Impact of Combined Wind Farm on Interconnected Distribution Systems

Transcription

1 International Journal on Power Engineering and Energy (IJPEE) Vol. (9) No. () ISSN Print ( ) and Online (234 73X) January 28 Impact of Combined Wind Farm on Interconnected Distribution Systems,2 Ahmed. M. M. Rashad, 2 Salah Kamel, 2 Ahmed El Badry, 3 Francisco Jurado. Department of Electrical Engineering, Faculty of Engineering, Aswan University, 8542 Aswan, Egypt 2 Dept. of Electrical EngineeringUniversity of Jaen, Jaen, Spain ahmedmmrashadar@gmail.com,skamel@aswu.edu.eg,ahmedalbadry48@yahoo.com, fjurado@ujaen.es Abstract This paper presents a comparison between the impact of two different types of wind farms on the distribution system. The first wind farm based on a combination of squirrel cage induction generator wind turbines (SCIGWT) and double feed induction generators wind turbines (DFIGWT); called combined wind farm (CWF). The second wind farm is based on squirrel cage induction generator wind turbine (). The third wind farm is based on double feed induction generators wind turbine (). The impact of the three wind farms on the stability of interconnected electric distribution networks is comprehensively studied during single line to ground and double lines fault as examples of unsymmetrical and during three phase fault and three phase open circuit fault as examples for symmetrical.. KeywordsSCIG, DFIG, combined wind farm, distribution system. p.u. AC DC q d R S L m R L r Perunit Alternative current Direct current I. NOMENCLATURE Subscript refers to quadratic component of parameter Subscript refers to direct component of parameter Refer to the rotor side Refer to the stator side Magnetizing inductance of stator Resistance Selfinductance of stator Rotor Speed Air density Flux linkage II. INTRODUCTION Nowadays the wind energy became more widespread and penetrating into the distribution system. This increase in wind farm penetration has raised concerns of power system operators. As a result of this concern, the power system developed rules for installing wind farms. The main rule is the wind farms' ability to remain connected to the distribution system and at the same time take part of power stem stability []. The performance of wind farms interconnected grid is investigated in many such as [2, 3]. Due to the increase in wind energy technology, encourages the utilization of wind farms' features in the distribution system. There are a few articles discusses the issue of distribution system integrated with wind energy system. The impact of the wind farm placement on distribution system is investigated in [4]. The impact of wind farms on voltage stability of distribution system is discussed in [57]. The impact of wind farm integrated with fixable AC transmission system on distribution system is investigated in [8]. A comparison among the impacts of different types of wind turbines is accomplished in [9]. The previous efforts discussed the impact of wind farms based on the single type of wind turbines SCIG or DFIG. This paper studies the impact of wind farm based on a combination of SCIG and DFIG. The main aim is utilizing the reactive power injected by DFIG in compensating the reactive power demanded by SCIG, in addition, voltage regulation process during contingency conditions. In order to point out this aim, the performance of distribution system integrated with CWF during different types of faults is compared with distribution system integrated with and individually. III. WIND FARM S MODELING The mathematical modeling of wind turbine consists of drive train equations and generator equations. The drive train equations represent output power extracted from the wind by the hub of wind turbine and it is given [2]: P m T m T em s Mechanical power Mechanical torque applied to the rotor Electrical torque Synchronous speed () Where, is the air density (nominally.22kg/m3),a is the area swept by the turbine blades, v is the wind speed andc p is known as the power coefficient.c p is function on two other factors the tip speed ratio and pitch angle.the tip speed ratio is given by: (2) Where r rotor speed andl b the blade length or rotor radius. The equation that describes the C p is given by: Reference Number: JOP 85

