In Europe, Asia and the Americas grid operators are being challenged by fundamental changes in the production and use of electricity. Renewables, electric motors and electric vehicles are being connected to the grid at an ever-increasing pace, requiring support for voltage stability, reactive power and other ancillary services. Adapting to these changes is a strong case for the use of STATCOM technologies.
Right across the modern world a series of broad trends in the production and usage of energy are having profound effects on electricity transmission networks.
One of the biggest changes is the development and growth of modern renewable energies such as wind and solar photovoltaics (PV). Despite the dramatic expansion in its use over the last decade and more, this is a trend which is still gathering pace.
In California, ranked as one of the top ten global economies if taken alone, Governor Edmund Brown recently signed the so-called SB100 bill setting out a 100% target for renewables in electricity generation by 2045. This is in addition to the state’s existing renewables portfolio standard, which mandates 50% renewable electricity in 2025 – just 6 years.
Considering the growth in renewables worldwide, according to the energy policy network REN21’s Renewables 2018 Global Status Report renewable power accounted for 70% of net additions to global power generating capacity in 2017, the largest increase in modern history. And at 98 gigawatts (GW), solar PV additions alone equated to more than the new gas, coal and nuclear additions combined.
There are changes in consumption patterns too. Even in transport – a sector closely associated with fossil fuels – increasing electrification is offering possibilities for renewable energy uptake with around 1.2 million passenger electric cars sold in 2017, up about 58% from 2016.
However, as variable output renewables like wind and solar replace conventional thermal generation, the ability of the grid to respond to fluctuations in supply and demand is becoming far more difficult. There is, for instance, the potential for voltage excursions in solar heavy areas during changing cloud cover, but also if bigger loads or generation trips out.
An additional challenge comes with the increase in inductive grid loads from sources like motors and drives in HVAC, fans, pumps and compressors. New analysis from market researcher PS Research indicates the global market for electric motors will reach USD 135.2 billion by 2022 at a compound annual growth rate of more than 6%, for example.
The rapid electrification of energy demand and the rise of renewables will also lead to massive growth in electricity transmission and distribution systems, according to the classification society DNV GL’s Energy Transition Outlook 2018: Power Supply and Use report. They predict that the total installed power line length and capacity will more than triple by 2050.
However, DNV GL also concludes that the system operators’ tasks will become substantially more complex. Indeed, although renewable energy can supply reactive power, the challenges of exerting control over a vast number of small, widely distributed generation sources is not to be underestimated.
Faced with maintaining frequency and voltage across a far bigger and much more dynamic grid, transmission and distribution system operators are turning to new technologies that can support network stability.
Power electronics enabling new grid stability options
The rise of renewables, Electric Vehicle growth and other key energy trends has coincided with major breakthroughs in solid state power electronics, for example, the emergence of Flexible AC Transmission Systems (FACTS) based on modular multilevel voltage-sourced converters (VSCs).
Using relatively few robust and proven components – such as typical AC power transformers, reactors, capacitors and industrial insulated-gate bipolar transistors (IGBTs) – designs have emerged that can be operated as static synchronous compensators (STATCOM). Today, this technology has established itself as the state-of-the-art dynamic shunt compensator for reactive power control in transmission systems.
Precise control of reactive power is vital to the operation of the grid. Conventional synchronous generating facilities like gas-fired power stations have traditionally provided the required reactive power. However, as the ratio of non-synchronous renewable capacity to conventional synchronous power has changed, this capability is being limited. Today, if a large power thermal plant is decommissioned system studies have determined the amount of reactive power needed to replace this, typically of the order of several hundred Mega Volt Amps Reactive.
Siemens’ STATCOM – branded SVC PLUS – uses modular multilevel converter (MMC) architecture and offers the ability to supply reactive power within milliseconds, control the voltage precisely, damp power oscillations and support load flow control in AC transmission grids. Generating a near perfect sinusoidal waveform with an appropriate level of lead or lag in phase angle and completely independently of the connected AC system voltage, such systems can modulate the network voltage. These high-speed control and intervention capabilities ensure stability of the system, reducing network faults and malfunctions.
As a result of MMC harmonic performance, there is also no need for supplementary harmonic filtering. Providing voltage control and support, reactive power control, power oscillation damping, and increased power transfer capacity, a STATCOM dramatically improves the dynamic stability and power quality of transmission systems and reduces the risk of voltage collapse. For example, where large loads and generators are some distance apart a large change in either supply or demand can result in power oscillation across the grid. SVC PLUS can effectively damp that oscillation, making the system more robust when it comes to disturbances.
