this blocking reactor ‘the Main Reactor.

 

A single line diagram example of a utility SVC network

 

The new arrangement creates two different secondary buses – one for low and one for high voltage fluctuation. The bus between the transformer and the main reactor is the connection point for an auxiliary transformer, due to its more stable voltage. The bus after the main reactor is the connection point for TCR and harmonic filters.
1_Important benefits
"So the prime purpose of the main reactor (one per phase) is to increase the reactance between the SVC and the power grid,” Aho points out. “By doing so, it prevents the harmonic currents generated by the thyristor controlled reactors in the SVC from entering the grid. As the voltage variation at the SVC busbar increases, the voltage stress on the thyristor valves is reduced at maximum TCR power and thyristor valve current. At the same time, the Main Reactor also prevents disturbances emanating from the grid from entering the SVC. Depending on the scenario, harmonics can be reduced anywhere from 40 percent to 60 percent."
 
The advantages of the Main Reactor do not stop there. The inductive reactance of the Main Reactor between the grid and the SVC also helps in dimensioning the SVC components. This results in a more compact footprint, reduced losses and lower installation costs. “The combination of SVC and Main Reactor is therefore ‘among the greenest’ solutions currently available among active compensation systems,” Aho states.
 
The Main Reactor also reduces voltage stress and therefore the number of thyristor levels needed in the TCR. This, in turn, improves the reliability and availability of the valve, thanks to the stable voltage profile in the valve. As far as power losses are concerned, if the purpose of the SVC is to operate at zero output power most of the time and support the network only in fault situations, it is justifiable to produce part or all of the capacitive power with thyristor switched capacitors (TSC). This way, the continuous control range is retained and the total losses at 0 MVAr are small. However, the Main Reactor solution reduces the overall losses significantly compared with conventional SVCs without main reactors. “Savings in losses are gained with smaller TCR coils (part of the inductance is in the Main Reactor) and smaller valves,” adds Aho.
 
2_The Main Reactor in operation
Alstom Design Manager, Antero Kahkonen, who undertakes system studies for reactive power compensation projects, is the inventor of the Main Reactor concept, which has now been patented. “At the end of the last decade, the general requirements for SVC specifications exceeded the capabilities of conventional SVCs. The biggest obstacle was meeting grid codes in terms of harmonic performance. But also items such as losses, space and availability requirements demanded innovations. The conventional SVC needed a facelift.
 
"I was preparing the technical proposal for the Beauly substation SVC [see sidebar], when I realised we were facing a brick wall. We had examined all possible conventional configurations without finding an acceptable solution. That’s when I started to write equations for impedances and current amplifications. I modelled my calculations and found we could solve a number of problems by adding just one reactor to the topology. The concept was examined using commercial simulation tools, with very good results. Subsequently, our design tools were updated with the ability to calculate Main Reactor configurations. Based on recent experience, this SVC innovation is well suited to today’s FACTS market."
 
Indeed it is, if the Beauly substation in Scotland is to be taken as the example. The planned expansion of the substation (to operate a new 400 kV transmission line between Beauly and Denny) required the installation of a new SVC to help operate the transmission network within quality limits. A standard SVC could not meet the harmonics requirements. However, this innovative SVC configuration – complete with the Main Reactor – fully satisfied the criteria for this strategic substation. It has been in operation since the summer of 2013. Other countries such as Brazil and Saudi Arabia are now showing interest in this promising solution.
 

Three questions to Neil Thomson, Engineering Manager, SHE Transmission

Could you briefly describe the SHE Transmission network and the role of the Beauly substation?
Scottish Hydro Electric Transmission plc (SHE Transmission) is responsible for the electricity transmission network in the north of Scotland, which serves around 70 percent of the landmass of Scotland. SHE Transmission has to ensure there is sufficient network capacity for those seeking to generate electricity from renewable and other sources. Beauly substation is a major transmission hub in the Scottish highlands. It has recently had its 275 kV compound rebuilt and a 400 kV compound added as part of the Beauly-Denny project. Beauly is a strategic hub for power generated from renewables in the north of Scotland. The Beauly-Denny 400 kV replacement transmission line project is replacing the 132 kV transmission circuits between Beauly and Denny, in central Scotland, to allow a greater transfer of renewable power from the highlands to the load centres in the central belt and further south.
The need for voltage control at Beauly was identified at an early stage of the project. A +150/-150 MVAr SVC was installed and connected to the 275 kV busbars, along with tertiary connected 90 MVAr reactors on the supergrid transformers and two 45 MVAr mechanically switched damped capacitor networks (MSCDN) on the 132 kV busbar.
 
What was the attraction of the SVC with Main Reactor?
We know that SVCs fitted with TCRs generate harmonics, and our specification detailed limits on the generation of these harmonics and amplification of existing harmonics. When Alstom offered its Main Reactor as part of its SVC offer, we saw its advantages and did wonder why such a simple idea had not been thought of before! The detailed studies submitted showed that the SVC’s harmonic performance complied with our requirements, so, as a company that is keen to encourage innovation and open to new ideas, we saw no reason not to proceed with a Main Reactor solution.
 
What has been your experience with the Beauly SVC with Main Reactors?
The SVC has been in operation since early summer 2013 and we have had no issues with its operation to date. We have had several faults on the network over that time, which the SVC has responded to well within the response criteria we specified. The SVC is in operation 24/7 and has quickly become an important part of the transmission network in the north of Scotland.

 

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