With the growing amount of renewable energy along with burgeoning distributed generation, the modern-day power grid faces many challenges. One challenge is that under current network codes distributed generation sources are not required to supply the reactive power necessary to balance the grid.
It is a fact of life that each power grid is unique and has specific challenges especially in terms of power quality, reactive power compensation, and grid stability. To help with this, Flexible AC Transmission Systems, (FACTS) can dramatically help increase the reliability of AC grids. They improve power quality and transmission efficiency from generation through transmission down to the private and industrial consumers and better utilisation of the network.
Reactive power compensation
Reactive power compensation technology is a subset of FACTS. These systems can regulate the parameters that determine the operation of a power supply grid and account for the quality of transmission. Such parameters include transmission impedances, currents, voltages and phase angles between the different nodes.
Reactive power compensation is divided into parallel compensation and serial compensation. While series compensation is primarily used to increase the power transfer capability on transmission lines, parallel compensation systems such as Static Var Compensator (SVC) and Static synchronous Compensator (STATCOM) systems primarily regulate the voltage at the connecting point and thus facilitate a stable, reliable grid operation.
FACTS devices can also significantly increase the power transmission capacity of existing AC systems and extend maximum AC transmission distances by balancing the variable reactive power demand of the system. Reactive power compensation is used to control AC voltage, increase system stability, and reduce power transmission losses.
State-of-the-art FACTS devices include Fixed Series Compensators, switched capacitor banks and, Static VAR Compensators for dynamic shunt compensation.
The latest generation of Siemens dynamic compensation devices is called SVC PLUS, a multilevel STATCOM solution. These are highly standardised compact devices that can easily be implemented in demanding network environments; for example, to allow connection of large offshore wind farms.
The make-up of SVC
Large and variable loads with abrupt changes in active and reactive power are a challenge for power supply solutions. The rapid growth in the use of variable-speed drives increases the generation of harmonic distortions: FACTS solutions help improve industrial production.
Harmonic filter circuits provide capacitive power, which improves the power factor. By tuning the filter circuits to specific harmonic frequencies, compliance with respective utility regulations can be achieved. This enables a higher utilisation of transmission assets and saves energy.
The heart of the SVC system is the light-triggered thyristor valve system. Working alongside the controlled reactor, the filter circuits, and the control system Simatic TDC, a fast and dynamic compensation is created.
The system improves flicker values, balances and controls reactive power, and reduces harmonics. More than 100 SVC systems are in operation worldwide in industrial plants, including metals and mining operations, large compressor units, and other drive applications.
The latest generation
SVC PLUS uses voltage-sourced converter (VSC) technology based on a modular multilevel converter (MMC) design. It offers a high degree of flexibility in converter design and station layout. The direct connection to the load bus usually eliminates the need for a transformer and increases speed.
Robust and proven standard components ensure excellent reliability in the latest generation of STATCOM. A small physical footprint reduces turnkey costs and offers an ideal solution when limited space is available. With an extremely good flicker reduction in the range of 4.5 or more, low harmonic generation, and low losses, the SVC PLUS stabilises power supply even when fed by weak networks. These systems have been installed all around the world.
Very fast and accurate control of the compensating device is necessary in order to compensate fluctuating loads. The large number of series-connected sub-modules in the Modular Multilevel Converter provides the flexibility and response time to achieve an ideal waveform for this compensation task. The innovative concept enables a combination of high dynamic output behaviour with low switching losses. The sub-modules contain insulated-gate bipolar transistors, a capacitor in the DC link, and a bypass switch. Thanks to this solution, the safe and rapid bypass of a failed sub-module permits uninterrupted operation with inbuilt redundancy.
SVC in action
The demand for electrical power in India is growing rapidly. A new report, In the Dark: How Much Do Power Sector Distortions Cost South Asia?, published by the World Bank reported that in the last decade, total installed generation capacity in India increased from 154.7 GW in 2007 to 345.5 GW in 2018, making it the world’s third-largest producer of electricity, falling behind only China and the United States. More than 115 million people have gained access to electricity since 2013, increasing the share of population with access to electricity from less than 80% in 2013 to 86% in 2017.
A large portion of this growth comes from renewable energy with it making up just over 21% of the country’s total production. Despite this growth, the International Energy Agency (IEA) projects that electricity demand in India will almost triple between 2018 and 2040. In order to cover this growth in demand for electrical power and to improve the stability of the power grid, the Indian power supplier and grid operator Power Grid Corporation of India Limited (PGCIL) is modernising its high-voltage grid and preparing it for the challenges of the future.