To date, Siemens has supplied around 80 MMC installations around the globe. For example, realising the benefits of this kind of ancillary services capability, German network operator Amprion has already installed an SVC PLUS reactive power compensation system to stabilize the grid in the Greater Frankfurt Area. Located in Kriftel in the Rhine-Main region between Frankfurt and Wiesbaden, the unit is one of a number of dynamic compensation systems being used to stabilize the Amprion grid.
Maximising clean energy, ensuring grid stability
Applying MMC architecture and SVC technology across a number of different network areas, Siemens has developed solutions to other modern grid challenges, for example, the renewable resource location issue.
The best renewable energy resources – pumped storage hydropower say – are pegged to locations and geographical conditions. Typically, these resources are located far from demand centres. A large proportion of Germany’s wind power capacity is located offshore in the North Sea, for instance, while the country’s major industrial centres, and hence power demands, are in the South.
The solution is transmission, but with AC systems the losses are such that long distance transport is not an economically or technically feasible proposition beyond about 600 km via overhead lines.
An alternative solution comes from High Voltage Direct Current (HVDC). Developed over a number of decades – Siemens, for example, has commissioned more than 50 HVDC systems worldwide to date – in HVDC systems a converter station at either end of a DC line switches AC to DC and back again. HVDC enables efficient energy transfer over long distances, but the converters also effectively isolate unwanted network effects – eliminating frequency differences between networks for example.
Branded as HVDC PLUS, Siemens’ latest generation of HVDC power transmission technology also uses an MMC design. The world’s first such system – the Trans Bay Cable Link in San Francisco, California – was installed in 2010.
Using this design, the commutation processes in the converters run independently of the network voltage and both converter stations can be also used to support stable voltage and deliver both active and reactive power.
The demand for long distance inter-regional and cross-border interconnections is a key driver for new HVDC projects.
Several new interconnectors are also underway in Europe, boosting overall network resiliency and potentially addressing issues such as the negative power prices that have been seen in Germany when renewables are at peak capacity and exceed domestic demand. Nemo Link, a joint venture between the network operators National Grid (United Kingdom) and Elia (Belgium), is connecting both networks with an HVDC PLUS 140 km long underground link based on converter technology from Siemens, for instance. The project is in commercial operation since 31 of January 2019.
Following the development of MMC HVDC system architecture, last October Siemens debuted its Medium Voltage Direct Current (MVDC) transmission technology for power ranges of around 30 to 150 MW. Significantly, like its HVDC PLUS cousin, MVDC also uses a modular multilevel VSC design and offers the ability to control, optimize, and regulate load flows in medium-voltage AC grids.
The lower voltages seen in the MVDC approach allow a far more compact design with distances between conductors and earth potential far shorter, for instance. Smaller transmission towers are possible, avoiding the investment associated with stepping up the voltage, while medium voltage components are naturally cheaper than their high voltage equivalents, too.
In addition, HVDC developments tend to be precisely designed for specific applications. Siemens has developed MVDC as a product solution, offering the system in three standard power transfer capacities – reducing the lead time as well as the cost of additional engineering.
Suitable for connecting and stabilizing distribution grids, regardless of their voltage and frequency, applications include connecting small communities in sparsely populated regions, urban grid reinforcement, microgrids and a host of others. Distances of up to 200 km or more can be bridged with Siemens’ compact MVDC PLUS technology at voltages of 20-50 kV. By enabling regulated power exchange between networks, MVDC also builds stability and ensures security of supply.
Even more recently, Siemens launched its SVC PLUS Frequency Stabilizer system which offers voltage control but also supports frequency stabilisation by combining SVC PLUS technology with supercapacitors to deliver up to 200 MW of stored energy. By using supercapacitors in bulk, the new SVC PLUS Frequency Stabilizer is a cost-efficient, compact solution that can emulate system inertia by boosting high active power into the grid when needed. This results in frequency stabilization.
New tools for new needs
The fundamental transition of the energy system is already in full swing and even more dramatic changes in electricity supply and use are anticipated as technologies like battery storage and electric vehicles mature. Meanwhile, megatrends like ever greater volumes of renewables, increasing electrification and more motors, drives and other energy demands are changing network function for ever.
In order to ensure the future grid meets our evolving needs, mechanisms for control that can flexibly and rapidly respond to high levels of distributed and fluctuating generation and ever-changing consumption patterns are required. Voltage-Sourced Converters are a key part of a suite of new solutions that can help achieve the primary goal of network stability.
Tobias Tepe is Head of Technical Sales for FACTS and Substations at Siemens Gas and Power and is based in Erlangen, Germany.