3. Modeling of Derna wind farm

Darnah wind farm turbines are located in Derna city, which is located on the coastline of Mediterranean around (320 29<sup>0</sup> 16.728<sup>00</sup> N—latitude 200 49<sup>0</sup> 54.26400—longitude), in the eastern part of Libya as shown in Figure 2 with population of 80,000 [1].

#### 3.1. Modeling of wind turbine

The wind energy system transfers the kinetic energy extracted from wind into mechanical energy through rotor blades of the wind turbine, and the permanent magnet synchronous generator (PMSG) transforms the mechanical energy in the rotor blades into electrical energy.

Figure 2. The geographic situation of Derna-Libya.

The power extracting from wind depends on the covered area A of the rotor and the wind velocity Vw and the air density p. The generated mechanical power Pmech is generally computed form wind energy using the coefficient of power Cp as follows [4]:

$$P\_{mch} = \left(\frac{1}{2}\right) \mathbb{C}\_{\mathbb{P}}(\lambda, \theta) A \rho V\_w^3 \tag{1}$$

The performance coefficient Cp is a function of the A and θ that depends on the wind velocity Vw, the rotational speed of the shaft wr and the rotor radius Rr.

$$
\lambda = \left[\frac{\alpha\_r R\_r}{V\_w}\right] \tag{2}
$$

(r.p.m) tends for speed of the rotor, Rr stands for e rotor (m) and Vw stands for velocity of wind (m/s). The power of the wind turbine versus wind speed and aerodynamic coefficients are shown in Figure 3.

#### 3.2. Modeling of drive train

requirements of the facilities. Therefore, the characteristics of the grid must be evaluated properly after the wind energy systems are connected to the grid. Prior knowledge of wind system characteristics must be adequately defined to avoid the drawbacks of connecting such sources. The electrical characteristics of wind turbines are usually specified by the manufacturer, not by specific site location. For this reason, when the electrical characteristics of a particular wind turbine are known, their impact on the power quality when connected to a particular location in the network can be predicted and calculated as a set of units. The necessity for quality requirements, detailed and applicable documentation on the power quality of wind sources is required. The International Electrotechnical Commission (IEC) started work to facilitate this in 1996. As a result, IEC 61400-21 was developed and, today, most large wind turbine manufacturers provide power quality characteristic data accordingly [18, 19].

Darnah wind farm turbines are located in Derna city, which is located on the coastline of Mediterranean around (320 29<sup>0</sup> 16.728<sup>00</sup> N—latitude 200 49<sup>0</sup> 54.26400—longitude), in the eastern

The wind energy system transfers the kinetic energy extracted from wind into mechanical energy through rotor blades of the wind turbine, and the permanent magnet synchronous generator (PMSG) transforms the mechanical energy in the rotor blades into electrical

3. Modeling of Derna wind farm

42 Stability Control and Reliable Performance of Wind Turbines

3.1. Modeling of wind turbine

Figure 2. The geographic situation of Derna-Libya.

energy.

part of Libya as shown in Figure 2 with population of 80,000 [1].

The behavior of the drive train dynamics is considered by taking three different model approaches such as single mass, double mass, and three mass model design in order to know which of the methods are more noticeable in detaining the performance of the network [17]. The study of drive train models depends upon the complexity of the network. If a study takes interest about the torsion fatigue, it just has to consider the dynamics of all parts of the networks [20, 21]. For these aim, double lumped mass or more accurate models are required. For that, when application targets on the interaction between wind farms and connected loads, the considered drive train model considered being a single mass model for the sake of simplicity. Due to the direct connection of generator shafts of the turbine, the model of drive train can be defined as:

Figure 3. Power coefficient Cp (λ, θ) curves.

$$\frac{d\boldsymbol{\alpha}\_{mch}}{dt} = \left(\frac{1}{\dot{\jmath}}\right) (T\_{mch} - T\_{elec} - f\boldsymbol{\alpha}\_{mch}) \tag{3}$$

$$\int \omega\_{mech} = \theta\_{mech} \tag{4}$$

Now putting the value of ϕsd and ϕsq in Eqs. (5) and (6), so one can land up with the expression

