Reactive Power Management of a Wind Farm to Prevent Voltage Collapse of an Electric Power System

Transcription

1 Reactive Power Management of a Wind Farm to Prevent Voltage Collape of an Electric Power Sytem R. M. Monteiro Pereira Intituto Superior Engenharia de Coimbra, Portugal rmfm@iec.pt C. M. Machado Ferreira Intituto Superior Engenharia de Coimbra and INESC Coimbra, Portugal cmacfer@iec.pt F. P. Maciel Barboa INESC TEC and Faculdade Engenharia da Univeridade do Porto, Portugal fmb@fe.up.pt Abtract- Nowaday, the large penetration of wind power generation poe new challenge for dynamic voltage tability analyi of an electric power ytem. The practical importance of dynamic voltage tability analyi i to help in deigning and electing counter-meaure in order to avoid voltage collape and enhance ytem tability. The impact of wind integration on reactive reerve requirement i a current area of interet for renewable integration tudie and power ytem operator. In thi paper i tudied a new wind power plant model with reactive power management. The active power and the frequency management are taken into account too. The developed model can be ued to repreent, in a implified way, an entire wind farm in order to imulate the dynamic voltage tability of the ytem, whatever the technology involved in the wind turbine. The ytem i completely modelled by a ingle dynamic converter model with appropriate control loop intended to reproduce the overall repone of a wind farm for different grid event, uch a fault or voltage and reactive power management at the point of common coupling. Index Term Dynamic Voltage Stability, Reactive Power Management, Voltage Collape, Wind farm. I. INTRODUCTION The voltage collape normally involve large diturbance. The mot important evidence of voltage collape are low voltage profile, heavy reactive power flow in tranmiion line, heavily loaded ytem and inufficient reactive upport [1]. The voltage collape often require long ytem retoration, while large group of cutomer are left without upply for extended period of time. Reactive power management and voltage control are playing an increaingly important role a electricity network experience higher flow and greater variability [2]. With the increaed preence of renewable energy ource in the electric power ytem during the lat year, epecially wind energy, many countrie have etablihed or are creating a et of pecific requirement, grid code, concerning grid upport during teady-tate operation and grid fault [3]. The aim of thee grid code i to enure that the continued expanion of wind power generation doe not compromie the power quality, a well a the ecurity and reliability of the electric power network [4]. Conequently, the rik of loing a large portion of wind power generation during fault event decreae and the Tranmiion Sytem Operator (TSO) can maintain an efficient, reliable and ecure ytem operation even with high wind power penetration level [5]. During the diturbance period, the wind farm are requeted to retore the voltage level back to the nominal value at the connection point, by injecting the required amount of reactive current. The reactive current control of the wind turbine mut be ued to upport the voltage [6]. Currently, there are different model that allow to tudy the dynamic behaviour of wind farm, uch a: wind turbine equipped with a Doubly-Fed Induction Generator (DFIG), wind turbine coupled with an aynchronou generator (with and without pitch control) and variable-peed wind turbine with direct driven ynchronou generator. In thi paper i tudied a new wind power plant model with active and reactive power management and frequency management. The model wa implemented in a tet ytem with a wind farm and conventional generation. The wind farm i repreented by a model of the wind turbine equipped with pitch control, coupled with a Fixed Speed Induction Generator (FSIG) [7]. The Automatic Voltage Regulator (AVR), the OvereXcitation Limiter (OXL) of the generating unit and the turbine Speed Governor (SG) were modelled. Different load model were ued and the Under Load Tap Changer (ULTC) wa taken into account too. Significant diturbance in the tet ytem were modelled, uch a: an increae of the wind peed and the tripping of the overhead tranmiion line were imulated in the tudie, uing the EUROSTAG oftware package. Finally, ome concluion that provide a better undertanding of how the reactive power management in a wind farm can be ued to improve the voltage tability of an electric power ytem are pointed out. II. WIND POWER PLANT MODEL The developed model can be ued to repreent in a implified way, an entire wind farm for ue in ytem impact tudie, whatever the technology involved. In thi tudy the technology involved i a wind turbine equipped with pitch control coupled with a FSIG. The whole ytem i modelled by a ingle dynamic converter model with appropriate control loop intended to reproduce the overall repone of a wind farm for different grid event, uch a hort-circuit or voltage and reactive power management at the point of common /15/$ IEEE

