Smart Transformers as the Central Control Point in Distribution Grids


Prof. Marco Liserre, Dr. Rongwu Zhu, Dr. Giovanni De Carne, Chair of Power Electronics Christian-Albrechts-University of Kiel; Anthony Donoghue, Scottish Power Energy Networks, UK; Ali Kazerooni, WSP, UK

The Smart Transformer represents a central control point in the distribution grid: the high controllability that it introduces can defer the grid infrastructure upgrades and, at the same time, offer ancillary services.

 

The Definition of a Smart Transformer

Distributed generators (DGs) and EV charging stations are steadily increasing their presence in the distribution grid. It has been proven that it can be more efficient to supply DGs—such as PV panels, fuel cells and micro-wind turbines—and DC loads—such as EV—with a DC network. This may attract the attention towards hybrid AC/DC distribution grids. However, the conventional distribution grid experiences a series of challenges in terms of voltage limit violations (upper and lower ones) and overload of network assets (e.g., circuits and transformers). In order to alleviate these issues, distribution system operators conventionally reinforce the grid by increasing the ratings of existing components, like larger size cables or larger capacity transformers. However, these upgrades are costly, time-consuming and may lead to disruptions for customers.

Currently, there is not a comprehensive solution to the distribution grid's needs. Due to widely distributed power converters, the research trends in smart grids are towards a decentralized scenario, which leads to many solutions for system control, as shown in Figure 1(a). These solutions include STACOM, storage, DC distribution converter, inter-area connection converter, and DGs injecting reactive power. However, coordinating all these smart solutions can be challenging, with the need of an extensive fast communication infrastructure.

 

Configuration of current distribution systems

Figure 1a. Configuration of current distribution systems

 

Configuration of ST-based distribution system

Figure 1b. Configuration of ST-based distribution systems

 

The Smart Transformer (ST) solution (Figure 1(b)), a solid-state transformer-based transformer, introduces higher grid controllability and provides DC connectivity to loads and generators and at the same time saves the infrastructure cost avoiding extensive grid upgrades [1].

Compared to the current distribution grid scenario (Figure 1(a)), all the functions, such as DC distribution, STATCOM/ Storage integration making unneeded or minimize the reactive power integrations from integration and enabling inter-area connection, can be integrated inside the ST. Moreover, the ST represents a solution to implement a semi-decentralized control of the electric grid, where the ST receives the information from the downstream grid and acts as a unique control point for the main grid. This avoids the drawbacks of an extreme-decentralization, and thus the complexity of managing large data inputs, actors, controls and decision-making options.

Several ST architectures are possible for the MV-front-end stage: low-frequency transformer combined with an AC/DC converter (T1); AC/ DC converter combined with an isolated DC/DC converter (T2) and isolated AC/DC converter (T3), which are selected respect to the DC network demands in real electric system, as shown in Figure 2.

 

Detailed structure of ST: (T1) low frequency transformer and back-to-back solution; (T2) AC/DC converter and DC/DC stage; (T3) isolated AC/DC converter.

Figure 2. A detailed structure of ST: (T1) low-frequency transformer and back-to-back solution; (T2) AC/DC converter and DC/DC stage; (T3) isolated AC/DC converter.

 

Smart Transformer Services

The ST acts on three different levels: on MV grid, on LV grid and on DC grids. In this case, a 3-stage topology ST has been considered, in order to show the full extension of services that it can provide [1]. In MV grid, the ST controls the AC active current in order to maintain the voltage at the nominal value in the MV DC link. This implies that the ST can regulate the reactive power injection independently from the active power, always respecting the converter ampacity limits. This allows to support the voltage amplitude in the MV grid, and eventually alleviating the overload of the main transformer, reducing the import of reactive current from the HV grid. Furthermore, it can inject higher frequency currents, both active and reactive, working as a harmonic compensator for the MV grid.

The DC/DC converter regulates the power flow from the MV to the LV DC link in order to control the voltage in the LV DC link. The presence of DC links can be used as first step for a DC grid infrastructure, where larger DC loads (fast-charging electric vehicles stations) and generators (wind farms) can be connected directly in MV DC and small resources, such as household PV, DC street lighting, and slow charging electric vehicle stations, can be connected in the LV DC link.

The LV side converter controls the AC voltage waveform to be sinusoidal and balanced. The ST, in the LV grid, has large possibility to provide services. The possibility to vary the voltage amplitude and frequency enables to shape the fed loads consumption and generators production, allowing fast load control dynamics. For example, it can reduce rapidly the load consumption by means of voltage reduction with the Soft Load Reduction control, offering an alternative to the firm load shedding during large power system perturbations. Alternatively, the ST can operate with the frequency, interacting with the droop characteristics of the local generators, in order to reduce (or increase, if possible) their power production. It has been shown that the reverse power flow from LV to MV grid can be avoided using the frequency control in the LV grid.

The ST can offer also advanced protection features, clearing rapidly a fault (<10ms) and continuing to supply the loads not affected by the fault. For example, in case of single-phase fault, the ST can continue to energize the remaining two phases at nominal voltage, despite one phase is cleared out.

Furthermore, the ST can work as active damper in LV grids. In presence of many small DGs, resonance phenomena may occur. The ST can actively damp these resonances, acting on its own control, without the need of additional hardware in the grid (e.g., active dampers).

 

A Business Case for the ST: the LV-Engine Project

In the UK, the distribution networks are already experiencing growing connections of electric vehicles, heat pumps and photovoltaics. This has caused existing network assets, e.g., distribution transformers, reach their thermal ratings or voltage violation from statutory limits are experienced in the LV network. Over-voltage and under-voltage conditions may be experienced at different times of the day depending on different levels of demand/generation.