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:…

dt LdIds <sup>þ</sup> <sup>ϕ</sup><sup>m</sup>

So, again resolving the above mentioned equation we can land up with following equation:

d

d

Therefore, in order to solve for the d and q axis stator currents, the above mentioned equation

Vqs � RsIqs � <sup>ω</sup>elecð Þ� LdIds <sup>ω</sup>elecϕ<sup>m</sup>

where Rs stands for the stator winding resistance, Ld stands for the stator inductance in direct axis, Lq stands for the stator inductance in quadrature axis, Vds stands for the direct axis stator voltage, Vqs stands for the quadrature axis stator voltage, Ids stands for the direct axis stator current, Iqs

Since the permanent magnet synchronous generator (PMSG) is a machine similar to wound rotor machine which is better for surface-seated applications, the generated electric torque by

<sup>P</sup> <sup>ϕ</sup>mIqs <sup>þ</sup> Ld � Lq

However, if permanent magnet synchronous generator is surface seated, then it is conceivable

In steady-state positions, the active power generated e from permanent magnet synchronous

stands for the quadrature axis stator current, Welec = Pwmech stands for the speed [19, 24].

dt LqIqs <sup>þ</sup> <sup>ω</sup>elec LdIds <sup>þ</sup> <sup>ϕ</sup><sup>m</sup>

dt Iqs <sup>þ</sup> <sup>ω</sup>elec LdIds <sup>þ</sup> <sup>ϕ</sup><sup>m</sup>

� <sup>ω</sup>elec LqIqs (9)

dt Ids � <sup>ω</sup>elec LqIqs (11)

Vds � RsIds <sup>þ</sup> <sup>ω</sup>elec LqIqs (13)

(14)

IdsIqs (15)

<sup>P</sup> <sup>ϕ</sup>mIqs <sup>þ</sup> ð Þ Ld � Ld IdsIqs (16)

<sup>P</sup> <sup>ϕ</sup>mIqs (17)

(10)

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45

(12)

d

d

Vds ¼ RsIds þ Ld

Ld

Vqs ¼ RsIqs þ Lq

can be formed in the following fashions:

the PMSG can be defined as follows:

generator is given by:

d dt Iqs <sup>¼</sup> <sup>1</sup>

d

dt Ids <sup>¼</sup> <sup>1</sup>

Lq

Telec <sup>¼</sup> <sup>3</sup> 2

Telec <sup>¼</sup> <sup>3</sup> 2

So, one can land up with the equation as follows:

to favor the assumption of Ld = Lq. Then torque can be expressed as follows:

Telec <sup>¼</sup> <sup>3</sup> 2

Vds ¼ RsIds þ

Vqs ¼ RsIqs þ

as below:

where Tmech stands for the mechanical torque generated by the wind turbine, Telec stands for the generated electromagnetic torque by the permanent magnet synchronous generator which can also be represented as a Tgen, j stands for the inertia moment and f stands for the viscous friction coefficient that cannot be considered in a medium-scale wind turbine. In order to have the voltage in a-b-c frame from the d-q frame, one can need the angle θ that obtained after integrating the mechanical speed of the rotor h.

#### 3.3. Modeling of permanent magnet synchronous generator

The model used for modeling the synchronous generator, which is based on permanent magnet (PMSG) is developed on the d-q axes 'park' model as shown in Figure 4. The mathematical model of PMSG is implemented using two-phase synchronous rotating reference frame theory in which the q-axis is in 90 degrees with the d-axis with reference to the direction of rotation. All the quantities in the rotor are referred to the stator, and it is given as follows [22, 23]:

$$V\_{ds} = R\_s I\_{ds} + \frac{d\phi\_{sd}}{dt} - \omega\_{elec} \phi\_{sq} \tag{5}$$

$$V\_{qs} = R\_s I\_{qs} + \frac{d\phi\_{sq}}{dt} - \omega\_{elec} \phi\_{sd} \tag{6}$$

where the stator fluxes are computed by following equation:

$$
\phi\_{sd} = \mathcal{L}\_d I\_{ds} + \phi\_m \tag{7}
$$

$$
\phi\_{sq} = L\_q I\_{qs} \tag{8}
$$

Figure 4. PMSG model in steady state condition with respect to the rotor flux reference frame: (a) direct axis and (b) quadrature axis.

