Review on various voltage stability issues in AC distribution network

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INTRODUCTION
The utilization of renewable energy sources (RESs) for generation of power has drawn in impressive consideration all throughout the world.This is because of the negative natural effect of consuming petroleum for energy generation, which delivers a tremendous measure of carbon dioxide and other toxic gases in the atmosphere [1].The expanding interest for energy and the requirement for atmospheric safety calls for effective and harmless power generation system.The use of environmental friendly power is generally considered as a promising option in contrast to customary non-renewable energy source framework and in this way draws more consideration [2]- [7].Keeping up voltage balance of the power system is one of the serious issues because of the voltage failure caused by various disturbances in the power system.Since past few decades the stability of the power system has proved to be a major problem.Various studies has been performed and analysis is done to study the effect of PV system injection in power system stability [8], [9].Fast improvement has enabled presence of extensive knowledge related to general power system control and stability.Number of IEEE standards and books [10]- [13] cover modelling issues in depth [13].High injection of PV generation on the power system may cause negative impacts in the power system operation due to variable voltages [8]- [14].New research on the smart grids considers voltage balance of the power system of very much importance.The variable nature of the renewable energy may cause instability in the power system and this may lead to power quality issues [15], [16].A review paper is written on the distributed energy generation and its control techniques, which focus on microgrid control, and voltage control methods [17].Now a days the distributed energy generation is getting much more acceptance.Although the cost of electricity production is very high, but it provides the power near the customer end rather than a faraway generator [18]- [22].Distributed energy resources (DERs) are getting so much reputation in the grids and these sources are utilized in achieving so many objectives [23], [24].Chakravorty and Das in [25] to express the sensitivity of buses, a voltage stability index for distribution network planning was proposed.Murty and Kumar in [26], for DER optimal settlement a voltage stability margin index was expressed and also in this paper evaluation of voltage stability improvement was performed.
Voltage stability problem was very important topic for the power distribution networks and was broadly investigated in 1990s [27], [28].Voltage stability studies have been extended to the distribution systems just with the arrival of new DG expertise [23].In this paper the long term voltage dependability will be considered to uncover the system's capacity to keep up consistent voltage followed by variable changes in the load [29].In this review paper a comprehensive analysis is done on effects of DGs injection on the power system voltage stability.

RENEWABLE ENERGY RESOURCES
Due to the world's population's rapid growth, energy demand is rising quickly.The production of energy becomes a significant global issue.A significant development in energy generation is the utilization of renewable energy sources.Energy that is produced using resources that are replenished naturally is known as renewable energy.Water, wind, solar, and biomass are the most popular renewable energy sources.These energy sources have historically been used for transportation fuels, heating, and power generation.Because they can be sustained, renewable energy sources are appealing.As a result, there is now an alternative to the declining use of traditional energy sources including coal, oil, and nuclear power.Compared to conventional energy sources, renewable energy sources are clean energy sources with significantly reduced environmental impact [30].
The microgrid uses photovoltaic and wind power as promising sources of energy to generate electricity.The microgrid's ability to generate electricity is dependent on the strength of the sun and the wind, which creates some difficult requirements for the technology used to regulate the microgrid [31].Figure 1 shows the percentage of renewable energy, which represents the condition of the rate at which energy has been produced from renewable sources in the past, present, and future [32].Overall, conventional fossil fuels are a sort of nonrenewable energy that could be dangerous due to resource depletion [33].The most advanced RESs for electrical power generation are solar photovoltaic (PV) and wind, which have been extensively connected with the primary power grid in numerous locations across the world [34].However, due to their intermittent availability, wind, and solar energy sometimes required energy storage devices [35].The utility grid's quality, stability, and dependability may be impacted by the incorporation of RESs that are weather-dependent.The grid codes (GCs) and other standards of many nations have so imposed and updated a variety of rules, restrictions, and regulations regarding the functioning and connection of these RES [36].GC contains a variety of technical specifications that describe important rules and restrictions associated with the generation units and their integration into the power grid to ensure the stable and appropriate operation of the power system [37].

