Innovation in Voltage Control System for an Optimal Wind Turbines Electrical Integration

Transcription

1 Innovation in Voltage Control System for an Optimal Wind Turbines Electrical Integration José Luis Iribarren, Raquel Ferret, Javier Olarte C/ Portal de Gamarra, 28, Vitoria-Gasteiz (Alava) Spain / Zigor Corporación S.A / jiribarren@zigor.com, rferret@zigor.com, jolarte@zigor.com Abstract There is an international trend, supported in most of the countries by approved regulations, to reach in the next years ambicious targets for the amount of renewable energy connected to the power system. The achievement of this target depends on the capacity of power system to tolerate this amount of renewable wind energy. owadays, it is well known that several wind turbines technologies connected to grid respond to voltage sag or interruption events in the network by shutting down automatically faced with a drop of voltage. Therefore, there is challenge to find in the market a commercial solution. Driven from the need to solve this problem ZIGOR CORPO- RACIÓ has developed a number of power conditioning solutions specifically designed for wind farms. Among the different FACTS (Flexible Alternating Current Transmission Systems) topologies techniques developed, namelly series and parallel, centrallized and distributed, the SE- PEC EÓLICO is presented as the optimun topology. This topology complemented with SET Q, as an active filter, provides a general purpose ride-through topology for most of grid codes. These solutions have been validated for the Spanish Grid Code by the Procedimiento Particular of the PO12.3 in a M750 wind turbine. Index Terms Spanish Grid Code, Voltage Sags, Wind Turbine, FACTs Topologies, Renewables Integration. I. THE PROBLEM According to the European Wind Energy Association (EWEA) Anual Report, the installed wind power capacity increased by 18% in 2007 to reach a level of MW (Megawatts) [1]. The European Commission estimates that, wind energy s share of electricity must climb from its current level of 3,7% to 12%. Although there has been a huge effort in technology development by major wind turbine manufacturers, still some already operational wind turbine technologies connected to electricity networks respond to voltage sags or interruptions in the network by shutting down automatically in the case of a drop in voltage. When the increase in the wind power generated in an area exceeds a value with regard to the power from other forms of generation, these wind turbines are required to continue generating during voltage sags, following the time/depth pattern represented in the Figure 1 for values established by the country grid code. Voltage (pu) Figure.1. Spanish Voltage-Time Curve Grid Code II. TOPOLOGIES EXPERECED Tim e (s) As mentioned before, when existing wind turbines could not be adapted to the new grid codes by the original manufacturer, some additional equipment, FACTs, have to be installed in the wind farm in order to comply with the new ride-through imposed. Recently, several solutions are being investigated to fulfil the established requirements[2]-[4]. Many of them are based on FACTS devices, whose correct placing and selection is critical for an effective and economical operation. The topologies described herein has been developed and tested by ZIGOR in order to offer the most adequate solution to each problem. All data shown in this paper comes from the test results obtained from equipments designed and manufactured by ZIGOR. The advantages and disadvantages presented in this paper, are obtained from our own developments and do not refer directly or indirectly to any solution from other manufactures. The following products/topologies have been analyzed: SET DVR LV/Distributed/Series SET DVR MV/Centralized/Series SET Q/Parallel/Distributed SEPEC EOLICO/Optimal/Distributed A. SET DVR LV The SET DVR LV (SET Dynamic Voltage Restorer Low Voltage) belongs to a series type of Dynamic Voltage Restorer operating at Low Voltage. So far, different topologies and products have been developed and tested in the industry to solve fast voltage regu-

2 lation problems. Nevertheless many of these traditional technologies are not adequate for the time frame and time response required for dips, as defined previously. Some products use ultracapacitors or batteries as energy backup to compensate the dips. Most topologies use a standby strategy, that is, the active compensation components of electronics operate only during the dip. The design target for this family of equipments was to solve some limitations of existing topologies, namely: Free of any energy storage components. Longer time frame for repetitive dips events. Allow continuous operation to offer very high stabilization accuracy. Allow bi-directional energy flow. Improve time response to allow permanent voltage distortion filtering. As shown in Figure 2 and based on a booster transformer plus a set of a reversible rectifier, plus an inverter, the device builds up a flexible energy injection/absorber compensator, which is capable to correct input voltage deviations, offering an extremely stable output voltage with a very fast response. The designed topology offers the following disturbances compensation capabilities: Voltage dips and swells. Voltage variations. Voltage distortion. Voltage flicker. Voltage unbalance. Some level of transient overvoltages. Figure 2 - Topology of the SET-DVR LV. to 10 or more units. Additionally, the proposed topology allows building custom-made solutions for medium voltage (MV) by changing the booster transformer. Figure 3.SET DVR 300 KVA. The proposed system has the following main features: No battery or alternative energy storage required, minimizing maintenance cost and increasing reliability. Continuous voltage regulation within the ± 0.5%. Compensation of long lasting dips (-50% up to 30 sec). Avoids relays and brushes. Time response less than 3 milliseconds. Capable to operate with industrial regenerative loads (e.g. four-quadrant converter). Improves the voltage distortion. Flicker compensation. Non stop of process operation in case of failure. Easy to parallel additional equipment. Independent phase compensation. Voltage balancing capability. Balanced and unbalanced dip compensation. Automatic Bypass Overload capacity: 150% during 1 second. Figure. 4 shows the compensation capabilities of the estandard system. A sample dip compensation (5 cycles) is represented in Figure. 5. The master unit is shown in figure 3. Higher power units can be built by paralleling slave units to the master one up