Now putting the value of ϕsd and ϕsq in Eqs. (5) and (6), so one can land up with the expression as below:

dωmech dt <sup>¼</sup> <sup>1</sup> j � �

integrating the mechanical speed of the rotor h.

44 Stability Control and Reliable Performance of Wind Turbines

3.3. Modeling of permanent magnet synchronous generator

where the stator fluxes are computed by following equation:

quadrature axis.

ð

where Tmech stands for the mechanical torque generated by the wind turbine, Telec stands for the generated electromagnetic torque by the permanent magnet synchronous generator which can also be represented as a Tgen, j stands for the inertia moment and f stands for the viscous friction coefficient that cannot be considered in a medium-scale wind turbine. In order to have the voltage in a-b-c frame from the d-q frame, one can need the angle θ that obtained after

The model used for modeling the synchronous generator, which is based on permanent magnet (PMSG) is developed on the d-q axes 'park' model as shown in Figure 4. The mathematical model of PMSG is implemented using two-phase synchronous rotating reference frame theory in which the q-axis is in 90 degrees with the d-axis with reference to the direction of rotation. All

dϕsd

dϕsq

Figure 4. PMSG model in steady state condition with respect to the rotor flux reference frame: (a) direct axis and (b)

the quantities in the rotor are referred to the stator, and it is given as follows [22, 23]:

Vds ¼ RsIds þ

Vqs ¼ RsIqs þ

ð Þ Tmech � Telec � f ωmech (3)

ωmech ¼ θmech (4)

dt � <sup>ω</sup>elecϕsq (5)

dt � <sup>ω</sup>elecϕsd (6)

ϕsd ¼ LdIds þ ϕ<sup>m</sup> (7)

ϕsq ¼ LqIqs (8)

$$V\_{ds} = R\_s I\_{ds} + \frac{d}{dt} \left( L\_d I\_{ds} + \phi\_m \right) - \omega\_{\text{clcc}} \left( L\_q I\_{qs} \right) \tag{9}$$

$$V\_{qs} = R\_s I\_{qs} + \frac{d}{dt} \left( L\_q I\_{qs} \right) + \omega\_{\text{elec}} \left( L\_d I\_{ds} + \phi\_m \right) \tag{10}$$

So, again resolving the above mentioned equation we can land up with following equation:

$$V\_{ds} = R\_s I\_{ds} + L\_d \frac{d}{dt} I\_{ds} - \omega\_{\text{elec}} \left( L\_q I\_{qs} \right) \tag{11}$$

$$V\_{qs} = R\_s I\_{qs} + L\_q \frac{d}{dt} I\_{qs} + \omega\_{\text{elec}} \left( L\_d I\_{ds} + \phi\_m \right) \tag{12}$$

Therefore, in order to solve for the d and q axis stator currents, the above mentioned equation can be formed in the following fashions:

$$\frac{d}{dt}I\_{ds} = \left(\frac{1}{L\_d}\right) \left[V\_{ds} - R\_s I\_{ds} + \omega\_{\text{clec}} \left(L\_q I\_{qs}\right)\right] \tag{13}$$

$$\frac{d}{dt}I\_{qs} = \left(\frac{1}{L\_q}\right) \left[V\_{qs} - R\_s I\_{qs} - \omega\_{elec} (L\_d I\_{ds}) - \omega\_{elec} \phi\_m\right] \tag{14}$$

where Rs stands for the stator winding resistance, Ld stands for the stator inductance in direct axis, Lq stands for the stator inductance in quadrature axis, Vds stands for the direct axis stator voltage, Vqs stands for the quadrature axis stator voltage, Ids stands for the direct axis stator current, Iqs stands for the quadrature axis stator current, Welec = Pwmech stands for the speed [19, 24].

