Voltage Profile Improvement by Integrating Renewable Resources with Utility Grid

Abstract

There are three main parts of an electric power system—power generation, transmission, and distribution. For electric companies, it is a tough challenge to reduce losses of the power system and deliver lossless and reliable power from the generating station to the consumer end. Nowadays, modern power systems are more complex due to gradually increasing loads. In the electrical power system, especially in transmission and distribution networks, there are power losses due to many reasons such as overloading of the line, long distribution lines, low power factors, corona losses, and unsuitable conductor size. The main performance factor of the power system is reliability. Reliability means continuity of the power supply without any interruptions from the generating station to the demand side. Thus, due to these power losses, there are voltage stability problems and economic losses in the electrical system. The voltage stability of the power system can be increased by improving the voltage profile. In this paper, different techniques are analyzed that include the integration of wind power, the integration of photovoltaic power, and reactive power injection by integrating FACTS devices. These techniques are applied to the IEEE 57 bus system with standard data using simulation models developed in MATLAB. Thus, the results of the analysis of these techniques have been compared with each other.

1. Introduction

Power generation, distribution, and transmission are the three main parts of the electric power system. It is quite challenging for electric companies to reduce losses in the electrical power system and deliver lossless and reliable power from the generating side to the consumer side [1]. Nowadays, the modern power system is more complex due to the everyday increase in load. The main factor of the power system is reliability. Reliability means continuity of the supply (power supply) without any interruption from the generating station to the demand side. Thus, power is delivered with the help of the transmission and distribution system, and losses of transmission and distribution systems are reduced to increase system reliability. In the transmission system, there is a major loss called corona loss due to the flow of power on the conductor’s surface. Thus, corona loss is reduced by using a bundle of conductors. The transmission system starts after the generating station and ends at the feeder. However, the distribution system begins from the feeder to the consumer end. The primary distribution system initiates from the feeder and ends at a distribution transformer, with a voltage level of 11 kV. The secondary distribution system starts from the distribution transformer to the consumer end, with a voltage level of 220 and 440 V [2,3].

The essential function of the distribution system is to increase reliability and smooth power transfer to consumers.

There are two types of distribution systems:

(a)

Radial distribution systems, and

(b)

Ring central distribution systems.

In the radial distribution system, a source comes from one side. In the early days, radial distribution systems were used. The substation is directly attached to the primary distribution, and then the primary distribution is connected with the secondary distribution. Therefore, the reliability of radial distribution was poor because there was only one source; and in the case of failure, there were no backup sources. On failure, consumers cannot obtain power until the recovery of the source. To overcome this problem, the ring main distribution system was initialized. All buses are connected in the loop in the ring main distribution system, and more than one source is used in this system. In the distribution system, losses are due to an increase in load, which draws more current from the system that causes a voltage drop. The distribution system requires reactive power in the system, and thus it can be compensated by capacitor banks, but we cannot place capacitor banks at different places. Thus, the loss occurs in the distribution system due to insufficient reactive power. Some other losses in the transmission and distribution system are called technical losses, which occur due to long distribution lines, decentralized distribution transformers, unsuitable conductor size, overloading lines, and low power factors. The constant increment of non-linear load demand increases the complexity of the power system. Some security issues might be associated with it, affecting power system reliability [4].

A significant loss in the electrical power system occurred due to a voltage drop. Therefore, renewable energy is integrated with the existing grid to improve the voltage profile. The main difficulty in renewable energy is instability because the output power in solar and wind depends upon solar radiation intensity and the speed of the wind, respectively [5]. Some controllers can be used to stabilize the output power after integrating renewable resources. Photovoltaic and wind farms are mainly used to enhance voltage stability [6,7]. Thus, a new smart grid concept is developing, and a smart grid is a digitally based power network that uses two-way digital communication to deliver electricity to customers. To increase efficiency, lower energy consumption, and costs, and increase the transparency and dependability of the energy supply chain, this system enables monitoring, analysis, control, and communication throughout the supply chain. The smart grid was created by utilizing intelligent net meters to address the flaws in traditional electricity grids. The visualization of the conventional and smart grids is shown in Figure 1. Still, there might be some tough challenges to integrating renewable resources with the utility grid because renewable resources such as wind energy generate voltage fluctuation. The power depends upon the speed of the wind, which is not constant throughout the day and night. So, some controllers must stabilize the voltage level and reduce security issues. Hence, with the help of controllers, renewable resources are perfectly integrated with the entire grid system. Renewable resources are photovoltaic and wind farms; the voltage profile and power quality increase [8,9,10].