2 coupling. Thi model include wind farm equipped with a power/frequency control for primary reerve participation [8]. A. Active power management The active power management of the wind farm i plit in three part. The firt part concern the calculation of the mechanical power. The performance coefficient (Cp) i determined by the aerodynamic law and it i multiplied by the cube of the wind peed. Contant loe and loe proportional to the rotor peed are taken into account. The econd part concern: The calculation of the peed et point according to the active power delivered in the grid. If the active power i above 0.75 pu, the peed et point correpond to the nominal peed. Under 0.75 pu, the peed i adapted to extract a maximum power a poible from the wind. The mechanical equation calculation to obtain the rotor peed, in function of the active power, the mechanical power and the inertia. The active power et point calculation in function of the rotor peed, with limitation on the abolute value of the active power and it variation rate. The pitch angle calculation in function of the rotor peed, alo with limitation on the abolute value and it variation rate. Third part concern the low voltage active current management (LVACM) function. The maximum total current (I max ) admitted by the model i et to 1.1 pu. Equation (1) allow to calculate the total current and at normal operation condition the active current ha priority on the reactive current. 2 2 t p q I = I + I (1) where It i the total current, Ip and Iq tand for active and reactive current repectively. If the voltage drop, the LVACM function command the bridge to radically decreae the maximum active current. The goal i to avoid injecting too much active current in a weakened grid becaue it doe not provide the deired active power due to the lowered voltage but, it woren the tability of the network during the fault and it low down the voltage recovery after the fault too. When the voltage recover, the active current increae rate i limited to avoid making the voltage recovery more difficult [8]. Another advantage of thi LVACM function i to increae the maximum limit of the reactive current (priority i given to the active current but it maximum limit i decreaed, leaving more margin for the reactive power), to inject a much reactive power a poible and upport the voltage. Therefore, the maximum accepted reactive current depend on the active injected current. B. Reactive power management The voltage and reactive power management comprie two part. In the firt part the compenated voltage i calculated in order to provide the neceary reactive compenation to meet the requirement at the grid connection point. The difference i made with the compenated voltage et point and enter a proportional and integral regulator with limitation that compute the reactive power et point. Subequently, the comparion i made with the meaured reactive power. The limitation on the reactive power et point are dependent on the voltage output of the wind turbine. If the voltage i above 0.8 pu, it i uppoed that the operation i almot at teady-tate and the limit applied are thoe from the capability curve of the wind turbine. Fig. 1 and Fig. 2 how, repectively, the capability curve PQ and VQ ued by default in the model. Fig. 1. Capability curve PQ of the wind turbine ued by default in the model [8]. In the econd part of the reactive power control, the error on reactive power enter another proportional and integral regulation to yield the reactive current. Two limitation are applied. In the firt one, monitoring the active current and limit the reactive current, o that the total current doe not exceed 1.1 pu. The econd one i the High Voltage Reactive Current Management that limit the injection of reactive current if the voltage reache a certain limit. Fig. 2. Capability curve VQ of the wind turbine ued by default in the model [8]. C. Frequency management The model of converter ued to repreent the wind farm require a few block that give the active and reactive current and the frequency. A phae lock loop wa then built and implemented in thi model to fix the commutation frequency of the converter model [8].