Since the permanent magnet synchronous generator (PMSG) is a machine similar to wound rotor machine which is better for surface-seated applications, the generated electric torque by the PMSG can be defined as follows:

$$T\_{elec} = \left(\frac{3}{2}\right) P \left(\phi\_m I\_{qs} + \left(L\_d - L\_q\right) I\_{ds} I\_{qs}\right) \tag{15}$$

However, if permanent magnet synchronous generator is surface seated, then it is conceivable to favor the assumption of Ld = Lq. Then torque can be expressed as follows:

$$T\_{elec} = \left(\frac{\mathfrak{Z}}{2}\right) P \left(\phi\_m I\_{qs} + (L\_d - L\_d) I\_{ds} I\_{qs}\right) \tag{16}$$

So, one can land up with the equation as follows:

$$T\_{elec} = \left(\frac{3}{2}\right) P\{\phi\_m I\_{qs}\} \tag{17}$$

In steady-state positions, the active power generated e from permanent magnet synchronous generator is given by:

$$P\_s = V\_{ds}I\_{ds} + V\_{qs}I\_{qs} \tag{18}$$

Figure 6. 60 MW wind farm connected to Derna medium voltage network.

Table 1. Parameters of electric network and wind energy system.

Wind bus Isc-3Ø = 10.02a83.5

220/30 KV 63 MVA main transformer Positive sequence data:

575/30 KV 2 MVA transformer Positive sequence data:

Bear 325 mm<sup>2</sup> 30 kV transmission line Z+ = 0.1162 + 0.385 j Ω/km

Wind turbine source Rating: 1.65 MW, 0.69 KV, PF = 0.9

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:…

Isc-1Ø = 7.99a 78.9 X0/X1 = 1.73

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47

X+/R + = 34.1 Zero sequence data: impedance voltage = 12%

X0/R0 = 34.1

Swept area = 2828 Rotor diameter = 80 m Pitch angle = 1 Air density = 1.225 kg/m<sup>3</sup>

impedance voltage = 6.25%

Z0 = 0.3486 + 1.155 j Ω/km

RPM = 15

X+/R + = 6 Zero sequence data: impedance voltage = 6.25%

X0/R0 = 6

impedance voltage = 12.76%

Turbine rated wind speed = 15 m/s Minimum wind speed = 4 m/s Maximum wind speed = 25 m/s

The wind farm is connected to the grid via step-up transformer to the 30 kV busbar [25, 26].

#### 4. Simulation results

Figure 5 shows a single-line diagram of a low-voltage network of Derna City simulated in NEPLAN [27]. The electric grid as shown in Figure 6 consists of different voltage level busbars. The network feeds from the 220 KV busbar which connects it to the rest of the Libyan network through different capacity power transformers. Also, wind turbines with a total capacity of 60 MW, consisting number of turbines with a rated power of 1.65 MW, is connected to the simplified network in our study. Table 1 shows data for different components in the network and details of the wind turbines system.

#### 4.1. Load profile

One of the most significant purposes for the integration of renewable energy sources into distribution networks is to reduce the costs of electricity to the consumers derived from charging line losses in this cost. The amount of line losses depends on the distance required to transfer the electric power as well as the value of the drawn current by consumers and thus affect the optimal economic dispatch based on the network configuration. Distribution network operators need to apply simple methods to predict the power flow in the network to ensure the balance of energy demand. Proper integration of the renewable energy sources in the distribution network will reduce the power losses in the transmission lines to a certain

Figure 5. Single-line diagram of a low-voltage network of Derna city simulated in NIPLAN.

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:… http://dx.doi.org/10.5772/intechopen.74796 47

Figure 6. 60 MW wind farm connected to Derna medium voltage network.

Ps ¼ VdsIds þ VqsIqs (18)

The wind farm is connected to the grid via step-up transformer to the 30 kV busbar [25, 26].

Figure 5 shows a single-line diagram of a low-voltage network of Derna City simulated in NEPLAN [27]. The electric grid as shown in Figure 6 consists of different voltage level busbars. The network feeds from the 220 KV busbar which connects it to the rest of the Libyan network through different capacity power transformers. Also, wind turbines with a total capacity of 60 MW, consisting number of turbines with a rated power of 1.65 MW, is connected to the simplified network in our study. Table 1 shows data for different components in the network

One of the most significant purposes for the integration of renewable energy sources into distribution networks is to reduce the costs of electricity to the consumers derived from charging line losses in this cost. The amount of line losses depends on the distance required to transfer the electric power as well as the value of the drawn current by consumers and thus affect the optimal economic dispatch based on the network configuration. Distribution network operators need to apply simple methods to predict the power flow in the network to ensure the balance of energy demand. Proper integration of the renewable energy sources in the distribution network will reduce the power losses in the transmission lines to a certain

Figure 5. Single-line diagram of a low-voltage network of Derna city simulated in NIPLAN.

