Abstract

Application of micro-grids (MGs) as a solution for future energy systems are significantly increasing in recent years. On the other hand, more stringent requirements are added to grid codes (GCs) of low-voltage distribution networks. Accordingly, this paper proposes a low-voltage ride-through (LVRT) control scheme for four-wire multi-source MGs. The novel strategy is composed of two layers for controlling each phase of the grid-connected sources independently. The primary layer contains a reverse-droop function for each phase and fractional-order sliding-mode-control (FOSMC). The secondary layer determines the total requested reactive power for each phase during symmetrical and asymmetrical voltage drops. Then, the mentioned power is shared among the phases of each inverter based on the reactive power-sharing strategy and GC requirements. Based on the proposed grid-following controller, the voltage of faulty phases is compensated by reactive power injection. In addition, power quality indexes are kept in acceptable ranges during abnormalities, and the FOSMC performs better than the conventional methods. The effectiveness of the proposed scheme is verified through offline simulations in MATLAB/Simulink as well as validation by real-time results.

Introduction

1.1. Background
The utilization of small-scale distributed energy resources (DERs) in low and medium-voltage networks is increasing, and as a result, their sudden and collective disconnection from the power system might cause voltage collapse on the grid during severe voltage perturbations. Therefore, the severe impact of the DERs cannot be ignored since their contribution to the power supply is considerable. Thus, the incremental usage of DER systems has become leverage for setting new requirements for power systems. These regulations are called grid codes (GCs), which vary from country to country and cover some grid-connected power sources such as wind turbines and photovoltaic (PV) systems [1]. The mentioned regulations force the sources to stay connected to the main grid during temporary voltage sags and support it by injecting reactive current. An essential requirement of the GC, which has gained much attraction in recent studies, is the low-voltage ride-through (LVRT) capability of the distributed generators (DGs). LVRT curves of the German and Danish GCs are highlighted in Fig. 1a [2]. Besides, in Fig. 1b amount of the reactive power injection per voltage drop is depicted for different scenarios [3]. Most studies cover symmetrical voltage disturbances that only include a minor percentage of the faults such as [4,5], etc. However, asymmetrical faults such as single-phase short-circuits are more probable with an 85 % occurrence rate [6]. In most cases, the conventional three-phase control methods for compensating voltage drops are not applicable for asymmetrical faults since they might affect the operation of safe phases by deteriorating voltage imbalance. Also, reactive power-sharing and reference power tracking in multi-source MGs are important operation factors, which has not been addressed sufficiently in previous studies.

Conclusion

MGs integration into power systems has boosted the system’s reliability and caused operational challenges for the network operators. Hence, some countries have considered a set of requirements to improve the functionality of their power systems. For this purpose, in this paper, an LVRT scheme is proposed for four-wire MGs, which can control each phase of the DGs independently through a grid-following method. The local controller of the DGs, known as the primary layer, utilizes reverse-droop control for voltage and frequency regulation and a nonlinear FOSMC. On the other hand, the secondary layer fulfills the modern GC requirements by calculating the required reactive power for the faulty phases. Next, this amount is shared between the corresponding phases of the DGs based on their free capacity. The effectiveness of the presented controller is proved by analyzing the results obtained from implementing 30 % single-phase, two-phase, and three-phase voltage sags. Accordingly, each DG phase injects reactive power during voltage sags according to the determined amount. Also, comparing the injected reactive power with the amounts of German GC demonstrates that the reactive power injection is done with acceptable accuracy. The results point out that during the external sags, the voltage of PCC, inverter terminals, and load terminals are restored to their nominal values, 30 % compensation, as if there is no perturbation. Consequently, the presented controller can realize the GC requirements and ride through during severe voltage conditions. Moreover, this control method keeps the power quality indices like THD and voltage imbalance factors in acceptable ranges during the sags. Also, the superiority of designed FOSMC over conventional current controllers such as PI and SMC are demonstrated. This outperformance is highlighted with several aspects such as MPAE index, THD values, voltage compensation, and reactive power tracking during the imposed voltage sags. It is shown that the best value for the alpha is 0.76, by which the response of the controller is fast and accurate. Finally, the simulations are validated through real-time results. Our future work is focused on the LVRT capability of the MGs including both grid-forming and grid-following inverters. In addition, the effect of grid strength and R/X ratio of the power lines can be discussed in the future investigations, which has not been addressed sufficiently in this literature.

