Real-time coordinated voltage control of PV inverters and energy storage for weak networks with high PV penetration

There are more large-scale PV plants being established in rural areas due to availability of low priced land. However, distribution grids in such areas traditionally have feeders with low X/R ratios, which makes the independent reactive power compensation method less effective on voltage regulation. Consequently, upstream Step Voltage Regulator (SVR) may suffer from excessive tap operations with PV induced fast voltage fluctuations. Although a battery energy storage system (BESS) can successfully smooth PV generation, frequent charge/discharge will substantially affect its cost effectiveness. In this paper, a real-time method is designed to coordinate PV inverters and BESS for voltage regulation. To keep up with fast fluctuations of PV power, this method will be executed in each 5s control cycle. In addition, charging/discharging power of BESS is adaptively retuned by an active adjustment method in order to avoid BESS premature energy exhaustion in a long run. Finally, through a voltage margin control scheme, the upstream SVR and downstream PV inverters and BESS are coordinated for voltage regulation without any communication. This research is validated via an RTDS-MatLab co-simulation platform, and it will provide valuable insights and applicable strategies to both utilities and PV owners for large-scale PV farm integration into rural networks

Simulating Distributed Battery and Solar Array Placement for Voltage Regulation

Energy storage has been around for many years in the US, mainly in the form of pumped hydro. However, in recent years, other storage technologies have developed quickly, with lithium ion batteries receiving significant investment and delivering technological and price improvements. Electricity storage has the potential to assist the US in transitioning to a smarter grid, as well as enabling increasing amounts of renewable generation to connect to the grid, without costly reinforcement works. Electricity storage can be co-located with power generation assets, installed along distribution systems for network services, or placed behind the meter, i.e., on the customer’s premises. This thesis focuses on the latter case. Modern day electrical grids are complex and varied. Using a representative of a large number of grids we can simulate real world conditions and show how the system reacts to distributed solar arrays but can also show how the system can recover from voltage failures using residential sized distributed battery banks. It is hypothesized that through distributed use of battery systems that energy grids can facilitate a larger amount of renewable energy in regard to voltage and current limitations. The tasks to be performed include the following: • Establish a base case network using the IEEE test feeder with local TMY data and local load data with Gridlab-D • Establish a distributed and isolated number of solar arrays that real world outputs to cover how the grid would begin to fail relative to voltage and current limitations • Study the ability of the grid to recover from voltage violations with the use of residential sized distributed battery systems using three utilization variations. Time of use shifting, peak shaving, and negative power shifting. Based upon the found data we can discuss the added benefits of distributed battery systems and how they can be used to harden the grid against voltage failures

Voltage Management for Large Scale PV Integration into Weak Distribution Systems

In long distribution feeders, step voltage regulators (SVRs) with the line drop compensation have been widely implemented to control voltage profiles. After integration of photovoltaic (PV) systems, reactive power support from PV inverters can also he utilized in voltage regulation. Although both SVR and reactive power support can he effective to manage system voltage without coordination, problems such as large voltage variations and excessive SVR tap operations still exist in some strong PV power fluctuating days. In order to solve these issues, SVR and reactive power support should be assigned to different voltage regulation tasks according to their voltage regulation characteristics. Specifically, in a distribution system, an SVR should mainly deal with slowly changing quantities (e.g., load, upstream voltage), while the limited reactive power support should be used to counter fast fluctuating PV power. In this paper, the power factor droop parameters applied on PV inverters are optimally selected to achieve such coordination, so that voltage problems and excessive SVR tap operations can he successfully mitigated. The effectiveness of the proposed method is demonstrated via case studies. Future PV integration project in weak distribution systems can benefit from the innovative and practical methodology proposed in this paper

Stability of microgrids and weak grids with high penetration of variable renewable energy

Autonomous microgrids and weak grids with high penetrations of variable renewable energy (VRE) generation tend to share several common characteristics: i) low synchronous inertia, ii) sensitivity to active power imbalances, and iii) low system strength (as defined by the nodal short circuit ratio). As a result of these characteristics, there is a greater risk of system instability relative to larger grids, especially as the share of VRE is increased. This thesis focuses on the development of techniques and strategies to assess and improve the stability of microgrids and weak grids. In the first part of this thesis, the small-signal stability of inertia-less converter dominated microgrids is analysed, wherein a load flow based method for small-signal model initialisation is proposed and used to examine the effects of topology and network parameters on the stability of the microgrid. The use of a back-to-back dc link to interconnect neighbouring microgrids and provide dynamic frequency support is then proposed to improve frequency stability by helping to alleviate active power imbalances. In the third part of this thesis, a new technique to determine the optimal sizing of smoothing batteries in microgrids is proposed. The technique is based on the temporal variability of the solar irradiance at the specific site location in order to maximise PV penetration without causing grid instability. A technical framework for integrating solar PV plants into weak grids is then proposed, addressing the weaknesses in conventional Grid Codes that fail to consider the unique characteristics of weak grids. Finally, a new technique is proposed for estimating system load relief factors that are used in aggregate single frequency stability models