Droop Control of Distributed Electric Springs for Stabilizing Future Power Grid

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Use of Hooke's law for stabilizing future smart grid - the electric spring concept

Hooke's law for mechanical springs was developed in the 17th century. Recently, new power electronics devices named electric springs have been developed for providing voltage regulation for distribution networks and allowing the load demand to follow power generation. This paper summarizes recent R&D on electric springs and their potential functions for future smart grid. Electric springs can be associated with electric appliances, forming a new generation of smart loads which can adapt according to the availability of power from renewable energy sources. When massively distributed over the power grid, they could provide highly distributed and robust support for the smart grid, similar to the arrays of mechanical springs supporting a mattress. Thus, the 3-century old Hooke's law in fact provides a powerful solution to solving some key Smart Grid problems in the 21st Century. © 2013 IEEE.published_or_final_versio

Electric spring and smart load: technology, system-level impact and opportunities

Increasing use of renewable energy sources to combat climate change comes with the challenge of power imbalance and instability issues in emerging power grids. To mitigate power fluctuation arising from the intermittent nature of renewables, electric spring has been proposed as a fast demand-side management technology. Since its original conceptualization in 2011, many versions and variants of electric springs have emerged and industrial evaluations have begun. This paper provides an update of existing electric spring topologies, their associated control methodologies, and studies from the device level to the power system level. Future trends of electric springs in large-scale infrastructures are also addressed

DC electric springs: an emerging technology for DC grids

There is widespread attention on integrating renewable energy sources, such as the solar power, to DC distributed power systems and DC microgrids. The voltage stability and the power quality issues are of concern if a large proportion of power sources in these DC power systems are generated by intermittent renewable energy sources. This paper presents an electric active suspension technology known as the DC electric springs for stabilizing and improving the quality of the power distributions in DC power grids. The basic operating modes and characteristic of a DC electric spring under different types of serially-connected non-critical loads will first be introduced. Then, various potential issues that affect the power quality of the DC power systems, namely the bus voltage instability, voltage droop, system fault, and harmonics, are briefly addressed. Laboratory-scale experiments validated that the aforementioned quality issues can be mitigated using the proposed DC electric spring technology. © IEEE.published_or_final_versio

A review of microgrid development in the United States – A decade of progress on policies, demonstrations, controls, and software tools

Microgrids have become increasingly popular in the United States. Supported by favorable federal and local policies, microgrid projects can provide greater energy stability and resilience within a project site or community. This paper reviews major federal, state, and utility-level policies driving microgrid development in the United States. Representative U.S. demonstration projects are selected and their technical characteristics and non-technical features are introduced. The paper discusses trends in the technology development of microgrid systems as well as microgrid control methods and interactions within the electricity market. Software tools for microgrid design, planning, and performance analysis are illustrated with each tool's core capability. Finally, the paper summarizes the successes and lessons learned during the recent expansion of the U.S. microgrid industry that may serve as a reference for other countries developing their own microgrid industries

Smart Loads for Voltage Control in Distribution Networks

This paper shows that the smart loads (SLs) could be effective in mitigating voltage problems caused by photovoltaic (PV) generation and electric vehicle (EV) charging in low-voltage (LV) distribution networks. Limitations of the previously reported SL configuration with only series reactive compensator (SLQ) (one converter) is highlighted in this paper. To overcome these limitations, an additional shunt converter is used in back-to-back (B2B) configuration to support the active power exchanged by the series converter, which increases the flexibility of the SL without requiring any energy storage. Simulation results on a typical U.K. LV distribution network are presented to compare the effectiveness of an SL with B2B converters (SLBCs) against an SLQ in tackling under- and over-voltage problems caused by EV or PV. It is shown that SLBCs can regulate the main voltage more effectively than SLQs especially under overvoltage condition. Although two converters are required for each SLBC, it is shown that the apparent power capacity of each converter is required to be significantly less than that of an equivalent SLQ

Rapid Frequency Response From Smart Loads in Great Britain Power System

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Distributed voltage-driven demand response: flexibility, stability and value assessment

The need for operating reserve from energy storage, demand reduction (DR) etc. is expected to increase signifcantly in future low-carbon Great Britain (GB) power system with high penetration of non-synchronous renewable generation. One way to provide the reserve is to use power electronic compensators (PECs) for point-of-load voltage control (PVC) to exploit the voltage dependence of loads. This thesis focuses on the quantifcation of DR capability from PVC in the domestic sector using high-resolution stochastic demand models and generic distribution networks in GB. The effectiveness of utilising PVC in contributing to frequency regulation is analysed and demonstrated through time domain simulations. The techno-economic feasibility of such technology is evaluated considering the investment cost of the PEC deployment as well as the economic and environmental benefts of using PVC. The payback period varies between 0.3 to 6.7 years for different future scenarios considering a range of converter price. It is demonstrated that PVC could effectively complement battery energy storage system towards enhanced frequency response provision in future GB power system. For practical application of PVC for flexible demand and voltage regulation in future distribution networks/microgrids, it is important to investigate the overall small signal stability of the system. In this thesis, the linearised state space model of a distribution network/isolated microgrid with converter-interfaced distributed generators (CDGs) working in grid following mode along with loads with PVC is developed. The stability performance is revealed through both modal analysis and time domain simulations. It is shown that multiple loads with PVC for voltage regulation in distribution networks are not likely to threaten the small signal stability of the system. In the case of a microgrid, the introduction of PVC is shown to have marginal impact on the low frequency modes associated with the droop control of the CDGs. However, there is a trade-off when choosing the droop gain of the loads with PVC. Lower droop gains could ensure better frequency regulation in face of intermittent renewables but at the expense of a lower stability margin for an oscillation mode at a frequency slightly higher than 20Hz.Open Acces

Small Signal Stability Analysis of Distribution Networks with Electric Springs

This paper presents small signal stability analysis of distribution networks with electric springs (ESs) installed at the customer supply points. The focus is on ESs with reactive compensation only. Vector control of ES with reactive compensation is reported for the first time to ensure compatibility with the standard stability models of other components such as the interface inverter of distributed generators (DGs). A linearized state-space model of the distribution network with multiple ESs is developed which is extendible to include inverter-interfaced DGs, energy storage, active loads etc. The impact of distance of an ES from the substation, proximity between adjacent ESs and the R/X ratio of the network on the small signal stability of the system is analyzed and compared against the case with equivalent DG inverters. The collective operation of ESs is validated through simulation study on a standard distribution network

Improved Synchronverters with Bounded Frequency and Voltage for Smart Grid Integration

Synchronverters are grid-friendly inverters that mimic conventional synchronous generators and play an important role in integrating different types of renewable energy sources, electric vehicles, energy storage systems, etc., to the smart grid. In this paper, an improved synchronverter is proposed to make sure that its frequency and voltage always stay within given ranges, while maintaining the function of the original synchronverter. Furthermore, the stability region characterised by the system parameters is analytically obtained, which guarantees that the improved synchronverter is always stable and converges to a unique equilibrium as long as the power exchanged at the terminal is kept within this area. Extensive OPAL-RT real-time simulation results are presented for the improved and the original self-synchronised synchronverters connected to a stiff grid and for the case when two improved synchronverters are connected to the same bus with one operating as a weak grid, to verify the theoretical development