In this paper, a distributed optimal reactive power control (DORPC) scheme is proposed for minimizing the total losses of doubly fed induction generator (DFIG)-based wind farms (WFs), including the losses of generators, converters, filters, and networks. The DORPC minimizes total WF losses by optimally coordinating reactive power outputs of the DFIG stator and the grid-side converter. The optimal control problem is solved in a distributed manner by using the consensus alternating direction method of multipliers (ADMM). With the consensus ADMM, the total WF loss optimization problem is transformed into a distributed optimal power flow problem considered with DFIGs’ optimal operation. The optimization problem with local constraints considers the reactive power limit of DFIG-based wind turbines (WTs) and the voltage limits at all WT terminal buses inside the WF. In the DORPC, the optimal control problem is solved by the collector bus station controller and WT controllers in parallel, only with the information exchange between immediate neighbors. It eliminates the need of a central controller and centralized communication, implying better robustness and plug-and-play capability. A WF with 20 DFIG-based WTs was used to validate the proposed DORPC scheme.

Introduction

Wind power has become a widely used renewable energy source (RES) with substantial potential and mature technology. With wind power generation expanding, the intermittency of wind power and the interaction between wind farms (WFs) and power systems introduce challenges [1]. The doubly fed induction generator (DFIG)-based wind turbine (WT) has been widely used in modern WFs due to high controllability and small converter rating [2]. With power electronic converters, DFIG-based WFs can regulate reactive power independently and provide reactive power support for power systems [3]. Voltage and reactive power control of WFs has motivated numerous studies. The WF is required to maintain the power factor within the limit at the point of connection (POC) [4,5] or provide reactive power support for power systems while tracking the dispatch command from the transmission system operator (TSO) [6]. Dynamic power electronic devices, such as static var compensators (SVC) and static synchronous compensators (STATCOM), are used in WFs for providing rapid reactive compensation and voltage control [7,8]. In DFIG-based WFs, each DFIG-based WT is equipped with power electronic converters. The DFIG-based WF can utilize the capabilities of the DFIG-based WTs for providing reactive power support to meet grid code requirements. The most widely used reactive power control scheme in WFs is the proportional dispatch (PD) scheme, which is simple, easy to implement, and considers the reactive power margin of each DFIG-based WT [9,10]. However, without the optimizing reactive power references for individual WTs, the WF controller cannot achieve WF optimal operation. In [11], the particle swarm optimization (PSO) was adopted to dispatch reactive power of WTs by minimizing total active power losses along the cables and the transformers of WTs.

1- Introduction

2- Problem statement

3- Reactive power control

4- Volt/Var control

5- Conclusions

References

Abstract

When relatively high-capacity renewable photovoltaic (PV) systems are connected to a grid, they can increase or decrease the voltage along feeders because of reverse power flow, even exceeding ± five percent of the rated voltage. Nowadays, grid-connected inverter-based PV systems that can control reactive power can alleviate such an increase or decrease in voltage by adjusting the reactive power, which is referred to as Volt/Var control and management. Therefore, the objective of this paper is to (a) perform case studies to analyze the steady-state response of a large distribution network (i.e., with more than 1000 buses) with high-capacity PV systems that can control Volt/Var (i.e., either producing or consuming reactive power) and (b) present a Volt/Var-control method for three-phase voltage regulation that uses the positive-sequence sensitivity impedance matrix with power-factor constraints. This method is verified in the IEEE 34-bus test feeder. Thus, the proposed methods can be used to regulate the voltage of a bus to which a PV system is connected if the system controls reactive power. These proposed methods can also be used for various impact studies for the operation or planning of distribution systems with such PV systems.

Introduction

Photovoltaic (PV) systems with a capacity from tens to thousands of kW that are connected to a distribution network below 30 kV can reduce losses and daytime peak demand, which is known as peak load shaving. However, if these systems are not adequately regulated, they can increase or decrease the voltage because of reverse power flow. Thanks to inverters that can control reactive power, modern inverterbased PV systems that are connected to grids can actively control reactive power, which is referred to as Volt/Var control and management. However, the connection of such PV systems to grids requires mutually well-coordinated voltage-regulation agreements between utilities and PV owners (or PV operators) if the PV systems regulate voltage. In fact, variations in the power output from PV systems can be regulated by utilities so that the feeder voltage can be maintained within a set range of the rated voltage (i.e., ANSI C84.1–2016 Range A [1] or EN 50160 [2]). In addition to voltage regulation, a distribution network that hosts PV systems should be able to detect abnormal or limited conditions according to the voltage ranges and clearing times, as presented in [3,4]. Furthermore, settings for dynamic or static voltage regulation can be allowed only under mutual agreement between PV owners and utilities [3,4].

