Investigation of Voltage Profile of Grid Integrated with Wind Turbine Generator

Investigation of Voltage Profile of Grid Integrated with Wind Turbine Generator

Kavita Yadav

Student

Electrical and Instrumentation Engineering Department Thapar University, Patiala

Manbir Kaur

Associate Professor

Electrical and Instrumentation Engineering Department Thapar University, Patiala

Abstract In a power system network, identification of weaker buses is critical and it is vital to improve the voltage profile and to minimize losses for its steady state stability. A comprehensive analysis is conducted about the optimum bus location of wind turbine generator in an IEEE 9 bus system. In this paper the impact of wind source penetration on voltage profile and losses is investigated on the system with load variation from 25% to 100%. The uncertain nature of wind speed is mathematically modeled and load demand pattern is considered for 24 hour. The result obtained shows that the lower wind penetration supports the voltage profile under the variations in load demand.

Keywords Squirrel Cage Induction Generator, Voltage Profile.

1. INTRODUCTION

   In the era of global energy crisis, the volatility of incessant oscillation of fossil fuel prices and complications of suffering ecological balance has acquired an accelerating significance and awareness towards alternative sources of energy. Wind power generation has thrived rapidly worldwide among distinctive competitive alternative sources of energy. Although, wind power generation may cause complications to the existing grid regarding power quality issues. One of the major concern for power system utilities is voltage stability which occurs due to several events in power systems such as generator reaching reactive power limits, action of tap changing transformers, and increase in loading, load recovery dynamics and line or generator outages. They

   cage induction wind turbine generators (SCIGs) are widely used in power systems. In this system, rotor speed is almost constant, thus it is sometimes called a fixed-speed wind turbine system. As wind generators can be integrated on transmission or distribution system. The impact of wind generators on the distribution system has been studied in [4][5] which shows that the integration of wind generator on the distribution system leads to improvement of the voltage profile and reduction in the losses. This is due to the characteristics of distribution system. Alternatively, when wind turbine is connected to transmission system leads to decline of the voltage profile and increase in the system losses which has been suggested in [6][7]. This is due to the interaction of power flow on the transmission system.

2. WIND GENERATION

   1. Model of Wind source

      Wind turbines produce electricity by using wind power to drive an electrical generator. Passing over the blades, wind generates lift and exerts a turning force. The rotating blades turn a shaft inside the nacelle, which goes into a gearbox. The gearbox adjusts the rotational speed to that which is appropriate for the generator, which uses magnetic fields to convert the rotational energy into electrical energy [7][8]. The wind turbine extracts kinetic energy from the swept area of the blades. The wind power is demonstrated as kinetic energy of the flowing air mass per unit time given by,

      may cause a progressively and uncontrolled fall of voltages leading to voltage instability or voltage collapse. The

      Pwind

      1 Av3

      2

      (1)

      integration of the wind production unit in the network causes difficulties such as the absence of voltage adjustment and the sensitivity of voltage drops [1].

      The major problem of wind is its intermittent nature that

      The power coefficient (Cp) describes the efficiency of a turbine that converts kinetic energy in the wind into rotational power. Cp values varies between 0.3-0.45. [9] Therefore, power output of the turbine is given by,

      is great variability of its production and especially the difficulty in predicting the production precisely several hours

      Pwind

      1 Av3 Cp (2)

      2

      in advance. According to the intensity and rate of change, the difficulties with the frequency and the voltage control could

      The variation of wind power output with wind speed is expressed in equation (3),

      seem making a direct impact on the level of the provided

      0,

      v vcin

      electric power quality. Mostly, the stability and reliability

      1

      1

      2

      2

      Av3C , v

      v v

      (3)

      studies are carried out whenever wind power is connected to power system to predict severe consequences on the power

      Pw

      r p cin r cou

      system. The proposed works in literature [2][3] suggest that the connection of wind turbines to the electric grid may affect stability of the system due to the random nature of the wind

      where,

      P

      0,

      v vcou

      and the characteristics of the wind generator. Because of the simple design and cost-effective performance the squirrel

      w is the wind power output (watts),

      is the air density (1.225 kg/m3 at 15C and normal pressure), A is the swept area (m2),

      vcin is cut in speed (m/sec), vr is rated speed (m/sec), vcou is cut out speed (m/sec)

      Figure 1 shows the wind power obtained for the variation

      in wind speed.

      Fig. 1. Wind power output

   2. Modeling of Wind source Generator

      Induction machines are used extensively in the system as motors, but not generators. It is mainly due to the dened relationship between the export of active power and absorption of reactive power, (Q). But, the benet of induction generator to provide large damping torque in the prime mover, makes it suitable for the application in xed speed wind turbines[10][14]. The SCIG is modeled as a PQ bus with the real power and reactive power demand specied.