3 III. APPLICATION EXAMPLE In Fig. 3, it i hown the ingle line diagram of the electric power network that wa ued in thi tudy. The imulation were carried out conidering the network data preented in [9], [10]. Generator G2 i conidered a an infinite bu. The AVR and OXL of the generating unit and the turbine peed governor were taken into account. The ULTC action of the power tranformer between bue N3 and N4 (380/1 kv) are repreented conidering a time delay and a dead-band. Time delay for ULTC operation were aumed to be 30 for the firt tap movement and 5 for ubequent tap movement. In thi tudy, the operating point aumed correpond to a load level of 1600 MW and 8 MVAr. In bu N3 it i aumed a load level of 600 MW and 5 MVAr wherea in bu N4 the active power load i 0 MW and reactive power load i 300 MVAr. In bu N4 the load wa aumed a contant impedance and in bu N3 wa aumed a contant power. The wind farm i connected at bu N6 by a three winding tranformer 1/0.69/0.69 kv. G1 N1 N2 N3 N5 G2 8 IV. RESULTS Fig. 4 how, for cae I and cae II, the voltage variation in bu N4, the field current of generator G1 and the change in the tranformer tap, correponding to the power device connected between bue N3 and N4. In cae I, after the occurrence of the tripping of the 380 kv overhead tranmiion line between bue N3 and N5 at, the voltage in bu N4 (Fig. 4.a) decreae and, conequently, G1 increae the generation of reactive power. The ULTC change it poition (Fig. 4.c) to increae the voltage in bu N4.Thi phenomena yield to an additional increae in the generation of the reactive power in G1. The OXL of G1 operate and the field current change to it maximum value of 3.03 pu, at 110 (Fig. 4.b). In thi cae the power network collape at approximately a) VOLTAGE VARIATION IN BUS N4 (CASE I) Unité : a) VOLTAGE VARIATION IN BUS N4 (CASE II) Unité : 3.0 Fig. 3. Single line diagram of the power network. L1 N4 L2 Wind Farm N6 T3 NWINDS NWINDR The wind farm ha 80 wind turbine each with 2 MVA and i repreented by an aggregated equivalent model. The wind farm wa modelled conidering that the wind turbine were equipped with pitch control coupled with a FSIG and a hunt capacitor bank with 127 MVAr. In thi model the wind peed to obtain a maximal power i 11.5 m/. In thi tudy two cenario were imulated and analyzed. In the firt one (cae I) the wind farm wa modelled conidering that the wind turbine were equipped with pitch control coupled with a FSIG and a hunt capacitor bank, without reactive and active power management and frequency control. In the econd ituation (cae II) the wind farm wa modelled conidering that the wind turbine were equipped with pitch control coupled with a FSIG and a hunt capacitor bank, with reactive and active power management and frequency control. In the two cae the following event were imulated: an increae of the wind peed from 8.5 to 10.5 m/, from 20 to 30 ; the tripping of the 380 kv overhead tranmiion line between bue N3 and N5 at b) FIELD CURRENTS OF G1 (CASE I) Unité : b) FIELD CURRENTS OF G1 (CASE II) Unité : tap c) TRANSFORMER TAP : N3 - N4 (CASE I) Unité : tap c) TRANSFORMER TAP : N3 - N4 (CASE II) Unité : tap Fig. 4. a) Voltage variation in bu N4, b) Field current of G1, c) ULTC poition of tranformer between bue N3 and N4. Fig. 5 preent, for cae I and cae II, the voltage variation in bu N6, the active power produced by the FSIG, the reactive power conumed by the FSIG and the reactive power injection of the capacitor bank in bu N6. After the occurrence of the contingency the reactive power injection decreae, due to the voltage dip at bu N6 (Fig. 5.d). The capacitor bank connected in parallel are not effective to prevent the voltage collape, ince the reactive power production diminihe with the terminal voltage decreaing. The FSIG wind turbine uing a quirrel-cage induction generator, uually have the ability to withtand high current during a voltage dip, due to it high thermal capacity. In thi ituation, the magnetization mut be fat enough in order to prevent the overpeed protection tripping. Thee machine do not poe the ability to participate in voltage regulation.

4 On the contrary, thi type of machine conume reactive power (Fig. 5.c), which can lead to a voltage collape ituation a hown in cae I a) VOLTAGE VARIATION IN BUS N6 (CASE I) Unité : a) VOLTAGE VARIATION IN BUS N6 (CASEII) Unité : MW 0.8 a) VOLTAGE VARIATION IN BUS N6 (CASE II) Unité : b) ACTIVE POWER PRODUDED BY THE FSIG (cae I) Unité : MW b) ACTIVE POWER PRODUDED BY THE FSIG (cae II) Unité : MW c) REACTIVE POWER CONSUMED BY THE FSIG (cae I) Yca=-1. Unité : c) REACTIVE POWER CONSUMED BY THE FSIG (cae II) Yca=-1. Unité : d) REACTIVE POWER INJECTION OF THE SHUNT CAPACITOR BANK IN BUS N6 (cae I) Unité : d)reactive POWER INJECTION OF THE SHUNT CAPACITOR BANK IN BUS N6 (cae II) Unité : Fig. 5. a) Voltage variation in bu N6, b) Active power produced by the FSIG c) Reactive power conumed by the FSIG, d) Reactive power injection of the hunt capacitor bank in bu N6. Fig. 6 how, for cae I and cae II, the frequency variation at bu N Hz FREQUENCY IN BUS N6 (cae I) Unité : Hz FREQUENCY IN BUS N6 (cae II ) Unité : Hz Fig. 6. Frequency at bu