1- Introduction

2- Proposed structure

3- Simulation results

4- Conclusion

References

Abstract

In this paper, a new two-stage approach has been presented for the reactive power control of power systems. In the first stage, the transmission network is divided into several parts using a partitioning approach based on graph concept. In the second stage, a hierarchical distributed framework based on a System of Systems (SoS) concept has been proposed for optimal reactive power dispatch. In this structure, every section of the grid is controlled by a smart agent. Agents are interconnected and exchange the required data via a telecommunication network. In this paper, the amounts of active and reactive power exchanged between agents are considered as boundary and common parameters. Our proposed method is implemented on the IEEE 118-bus network connected to 7 active power distribution networks. The results are compared to the ones obtained from a distributed method based on an Incident Command System (ICS) and a centralized control method. It turns out that the proposed method outperforms the other two competing methods.

Introduction

The ever-increasing advances in power electronics and utilization of electricity in industry necessitated alterations in power distribution systems in various countries, due to altering the operational strategies. Consequently, energy management is addressed at the highest levels of technology and engineering. It is studied as a major economic investment and commodity. Thus, one of the main objectives for power systems operators is the economic and safe operation. To this end, optimal scheduling and performance of the power systems are required. They have recently attracted the attention of many researchers. One of the tools to achieve the optimal performance and utilization of power systems is the Optimal Reactive Power Dispatch (ORPD). The problem of ORPD is a part of power system optimization problems in which, based on a series of constraints and control variables, specific objective functions are optimized. A review of the literature reveals three objective functions, as follows [1–4]: (i) improving the voltage profile; (ii) increasing the voltage stability margin; (iii) reducing active power losses. Thus, it is evident that reactive power significantly affects the major parameters of power systems operation. There are different methods to solve ORPD optimization problems. Typical classification of these methods is as follows: graphical, analytic or classic, specific, numerical, dynamic planning, heuristic methods. The details for each one of these subdivisions are presented in [5,6]. Heuristic methods are suitable to solve such kind of problems. Some examples for these methods include Genetic Algorithm (GA) [7,8], Tabu Search (TS) [9], Particle Swarm Optimization (PSO) [10,11], Gravitational Search Algorithm (GSA) [12], Artificial Bee Colony (ABC) aided by Differential Evolution (DE) [13], and Seeker Optimization Algorithm (SOA) [14].

1- Introduction

2- VSC-HVDC multi-terminal systems

3- Q control for transient stability improvement using local measurements

4- Illustrative example

5- Case study

6- Conclusion

References

Abstract

This paper proposes a new reactive-power control strategy for High Voltage Direct Current multi-terminal systems with Voltage Source Converter stations (VSC-HVDC) to improve transient stability in electric power transmission systems. The proposed algorithm uses local measurements to estimate, in each converter station, a weighted average of the frequencies seen by the VSC stations of the HVDC multi-terminal system. This estimation is carried out making use of a (small) auxiliary local active-power modulation along with the DC-side voltage droop control in the VSC stations, where the latter also uses local measurements only. The estimated frequency is used as the set point for supplementary reactive-power control at each converter station. The proposed control law has been simulated in a test system using PSS/E and the results show that it improves transient stability, significantly, producing similar results to those obtained controlling reactive power using global measurements.

Introduction

Electrical grids in Europe are expected (a) to ease the integration of local distributed generation, (b) to integrate remote renewable generation such as a large amount of offshore wind power from the North Sea and (c) to connect to neighbouring electrical systems such as the Baltic Energy Market or those in African Countries [1]. To make this possible, it is necessary to increase the transmission capability of the system, interconnect power systems of different countries (even asynchronous) and be able to transmit, efficiently, distant offshore energy to the onshore grid through submarine cables. Meanwhile, the reinforcement of the existing infrastructure with new overhead AC lines raises important environmental concern and public opposition and Transmission System Operators (TSOs) are often driven to plan for underground connections. High Voltage Direct Current (HVDC) technology has a great potential to tackle these challenges, above all if submarine or underground cables are required. HVDC based on Line Commutated Converters (LCC-HVDC) is already a mature technology while multiterminal HVDC systems based on Voltage Source Converters (VSCHVDC) have many advantages as pointed out in [2], for example. Unfortunately, the latter is less mature and more expensive than the former and it is necessary to investigate further whether its potential advantages can overweight its investment costs. When planning to stretch power systems to their transmission limits, transient stability (i.e., the ability of the generators to remain in synchronism after large disturbances [3]) is one of the most important limiting factors. Although bulk power transmission should remain the main purpose of any multi-terminal VSC-HVDC system (VSC-MTDC, for short) to be built, they are also a very attractive alternative to improve transient stability, due to the fast control of active- and reactive-power injections of the VSC stations at different (often distant) points [4,5]. In fact, previous publications have showen that transient stability could be greatly improved using VSC-HVDC point-to-point links [6–9] and VSC-HVDC multi-terminal systems [10–14]. A complete review of previous work on control strategies in VSC-MTDC systems with a detailed description of each strategy is presented in Section 1.1.