Due to the ongoing rise in electricity demand, modern power networks are enormous and complex. The fundamental goal of generation, transmission, and distribution is to provide customers with efficient, lossless power. However, this poses a severe problem for the entire power grid. The researchers are attempting to minimize loss. To address this issue, several alternatives have been put up. To properly illustrate the contributions of other researchers and the work in this research, a brief explanation of the current solutions is required. As a result, numerous factors, including long distribution lines, overloading, poor power factors, and unsuitable conductor size, contribute to power losses in the electric system, particularly in transmission and distribution. Photovoltaic (PV) dynamic modeling is utilized to create an accurate simulation. As a result, MATAB’s Power System Analysis Tool (PSAT) is the best choice for this purpose. As a result, this software can be used to determine the load flow of an IEEE 14 bus system with three breakers. Afterward, the electric distribution network can be integrated using photovoltaic technology. It has been determined that this system’s load flow analysis enhanced power quality and voltage stability and minimized losses. Thus, by integrating renewable resources (PV) with the current utility grid, power quality is improved while losses are minimized, and voltage stability is increased [3]. The IEEE 9 bus system is used in this study, and by integrating various FACTs (Flexible AC Transmission systems) such as a static VAR compensator (SVC), a static synchronous compensator (STATCOM), and a static synchronous series compensator (SSSC), voltage stability and reactive power sustainability are improved. The MATLAB software’s PSAT tool is used for simulation [2]. The basic purpose of this work is to improve the voltage profile and power quality for loss minimization in the electrical distribution network. Hence, the IEEE 57 bus system with standard data is chosen, and load flow is calculated by using a mathematical method (Newton–Raphson) in Power System Analysis Tool (PSAT) in MATLAB software [11].

Therefore, it is noted from the literature review that no one uses the IEEE-57 bus system to integrate renewable energy resources. In previous studies, the IEEE-57 bus system is only used to calculate the load flow. In this paper, different techniques are integrated, such as renewable resources (wind turbine and photovoltaic) and FACTs devices, with the injection of reactive power in the distribution system and drive results and graphs. Thus, the analysis of these techniques has been compared with each other. By comparing results, suitable strategies can be identified for improving the voltage profile and power quality with low loss in the desired Distribution Generation System.

1.1. The Newton–Raphson Method

The modern electric grid was created as a vertical structure with the generation, transmission, distribution, and sophisticated control support equipment for efficiency, reliability, and stability. The Newton–Raphson approach can be used to resolve power flow issues to preserve system stability as the size of the electric network grows. The square sum of the nodal capacity discrepancies is minimized in the Newton–Raphson method’s calculation steps. When calculating power flow, the Jacobian matrix determinant has a negative value while the matrix determinant is positive around zero. The fundamental advantage of the Newton–Raphson approach is its simplicity in handling PV bus issues. The Newton–Raphson method for load flow analysis has benefits since it is more efficient, accurate, and dependable than any other method for problems of any size or type [12,13]. The load flow chart of the Newton–Raphson method is shown in Figure 2.

1.2. Objective

The mandatory objectives of this work are as follows:

• Design, simulation, and load flow by the Newton–Raphson method’s standard data IEEE-57 bus system with three different load configurations.
• By performing the fast voltage stability index (FVSI) calculation, identification of the critical buses in the system is achieved.
• The integration of renewable (diesel generator), non-renewable techniques (wind energy and photovoltaic), and FACTS devices (SVC and STATCOM).
• Optimal solution for the utility grid by integrating renewable resources to reduce losses and improve the voltage profile.
• Voltage profile improvement enhances voltage stability and leads to an increase in power stability.

1.3. Problem Statement

In the power system, the losses occur due to low power factors, overloading of the line, unsuitable conductor size, and corona losses. Therefore, due to losses and overloading of the power system, along with environmental and economic constraints, voltage instability becomes a severe problem in both transmission and distribution systems. The voltage stability of the power system can be improved by improving the voltage profile. This paper explores different possible techniques (integration of renewable and non-renewable resources and FACTS devices) to enhance voltage profiles.

2. Voltage Stability in Power System

Voltage stability means that the system maintains its voltage at its required level. Due to a deficiency in voltage level, voltage instability occurred in the system. Voltage stability is “the ability of an electrical system to maintain all the bus voltages at their normal state after the small disturbances occur in the system.” Load stability is also known as voltage stability. However, voltage stability causes a severe problem in the electrical system and especially causes blackouts. Therefore, for a stable system, stability must be improved in the system.

Voltage Stability Indices

Voltage stability is considered the backbone of the system because, ideally, we want the system to have higher voltage stability. The stable system is identified with the help of voltage stability. Hence, using the voltage stability index, system voltage stability is characterized. The fast voltage stability index identifies the critical buses and lines where voltages collapse. In both offline and online studies, voltage stability indices are used.

