Grid 4.0 – Leveraging Power Electronics
Published on : Thursday 03-12-2020
Increasing penetration of power electronics based converters and a host of other factors is leading to an autonomous and self-healing Grid 4.0, says P K Agarwal.
The electric power grid is the largest man-made machine in the world. In any country, the grid is made of thousands of generating machines, many kilometres of transmission lines and many distribution feeders to supply electricity to its people.
The seed of the present electric grid was born with the beginning of the first power plant with six generators (called dynamo) on 4th September 1882 at Pearl Street in New York. The distribution system was of about 80,000 ft of underground cable installed in manhole and conduit and comprising two wires in the form of twin half-moon shaped copper conductors for lighting incandescent lamps. Similar to this, DC low-voltage central-station electric systems were built in other parts of New York City, and many were licensed for installation in cities and towns throughout North America, Europe, South America and Japan during the next decade. With this, Version One of electricity, i.e., Grid 1.0 born.
However, a low-voltage DC system has inherent disadvantages, the chief one being high line losses that limit the distance that the DC electric power can be economically transmitted. By the mid-1880s, alternating current (AC) systems competed with Edison's DC system. The invention of the AC transformer permitted the economical long-distance transmission of electric power at high voltages, resolving the major disadvantage of low-voltage DC systems. By the end of 1887, Westinghouse had 68 alternating current power stations. The advantages of AC electric utility service became obvious, and, by the end of the 19th century, DC systems began a gradual and inevitable decline. The AC systems of generation, transmission and distribution along with direct AC consumption or converted to DC consumption remain prevalent for many years.
In the 20th century, local grids grew over time and were eventually interconnected for economic and reliability reasons. By the 1960s, the electric grids of developed countries had become very large, mature and highly interconnected, with thousands of 'central' generation power stations delivering power to major load centres via high-capacity power lines which were then branched and divided to provide power to smaller industrial and domestic users over the entire supply area. This was the era of Grid 2.0.
Towards the end of the 20th century, the advantage of information and communication technologies (ICT) for technical and economic advantage of managing the grid become apparent. The ICT enabled the managing peak demand with without additional requirement of low utilisation peaking generating units. Application of ICT in managing the metering and accounting of the electricity proliferated the automated meter reading later on automated metering infrastructure. The grid transformation started taking place towards Smart grid and thus an era of Grid 3.0 begun.
Worldwide growing concerns over environmental damage from fossil-fired power stations led to a desire to use large amounts of renewable energy since mid-20th century. Generation from wind and solar, being highly variable, necessitated more sophisticated control systems to facilitate the connection of sources to the otherwise highly controllable grid. The rapidly falling costs point to a major change from the centralised grid topology to one that is highly distributed, with power being both generated and consumed right at the edge of the grid. Along with generation from wind and solar, other forms of clean energy resources like battery storage, electric vehicles, biomass, etc., are sprouting everywhere in the quest of combating the climate change.
Most wind turbines generate electricity at variable frequencies and require power electronic converters to interface the generation with the grid. Photovoltaic solar panels generate DC, which needs to be converted into AC to make it compatible with the existing grid. Similarly, electric vehicles and battery storage systems require power electronic converters to integrate their generation and consumption with the grid.
The present power grid is in a transition state. It is transforming from information and communication technology dominated grid to the grid where the majority of elements will be connected via power electronics. This transition is posing many challenges, as the conventional grid was never designed for non-synchronous connected DERs (distributed energy resources) and loads. Reduction in inertial response capability of the grid is affecting its robustness against frequency variation. Higher fault levels are associated with stronger power systems because of generation sources contributing fault current. But PE (power electronics) connected sources does not contribute to fault current, resulting in a reduction in system strength. The power injected by conventional synchronously connected machines maintains synchronism and damps mechanical oscillations through their synchronising and the damping torque components of the total electric torque. Because of increasing penetration of the non-synchronous connected machines decrease in total synchronous torque from synchronous machines poses challenges in mitigation of the large active and reactive power imbalances in the grid.
Presently, approximately 30% of all electric power generated utilises power electronics somewhere between the point of generation and its end use. By 2030, it is expected that perhaps as much as 80% of all electric power will use power electronics somewhere between generation and consumption. Putting all the above together, future power systems will be power electronics based, instead of electric machines based, with a huge number of relatively small and non-synchronous players at the supply side, inside the network and at the demand side.