PROBLEMS ASSOCIATED WITH RENEWABLE ENERGY RESOURCES
Power quality issues are related to the main challenge in integrating RESs with the traditional grid.High levels of solar PV and wind system penetration could pose serious problems for system stability overall.Voltage variations, flickering, unbalanced voltage and current harmonics, and frequency fluctuation are some of the issues with power quality.These issues are brought on by the switching elements (firing angle control) utilized in power electronics devices, which are now an essential part of RESs technology.The placement of the PV module, the distributed system configuration, and the level of solar PV penetration all affect how strong these problems are in the context of solar PV.In the case of wind, it is based on the kind of turbine, wind density, and wind speed.Power quality problems have been discovered in the literature to affect all RES integrated networks on the distribution, generating, and transmission sides.Rapid changes in sunshine and clouds produce voltage fluctuations and unbalance in solar PV, which has intermittent characteristics.In contrast, fluctuating wind speeds have an impact on wind power output [38].Sensitive electrical and electronic equipment have a shorter lifespan due to voltage fluctuations.
Voltage fluctuations and unbalance, current and voltage harmonics, grid islanding protection, and other power quality issues, such as flicker and stress on distribution transformer, could be identified as the main effects of PV integration.These effects can be categorized as either dynamic or steady-state in nature: i) Variation of voltage in the feeder, which includes imbalanced voltage and voltage surge or fall; ii) Equipment used to regulate voltage, such as capacitor banks, on-load tap changers, and line voltage regulators, operating improperly; iii) The potential for distribution feeder overload; iv) Islanding identification and operation in the event of grid disconnect; and v) Distribution system security and dependability.
Power systems often have spinning reserves to avoid activation of load shedding.This will result in the production of high-quality sustainable energy.Power electronics converters, which are used to produce electricity from renewable energy sources, produce harmonics.We first talked about the features of renewable energy.Due to the semi-unpredictable characteristics and uncontrollable changes of wind and solar PV, both wind and PV power generation are prone to periodic variations that might cause deviations in performance.One way to lessen some of the unexpected aspects of wind and solar resources is to improve weather and power generation forecasting systems targeted at more precisely predicting the weather and power generation output of wind and solar resources at various time scales [39].
IEEE standard 1547a updated its voltage sags requirements in 2014.This included updated settings that make it easier for devices to withstand voltage drops steadily.These settings, which are listed in Table 1, are intended to reduce power quality difficulties brought on by disturbances brought on by tripping as a result of voltage dips [40].

CONTROL STRATEGIES USED IN DISTRIBUTION NETWORKS
SCADA are frequently used for power distribution system monitoring, while distribution management systems and energy management systems are employed for connecting to and communicating with remote devices.The main explanation for this move is the requirement for a significant increase in distributed generation, especially renewable energy sources, in order to achieve ecological priorities for emission reduction and sustainability.The integration of distributed generation (DG) into distributed networks has gained significant significance in the modern period in terms of the progress of quality and effectiveness.In any event, the large-scale penetration of DG into the current distribution systems was not envisaged for.Due to a growth in the injection of distributed generation into the system, numerous technical issues, including voltage management, power quality, and system protection, arise.In order to examine the distribution system and the impact of DG on the distribution system, additional components had to be combined with outdated distribution system devices.Significant efforts have been made in recent years to develop the optimal advanced control distribution system plan.The main goal of power distribution planning (PDP) is to build a power system that will enable cost-effective, dependable, and efficient operation.
Recently, a few theories and tactics have been put out for the solution of sophisticated PDP problems [41].Generally, there are total three types of control configurations: centralized control, distributed control, and decentralized control.All these controls are comparatively analyzed in Table 2.No need to shut down whole system.
Whole system is to be shut down Easily managed