Siemens supported PGCIL on this task with the design, engineering, installation and commissioning of three large Static Var Compensators of the SVC Classic series. The first and largest of the three SVC systems went into operation at the Ludhiana substation in the Indian state of Punjab, followed by a second SVC with a slightly smaller reactive power control range at the Kankroli substation in Rajasthan, both in 2016, with the third system at the New Wanpoh station in the state of Kashmir following in 2017.
The three 400 kV SVC systems at the Ludhiana (-400/+600 MVAr), Kankroli (-300/+400 MVAr) and New Wanpoh (-200/+300 MVAr) substations provide inductive or capacitive reactive power for the high-voltage grid in normal static operation. Depending on the grid fault situation, they also dynamically supply the reactive power required in order to reliably counteract grid undervoltage or overvoltage in the shortest possible time.
The SVC systems also provide the reactive power to enable the transmission voltage in the grid and in the substations to be maintained at an optimum value. The SVCs thus enhance the transient and dynamic stability of the power supply grid.
Making it mobile
Sometimes greater mobility is required, especially in grids that are susceptible to disruption. Such was the case for Dominion Energy in the US who are using Siemens SVC PLUS Mobile (Mobile STATCOM) solution that can be located anywhere in order to rapidly restore power grids.
The mobile STATCOM is Siemens’ multi-tool for transmission grids that enables temporary grid support and grid resilience against emergencies. The preconfigured SVC PLUS container combines major components in one housing and enables plug and play cable interconnection. A reactor container that contains the reactor coils including conventional bus bar arrangement complements it. It is the optimal solution for rapid service restoration after severe incidences, line rework, and grid changes, for example during power plant shutdowns.
The solution keeps the grid stable during disturbances and faults by providing fast and controlled reactive power. Mobile STATCOM gives the utility an unprecedented level of flexibility to move the device anywhere grid support is needed, especially in cases of unexpected outages from extreme weather, unplanned events or simply quick power plant retirements. Instead of several years of planning and executing a permanently installed substation project, the technology can be moved within days.
The flexible nature of the substation technology will also help Dominion Energy manage the growing number of renewable and distributed energy resources on its power grid. As these generation sources come online, they often change how a traditional grid operates. Temporary STATCOM installation helps keep the grid stable while the utility can allow enough time to plan for renewables on its system. The substation technology also responds to any faults on the transmission network within milliseconds.
There are numerous other examples from around the globe, across a variety of sectors. In Oman, Sohar Steel utilised SVC PLUS (STATCOM) modular multilevel converter technology to eliminate flicker. They had replaced an old arc furnace with a new one that was twice as powerful, and they required dual flicker reduction. SVC reacts very fast to abrupt load variations of large consumers to eliminate undesirable disturbances caused by the load back into the grid directly at the source. Sohar achieved a flicker reduction factor of between six and eight.
Statnett, the system operator of the Norwegian power system, required fast compensation to overcome instabilities on the existing high-voltage line. By using the containerised SVC Plus they minimised local construction and installation activities. The self-contained structure also provided optimal protection from the harsh environment, including vermin attacks.
In New York, the Long Island Power Authority used hybrid SVC technology, a combination of STATCOM and SVC technology, to stabilise the voltage and provide reactive power support to the grid in Long Island. The system allowed them to meet severe undervoltage performance requirements during summer peaks while achieving very low electrical losses and significant space savings. In order to extend the symmetrical operating range of SVC PLUS solutions, Siemens has developed the hybrid STATCOM, which allows asymmetrical operating ranges thanks to the additional thyristor switched reactor or thyristor switched capacitor.
Meeting future demand
According to the IEA, global electricity demand in 2018 increased by 4% or 900 TWh, growing nearly twice as fast as the overall demand for energy. This was also the fastest increase since 2010, when the global economy recovered from the financial crisis. Together, renewables and nuclear power met most of the increase in power demand.
With that demand predicted to continue the power grid will be stretched to its limits with maximum demand for reliability. Whenever an innovative and universally applicable solution for grid enhancement is required, SVC PLUS provides the ideal solution.
The advanced STATCOM with MMC technology provides additional benefits compared to conventional solutions for reactive power compensation. While conventional SVC installations already provide a highly dynamic solution for voltage regulation in the power grid, the modular multilevel system SVC PLUS is even faster and offers additional benefits.
Bernd Niemann is responsible for Business Development for FACTS systems at Siemens Energy and is based in Erlangen, Germany.