2 International Journal on Power Engineering and Energy (IJPEE) Vol. (9) No. () ISSN Print ( ) and Online (234 73X) January 28 (8) (3) The mechanical torque is given by. (4) Where H r is inertia of wind turbine rotor shaftandt wt is the mechanical torque of wind turbine rotor shaft. The pitch angle control system is used to keep the output power within its allowable limits and is shown in Fig. [,, 3, 4]. The general equivalent circuit of induction generators is shown in Fig. 2 [,, 3, 4]. Fig. (2): Equivalent circuit of IG generators The electrical torque of DFIG and SCIG is given by: For DFIG (9) For SCIG () Fig.3. shows the schematic diagram of DFIG and SCIG interconnected grid [4]. Fig. (): Block diagram of Pitch angle control system The actual rotor speed r is compared the reference rotor speed ref at the applied wind sped that is obtained from the power characteristic curve. The error is injected to pitch angle actuator to produce the new pitch angle new that can extract the maximum power at the applied wind speed [,, 3, 4]. The mathematical modeling of induction generators is given by [,, 3, 4]. (a) (5) (b) Fig. (3): Single line diagram of (a) DFIG and (b) SCIG (6) (7) It is already known that the rotor voltage is zero because the rotor of SCIG is short circuit. The rotor voltage of DFIG cannot be neglected because the rotor of DFIG is wound rotor and transfers its active and reactive power to the grid through the three winding transformer and AC/DC/AC converter. The AC/DC/AC converter composes of two voltage sources connected together through DC bus. The first voltage source is connected from rotor side (RSC). The first voltage source is connected from grid side (GSC). RSC is connected to GSC are through DC bus [,, 3, 4]. Reference Number: JOP 86

3 International Journal on Power Engineering and Energy (IJPEE) Vol. (9) No. () ISSN Print ( ) and Online (234 73X) January 28 The equivalent circuit of SCIG is shown in Fig. 4 (a) while the equivalent circuit of DFIG is shown in Fig. 4 (b). All the parameters are referred to the stator side [,, 3, 4] Voltagein pu.9972 V of B4 of CWF V of B3 of CWF.997 (c) Voltage at B3 and B4 of CWF of of (d) Voltage at B8 and B9 of (a) of of DGIGWF.9953 (e) Voltage at B8 and B9 of Voltge in pu of CWF of CWF.9945 (f) Voltage at B8 and B9 of CWF Activepower in MW P of P of (b) Fig. (4):Steady stat equivalent circuit of (a) DFIG and (b) SCIG. IV. DESCRIPTION OF STUDIED SYSTEM The studied system consists of distribution systemintegrated with different types of wind farms. The first wind farm is based on SCIG ().The second wind farm is based on DFIG (). The last wind farm is based on a combination of SCIG and DFIG (CWF). The studied distribution system composes of a 25 KV consists of four feeders buses andexports power to four loads of MW. The studied wind farmsare connected to bus B. The performance of the studied distribution systemintegrated with the studied wind farms is examined during different types of symmetrical and unsymmetrical faults. The Bus B6 is insecure due to the fault occurred at a time equal to 25 s and remains for.5 s. The studied electric distribution network is shown in Fig. 5. (g) Active power of at PCC Activepower in MW (i) Active power of CWF at PCC V of (k) Voltage of at PCC Reactivepower in MW Q of (m) Reactive power of at PCC (h) Active power of at PCC Voltae in MW V of (j) Voltage of at PCC Reactive powr in MVAr (l) Voltage of CWF at PCC Q of Time in pu (n) Reactive power of at PCC Fig. (5): Single line diagram of studied system V. SIMULATION RESULTS A. Impact of of single line to ground fault. Fig. 6 shows the impact of line to ground fault V of B4 of V of B3 of (a) Voltage at B3 and B4 of V of B4 of DFIG V of B3 of DFIG.998 (b) Voltage at B3 and B4 of (o) reactive power of CWF at PCC Fig. (6): Impact of single line to ground fault on the studied system From the results, it can be observed that the three wind farms improve the voltage of load buses. The voltage of nearest load buses B3 and B4 to the wind farms has been improved more than the furthest load buses B8 and B9. The CWF represents an intermediate case between and where has the lowest parameters' values (voltage, active power and reactive power at PCC) and DFIG WF has the highest parameters' values. Also, it can be observed that the improvement of wind farms depend on the type of generator that is used in wind farms. The improvement of CWF is much better than the improvement of and Reference Number: JOP 87