POWER SYSTEM INSTABILITY PROBLEMS
The broadest definition of power system stability is the capacity of a power system to return to a stable operating point after exposure to a physical disturbance.This section has covered four types of power system stability: Rotor angle stability, converter stability, frequency stability, and voltage stability.Two further classifications for this division are small disturbance angle/voltage stability and high disturbance angle/voltage stability.Figure 2 displays the commonly used classification of power system stability.Long transmission lines, which restrict the amount of power that can be transferred between generators and loads in conventional power systems, are a primary contributing factor to voltage instability.However, because the feeders in microgrids are very short, only a small voltage difference exists between their transmitting and receiving ends.Depending on the time scale, voltage instability may develop over a short or extended period of time.Short-term voltage instabilities are brought on by a lack of control coordination or swift dynamical changes in the active and/or reactive power imbalance.Figure 2. Classification of power system stability [44] Voltage stability can be split into two categories in the report: small disturbance voltage stability and high disturbance voltage stability.When a system maintains its ability to maintain voltage stability following a little disruption, such as an increase in system load, this is referred to as small disturbance voltage stability.It is impacted by the load characteristic as well as the continuous and discontinuous control impacts over the allotted time.Large disturbance voltage stability is the capacity of all system buses to retain voltage stability after the system experiences a significant disturbance such a system failure, cutting machine malfunction, or line fault.Due to this reason undamped power oscillations are produced in the system, which increase beyond allowable operating range.On contrary to this, undamped oscillations are generated in heavily loaded microgrids due to small load changes for long term [43].

COMMUNICATION-LESS (LOCAL) VOLTAGE CONTROL STRATEGIES IN DISTRIBUTION NETWORKS
A low-voltage network's functionality is altered when distributed energy resources (DERs) are present in the system.With a large number of photovoltaics, which were formerly just conceivable at advanced voltage stages, bidirectional power flows in the system may be possible.One result of this is an increase in voltage at the ends of the lines.This occurrence can raise the level of the electricity by minimizing voltage drops, but it can also, in bad situations, result in the highest allowable voltage limit being surpassed.The ability of the grid to host PV devices is constrained by these phenomena, also known as "overvoltage" or "voltage boosting".
Techniques for controlling voltage locally don't require any kind of communication.The point of common coupling (PCC) measurement data, including voltage, frequency, and other measurement information, as well as the inverter's internal settings and internal algorithms determine how the PV source will respond.The limitation of using local approaches is that the network as a whole cannot be coordinated.Local control, on the other hand, is quick and doesn't need a lot of money up front.Autonomy is the fundamental tenet of local voltage regulation techniques.Low-voltage networks in the Polish electricity system typically lack communication infrastructure.Because they won't require the system operator to bear the high initial expenditures of creating the full network infrastructure, local techniques may end up being the greatest solution to improve the network hosting capacity.Different communication less local voltage control strategies are compared at Table 3

COMMUNICATION-BASED VOLTAGE CONTROL OF DISTRIBUTION NETWORKS
Power is delivered to the loads from the substation and transformer in conventional distribution networks.Power flows in both ways when DER is linked to the network, increasing the voltage along the feeders.A voltage profile that satisfies legal requirements is advantageous for utilities and customers since it lowers network power losses and raises supply quality.The two main types of voltage control techniques are communication-based and local (autonomous) control.Local quantities taken at the point of common coupling (PCC) serve as the basis for autonomous control.There are five categories of communication-based schemes: local, centralized, decentralized, distributed, and hybrid (combination of centralized and local).Table 4 (see Appendix) depicting the comparative analysis of communication-based voltage control strategies.

CONCLUSION
In the paper thorough review on control of voltage stability issues have been carried out and found that decentralized and distributed controllers are viable to be implemented in the distribution system to tackle stability issues.Furthermore, it has been analysed that with the implementation of those controllers with availability of renewable energy resources the behaviour of such controllers has impact of distribution network.Hence, a detailed simulation will be carried out in MATLAB environment to exactly check the response of all the discussed controllers.
Int J Pow Elec & Dri Syst ISSN: 2088-8694  Review on various voltage stability issues in AC distribution network (Attaullah Khidrani) 405

Figure 1 .
Figure 1.Increasing rate of energy generation from renewable energy sources in present and future Int J Pow Elec & Dri Syst ISSN: 2088-8694  Review on various voltage stability issues in AC distribution network (Attaullah Khidrani) 407

Table 3 .
. Different local control techniques comparison