      Rs and Rr are stator and rotor resistance per phase respectively,

      Xs and Xr are stator and rotor leakage reactance per phase respectively,

      is the power factor angle.

      From the equivalent circuit, input impedance is expressed

      as

      Fig.2: Equivalent circuit of SCIG

      Figure 2 shows the per phase equivalent circuit of squirrel cage induction generator where,

   3. Load ow Analysis Method

   The general steady-state effect of connecting wind turbine generators to the grid can be analyzed through the load-ow analysis. It calculates the voltage drop on each feeder, the voltage at each bus, and the power-ow in all branch and feeder circuits. It determines if the voltages remain within specied limits under various contingency conditions, and whether equipment such as transformers and conductors are overloaded. Load-ow studies are often used to identify the need for additional generation, capacitive, or inductive VAR support, or the placement of capacitors and /or reactors to maintain system voltages within specied limits. It is also necessary for planning, economic scheduling, exchange of power between utilities and control of an existing system as well as planning its future expansion. Here, Newton Raphson method is used to carry out load ow analysis on IEEE 9 bus system with and without wind source.

3. REQUIREMENT OF REACTIVE POWER AND VOLTAGE PROFILE

   Under normal conditions, the acceptable limits of voltage ranges from 0.95 to 1.05 p.u. for a reliable power networks. It is quite difcult to maintain acceptable voltage limits without reactive power supply.

   1. Power system networks with conventional generators.

      Z R jX ( jX || ( Rr in s S m s

      • jXr ))

      (4)

      Synchronous generators are commonly used as a conven- tional generator connected to grid that can supply or consume reactive power. For these generators reactive power produc-

      Circuit parameters can be express as equation (5) to (8)

      tions are provided by automatic voltage regulator which in

      s

      s

      I Vs Zin

      Va Vs Is (Rs jXS ) Pw 3 |Vs || Is | cos Qw 3 | Vs || Is | sin

      (5)

      (6)

      (7)

      (8)

      turn will maintain the system voltage prole. During normal operation, variation in the generation and load prole affects the system voltage prole at different buses [15].

   2. Power system networks with wind generators

   When wind generators are connected to meet the increased load demand, the output of the conventional generator is op- timally used to meet the load demand which is tabulated in table I when wind source was connected at different locations. In case there is no output from the wind generator, demand will be met through conventional generators. In case of transmission system, the impact of

   wind power depend upon the location of wind generator in relation to load [16][17]. The power balance equations are given by

   1. Network with integration of the wind generators at different locations and varying load.

   2. Network with wind generator with variation in wind

   Pgen Pwind Pdemand Plosses

   Qgen Qdemand Qlosses Qwind

   Wind source at	Pcon	Pw	PD	PL	Qcon	Qw	QD	QL
   Bus 5	291.5	28	315	4.5	18.06	1.17	115	-95.80
   Bus 6	291.3	28	315	4.7	40.75	-18.27	115	-92.50
   Bus 8	29.1	28	315	5.1	53.62	-24.50	115	-85.90

   Wind source at	Pcon	Pw	PD	PL	Qcon	Qw	QD	QL
   Bus 5	291.5	28	315	4.5	18.06	1.17	115	-95.80
   Bus 6	291.3	28	315	4.7	40.75	-18.27	115	-92.50
   Bus 8	29.1	28	315	5.1	53.62	-24.50	115	-85.90

   Table-1

   (9)

   (10)

   power and load in a day at different penetration levels.

   1. Network without the wind generator with varying loading conditions

      The load ow analysis on the test system is carried out with variation in load demand from 25% to 100%. The result obtained is tabulated in table II.

      Bus	Standard load		25 % load increase
      	Voltage(p.u.)	angle(Deg)	Voltage(p.u.)	angle(Deg)
      1	1.040	0.00	1.040	0.0
      2	1.025	9.280	1.025	3.63
      3	1.025	4.665	1.025	-0.85
      4	1.025	-2.217	1.016	-4.69
      5	0.995	-3.989	0.977	-8.60
      6	1.012	-3.6870	0.997	-7.99
      7	1.025	3.7200	1.017	-1.98
      8	1.015	0.728	1.002	-5.52
      9	1.032	1.907	1.02	-3.57

      Bus	Standard load		25 % load increase
      	Voltage(p.u.)	angle(Deg)	Voltage(p.u.)	angle(Deg)
      1	1.040	0.00	1.040	0.0
      2	1.025	9.280	1.025	3.63
      3	1.025	4.665	1.025	-0.85
      4	1.025	-2.217	1.016	-4.69
      5	0.995	-3.989	0.977	-8.60
      6	1.012	-3.6870	0.997	-7.99
      7	1.025	3.7200	1.017	-1.98
      8	1.015	0.728	1.002	-5.52
      9	1.032	1.907	1.02	-3.57

      Table-2: Result of load flow analysis

4. TEST CASE AND RESULTS

   In this study, the voltage prole and losses in IEEE 9 bus system has been investigated for the optimal location of wind source in the system. The simulation is carried out using Powerworld simulator. [18] The single line diagram of IEEE 9 bus test system is shown in gure 3.

