The electric grid is continuously going through an evolutionary process with the growing penetration of intermittent renewable power, multi-directional power flow, and demand-side management based on real-time data. This means a transition from centralized generation-based unidirectional power flow grid to a dynamic and fast-responsive grid with distributed generation.
If we take the U.S. electricity network as an example, grid modernization started with the introduction of the smart grid concept by the U.S. Department of Energy (DOE) in 2007, which aimed to embed digital technology in the grid. Additionally, given the current scenario of the U.S. power grid where huge investments are required to upgrade the aged infrastructure, it is a perfect example to discuss grid modernization. The International Energy Agency (IEA), in its energy sector review of United States (2014), estimated that an investment of US$2.1 trillion is required by 2035 to revamp the grid infrastructure. The question remains: Where exactly in the grid can this investment can be injected? Based on current dynamic, there are three possibilities:
- Continue to invest in old-fashioned, conventional synchronous generation and passive control-style grid
- Invest in new storage and a distributed generation-based grid
- A hybrid solution that leverages current infrastructure to achieve dynamic control, i.e. modernizing the existing grid
The only option that could be acceptable for all major stakeholders, i.e. policymakers, utilities and OEMs, is to go for investment in modernizing the existing grid infrastructure.
Power electronics as a tool for grid modernization
If we a look at the current design of the grid, power electronics already exists in several stages, including long-distance power transmission and grid-edge applications like power quality/control and renewable generation integration. There is one key area in which power electronics has not been able to penetrate yet: electrical substations. Transformers that are the heart of a transformation substation and all the protection equipment around them, currently are designed to operate at line frequency.
Solid State Transformers:
Large power transformers are an extremely critical part of the grid. However, they are costly, bulky and are typically loaded 10-60% with an increased total ownership cost for consumers. These issues can be addressed with a Solid-State Transformer (SST) as they are compact, lightweight, faster in response and efficient at light load conditions. SST is not a new concept as the terminology was first introduced by Navy researchers in 1980. SST originally was targeted toward the special applications like submarine or traction applications.
However, with the advancements in the semiconductor field, especially Silicon Carbide (SiC)-based power electronics devices, the concept of SST is expected to penetrate in grid applications. There are some prototypes developed using SiC-based semiconductor devices. General Electric has developed a 13.8kV/265kV 1MVA SST, which uses 10 kV SiC MOSFETs operating at 20 kHz frequency. Other prototypes include SST based on 15kV SiC MOSFET [4] and 15kV IGBTs.
The biggest advantage of a SST over conventional transformer is the reduction in footprint which is directly correlated with extremely long lead times of conventional transformers. Additionally, conventional transformers are designed to operate with one directional power flow and do not have a fast response time when it comes to voltage regulation. SSTs on the other hand, have a faster response time and can coordinate with other power electronics converters in the grid e.g. converters installed with intermittent renewable energy sources. High-speed controllability of the power flow is another advantage of SSTs; however, this is not a critical requirement in today’s AC grids as there are other competing technologies which can serve the purpose (e.g. tap changers). However, SSTs applications would be justified under an MVDC and LVDC (e.g. microgrids) grid paradigm or niche applications (e.g. traction or subsea) where the size or weight limits are crucial.
SSTs are still facing challenges regarding cost and losses when it comes to the commercial production of the device. It costs five times more as compared to conventional transformers and its efficiency has not surpassed the efficiency levels of conventional transformers yet. Additionally, at a component level, high voltage SiC technology is still not mature enough and faces challenges like packaging and gate driver design.
Solid-State Substations:
Transformers are not the only piece of equipment in a substation that could be impacted by these advances in power electronics. This concept extends beyond transformers toward other protection equipment in a substation, like circuit breakers and filters. A solid-state substation is defined as a substation that uses capabilities of high-power semiconductor devices to support the requirements of a modern grid with distributed energy resources. Such a substation would have the capability to channel energy bidirectionally while ensuring enhanced efficiency, grid security and better integration of DER in the network. The possible entry point of high-voltage power electronics in substations is at distribution level where already a hybrid AC-DC grid exists because of DER integration.
In the start, the role of such distribution substations would be supporting the existing infrastructure through power quality improvement and maintenance of critical grid parameters like voltage and frequency. Such ancillary services could prove more critical in the wake of Electric Vehicles (EVs) connecting with the distribution grid where power quality and a dynamic supply-demand response would be crucial for grid stability. This distribution grid infrastructure could also support peer-to-peer energy trading with the possibility of flexible energy routing at extremely faster response time compared to conventional grid infrastructure. Once applied at the distribution scale, penetration of power electronics could trickle-up and impact grid topologies at sub-transmission and transmission scale and provide grid resilience and support to relatively weaker networks.
