A novel method to investigate voltage stability of IEEE-14 bus wind integrated system using PSAT

Abstract

The maximum demand of power utilization is increasing exponentially from base load to peak load in day to day life. This power demand may be either industrial usage or household applications. To meet this high maximum power demand by the consumer, one of the options is the integration of renewable energy resources with conventional power generation methods. In the present scenario, wind energy system is one of the methods to generate power in connection with the conventional power systems. When the load on the conventional grid system increases, various bus voltages of the system tend to decrease, causing serious voltage drop or voltage instability within the system. In view of this, identification of weak buses within the system has become necessary. This paper presents the line indices method to identify these weak buses, so that some corrective action may be taken to compensate for this drop in voltage. An attempt has been made to compensate these drops in voltages by integration of renewable energy systems. The wind energy system at one of the bus in the test system is integrated and the performance of the system is verified by calculating the power flow (PF) using the power system analysis tool box (PSAT) and line indices of the integrated test system. The PF and load flow results are used to calculate line indices for the IEEE-14 bus test system which is simulated on PSAT.

Keywords

voltage stability / line indices / power system analysis tool box (PSAT) / wind system / line loading / power flow (PF)

Introduction

Voltage stability has been a major concern in the area of power system because of possible voltage collapse. Due to increased power demand, more and more attention is being paid to meet this challenge by monitoring voltage profile and voltage regulation of the system. Environmental concerns from fossil-fuel-based generation have propelled the integration of less-polluting energy sources in the generation portfolio and simultaneously have motivated increased energy conservation programs [¹]. Renewable energy emerges from sources that are naturally inexhaustible, like water (water energy), the sun (solar energy) and the wind (wind energy), or from the sources that are exhaustible provided they are well managed i.e., the heat from the earth (geothermal energy) and biomass from organic waste and wood, etc. Usage of two or more different renewable energy sources in an embedded energy system is called a hybrid energy plant [²]. When the load on the conventional power generation system increases to meet the demand of the consumer, the voltage profile of the system deteriorates for various reasons [³]. This may be result from the decrease in the reactive power of the lines. Power system utilities concerns have preferred focus mainly on stability problems over power quality issues for better and effective wind power integration to boost overall system performance [⁴,⁵]. Voltage stability analysis is mainly conducted to predict the point of voltage collapse or possible fault within the system. Various line voltage stability indices have been proposed to identify the weak bus in the system [⁶,⁷]. Power quality and stability may also be affected due to wind power integration. Higher and fluctuating wind penetration also causes the drop in power transfer capacity of the system [⁸].

One method of determining voltage stability of wind integrated system is to evaluate the derived voltage stability indices. The values of the voltage stability indices would indicate the distance of voltage collapse for a given line loading condition [⁹]. In the present scenario, small-size wind systems made up of doubly fed induction generators (DFIG) located within the distribution system are increasing rapidly [¹⁰]. Small signal stability study and impact of squirrel cage induction generator (SCIG) and DFIG on the power system, have already been conducted on the IEEE-14 bus system with SCIG, DFIG wind turbine systems. The simulation result shows that fixed speed wind generators are simpler to operate and reliable but they are limited as the energy output of a wind turbine. It is also depicted that variable speed wind generators of similar rating can improve the stability of the system effectively [¹¹].

Voltage stability analysis

Voltage stability is the ability of the power system to maintain steady acceptable voltages at all the buses in the system under normal operating conditions and after being subjected to a disturbance [¹¹]. Voltage stability analysis can be done either online or offline, but offline study is preferred because it is less risky [¹²]. Offline voltage stability analysis is normally conducted at the planning stage and is capable of giving early indication of the system load status. Thus necessary precaution and compensation techniques must be employed to avoid such incidents or occurrence of various undesirable faults within the system.

Many major blackouts caused by power system instability have been studied so far, which illustrates the importance of these phenomena [¹³,¹⁴]. Various types of voltage stabilities, their classification and transient stability have been studied to make voltage instability problem more confined and conceptual [¹⁵].

Wind power integration

Integration of wind power causes some improvement in the voltage profile of the system [¹⁶]. Voltage level stabilizes itself when the bus, which is most prone to voltage instability or possible voltage collapse point, is integrated with the wind system [¹⁷]. The wind energy system, available in PSAT Simulink library with different configurations of induction generators (IG) integrated with the IEEE-14 bus system is shown in Fig. 1. The voltage stability of the power system would be deteriorated with the steady increase of wind energy penetration, considering the large variation range of power output from wind farms [¹⁸]. Power system security and stability may also be affected by higher wind power penetration [¹⁹].