   Fig.3: IEEE 9 bus

   In order to arrive at the optimum bus location the following system parameters are investigated:

   1. Steady-state voltage magnitudes and angle at each bus.

   2. Total transmission losses.

      The outcomes are yielded for the following network states:

      i). Network without the wind generators with standard load and varying loading conditions.

      Bus	50 % load increase		100 % load increase
      	Voltage(p.u.)	angle(Deg)	Voltage(p.u.)	angle(Deg)
      1	1.040	0.00	1.040	0.0
      2	1.025	-2.36	1.025	-12.63
      3	1.025	-6.75	1.025	-17.4185
      4	1.002	-7.26	0.971	-11.34
      5	0.95	-13.45	0.908	-20.62
      6	0.975	-12.60	0.924	-21.00
      7	1.00	-8.02	0.984	-18.43
      8	0.986	-12.16	0.949	-24.13
      9	1.01	-9.49	<>0.99	-20.21

      As the load demand progressively increased from 25% to 100% buses 5, 6 and 8 suffer from voltage instability.

   1. Network with the wind generators at different locations with varying load conditions

      Fig.4: Voltage prole with WG at bus 5

      When wind generator was connected at bus 5 with standard load and varying load condition the change in voltage prole in the buses is shown in gure 4 for xed wind speed. Voltage at bus 5 reaches to 0.97 p.u.

      Fig.5: Voltage prole with WG at bus 6

      When wind generator was connected at bus 6 with standard load and varying load condition the change in voltage prole in the buses is shown in gure 5. Voltage at bus 5 reaches to 0.908 p.u.

      Fig.6: Voltage prole with WG at bus 8

      When wind generator was connected at bus 8 with standard load and varying load condition the change in voltage prole in the buses is shown in gure 6. It can be shown that voltage prole at bus 5 reaches to 0.88 p.u.

      It was observed from gures 4, 5 and 6 that voltage prole of the buses are better when the wind generator was connected at the bus 5 wherein, connected at buses 6 or 8. In order to enhance the voltage stability, the best location of wind turbine is the weakest bus which contains the largest load and that is bus 5.

   2. Network with wind generator with variation in wind power and load in a day at differenet penetration level

      The impact of wind generation in transmission networks is considered for increased in load demand up to 150% and

      wind source penetration is being investigated and results are depicted in gures 7 and 8.

      Fig.7: Voltage prole with 10% penetration level of wind

      Fig.8: Voltage prole with 30% penetration level of wind

      It can be analyzed from gure 7 and 8 that the system is within the limit at lower wind penetration level but become unstable at higher wind penetration level.

   3. Comparision of losses.

   With respect to change in location of wind generator relative to the load, there is increase or decrease in the losses. This is due to the relation between wind power production and load consumption in the transmission system.

   Cases	MW	MVar
   Without WG	4.6	-92.2
   WG at bus 5	4.5	-93.2
   WG at bus 6	4.7	-91.2
   WG at bus 8	5.1	-88.6

   Cases	MW	MVar
   Without WG	4.6	-92.2
   WG at bus 5	4.5	-93.2
   WG at bus 6	4.7	-91.2
   WG at bus 8	5.1	-88.6

   Table-3: Active and reactive power losses

   It can be analyzed that from table III that there is increase in losses when wind turbine was connected at buses 6 and 8 and decrease in losses when connected at bus 5. So, bus 5 can be considered as optimal location for the connection of wind turbine generator.

5. CONCLUSION

This paper investigates the impact of wind generation on power system network particularly on voltage prole and losses of the system. The load-ow analysis has been performed on the IEEE 9 bus system without wind generator and with wind generator at distinct locations and with varying loading condition. It can be derived from the tabulated results that the voltage prole obtained at bus 5 is more prominent than it is at bus 6 and bus 8 at different loading condition. The voltage prole of the buses are in limit at lower penetration level than higher penetration levels. The losses in the system with and without wind generator at distinct bus location are assessed. Location of wind generator at Bus 5 can be considered as the optimum choice. It can thus be concluded that optimum location and penetration level of wind generator are to be consider for a better system voltage prole.

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