The voltage stability index is derived from Mustrin for the transmission lines in the power system in a single-line diagram. The formula used for the fast voltage stability index (FVSI) is given in Equation (1).

$$FVS{I}_{line}=\frac{4Z^2{Q}_j}{V{i}^{2}X}$$

(1)

In Equation (1), the parameter Z is line impedance, X line reactance, Q_j receiving-end reactive power, and Vi² sending-end voltage. Hence, those buses or lines with a value close to 1 are considered more critical. Hence, the critical buses in the whole system can be identified by the voltage stability index [14].

3. Techniques to Address Stability Issues

To improve stability issues, non-renewable or conventional resources have been used for a long time ago. However, with time, renewable resources became dominant because traditional resources, due to the emission of carbon dioxide, affect the environment, and on the other hand, renewable resources are a clean source of energy [15].

3.1. Conventional Techniques

Conventional resources are called non-renewable resources because they are in limited form. For example, natural gas, coal, crude oil/petroleum, natural gas, and nuclear fuels are non-renewable resources. Thus, conventional resources were used in the past to improve the stability of the power system [16].

3.1.1. Installation of New Generators

Stability was improved sharing the load by installing new generators. Therefore, diesel generators were commonly used. However, the cost of fuel was much higher than the production of electricity. The electricity demand increased daily, so installing new generators gave stability to some extent, but in terms of environmental aspects and cost statistics, these generators are not good enough [17].

3.1.2. Injection of Reactive Power

In the electrical power system, voltage control is mandatory for electrical appliances to prevent damage. Therefore, losses occur in the transmission and distribution lines due to low voltage. Hence, a decrease in reactive power causes an increase in voltage and vice versa. So, the previous strategy to improve the voltage profile is injecting reactive power. Reactive power is injected with the help of the capacitor bank. The capacitor bank combines various series and parallel capacitors to store energy. Hence, when reactive power is in excess, the capacitor banks store energy and release it when there is less reactive power. However, stability was improved by the injection of reactive power [18].

3.1.3. Load Shedding

Load shedding was also used for voltage stability. Load shedding was used only when the generator had a low output voltage, but demand was greater. Due to a gradual rise in demand, the generator tries to manage load, but the generator can be damaged instead of balancing demand. Hence, one factor is frequency regulation. In short, to reduce the burden on the generator, some areas are shut down, and this is called load shedding. So, load shedding was also used to maintain stability in the system [18].

3.1.4. Reduction in System Reactance

Transmission line reactance was kept low to improve stability. The power transfer capacity is inversely proportional to the reactance of the transmission line. A low transmission line reactance causes higher voltage stability, and a high transmission line causes low voltage stability [19,20].

3.1.5. Injection of Hydropower

Hydroelectricity is also considered a source of renewable energy. Water is used as input in hydropower to convert mechanical energy into electrical energy. In the process of hydropower, water comes through penstock with high pressure and, from the top level, falls on the turbine’s blades. Thus, the turbine rotates, and the turbine shafts are connected with generators that convert the mechanical energy into electrical energy [19].

3.2. Renewable Technologies

Wind energy is not a new technology of renewable resources. Wind energy has been used to pump water on many farms for years. However, now, in the current century, wind turbine technology is different from that used to generate electricity [21]. Wind energy depends upon wind turbines and wind generators.

There are two types of wind turbines:

(a)

Horizontal-axis wind turbines, and

(b)

Vertical-axis wind turbine.

There are three types of wind generators:

(a)

Direct current (DC) generators,

(b)

Alternating current (AC) synchronous generators, and

(c)

Alternating current (AC) asynchronous generators.

Mainly variable speed generators are used in the wind turbine. In fixed-speed generators, the rotor speed is fixed and has a constant revolution per minute. The turbine speed is variable because the wind speed varies with time, so the fixed-speed generator causes stress on the drive train. Therefore, a mostly variable-speed wind turbine is used. AC asynchronous generators have variable speeds, but the modern wind turbine uses induction generators [22]. In hydropower stations, the formula used for calculating the production of electric power is given in Equation (2).

P = −n (mg ΔH) = −n (ρ v)(g ΔH)

(2)

In Equation (2), the parameter P is power, ρ is water density, v is flow rate, n is the efficiency constant, m is flow rate mass, ΔH is change in height, and g is acceleration gravity.

Induction generators have two types: squirrel cage induction generators and double-fed induction generators.

3.3. The Integration of a Double-Fed Induction Generator

A double-fed induction generator (DFIG) is an electric machine that can feed AC both ways (rotor and start winding), as shown in Figure 3. Three-phase wound-rotor generators are mostly double fed and currently used in the industry. DFIG are widely used in wind turbines to generate electricity.