Grid Inertia:
Inertia can be viewed as stored rotational energy hidden in the network and it plays an essential role in rescuing in case of system loss.
Right now, roughly 70% of system inertia is provided by conventional power generation units in a typical stable network. With increasing share of renewables, inevitably there will be less contribution from conventional power plants towards rotational inertia. A decrease in grid inertia means dynamic system changes requiring adaptive protection systems for the grid.
Experience of System Operators
North America:
NERC reported a decline in system inertia because of the increased number of inverter-based generation interconnections. In the United States, RTOs have also demonstrated disturbance in system frequency due to increasing power electronics-based distributed generation. CAISO suggested that with increasing penetration, the level of DER (roughly 30%) will drive the system inertia low to the level where system faults can force the grid frequency lower than standard. Similarly, ERCOT studies have shown lower system inertia because of high wind generation, which has replaced conventional generation sources.
Europe:
ENTSO-E has analyzed the impact of reduced system inertia on grid operation in EU and has listed this issue as one of the three focus areas for R&D. National Grid also carried out studies to understand the impact of a large amount of wind power connected with UK’s power system and concluded that there has been a reduction of inertia because of fewer number of synchronous generators. These case studies and simulation results from system operators show that system inertia is a concern for many transmission grids with a high penetration of renewables (specifically wind generation).
Synthetic Grid Inertia
To mitigate the loss of inertia, power electronics can be used to mimic the operation of conventional power plants and provide 'virtual' or 'synthetic' gird inertia. Grid inertia can be modeled as stored mechanical energy, so energy storage devices like batteries, flywheels, and supercapacitors, along with power electronics-based inverters can be used to fulfill the missing rotational inertia specifically in solar applications. In the case of wind turbines, where kinetic energy exists at variable speed, advanced control techniques are proposed to emulate the behavior of synchronous generators. Grid inertia support through power converters can change the perception of renewables from a liability to a contributor toward grid stability.
Recent advancements in materials promise substantial enhancements in operational capability of traditional power electronics. However, improvements in design and manufacturing process are required for new materials to compete with existing technologies in terms of cost. Utilities around the globe seem to have an open-minded approach toward innovative ideas around the field of power electronics, which has resulted in joint initiatives involving utilities, OEMs and academia/R&D institutes to cope with design bottlenecks. However, PTR believes that due to the conservative nature of T&D market, the journey from the ‘pilots’ to the field application (substations) for SiC devices will take more time than its penetration time in other applications such as traction, grid-tied converters and electric vehicle charging infrastructure. Utilities and system operators will sit-back and observe the success and reliability of SiC based power electronics in these applications before moving forward with its widespread installation in grid systems.
This article is an extract of analysis from Power Technology Research’s grid equipment market research.
Power Technology Research is a T&D World media partner.
References
[1] Department of Energy, "Enabling modernization of the electric power system," 2014.
[2] Solid state transformer concept development, Naval Material Command, Civil Engineering Laboratory, 1980.
[3] "Overview of high voltage sic power semiconductor devices: development and application," CES Transactions on Electrical Machines and Systems, vol. 1, no. 3, pp. 254-264, 2017.
[4] "Medium voltage solid state transformers based on 15 kV SiC MOSFET and JBS diode," in Industrial Electronics Society, IECON 2016-42nd Annual Conference of the IEEE, IEEE, 2016, pp. 6996--7002.
[5] "Solid-state transformer and MV grid tie applications enabled by 15 kV SiC IGBTs and 10 kV SiC MOSFETs based multilevel converters," IEEE Transactions on Industry Applications, vol. 51, no. 4, pp. 3343--3360, 2015.
[6] "Silicon Thyristors for Ultrahigh Power (GW) Applications," IEEE Transactions on Electron Devices, vol. 64, no. 3, pp. 760-768, 2017.
[7] "20 kV, 2 cm 2, 4H-SiC gate turn-off thyristors for advanced pulsed power applications," in Pulsed Power Conference (PPC), 2013 19th IEEE, IEEE, 2013, pp. 1-4.
[8] "2.4 Report. Operating Practices, procedures and tools," North American Electric Reliability Corporation (NERC), 2011.
[9] "System inertial frequency response estimation and impact of renewable resources in ERCOT interconnection," in Power and Energy Society General Meeting, 2011 IEEE, IEEE, 2011, pp. 1-6.
[10] ENTSO-E, "Frequency Stability Evaluation Criteria for the Synchronous Zone of Continental Europe," European Network of Transmission System Operators for Electricity, 2016.