Good news is that inverters allow for the control of active and reactive power independent from each other. With proper design of controller, they can also provide synthetic inertia like response. Wind turbines are capable of injecting additional active power into the grid by extracting the energy stored in the rotating mass of blades and generators. PV inverters can also provide inertia-like response. Energy storage can also be programmed to modulate its active power to mimic the inertial response of rotating machines. In addition, inverter-based generators have superior fault ride-through performance. With the proper converter design, wind, PV, and storage inverters can ride through various types of balanced and unbalanced under and over voltage faults and frequency excursions, thus improving the overall reliability of a power system. If desired, they can also inject desired levels of reactive current during the fault to assist in faster post fault voltage recovery.
Further, the power electronics inverter connected DERs can provide many ancillary services from which the grid is being deprived in transition state. The following and many more other ancillary services could be provided by improving the DER inverter and its control methods:
1. Voltage control and reactive power compensation: DER can be used for injection or absorption of reactive power to maintain transmission system voltages within required ranges.
2. Frequency regulation: DER current control can provide higher amounts of real power as the frequency of the system decreases, or smaller amounts of real power if the system frequency exceeds the nominal frequency.
3. Spinning reserve: Spinning reserve is capacity margin in the synchronised generations to the grid, so that the generating equipment can increase or decrease the output immediately in response to changes in frequency. Most DER can perform spinning reserve and respond in less than 10 seconds.
4. Non-spinning reserve: Offline fast start generation capacity available to correct generation/load imbalance is called non-spinning reserve. Most DER systems can respond in just 2 or 3 minutes from a completely turned off state.
5. Network stability: Network stability is the use of special equipment like power system stabilisers or dynamic resistors to help maintain transmission system reliability. Control of FACTs, energy storage, DER import/export, could provide network stability functions.
In addition, power electronics devices in transmission like HVDC and FACTs are already being used for increasing transmission capacity, reactive power compensation, etc.
Increasing domination of phasor measurement units and new research in Wide Area Monitoring, Protection and Control (WAMPAC) and AI/ML for monitoring and control of the grid from central control room based on a wide view of the grid will further contribute towards a next version of power Grid 4.0
Further, research is underway to mimic the fundamental synchronisation mechanism of a synchronous machine while maximising the benefits of power converters. This will enable fast responses and flexible controllability by emulation of conventional synchronous machine inside the inverters connecting DERs. Research in smart technologies at the consumer level is driving dramatic changes to the power system that will significantly transform how power is made, delivered, and used.
The continuous increase in penetration of power electronics based converters and leveraging its inherent capability of emulating conventional grid services along with use of information and communication technology for controlling these services from remote is leading to an autonomous and self-healing grid called Grid 4.0.
References
1. https://ethw.org/w/images/a/ae/Edison_and_Pearl_Street%2C_Text%2C_031410.pdf, retrieved on 03-Nov-2020
2. https://en.wikipedia.org/wiki/Smart_grid, retrieved on 05-Nov-2020
3. Kroposki et al., "Achieving a 100% Renewable Grid: Operating Electric Power Systems with Extremely High Levels of Variable Renewable Energy," in IEEE Power and Energy Magazine, vol. 15, no. 2, March-April 2017, pp. 61-73,
4. HALLEY et al, “Effects of increasing power electronics based technology on power system stability: performance and operations” Cigre Science & Engineering • N°11 June 2018, pp 5-15.
5. Qing-Chang Zhong, Power-Electronics-Enabled Autonomous Power Systems: Architecture and Technical Routes, IEEE Transaction on Industrial Electronics, Vol. 64, N0. 7, July 2017
Praveen Kumar Agarwal, former Director & CISO, POSOCO Ltd, has 39 years of experience in diverse areas of power sector, of which 24 years in electricity market design and operations, systems automation, WAMS & SCADA system integration and cyber security with active involvement in project execution and management of Supervisory Control and Data Acquisition (SCADA) system, and Integrated National Control Centre’s SCADA with regional control centres and with control centres of countries like Bhutan and Bangladesh. Agarwal pioneered synchrophasors technology (WAMS) in Indian power system in the year 2009.
Among other achievements Agarwal played a key role in designing the Unified Real Time Dynamic State Measurement Scheme and setting up of Renewable Energy Management Centres in India. He has written and published over 50 technical papers and articles, and has contributed chapters in power system books published by international publishers. He has also delivered talks in many International and National conferences, chaired panel sessions, etc.