Double-fed induction generators have the main advantage of maintaining output voltage magnitude and frequency at a constant level without depending upon the variable speed of wind turbines. Secondly, they can control the power factor. When the rotor magnetic field rotates in the direction of the generator rotor, then the speed of rotor N (rotor) and the magnetic field of rotor subtract from each other. Hence, the frequency f_stator can be calculated using Equation (3) [22].

$$f_{stator}=\left(\frac{N_{stator}\times P}{120}\right)+f_{rotor}$$

(3)

In contrast, when the magnetic field rotates in the opposite direction of the generator’s rotor, the frequency f_stator can be calculated by the following Equation (4).

$$f_{stator}=\left(\frac{N_{stator}\times P}{120}\right)-f_{rotor}$$

(4)

The principle of electric generators is the same as that of a double-fed induction generator. However, a DFIG is preferred because it can run slightly slower or faster than its natural speed (synchronous speed). Thus, rotor winding of a DFIG is directly connected to the grid with the help of slip rings. Hence, rotor and grid currents are controlled by the back-to-back voltage converter. Thus, active and reactive power can be controlled by integrating a DFIG into the electrical system. The double-fed induction generator gave power resulted in an improvement in voltage that led to an improvement in voltage stability [23].

3.4. The Integration of a Squirrel Cage Induction Generator

A squirrel cage induction generator (SCIG) is a constant-speed generator that can provide wind power in the form of a voltage boost, as shown in Figure 4. Therefore, wind energy has low efficiency, and a heavy three-stage gearbox is connected to it, making this technique less efficient. However, a SCIG absorbs reactive power for the excitation current and takes this reactive power for the grid, which can affect the power quality of the whole electrical system.

Due to these problems, a SCIG is less efficient than a DFIG. Hence, we used a DFIG instead of a SCIG due to high efficiency [23]. In wind energy conversion, aerodynamic losses, electrical generator losses and converter losses occur as shown in Figure 5.

3.4.1. The Integration of Photovoltaic Power

Renewable technology has some integration issues but also enhances the system voltage stability. In recent years, the integration of photovoltaic energy has gradually increased, and so installation has doubled. New world technology is directed towards distributed energy generation. To integrate photovoltaic power, the photovoltaic PV module or PV cell should be studied. PV cells combine to make an array. Thus, the combination of a series and a parallel array is called a module [24].

The Integration of photovoltaic power with a grid is shown in Figure 6. The efficiency of photovoltaics is less than 40%. The main disadvantage of solar energy is that it depends upon the availability of solar radiation. Hence, the AC–DC conversion also causes losses in the system [25,26].

3.4.2. Integration of FACTs Devices

FACTs devices are called flexible AC transmission stem devices, used to improve power quality in the electrical system by injecting reactive power. FACTs devices are a combination of power electronics-based devices that work together to control the flow of power with its capacity [27]. FACTs devices are used for stability in the case of any faults, and the control mechanism is shown in

Table 1. Control mechanism of FACTs devices.

Control Devices	FACTS Devices
	SVC	STATCOM	TCSC	SSSC	IPFC	TCPST	UPFC
Voltage control device	✓	✓	X	✓	✓	X	✓
Impedance control device	X	X	✓	✓	✓	X	✓
Angle control device	X	X	X	✓	✓	✓	✓

Note: SVC: static VAR compensator, STATCOM: static synchronous compensator, TCSC: thyristor controlled series capacitor, SSC: static synchronous series compensator, IPFC: interline power flow controller, TCPST: thyristor controlled phase shifting transformer, and UPFC: unified power flow controller.

.

3.5. The Integration of a Static VAR Compensator

A static VAR compensator absorbs or releases reactive power used to improve stability. The static VAR compensator supplies the capacitive or inductive current that controls the voltage of the bus. The thyristor is used in a SVC (static VAR compensator), which contains switching purposes without turn-off capability. In the SVC model, there are two thyristor connected with the series in the inductor and with a parallel capacitor [27].

3.6. Integration of a STATCOM

A STATCOM is a synchronous compensator, used to enhance power quality by injecting reactive power, but with a higher efficiency than a static VAR compensator. In a STATCOM, a voltage source converter is used, and a STATCOM is connected to the primary side of the transformer. A STATCOM acts like an inductor or capacitor, depending on the voltage level of the system. A STATCOM absorbs reactive power when there is an excess amount of reactive power in the design and acts as an inductor. In contrast, a STATCOM delivers reactive power when there is less reactive power in the system and acts as a capacitor [27].

4. System Models and Simulation Scenarios

Different techniques are implemented for improvement in voltage stability. All techniques are applied to the standard IEEE 57 bus system model with three loads.

