Historically, centrally computed algorithms have been the primary means of power system optimization and control. With increasing penetrations of distributed energy resources requiring optimization and control of power systems with many controllable devices, distributed algorithms have been the subject of significant research interest. This paper surveys the literature of distributed algorithms with applications to optimization and control of power systems. In particular, this paper reviews distributed algorithms for offline solution of optimal power flow (OPF) problems as well as online algorithms for real-time solution of OPF, optimal frequency control, optimal voltage control, and optimal wide-area control problems.

https://ieeexplore.ieee.org/ielaam/5165411/8075182/7990560-aam.pdf

A Survey of Distributed Optimization and Control Algorithms for Electric Power Systems

Load modeling has significant impact on power system studies. This paper presents a review on load modeling and identification techniques. Load models can be classified into two broad categories: 1) static and 2) dynamic models, while there are two types of approaches to identify model parameters: 1) measurement-based and 2) component-based. Load modeling has received more attention in recent years because of the renewable integration, demand-side management, and smart metering devices. However, the commonly used load models are outdated, and cannot represent emerging loads. There is a need to systematically review existing load modeling techniques and suggest future research directions to meet the increasing interests from industry and academia. In this paper, we provide a thorough survey on the academic research progress and industry practices, and highlight existing issues and new trends in load modeling.

Load Modeling—A Review

In this paper, distributed energy-storage systems (ESSs) are proposed to solve the voltage rise/drop issues in low-voltage (LV) distribution networks with a high penetration of rooftop photovoltaics (PVs). During the peak PV generation period, the voltages are mitigated by charging the ESSs, and the stored energy is discharged for voltage support during the peak load period. The impact of storage devices integrated with the PV source on feeder voltages is investigated in detail. A coordinated control method, which includes distributed and localized controls, is proposed for distributed ESSs. The distributed control using the consensus algorithm regulates the feeder voltages within the required limits, while the localized control regulates the state of charge (SoC) of each ESS within the desired SoC range. The entire control structure ensures voltage regulation while effectively utilizing storage capacity under various operation conditions. The proposed control method is evaluated in LV distribution networks, and the simulation results validate the effectiveness of this method.

/pdf/coordinated-control-of-distributed-energy-storage-systems-hldir545co.pdf

Coordinated Control of Distributed Energy-Storage Systems for Voltage Regulation in Distribution Networks

In this paper, the overvoltage problems that might arise from the integration of photovoltaic (PV) panels into low-voltage (LV) distribution networks is addressed. A distributed scheme is proposed that adjusts the reactive and active power output of inverters to prevent or alleviate such problems. The proposed scheme is model-free and makes use of limited communication between the controllers in the form of a distress signal only during emergency conditions. It prioritizes the use of reactive power, while active power curtailment is performed only as a last resort. The behavior of the scheme is studied using dynamic simulations on a single LV feeder and on a larger network composed of 14 LV feeders. Its performance is compared with a centralized scheme based on the solution of an optimal power flow (OPF) problem, whose objective function is to minimize the active power curtailment. The proposed scheme successfully mitigates overvoltage situations due to high PV penetration and performs almost as well as the OPF-based solution with significantly less information and communication requirements.

/pdf/active-management-of-low-voltage-networks-for-mitigating-485lx5mwzy.pdf

Active Management of Low-Voltage Networks for Mitigating Overvoltages Due to Photovoltaic Units

A constant or voltage-dependent load model is usually assumed in most distributed generation (DG) planning studies. However, this paper proposes several different types of time-varying voltage-dependent load models to determine the penetration level of photovoltaic (PV) units in a distribution network. Here, a new analytical expression is first proposed to size a PV unit, which can supply active and reactive powers. This expression is based on the derivation of a multiobjective index (IMO) that is formulated as a combination of three indices, namely active power loss, reactive power loss and voltage deviation. The expression is then adapted to allocate PV units while considering the time-varying load models and probabilistic PV generation. The effectiveness of the proposed approach was validated on 69- and 33-bus test distribution systems. The results showed that PV allocation with different types of time-varying load models can produce dissimilar penetration levels.

Determining PV Penetration for Distribution Systems With Time-Varying Load Models

High penetrations of photovoltaic (PV) systems in distribution grids have brought about new challenges such as reverse power flow and voltage rise. One of the proposed remedies for voltage rise is reactive power contribution by PV systems. Recent German Grid Codes (GGC) introduce an active power dependent (APD) standard characteristic curve, ${\rm Q}({\rm P})$ , for inverter-coupled distributed generators. This study utilizes the voltage sensitivity matrix and quasi-static analysis in order to locally and systematically develop a coordinated ${\rm Q}({\rm P})$ characteristic for each PV system along a feeder. The main aim of this paper is to evaluate the technical performance of different aspects of proposed ${\rm Q}({\rm P})$ characteristics. In fact, the proposed method is a systematic approach to set parameters in the GGC ${\rm Q}({\rm P})$ characteristic. In the proposed APD method the reactive power is determined based on the local feed-in active power of each PV system. However, the local voltage is also indirectly taken into account. Therefore, this method regulates the voltage in order to keep it under the upper steady-state voltage limit. Moreover, several variants of the proposed method are considered and implemented in a simple grid and a complex utility grid. The results demonstrate the voltage-regulation advantages of the proposed method in contrast to the GGC standard characteristic.

Coordinated Active Power-Dependent Voltage Regulation in Distribution Grids With PV Systems

Multi-objective coordinated droop-based voltage regulation in distribution grids with PV systems

High penetrations of photovoltaic (PV) systems within load pockets in distribution grids have changed pure consumers to prosumers This can cause technical challenges in distribution and transmission grids, such as overvoltage and reverse power flow Embedding voltage support schemes into PVs, such as standard ${\cos \phi (P)}$ characteristic proposed by the German grid codes, may cause more changes in the steady-state behavior of distribution grids and, in turn, the transmission side Accordingly, it is important to properly model active distribution grids to analyze the system impacts of these changes to plan and operate future smart power grids However, due to the high dimension of distribution grids, considering a detailed distribution grid to study the transmission side or a fraction of the distribution grid is either cumbersome or impractical Therefore, it is required to develop a reasonable equivalent that can fairly capture the dominant behavior of the distribution grids The aim of this paper is to use gray-box modeling concepts to develop a static equivalent of distribution grids comprising a large number of PV systems embedded with voltage support schemes In the proposed model, the PV systems are aggregated as a separate entity, and not as a negative load, which is traditionally done The results demonstrate the superior quality of the proposed model compared with the model with PV systems as the negative load

Static Equivalent of Distribution Grids With High Penetration of PV Systems

Reactive Power Dynamic Assessment of a PV System in a Distribution Grid

Photovoltaic systems continue to be deployed at increasing levels and their impact on the electric grid needs to be evaluated more accurately. This includes the impact both in the local grid where they are connected and the impact on the operation of the whole system. As a consequence, adequate models of panels, inverters, and the rest of the grid are required. Models of photovoltaic systems need to characterize their dominant characteristics and effects on the electric grid for the different types of studies, i.e. load flow, harmonic distortion, voltage stability and electromagnetic transient studies. This paper gives an overview of models that are currently in use for different types of grid impact studies, and points to their applications and limitations.

Afshin Samadi

Papers

Coordinated Active Power-Dependent Voltage Regulation in Distribution Grids With PV Systems

Multi-objective coordinated droop-based voltage regulation in distribution grids with PV systems

Static Equivalent of Distribution Grids With High Penetration of PV Systems

Reactive Power Dynamic Assessment of a PV System in a Distribution Grid

Considerations on the Modeling of Photovoltaic Systems for Grid Impact Studies