The conventional approach practiced by Distribution Network Operators (DNOs) is to reinforce networks to alleviate network issues and comply with the grid code requirements for quality and continuity of the supply to customers. The traditional reinforcement base case includes the replacement of the transformer with a larger one (e.g., replace a 500kVA with an 800kVA transformer), which is capable of satisfying the additional demand.

In addition, the LV cables should be replaced with larger size cables, in order to maintain the voltage within the statutory limits. Whilst these solutions will facilitate the load growth, they are prohibitively expensive, time-consuming and may cause disruptions due extensive costly excavation of public roads and pavements.

 

The LV-ENGINE project: possible substations interconnection strategy developed in the project

Figure 3a. The LV-ENGINE project: possible substations interconnection strategy developed in the project

 

LV-ENGINE project: the number of foreseen STs in the UK grid.

Figure 3a. LV-ENGINE project: the number of foreseen STs in the UK grid.

 

As an alternative approach, SP Energy Networks, one of the UK DNOs, is planning to trial ST within 11kV/0.4 secondary substations [2]. This project, LV Engine, aims to demonstrate that ST can be used for overcoming the overload and voltage issues within LV networks. The ST, providing higher controllability and more services in the distribution grid, introduces several advantages:

  • Reduction in network charging costs: the ST will reduce the network charging costs imposed on customers by avoiding and deferring the costly network reinforcement required in both the LV and MV grids.
  • Facilitate access to low-cost energy: the ST acts as an enabler of PV connection, due to its voltage regulation feature and availability of DC grid connection.
  • Providing scalability to secondary substations: the modular nature of ST allows to increase the substation capacity with limited cost and disruption to customers, by adding additional hardware blocks to meet the demand increase.
  • Enabling the transition to DSO: the ST increases the flexibility and adaptability of the LV network. This provides DSO with the tools requires to intelligently and efficiently operate the distribution grid and also delays the point at which the DSO is required to interact with customers to remove local constraints.

The initial estimation showed that there can be considerable deployment opportunity for STs within the UK distribution network, reaching to around deployment within 16% of existing secondary substations by 2050. This estimation has considered other potential smart solutions that may deliver part or full functionalities of ST. The initial cost-benefit analysis showed that by deploying STs, there can be a total savings of £62m by 2030 and £528m by 2050 at the national level.

 

The 100kW Prototype

A 100kW prototype (Figure 4(a)) has been built in the laboratory of the Chair of Power Electronics at Christian-Albrechts Universität zu Kiel. It consists of a Combined H-Bridge converter at the MV side, connected with quadruple-active-bridge (QAB) module in each phase as shown in Figure 4 (b).

 

100kW tested prototype in a laboratory

Figure 4a. 100kW tested prototype in a laboratory 

 

100kW tested prototype configuration

Figure 4b. 100kW tested prototype configuration

 

The LV DC/AC converter is not currently included in this prototype due to the higher focus on the challenges in the MV and DC/DC stage. Furthermore, the technology for the LV DC/AC converter is already largely available in the market, and thus ready to be integrated in the ST. The voltages of each DC/DC module adopted in this case are 0.8kV for the DC link and 1.5kV in MVAC. In case higher voltages are needed (10kV or higher), they can be reached by simply connecting more QAB modules in series in MV side. In LV side, these modules will be connected, instead, in parallel, in order to satisfy the higher power consumption (and thus higher current) need of the LV grid.

This prototype can be used to test the advanced control strategies, such as power routing in QAB for system lifetime extension, active thermal control for IGBT reliability improving, power reverse limitation, MVAC voltage support and harmonic compensation [3].

 

References

  1. M. Liserre, G. Buticchi, M. Andresen, G. De Carne, L. Costa, Z. Zou, The Smart Transformer: Impact on the Electric Grid and Technology Challenges, IEEE Industrial Electronics Magazine, Vol. 10, no. 2, pp:46-58, 2016
  2. SP Energy Networks
  3. Christian-Albrechts-Universität zu Kiel Industrial/PhD course

 

About the Authors

Professor Marco Liserre is Chair of Power Electronics: Christian-Albrechts-University of Kiel. He is the author of 310 technical papers (90 of them in international peer-reviewed journals), 3 chapters of a book and a book (Grid Converters for Photovoltaic and Wind Power Systems, ISBN-10: 0-470-05751-3 – IEEE-Wiley, also translated in Chinese) and a patent. 

Dr. Rongwu Zhu is a post-doctoral researcher at the Chair of Power Electronics, CAU, Kiel, Germany. He holds a Ph.D. from the Department of Energy Technology, Aalborg University, Denmark.

Dr. Giovanni De Carne is a Ph.D.student at the Chair of Power Electronics, CAU, Kiel, Germany. He holds a bachelors in electrical engineering from Politecnico of Bari, Bari, Italy. His main research at CAU is analyzing smart transformer features for electric distribution.

Anthony Donoghue is the senior innovation engineer at Scottish Power. He holds a masters degree from the University of Strathclyde in sustainable engineering and a bachelors degree from the University of Glasgow in aeronautical engineering. 

Ali Kazerooni is the Power Systems team lead at WSP, UK. He holds a masters degree in electrical power systems from Tarbiat Modares University and a Ph.D. in electrical power systems from the University of Manchester.

More information:    Source: Bodo's Power Systems, October 2018