4.1. The IEEE 57 Bus System Model

The line diagram of the IEEE-57 bus system model is shown in [11]. The network statistics are shown below.

(a)

57 buses,

(b)

63 lines,

(c)

17 transformers,

(d)

7 generators, and

(e)

42 loads.

4.2. Simulation Model of the IEEE 57 Bus System

The bus system model is designed in the PSAT Simulink library in MATLAB software as shown in Figure 7. In this model, there are 63 lines, 57 transformers with 7 generators, and 42 loads. The data used for simulation of the IEEE 57 standard bus system model are taken from [11].

The procedure for designing the 57 bus system model is shown below.

(a)

Draw a single line diagram of the 57 bus system model with IEEE standard data.

(b)

Select the components from Simulink library, such as the PQ bus, the PV bus, the slack bus, generators, transformers, transmission lines and loads.

(c)

Draw a single line diagram by dragging all the components in the editor window.

(d)

Adjust the values of each component according to the corresponding given standard data.

(e)

Adjust the fixed MVA based value for the whole system.

(f)

Save the model with the desired name in the PSAT tool.

4.3. Load Configuration

The IEEE 57 bus system model of the electrical system is used with three different static loads. The essential purpose is to analyze the behavior of other variable loads, as shown in

Table 2. Generation and load configuration.

Bus No.	Bus Voltage		Generation		Load I (100% Regulation)		Load II (70% Regulation)		Load III (80% Regulation)
	Voltage Magnitude (p.u)	Phase Angle	Real Power (MW)	Reactive Power (MVAR)	Real Power (MW)	Reactive Power (MVAR)	Real Power (MW)	Reactive Power (MVAR)	Real Power (MW)	Reactive Power (MVAR)
1	1.040	0.000	4.78	1.289	0.55	0.17	0.385	0.119	0.44	0.136
2	1.010	0.000	0.000	−0.008	0.03	0.88	0.021	0.616	0.024	0.704
3	0.985	0.000	0.4	−0.01	0.41	0.21	0.287	0.147	0.328	0.168
4	1.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
5	1.000	0.000	0.000	0.000	0.13	0.04	0.091	0.028	0.104	0.032
6	0.98	0.000	0.000	0.008	0.75	0.02	0.525	0.014	0.6	0.016
7	1.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
8	1.005	0.000	4.50	0.621	1.50	0.22	1.05	0.154	1.2	0.176
9	0.98	0.000	0.000	0.022	1.21	0.26	0.0847	0.182	0.968	0.208
10	1.000	0.000	0.000	0.000	0.05	0.02	0.035	0.04	0.04	0.016
11	1.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
12	1.015	0.000	3.10	1.285	3.77	0.24	2.639	0.168	3.016	0.192
13	1.000	0.000	0.000	0.000	0.18	0.023	0.126	0.0164	0.144	0.0184
14	1.000	0.000	0.000	0.000	0.105	0.053	0.0735	0.0371	0.084	0.0424

.

(a)

Load 1: 100% regulation,

(b)

Load 2: 70% regulation, and

(c)

Load 3: 80% regulation.

4.4. The Fast Voltage Stability Index of Load 1

The critical bus can be identified by the fast voltage stability index method. However, a line or a bus with a higher value index is more essential than others, and a lower value index point shows a strong bus in the system. Thus, a suitable location for a renewable resource can be identified by the fast voltage stability index. Thus, the graphical representation of the fast voltage stability index of Load 1 is shown in Figure 8. By visual representation, it can be seen that, out of 80 lines, the critical lines are 7, 11, 23, 28, 29, 30, and 31. However, the critical buses are 6, 8, 9, 10, 12, 14, 15, 18, 19, and 20.

4.5. The Fast Voltage Stability Index of Load 2

The graphical representation of Load 2’s fast voltage stability index is shown in Figure 8. By graphical representation, it can be seen that, out of 80 lines, the critical lines are 37, 38, 39, 40, 43, 44, 45, 67, and 68. However, the critical buses are 26, 27, 28, 29, 30, 31, 32, 33, 52 and 43.

4.6. The Fast Voltage Stability Index of Load 3

The graphical representation of the fast voltage stability index of Load 3 is shown in Figure 8. By graphical representation, it can be seen that, out of 80 lines, the critical lines are 29, 30, 35, 36, 37, 38, 39, 40, 42, 43, 44 and 45. However, the critical buses are 18, 19, 20, 25, 26, 27, 28, 29, 30, 31, 32 and 33.

5. Results and Discussion

In this section, techniques, including integrating renewable resources (wind energy and photovoltaic energy) and FACTs devices, are implemented on the IEEE-57 bus system model, and results are analyzed. All parameters are shown in

Table A1. Dataset, inputs, assumptions and other major parameters used in this study.