4. Simulation results

4.1. Load profile

and details of the wind turbines system.

46 Stability Control and Reliable Performance of Wind Turbines


Table 1. Parameters of electric network and wind energy system.

level. This will contribute to the postponement of network infrastructure promotion. Various methods to determine the optimal capacity and location of each renewable DG at the minimum line losses, among which is Branch power loss formula as given in Eq. (19) [28]. Figure 7 shows the loading profile of distribution transformers with different capacities through the day. Figure 8 shows the power supplied by the main feeder of the network in different wind farm penetration level.

PLoss <sup>¼</sup> <sup>X</sup><sup>n</sup>

voltage at bus i.

4.2. Voltage profile

where

units.

and improves the voltage profile on all busbars.

improvement can be evaluated as follows:

i¼1

where Pbi = Active power at branch i, Qbi = Reactive power at branch i, Vi j j = Magnitude of

The investigation of the load curve during the day shows a difference in loading ratios. The maximum rate of loading occurs at 20:30 due to switching lights in most homes. The investigation of energy feeding from the transmission system during the day showed the highest contribution from the grid 180 MW occur during the period of the sunset due to lighting loads, and since most loads in the network is a household loads. The electric supply from the grid decreases at the highest rate of integration of wind farm. This reduces the stress on the cables

Injection of renewable energy sources into the distribution network alters the direction of power flow in the grid. As a result, it enhances the voltage profile. The integration of such sources thus improves the voltage at feeder endings and thus improves the quality of the power fed to consumers as a whole [28]. The benefits of DG penetration on the voltage profile

> <sup>V</sup><sup>11</sup> <sup>¼</sup> <sup>V</sup><sup>11</sup>=wDG V<sup>11</sup>=w0DG

> > i¼1

i¼1

<sup>W</sup><sup>1</sup> <sup>¼</sup> <sup>W</sup><sup>2</sup> <sup>¼</sup> <sup>W</sup><sup>3</sup> <sup>¼</sup> Wn <sup>¼</sup> <sup>1</sup>

V<sup>11</sup>=wDG = General expression for voltage profile at bus I with the application of renewable DG

<sup>V</sup><sup>11</sup>=wDG <sup>¼</sup> <sup>X</sup><sup>n</sup>

<sup>V</sup><sup>11</sup>=woDG <sup>¼</sup> <sup>X</sup><sup>n</sup>

Xn i¼1

If all the loads at bus i are equally weighted, Wi expressed as

V11 = Voltage profile improvement benefits.

P2 bi <sup>þ</sup> <sup>Q</sup><sup>2</sup> bi

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:…

Vi j j<sup>2</sup> Ri (19)

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ViLPiWi (21)

Vi0LPiWi (22)

<sup>n</sup> (24)

Wi ¼ 1 (23)

(20)

49

Figure 7. The loading Profile of 20, 10 and 5 MW Distribution Transformers During 24 Hours.

Figure 8. MW power from main feeder with different wind farm penetration.

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:… http://dx.doi.org/10.5772/intechopen.74796 49

$$P\_{Loss} = \sum\_{i=1}^{n} \frac{P\_{bi}^2 + Q\_{bi}^2}{\left|V\_i\right|^2} R\_i \tag{19}$$

where Pbi = Active power at branch i, Qbi = Reactive power at branch i, Vi j j = Magnitude of voltage at bus i.

The investigation of the load curve during the day shows a difference in loading ratios. The maximum rate of loading occurs at 20:30 due to switching lights in most homes. The investigation of energy feeding from the transmission system during the day showed the highest contribution from the grid 180 MW occur during the period of the sunset due to lighting loads, and since most loads in the network is a household loads. The electric supply from the grid decreases at the highest rate of integration of wind farm. This reduces the stress on the cables and improves the voltage profile on all busbars.

