The electric grid is continuously going through an evolutionary process with 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 US electricity network as an example, grid modernization started with the introduction of the smart grid concept by US Department of Energy (DOE) in 2007, which aimed to embed digital technology in the grid. Additionally, given the current scenario of US electrical grid where huge investments are required to upgrade the aged infrastructure, it is a perfect example to discuss the grid modernization. The International Energy Agency (IEA) in its energy sector review of United States (2014) estimated that an investment of $2.1 Trillion is required by 2035 to revamp the grid infrastructure in the US. The question remains that where exactly in the grid this investment can be injected? Based on current dynamics of the US grid, there are three possibilities:
1- Continue to invest in old-fashioned, conventional synchronous generation and passive control style grid
2- Invest in new storage and distributed generation-based grid
3- A hybrid solution which 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. Additionally, replacements in the US grid is a necessity given some extreme weather events in the recent past. So, achieving resilience in the grid is an extremely crucial factor in achieving grid modernization.
Grid modernization and grid resilience:
Grid resilience is a hot topic for utilities these days; however, just like smart grids, grid resilience is an ambiguous term. Resilience is to plan and prepare the grid for low probability high impact events with an aim to isolate the affected area and keep the downtime to a minimum. The question arises of why it is being discussed among utilities and system operators? There is a need to re-evaluate the operational capacity of our grid because of increasing share of Distributed Energy Resources (DER) and aging grid infrastructure. Grid infrastructure is aging specifically at the transmission level. As described in a Department of Energy report [1], nearly 70% of power transformers are 25 years or older and roughly 60% of circuit breakers are 30 years or older. Additionally, some intense weather events in recent past which have extreme damages to the grid infrastructure is another factor causing grid resilience to be discussed frequently.
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 which are the heart of a transformation substation and all the protection equipment around them, currently are designed to operate at line frequency.
a) 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, light-weight, faster in response and efficient at light load conditions. But we will come back to the benefits of SST, first let’s look at what has already been achieved in this field. SST is not a new concept as the terminology was first introduced by Navy researchers in 1980 [2]. SST originally was targeted towards 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 10kV SiC MOSFETs operating at 20kHz frequency [3]. Other prototypes include SST based on 15kV SiC MOSFET [4] and 15kV IGBTs [5].
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 with intermittent renewable energy sources. Plus, oil-free nature of SST can help fulfil ECODESIGN regulations enforced by European Union. Research projects like ‘SPEED’ show the amount of investment poured in by EU to facilitate the development of new materials to achieve high efficiency in grid applications. 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.
b) Solid State Substations:
If we look at the overall T&D market, transformers are not the only piece of equipment in a substation which could be impacted by these advances in power electronics. This concept extends beyond transformers towards other protection equipment in a substation like circuit breakers and filters. A solid-state substation is defined as a substation that utilizes 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 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.
c) UHVDC and multiterminal DC grid:
From 2009 onwards started the era of Ultra High Voltage Direct Current (UHVDC) primarily because of multi GW HVDC transmission projects constructed by China. Roughly 60% of the HVDC projects installed after 2010 were 3GW or more in China. State Grid of China (SGCC) plans to spend roughly $90B just in UHVDC interconnections from 2009 to 2020, and it aims to make more than 20 UHVDC transmission links operational by 2030. The Changji-Guquan UHVDC link, which is expected to be commissioned by 2019, will set new records in voltage level, transmission capacity and transmission line length. This 1100 kV UHVDC link is expected to deliver 12GW of power through DC transmission lines spread across 3000 km. It has the capability to transfer 50% more power than 800kV UHVDC transmission links.
Uneven geographical distribution of renewable energy resources and long distances between generation and load centers is going to result in more bulk power transmission projects and cross-border grid connections in future. What role high power semiconductor devices are playing in breakthroughs of high power UHVDC? With current materials and technologies in use, is there an upper limit of voltage and capacity? Right now, state-of-the-art thyristor devices with highest power handling capability are utilized for UHVDC transmission. VSC based systems still lack in terms of on-state losses hence they still are not a strong contender for long distance UHVDC systems. Silicon based high power thyristors have served the industry for over four decades through a continuous evolution of voltage and current ratings. Now, thyristors with highest voltage withstand limit of 8.5 kV and current rating up to 6kA are used commercially [6]. However, Silicon-based thyristors are reaching the physical limit in terms of blocking voltage capability, current and rate of change of voltage. As discussed in the last section, wide-bandgap SiC is a promising alternative which can overcome the limitation of Si based devices. Theoretically, Silicon Carbide’s ten times higher breakdown electric field allows the switching devices to be designed at much higher breaking voltage. Highest blocking voltage that has been achieved by SiC-based devices is 22kV [7]. However, these high voltage devices are under development right now and are not available commercially. With such fast developments in SiC based devices, ultra-high voltage SiC thyristors are promising for next generation of HVDC converters.
d) Grid Inertia:
Traditionally most of the generation comes from conventional power plants based on rotating synchronous generators which provide rotational inertia to the grid. Gird frequency, which determines the stability of a grid, is directly coupled with net rotational inertia present in the grid. 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 i.e. opposing a change in system frequency resulting from an imbalance in production and consumption. Less grid inertia due to a higher share of inverter connected renewables in the modern grid can directly impact grid stability.
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. i.e. faults must be cleared much faster to comply with grid codes regarding frequency change.
Experience of System Operators:
North America:
The North American Electric Reliability Corporation (NERC) reported a decline in system inertia because of the increased number of inverter-based generation interconnections [8]. In US States, Regional Transmission Organizations (RTOs) have also demonstrated disturbance in system frequency due to increasing power electronics based distributed generation. California Independent System Operator (CAISO) suggested that with increasing penetration level of DER (roughly 30%), there will drive the system inertia low to the level where system faults can force the grid frequency lower than standard. Similarly, Electric Reliability Council of Texas (ERCOT) studies have shown lower system inertia because of high wind generation which has replaced conventional generation sources [9].
Europe:
ENTSO-E has analysed the impact of reduced system inertia and its possible impacts on grid operation in EU [10] and ENTSO-E 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 less 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 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. As discussed earlier that grid inertia can be modelled as stored mechanical energy, so energy storage devices like batteries, flywheels, and supercapacitors along with power electronics-based inverters can be utilised to fulfil the missing rotational inertia specifically in solar applications. In case of wind turbines, where kinetic energy exists at variable speed, advanced control techniques are proposed to emulate the behaviour of synchronous generators. Grid inertia support through power converters can change the perception of renewables from a liability to a contributor towards grid stability.
Conclusion:
This article encompasses the role of power electronics in the ongoing process of grid modernisation. 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 towards 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. Additionally, grid stability driven revised grid codes are also enforcing OEMs to implement better control techniques on existing power electronics-based systems.
References:
[1] Deprtment of Energy, “Enabling modernization of the electric power system,” 2014.
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[8] N. I. Task, “2.4 Report. Operating Practices, procedures and tools,” North American Electric Reliability Corporation (NERC), 2011.
[9] S. a. H. S.-H. a. S. N. Sharma, “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.