Power system analysis toolbox (PSAT)

PSAT is a Matlab toolbox for electric power system analysis and control, which is available online. With the use of PSAT, the power flow (PF), continuation power flow (CPF), optimal power flow, small signal stability analysis, and time domain simulation can be plot and analyzed to monitor various possible point of collapse within the system. This toolbox also provides a complete graphical interface and a Simulink-based online network editor. All operations can be assessed effectively by means of the graphical user interfaces (GUIs) of PSAT. In PSAT, a Simulink-based library provides an user friendly toolkit for network design. One advantage of using PSAT is that it allows drawing electrical schemes and multi machine networks by means of pictorial blocks [²⁰]. The IEEE-14 bus test system used in this paper is simulated using PSAT.

Voltage stability analysis using line indices

The purpose of voltage stability indices is to determine the point of voltage instability or possible voltage collapse within the system [²¹]. These indices are referred either to a bus or a line. In this paper, line stability indices for the IEEE-14 bus system are evaluated for each bus for base load to peak line loading condition. The load flow program in Matlab R2012a is developed to obtain the power flow (PF) solution for the system under test. The results obtained from the load flow program and PF using GUI of PSAT are used to calculate line indices values for the system. The values of these line stability indices with voltage and angle profile of buses would indicate the voltage stability or possible voltage collapse condition within the system for a variable loading condition as shown in Figs. 1 and 2.

Line voltage stability indices FVSI

The line voltage stability indices which exhibits FVSI very close to unity (1.0) implies that it is approaching toward point of instability. If FVSI is found to be greater than unity, this is the indication of voltage collapse in the system due to sudden drop in voltage [²²]. Line stability indices (FVSI), proposed in Ref. [⁵] can be defined further as,

(1){\mathrm{FVSI}}_{ij}={\displaystyle \frac{\mathrm{4}{Z}^{\mathrm{2}}{Q}_j}{{\left|V_i\right|}^{\mathrm{2}}X}},

where Z is the line impedance, V is the sending end voltage, X is the line reactance, and Q is the reactive power at the receiving end.

Line voltage stability indices L_mn

This line voltage stability indices is used to find the stability indices for each line connected between two bus bars in an interconnected network. As long as the stability indices L_mn remains less than 1, the system is stable. This voltage stability criterion is based on a power transmission concept in a single line [⁵] and can be defined as,

(2)L_{\mathrm{m}\mathrm{n}}={\displaystyle \frac{\mathrm{4}X{Q}_j}{{[V_i\sin (\theta -\delta )]}^{\mathrm{2}}}},

where Q is the reactive power at receiving end, q is the line impedance angle, V is the sending end voltage, X is the line reactance, and d is the angle between supply voltage and receiving end voltage.

Computation of line voltage stability indices

The test systems presented in Figs. 3 and 4 are simulated using PSAT version 7.5.1. It consists of 14 buses, 16 lines, 4 transformers, 5 generators and 11 loads. Loading of the buses and lines has been done using the standard IEEE-14 bus data format [²³]. The result of PF obtained for the IEEE-14 test bus system is presented in Tables 1, 2 and 3, respectively. Two line stability indices L_mn and FVSI have been calculated when line loading is varied from 25% to 100% of the base loading. Values of two line indices FVSI and L_mn for different line loading conditions are also presented in Tables 2 and 3. It is evident from the results that Buses No. 5, 7, 8 and 14 have line indices values close to unity when the line is fully loaded (maximum loading condition). So these buses can be treated as buses which may lead to voltage instability or possible voltage drop if loaded further. The integration of the wind system is done at Bus 14, which is most likely to cause voltage instability.

The integration of the wind system at Bus 14 is random, which can be integrated at any bus that is most likely to reach the voltage instability region. The PF and line indices values for the wind integrated system, given in Tables 4, 5 and 6 shows improvement in both, per unit values of stability indices as well as voltage and angle magnitude for all the buses of the system. The values of FVSI and L_mn, indicate that the integration of the wind system has improved the line indices value for Buses 5, 7, 10 and 14 up to significant level.

Results

Based on the results obtained from simulated model on PSAT and Matlab with and without integration of wind are shown in Figs. 5–8. Comparative study of plots shows that with the integration of wind, the two line voltage stability indices have improved significantly, indicating that the buses are less stressed or less prone to voltage collapse within the system.

Conclusions

A thorough study of voltage stability analysis for IEEE-14 bus wind integrated system based on line stability indices and PF is presented in this paper. The line stability indices are tested for variable line loading conditions. The power flow results using PSAT and line voltage stability indices are calculated with and without wind integration. It is also concluded that the integration of the wind system improves the voltage profile of the system. Besides, the two line stability indices FVSI and L_mn indicate better improvement close to stability limit with the integration of the wind system.

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