#### 4.2. Voltage profile

level. This will contribute to the postponement of network infrastructure promotion. Various methods to determine the optimal capacity and location of each renewable DG at the minimum line losses, among which is Branch power loss formula as given in Eq. (19) [28]. Figure 7 shows the loading profile of distribution transformers with different capacities through the day. Figure 8 shows the power supplied by the main feeder of the network in different wind

Figure 7. The loading Profile of 20, 10 and 5 MW Distribution Transformers During 24 Hours.

Figure 8. MW power from main feeder with different wind farm penetration.

farm penetration level.

48 Stability Control and Reliable Performance of Wind Turbines

Injection of renewable energy sources into the distribution network alters the direction of power flow in the grid. As a result, it enhances the voltage profile. The integration of such sources thus improves the voltage at feeder endings and thus improves the quality of the power fed to consumers as a whole [28]. The benefits of DG penetration on the voltage profile improvement can be evaluated as follows:

$$V\_{11} = \frac{V\_{11/wDG}}{V\_{11/wDG}}\tag{20}$$

$$V\_{11/wDG} = \sum\_{i=1}^{n} V\_i L P\_i W\_i \tag{21}$$

$$V\_{11/\text{woDG}} = \sum\_{i=1}^{n} V\_{i0} L P\_i W\_i \tag{22}$$

$$\sum\_{i=1}^{n} W\_i = 1 \tag{23}$$

If all the loads at bus i are equally weighted, Wi expressed as

$$W\_1 = W\_2 = W\_3 = W\_n = \frac{1}{n} \tag{24}$$

where

V11 = Voltage profile improvement benefits.

V<sup>11</sup>=wDG = General expression for voltage profile at bus I with the application of renewable DG units.

V<sup>11</sup>=woDG = General expression for voltage profile at bus I without the application of renewable DG units.


To investigate the fluctuation effects of wind energy source penetration, the voltage profile of different busbars is considered to display and show the excessive loading on the distribution busbars. The voltage profile for different busbars is illustrated in Figure 9. The influence of wind energy penetration on the voltage profile is slightly low, and this effect could be increased on radially connected busbars. The impact of wind generation will be noticeable at the busbar of point of common coupling (PCC) connecting the wind turbine to the electric grid. It is shown in Figure 10.

It is clear from the figures above, the voltages of all nodes have improved after interconnection of wind farm. The enhancement in voltage for busbars near from busbar of point of common coupling is better than the rest of other busbars in the network.

#### 4.3. Harmonic distortion

The harmonics are created when non-sinusoidal currents and non-sinusoidal voltages increase in the network, and these distortions are generally called harmonic distortion [29, 30]. The

basic conditions that lead to network consonances usually result from nonlinear loads, voltage imbalances. Power quality studies are carried out due to summation law [31]. The total harm-

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:…

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51

onic current distortion is given as:

Figure 10. Voltage profile of point of common coupling (PCC).

Figure 11. Simulated network with wind energy system.

Figure 9. Voltage profile for different busbars for 1 day.

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:… http://dx.doi.org/10.5772/intechopen.74796 51

Figure 10. Voltage profile of point of common coupling (PCC).

V<sup>11</sup>=woDG = General expression for voltage profile at bus I without the application of renewable

To investigate the fluctuation effects of wind energy source penetration, the voltage profile of different busbars is considered to display and show the excessive loading on the distribution busbars. The voltage profile for different busbars is illustrated in Figure 9. The influence of wind energy penetration on the voltage profile is slightly low, and this effect could be increased on radially connected busbars. The impact of wind generation will be noticeable at the busbar of point of common coupling (PCC) connecting the wind turbine to the electric grid.

It is clear from the figures above, the voltages of all nodes have improved after interconnection of wind farm. The enhancement in voltage for busbars near from busbar of point of common

The harmonics are created when non-sinusoidal currents and non-sinusoidal voltages increase in the network, and these distortions are generally called harmonic distortion [29, 30]. The

Vi<sup>0</sup> = Voltage at bus I per unit without renewable DG.

coupling is better than the rest of other busbars in the network.