Sr. No.	Inputs	Value	Source
1	Electrical Load
	Scaled actual load	Bus-specific	IEEE data
	Voltage and power rating	[400 kV, 100 MVA]	Not applicable
	Frequency	60 Hz	Not applicable
2	Tool Parameters
	PSAT tool	Not applicable	MATLAB library
	Version	2.2.11	Not applicable
	Base frequency	60 Hz	PSAT library
	Base power	100 MVA	PSAT library
	Simulation starting time	0 (s)	PSAT library
	Simulation ending time	20 (s)	PSAT library
	Power factor (PF) tolerance	0.0001	PSAT library
	Maximum PF tolerance	20	PSAT library
	Dynamic tolerance	0.0001	PSAT library
	Maximum Dynamic tolerance	20	PSAT library
	Time step	0.05 (s)	PSAT library
	Power flow solver	Newton–Raphson method	PSAT library
	Integration method	Trapezoidal rule	PSAT library
3	Wind Energy Resource
	DFIG	Built-in	Not applicable
	Voltage and power rating	[400 kV, 4 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Stator resistance (Rs) and reactance (Xs)	[0.01, 0.10] p.u	PSAT library
	Rotor resistance (Rr) and reactance (Xr)	[0.01, 0.08] p.u	PSAT library
	Magnetization reactance (Xm)	3.00 p.u	PSAT library
	Inertia constants (Hm)	8 kWs/kVA	PSAT library
	Pitch contrl gain (Kp)	10 p.u	PSAT library
	Time constant	3 (s)	PSAT library
	Power control time constant (Te)	0.01 (s)	PSAT library
	No. of poles and gear box ratio	[4, 1/89]	PSAT library
	Blade length	75.0 (m)	PSAT library
	No. of blades	3	PSAT library
	Pmax and Pmin	[1.0, 0.0] p.u	PSAT library
	Qmax and Qmin	[0.7, −0.7] p.u	PSAT library
	No. of wind generator	01	PSAT library
4	Wind Energy Resource
	SCIG	Built-in	Not applicable
	Voltage and power rating	[400 kV, 4 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Stator resistance (Rs) and reactance (Xs)	[0.01, 0.10] p.u	PSAT library
	Rotor resistance (Rr) and reactance (Xr)	[0.01, 0.08] p.u	PSAT library
	Magnetization reactance (Xm)	3.00 p.u	PSAT library
	Inertia constants [Hwr, Hm, Ks]	[2.5, 0.5, 0.3] kWs/kVA p.u	PSAT library
5	Diesel Generator Resource
	Diesel generator	Built-in	Not applicable
	Voltage and power rating	[400 kV, 4 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Active power	0.045 p.u	PSAT library
	Voltage magnitude	1.00 p.u	PSAT library
	Qmax and Qmin	[2.00, −1.40] p.u	PSAT library
	Vmax and Vmin	[1.1, 0.9] p.u	PSAT library
	Loss participation factor	1	PSAT library
6	Solar Energy Resource
	Minimum solar irradiance	4 MVA	PSAT library
	Average solar irradiance	8 MVA	PSAT library
	Maximum solar irradiance	12 MVA	PSAT library
	Active power	0.8 p.u	PSAT library
	Qmax and Qmin	[0.8, −0.2] p.u	PSAT library
	Vmax and Vmin	[1.1, 0.9] p.u	PSAT library
	Loss participation factor	1	PSAT library
7	Tap Changer Transformer
	Voltage and power rating	[400 kV, 100 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Resistance and reactance	[0.001, 1.195] p.u	PSAT library
	Fixed tap ratio	0.958	PSAT library
8	Fault Generation
	Fault starting time	3 (s)	PSAT library
	Fault clearing time	3.3 (s)	PSAT library
	Fault resistance and reactance	[0.0001, 0.0001] p.u	PSAT library
9	Static Var Compensator
	Voltage and power rating	[400 kV, 4 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Regular time constant Tr	10 (s)	PSAT library
	Regular gain Kr	100 p.u/p.u	PSAT library
	Reference voltage	1.00 p.u	PSAT library
10	STATCOM
	Voltage and power rating	[400 kV, 4 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Regular gain Kr	50 p.u/p.u	PSAT library
	Regular time constant Tr	0.1 (s)	PSAT library
	Imax and Imin	[1.2, 0.8] p.u	PSAT library
11	Line Parameters
	Voltage and power rating	[400 kV, 4 MVA]	PSAT library
	Frequency	60 Hz	PSAT library
	Resistance and reactance and susceptance	[0.023, 0.068, 0.0016] p.u	PSAT library

.