N6. Fig. 7 preent, for cae II, the voltage variation, the active power produced by the FSIG, the reactive and the active power management in bu N6. In cae II, uing the model with reactive and active power management and frequency control in the wind farm, the bu voltage value are much more table, the capacitor bank produced more reactive power (Fig. 5.d) when compared with cae I. The OXL of G1 do not operate (Fig. 4.b) and conequently avoiding voltage tability problem. In thi cae de frequency in the bu N6 i more table when compared with cae I (Fig. 6) MW b) REACTIVE POWER MANAGEMENT IN BUS N6 (cae II) Unité : b) ACTIVE POWER MANAGEMENT IN BUS N6 (cae II) Unité : MW Fig. 7. a) Voltage variation in bu N6, b) Reactive power management in bu N6 c) Active power management in bu N6. In cae II, when the voltage at bu N6 drop below 0.8 pu (Fig.7.a) there are reactive production (Fig.7.b) and during that moment the active power production decreae (Fig.7.c), a it wa dicued in ection two (Wind Power Plant Model). V. CONCLUSION Thi paper preent a tudy of the reactive power management of a wind farm to prevent voltage collape of an electric power ytem. In order to ae the power network voltage tability evere contingencie were imulated. The wind power plant model with active and reactive power management and frequency management i very ueful when ued in tranient or voltage tability tudie during the occurrence of evere diturbance that can originate the collape of the ytem. The model ha the advantage that can be ued in wind farm with generator of different technologie. In the imulation reult preented in thi tudy the model wa ued in a wind farm equipped with FSIG. Thi model can be ued in wind park with voltage and frequency control, o that when a diturbance occur, for example a hort-circuit the wind farm i not diconnected from the grid. The grid code concerning wind power generation are pecific of each country and cover ignificant technical regulatory iue. The aim of thee grid code i to enure that the continued expanion of wind power generation doe not compromie the power quality, a well a the ecurity and reliability of the electric power network. The udden diconnection of wind generator, due to a fault may produce an unbalance between the power upply and the power

5 demand. Low Voltage Ride Through i one requirement pecified by the grid code, where wind turbine have to tay connected to the network for typical contingencie. The new wind power plant model can be modified in order to comply with requirement of the different grid code. REFERENCES [1] R. M. Monteiro Pereira, C. M. Machado Ferreira, F. P. Maciel Barboa, Influence of the Reactive Power Management for Wind Power Plant in the Dynamic Voltage Stability, in Przeglad Elektrotechniczny (Electrical Review), R. 88 NR 1a/2012, pp Available online: [2] P. Sharma, D. Thukaram, Reactive power and voltage control in grid connected wind farm, in Proc. of the 7th IEEE International Conference on Indutrial and Information Sytem (ICIIS), Indian Intitute of Technology Madra, Chennai, India, 6-9 Aug [3] M. Tili, Ch. Patioura, S. Papathanaiou, Grid code requirement for large wind farm: a review of technical regulation and available wind turbine technologie, in Proc. of the European Wind Energy Conference, EWEC 08, European Wind Energy Aociation, Bruel, Belgium, Apr [4] Power Sytem Operation Corporation, Reactive Power Management & Voltage Control in North Eatern Region, Technical Report, North Eatern Regional Load Depatch Centre (NERLDC), Shillong, India, Nov [5] P. Panchal, B. Mehta, Enhancement of Reactive Power Capability of Doubly Fed Induction Generator, International Journal of Electrical Engineering & Technology (IJEET), Vol. 5, Iue 8, pp , Aug [6] Y. Wang, et al., Fat Coordinated Control of DFIG Wind Turbine Generator for Low and High Voltage Ride Through, Energie, Vol. 7, Iue 7, pp , Jun [7] R. M. Monteiro Pereira, C. M. Machado Ferreira and F. P. Maciel Barboa, DFIG Performance Aement during Low Voltage Ride through in the Dynamic Voltage Stability of an Electric Power Sytem, 46th International Univeritie Power Engineering Conference, UPEC2011, South Wetphalia Univerity of Applied Science, Soet, Germany, 5th-8th September [8] Tractebel Energy Engineering and Réeau de Tranport d Electricité (RTE), Eurotag Software uer Manual and Releae Note, Eurotag Package Releae 4.5, Jun [9] R. M. Monteiro Pereira, C. M. Machado Ferreira, F. P. Maciel Barboa, Comparative tudy of STATCOM and SVC performance on Dynamic Voltage Collape of an Electric Power Sytem with Wind Generation, IEEE Latin America Tranaction (Revita IEEE América Latina), Vol. 12, Iue: 2, pp , Mar [10] Tractebel Energy Engineering and Réeau de Tranport d Electricité (RTE), Eurotag Tutorial, Eurotag Package Releae 4.5,june 2010.