Vi = Voltage at bus I per unit with renewable DG.

N = Number of busses in the power system.

50 Stability Control and Reliable Performance of Wind Turbines

LPi = Load at bus I (per unit).

Wi = weighting factor for bus i.

It is shown in Figure 10.

4.3. Harmonic distortion

Figure 9. Voltage profile for different busbars for 1 day.

DG units.

basic conditions that lead to network consonances usually result from nonlinear loads, voltage imbalances. Power quality studies are carried out due to summation law [31]. The total harmonic current distortion is given as:

Figure 11. Simulated network with wind energy system.

$$I\_{\rm THD} = \sqrt{\sum\_{h=2}^{40} \frac{I\_h^2}{I\_1} 100} \tag{25}$$

in currents of the main busbar connecting wind farm for 20, 40, and 60 MW of penetration from wind farm. These results were extracted and implemented using MATLAB/SIMULINK after

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53

Figure 12 represents the harmonic current distortion for 20 MW penetration from wind energy system to the network. However, the total harmonic distortion shows slightly significant increase in the percentage of harmonic currents at PCC specially second and third harmonic

Figure 13 represents the harmonic current distortion for 20 MW penetration from wind energy system to the network. Moreover, the total harmonic distortion shows further slightly increase in the percentage of harmonic currents at PCC especially second and third harmonic currents

Figure 14 represents the harmonic current distortion for 60 MW penetration from wind energy system to the network. Moreover, the total harmonic distortion shows further highly increase

simplifying several wind turbines as one unit.

Figure 13. FFT output of total harmonic current distortion for 40 MW penetration.

currents to reach a value of 4.99% at PCC.

to reach a value of 10.07% at PCC.

Harmonics contributed by wind turbines in the network may cause a problem due to the existing harmonics in the network. The current waveform of wind turbines is non-sinusoidal and distorted due to low integer harmonics of second and fifth harmonics. The variable speed of wind turbine equipped with power electronic converters causes an increase in the harmonic distortion at the point of common coupling. Figure 11 shows the simulated model in MATLAB/SIMULINK.

Measurement of harmonics taken from node 575. Measurement of a number of harmonics in currents has been carried out for different penetration level to study the effect of the penetration on the amount of harmonics in the network. Figures 12–14 represent the harmonics values

Figure 12. FFT output of total harmonic current distortion for 20 MW penetration.

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:… http://dx.doi.org/10.5772/intechopen.74796 53

Figure 13. FFT output of total harmonic current distortion for 40 MW penetration.

ITHD ¼

Figure 12. FFT output of total harmonic current distortion for 20 MW penetration.

MATLAB/SIMULINK.

52 Stability Control and Reliable Performance of Wind Turbines

s

Harmonics contributed by wind turbines in the network may cause a problem due to the existing harmonics in the network. The current waveform of wind turbines is non-sinusoidal and distorted due to low integer harmonics of second and fifth harmonics. The variable speed of wind turbine equipped with power electronic converters causes an increase in the harmonic distortion at the point of common coupling. Figure 11 shows the simulated model in

Measurement of harmonics taken from node 575. Measurement of a number of harmonics in currents has been carried out for different penetration level to study the effect of the penetration on the amount of harmonics in the network. Figures 12–14 represent the harmonics values

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X<sup>40</sup> h¼2 I 2 h I1 100

(25)

in currents of the main busbar connecting wind farm for 20, 40, and 60 MW of penetration from wind farm. These results were extracted and implemented using MATLAB/SIMULINK after simplifying several wind turbines as one unit.

Figure 12 represents the harmonic current distortion for 20 MW penetration from wind energy system to the network. However, the total harmonic distortion shows slightly significant increase in the percentage of harmonic currents at PCC specially second and third harmonic currents to reach a value of 4.99% at PCC.

Figure 13 represents the harmonic current distortion for 20 MW penetration from wind energy system to the network. Moreover, the total harmonic distortion shows further slightly increase in the percentage of harmonic currents at PCC especially second and third harmonic currents to reach a value of 10.07% at PCC.