5.1. Scenario 1: The Integration of a Double-Fed Induction Generator (DFIG)

The integration of a double-fed injection generator with critical buses causes enhancement in voltage stability. In the simulation, the nominal wind speed is 15 m/s with an air density of 1.225 kg/m³. Hence, power is kept at 4 MVA with 60 Hz frequency, and the system voltage is 400 kV.

The Integration of a DFIG with Load 1, Load 2 and Load 3

The results of the voltage profile of the integration of a DFIG with the 27th bus system of the IEEE-57 bus system model of Load 1, Load 2 and Load 3 are shown in Figure 9, by taking the voltage magnitude per unit on the y-axis and the number of buses on the x-axis. It can be seen that the voltage magnitude is increased after the injection of a wind generator. The voltage magnitudes are shown in per-unit values. Thus, by improving the voltage, power stability increases through the reduction in voltage drop losses in the electrical system. The integration of a DFIG with a different wind speed profile is shown in Figure A1.

5.2. Scenario 2: The Integration of a Squirrel Cage Induction Generator (SCIG)

From the above scenario, it can be noted that in any case of load, the double-fed induction generator improves the voltage magnitude, which provides stability in the system. Now, we can take only Load 2 configurations and apply all other techniques. In this section, the squirrel cage induction generator is integrated with the 27th bus of the IEEE-57 bus system with a Load 2 configuration. The squirrel cage induction generator also provides stability in the system. Hence, power is kept at 4 MVA with 60 Hz frequency, and the voltage is 400 kV, with a nominal wind speed of 15 m/s and air density of 1.225 kg/m³. The graphical representation with per-unit values of voltage magnitudes is shown in Figure 10, by taking voltage magnitude per unit on the y-axis and the number of buses on the x-axis. Therefore, a SCIG with a different wind speed profile is shown Figure A2.

5.3. Scenario 3: The Integration of a Diesel Generator

In this section, a diesel generator is used instead of wind energy, and there is stability enhancement in the system. Hence, power is kept at 4 MVA with 60 Hz frequency, and the voltage is 400 kV. The graphical representation is shown below in Figure 11, by taking the voltage magnitude per unit on the y-axis and the number of buses on the x-axis.

5.4. Scenario 4: Comparison of DFIGs, SCIGs and Diesel Generators

The graphical representation of a comparison of DFIGs, SCIGs, and diesel generators is shown in Figure 12, by taking the voltage magnitude per unit on the y-axis and the number of buses on the x-axis.

It is clear from results that a diesel generator gave higher stability than a DFIG; and after the diesel generator, the DFIG showed higher strength. Hence, renewable energy is being adopted worldwide due to its positive environmental impact. A diesel generator has higher efficiency, but the cost is much higher, and the emission of gases due to fuel burning also impacts the environment. Thus, we can conclude that a DFIG shows greater enhancement in the voltage stability of the electrical power system than a SCIG or a diesel generator.

5.5. Scenario 5: The Integration of Photovoltaic Materials

From a literature review [3], it is clear that photovoltaic materials were used to meet the demand of the past decade. Therefore, in this section, a photovoltaic material is integrated to improve the voltage profile, enhancing power system stability. Thus, solar energy depends upon solar radiation, and solar radiation varies with time from day to evening. Therefore, we can relate power with solar radiation, divided into three categories, as shown in Figure 13—a solar generator gives 4 MVA power when there is cloudy weather or poor solar radiation, 8 MVA power when there is average solar radiation, and 12 MVA power when there is maximum solar radiation or at peak sunny day times [8].

5.5.1. Case 1: The Integration of a 4 MVA Photovoltaic Generator

Thus, a photovoltaic generator with 4 MVA power is integrated with the 27th bus of an electrical power system with Load 2. The graphical representation is shown in Figure 14, by taking the voltage magnitude per unit on the y-axis and the number of buses on the x-axis. It can be observed that it improves stability by injection of the voltage profile. So, there is an enhancement in voltage stability.

5.5.2. Case 2: The Integration of a 8 MVA Photovoltaic Generator

Now, a photovoltaic generator with 8 MVA power is integrated with the electrical system and considered the average solar radiation. The graphical representation is shown in Figure 15, taking the voltage magnitude per unit on the y-axis and the number of buses on the x-axis, and there is an improvement in the voltage profile that leads to voltage stability.

5.5.3. Case 3: The Integration of a 12 MVA Photovoltaic Generator

A photovoltaic generator with 12 MVA power is taken because when solar radiation is at a maximum, then maximum output power is delivered by the generator. The battery can store energy that can be used domestically, but there are losses from AC–DC conversion. Hence, preferably, photovoltaic power is used directly without storage. The graphical representation is shown in Figure 16 by taking the voltage magnitude per unit on the y-axis and the number of buses on the x-axis, which offers a significant rise in the voltage profile that improves voltage stability and reduces losses in the system. The overall efficiency and stability of the system increase.