4 International Journal on Power Engineering and Energy (IJPEE) Vol. (9) No. () ISSN Print ( ) and Online (234 73X) January 28 is near the improvement of. This is because the (a) Voltage at load buses of (b) Voltage at load buses of CWF collects the advantage of SCIG and DFIG. B. Impact of double line fault. Fig. 7. shows the impact of single line to line fault on the studied system. As it can be observed from Fig. 7, the impact of line to line is tougher than the impact of single line to ground fault on electric distribution networks. Despite the severity of the line to line fault, the three wind farms improve the voltage of load buses. The voltage of nearest load buses B3 and B4 to the wind farms has been improved more than the furthest load buses B8 and B (a)voltage at load buses of (c) Voltage at load buses of Q of 6 Q of (e) Reactive power of the three (b)voltage at load buses of P of P of (d) Active power of the three..9.7 V of V of (f) Voltage of the three wind farms at PCC Fig. (7): The impact of double line fault on the studied system CWF shows the best performance compared with SCIFWF and. The CWF has the highest values of active power and, voltage. The CWF does not suffer from complete disconnection such as or instantaneous disconnection such as. The CWF remains connected to electric distribution networks and injects its active power to the system during fault time. This is because the CWF collects the advantage of SCIG and DFIG. C. Impact of three phase fault. Fig. 8.(a; b and, c) show that voltage at B3 and B4 has the best improvement when the system is interconnected to CWF compared with and. This performance of voltage at B3 and B4 is due to the impact of generators that are used in the wind farm on the electric distribution networks. This impact can be explained by monitoring the performance of the three wind farms at the PCC..4.2 (c) Voltage at load buses of CWFWF 5 Q of 5 Q of (e) Reactive power of the three 5 P of P of 5 Time in pu (d) Active power of the three.4 V of.2 V of (f) Voltage of the three wind farms at PCC Fig. (8): The impact of three phase fault on the studied system Fig. 8. shows the impact of threephase fault on the studied system. As shown in Fig. 8. (d) the active power of both SCIG WF and drop to zero while the active power of CWF is still greater than zero. This means that the is disconnected from the system and suffers from instantaneous disconnection while CWF is never disconnected from the grid. From Fig. 8. (e and, f) it can be observed that CWF has the highest voltage's value at PCC because it has the highest injected reactive power at PCC. This reactive power is used to regulate the voltage and at the same time compensate reactive power demanded by SCIG wind turbines in CWF. This performance prevents CWF from complete or instantaneous disconnection from the system such as SCIG WF and. VI. CONCLUSIONS In this paper, the performance of disruption systems integrated with different types of wind farms is examined during different types of faults. The paper has presented three types of wind farms based on different combination of induction generators. The first wind farm based on SCIG wind turbines. The second wind farm based on DFIG wind turbines. The third wind farm based on a combination of SCIG and DFIG together. The results shown that the impact of grid faults on disruption system integrated with the wind farm depends on the types of induction generators used in wind farm. The results shown that the distribution system integrated with the combined wind farm provided the best performance especially with increasing severity of the fault. The distribution system integrated with the wind farms based on SCIG shown the worst performance especially with increasing severity of the fault. The distribution system integrated with the wind farms based on DFIG shows good performance during a light fault while it cannot withstand with the increasing severity of the fault. VII. REFERENCES Vof B8 [] S. Muyeen, R. Takahashi, T. Murata, and J. Tamura, "A variable speed wind turbine control strategy to meet wind Reference Number: JOP 88

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