Figure 14 represents the harmonic current distortion for 60 MW penetration from wind energy system to the network. Moreover, the total harmonic distortion shows further highly increase

and environmental operation of an electric network. A wind energy system interconnected to a real network was investigated. Loads were programmed by NEPLAN. Actual wind measurements were used in this study to measure the extent to which wind power can be provided while ensuring a reasonable power quality and commensurate with international standards. It can be noticed from the results that the influence of the wind energy system can be significant on point of common coupling, which is close to the wind farm. Investigation of the cables connected in a mesh is less affected by the fluctuation more than the cables connected in radial can be further performed. It is also noticeable that energy losses decrease with increased penetration of the wind farm. From the analysis of the harmonic currents in MATLAB/ SIMULINK, their impact on the power quality of the energy is insignificant, the effect of the harmonic currents on the grid increase with increasing penetration of wind farm. Finally, it can be said that the performance of the wind farm falls within the limits of international standards but may increase its impact on the voltage profile and energy losses, and power quality with

Power Quality and System Stability Impact of Large-Scale Distributed Generation on the Distribution Network:…

increased penetration of wind energy.

\*, Naser El Naily2

Hammamet, Tunisia; 2016. pp. 1-6

\*Address all correspondence to: smuftahndi@gmail.com

, Jamal Wafi2

1 College of Electrical and Electronics Technology-Benghazi, Benghazi, Libya 2 Electrical and Electronics Engineering Dept., University of Benghazi, Libya

4 Electrical and Computer Engineering Dept., Sultan Qaboos University, Muscat, Oman

[1] Asheibi A, Khalil A, Rajab Z. The economic feasibility of photovoltaic systems for electricity production in Libya. In: The 7th International Renewable Energy Congress (IREC'2016);

[2] Khalil A, Asheibe A. The chances and challenges for renewable energy in Libya. In: The

[4] Khalil A, Rajab Z, Asheib A. Modeling, Simulation, Analysis and Control of Stand-alone PV System at the Seventh International Renewable Energy Congress"IREC 2016"

[5] Asheibe A, Khalil A. The renewable energy in Libya: Present difficulties and remedies. In:

3 Authority of Natural Science Research and Technology, Tripoli, Libya

Proceedings of the Renewable Energy Conference; 2015

The Proceedings of the World Congress; 2013

[3] Libya Infrastructure Report 2013, Business Monitor International Ltd; 2013

, Faisal A. Mohamed<sup>3</sup> and Abdelsalam Elhaffar<sup>4</sup>

http://dx.doi.org/10.5772/intechopen.74796

55

Author details

Saad M. Saad<sup>1</sup>

References

Figure 14. FFT output of total harmonic current distortion for 60 MW penetration.

in the percentage of harmonic currents at PCC especially second and third harmonic currents to reach a value of 20.08% at PCC.

The simulation is done at the selected PCC of 33 KV busbar system. The simulation result of current THDs is having low values. Three phases waveform of output currents at 33 KV busbar system is shown. From the three graphs, the result summary of harmonic currents is within permissible limits as compared to IEEE standard values and comply with the grid requirements. Therefore, it can be said that the wind farm power quality is sufficient and did not affect the grid system power quality, except for the case of 60 MW.

#### 5. Conclusion

This chapter has interpreted the technical challenges of penetrating wind DG into the distribution system. Renewable DG can perform many significant functions in the economic, technical and environmental operation of an electric network. A wind energy system interconnected to a real network was investigated. Loads were programmed by NEPLAN. Actual wind measurements were used in this study to measure the extent to which wind power can be provided while ensuring a reasonable power quality and commensurate with international standards. It can be noticed from the results that the influence of the wind energy system can be significant on point of common coupling, which is close to the wind farm. Investigation of the cables connected in a mesh is less affected by the fluctuation more than the cables connected in radial can be further performed. It is also noticeable that energy losses decrease with increased penetration of the wind farm. From the analysis of the harmonic currents in MATLAB/ SIMULINK, their impact on the power quality of the energy is insignificant, the effect of the harmonic currents on the grid increase with increasing penetration of wind farm. Finally, it can be said that the performance of the wind farm falls within the limits of international standards but may increase its impact on the voltage profile and energy losses, and power quality with increased penetration of wind energy.