5.5.4. Case 4: Comparison of Photovoltaic Generators

The comparison of photovoltaic generators with 4, 8, and 12 MVA is shown in Figure 17. The graphical representation shows that a higher-power generator has a higher output voltage and higher stability. It can be concluded that generators under higher solar radiation have higher stability.

5.6. Scenario 6: Fault Generation and the Integration of FACTs Devices

The literature review [28] shows that FACTs devices can be used for stability purposes, mainly in the case of faults. When a temporary fault in the system causes the voltage of the electrical system to be unbalanced, FACTs devices are used for stability purposes. Thus, a fault is generated with a duration of 0.3 s (from 3 to 3.3 s), and a graphical representation is shown in Figure 18, showing that the voltage profile of the whole system is disturbed because this is a temporary fault, so the voltage magnitude is disturbed rather than there being a blackout.

5.6.1. Case I: The Integration of a Static VAR Compensator (SVC)

The graphical representation of the integration of the static VAR compensator is shown in Figure 19. A 4 MVA power SVC is integrated after the injection of the fault, and it is seen through the graph that there is an enhancement in the voltage profile throughout the system. Thus, in the case of a temporary fault, a static VAR compensator can be used to enhance voltage stability.

5.6.2. Case II: The Integration of a STATCOM

After the injection of a SVC, a STATCOM is now injected in place of a SVC. Thus, there is more improvement in voltage magnitude because a STATCOM has a higher efficiency than a static VAR compensator. The graphical representation is shown in Figure 20.

5.6.3. Case III: The Integration of a SVC and a STATCOM

In this section, a static VAR compensator and a STATCOM are compared. From the literature review, it is clear that FACTs devices are used for enhancement in voltage stability but only in the case of temporary fault generation. Therefore, compared to a SVC and a STATCOM, a STATCOM has a higher efficiency because the voltage source converter is connected to the primary winding of the transformer. Thus, stability can be improved by both a SVC and a STATCOM, but a STATCOM has a higher efficiency and reliability too. The graphical representation is shown in Figure 21. It can be noted that the voltage profile in the case of a STATCOM is higher than a SVC, thus there is more reliability and stability in the case of a STATCOM. Therefore, a STATCOM is the best and most suitable option for enhancement in voltage stability in the case of a temporary fault.

6. Conclusions and Future Work

In the electrical power system, stability is considered an essential factor. Therefore, the voltage profile of the power system must be improved to enhance stability. Hence, in recent years, much research has been performed in this area, mostly theoretically. In practice, voltage stability was improved by conventional techniques such as the injection of reactive power with the help of capacitor banks, the installation of new generators, and a reduction in system reactance. Thus, the main reason behind voltage instability was that due to non-centralized generation, the loads became unbalanced, and losses in the voltage occurred in the form of a voltage drop. Because generation is far away from loads, voltage instability occurs. Now, the use of a new renewable technology increases daily due to its clean energy and because it can be used domestically and commercially.

Integrating energy allows the generators to be installed near the loads, which helps with centralized distribution. Therefore, in this study, different techniques for the integration of renewable resources and FACTs devices are adopted. Further, in terms of renewable techniques, there is the integration of wind energy in which double-fed induction generators (DFIG) and squirrel cage induction generators (SCIGs) are used as well as the integration of photovoltaic materials with different ratings in generators. Lastly, FACTs devices (integration of a static VAR compensator and a static synchronous compensator) are used for stability in the case of a fault.

In conclusion, compared with integrating a double-fed induction generator and a squirrel cage induction generator, the DFIG shows a higher voltage profile improvement. So, we can conclude from the results that a DFIG is better than a SCIG to improve stability. Now, by the integration of photovoltaics, it can be concluded that the higher the solar radiation, the higher the power output of the solar generator and the greater the improvement in the voltage profile of the electrical system. In temporary faults, a static VAR compensator and a STATCOM are used; and from the results, it can be concluded that a STATCOM enhances stability more than a SVC during transient faults.

Lastly, on comparing PV generators, DFIGs and SCIGs, it can be concluded that DFIGs are the most suitable option when considering all the factors. Additionally, solar and wind availability varies with time and location. So, both can be used according to context, and a STATCOM can be used in the case of temporary faults.

7. Future Work

In future work, the enhancement of frequency stability should be investigated as the voltage stability has been in this study. Thus, by integrating other renewable resources such as tidal, ocean, geothermal, and fossil fuels, frequency stability could be improved along with voltage stability to enhance system efficiency.