3 DIP / SAG (% / Vin) 0 A. B.. C.. -30% -40% -50% A. Vin = ±30% of nominal / Vout = ± 0,5% of Vin Continuously B. Vin = 30 % to 40% of nominal / Vout = ± 0,5% of Vin during 30 seconds C. Vin = > 40% of nominal / Vout = Vin + 40% of Vin ± 0,5% during 30 seconds -X% Figure 4. SET-DVR dips compensation capabilities. Figure 7. Input versus Ouput during compensantion opertaion of the SET DVR MT C. SET Q Figure 5. Compensation sample for a 40% voltage dip. B. SET DVR MV The SET DVR MT is MV (Medium Voltage) evolution of the SETDVR LV. It is a state-of-the-art Dynamic Voltage Restorer operating at medium voltage for a nominal power of 2.1MVA. The simplified diagram is represented in Figure 6. Figure 6. Simplified diagram of SET DVR MT connection The basic operation funtion of the SET DVR is to maintain the output voltage within a maximum an minimum limits, independent of the main inputs variations. A sample of the operation plot is illustrated in the figure 7, where a sudden drop of the input voltage (in red color) is compensated to guarranty that the output voltage (in blue color) is kept stable. The well know Stat Com Topology has been originally developed as a complement to the next topology presented, the SEPEC EOLICO, due to the requirement to generate Q power durint the fault. This topology is very limited according to our experience and although it could be helpfull and complementary to other systems D. SEPEC EOLICO The SEPEC EOLICO is based on a StandBy system that only operates during the sags. There are two posible operation states: During normal operation (NO SAGS) where the SS (Static Switch), figure 8, is closed and the TPVS (Three Phase Voltage Source) is in stand-by mode without no operation and without any energy consumption. During sags operation, where the SS is open and the TPVS is operating and generating a voltage following a mains pattern (phase and magnitude) that guarranties that the generator does not stop. Durinng the sags the energy generated by the turbine is absorbed by the brake included in the TPVS in the intermeate DC BUS of the TPVS. As ilustrated in the the figure 8, the SEPEC EOLICO is connected in between the generator and the medium voltage transformer (690 VAC three wires). It should be noted that the SEPEC EOLICO is neither a standard SERIES FACTS nor a PARALLEL FACTS. This is a different topology from what is normally described in literature. It can be interpreted that the SEPEC EOLICO is a Parallel that only operate during the SAG. Summarising, the basic components of the SEPEC EO- LICO, it has two fundamental components: A Static Switch (SS) and the Three Phase Voltage Source (TPVS) as illustrated in the figure 8.

4 Static Switch Three Source Voltaje Source Figure 8 Basic SEPEC EOLICO DIAGRAM As an option, the SEPEC EOLICO could include another inverter, the previously refered as SET Q: This is similar to the TPVS but placed on the left hand side of the SS and connected to the TPVS via a DC BUS. This topology option is ilustrated in Figure 9. The optional inveter (that operates as controlled current source) could generate a pattern of reactive/active energy during the sags to comply with specific grid-codes. To test the behaviour of the SEPEC EOLICO described, several lab test have been carried out both in real field test wind turbines and in the laboratory Addiotnally, the standard voltage dip measurements to certificate that the system fulfils the Spanish grid code have been carried out. These tests have been done in a nacional wind park for the NM750 turbines. In the case of the proposed 750 KVA wind turbine the installation of 2 TPVS modules with a total power of 1400 KVA, has been required. Following the some relevant operation data are explained: 1. Response Time Two reponse Time has to be defined: The Total Response Time in Case of SAG, say SS_OFF. SS_OFF is the time since the Mains fails until the TPVS starts. This value is typically 1 milisecond. The Total Response Time when the SAG disapears, say SS_ON. SS_ON is a parameter that can be changed between 0 and 150 milisecond, typically 100 miliseconds. 2. Maximum and Minimum Voltage ar the Generator Terminal Figure 9 Full equiped SEPEC EOLICO Mains-Side Current Inverter Considering that the SEPEC EÓLICO is interconnected between the generator and the network, in normal network regime it behaves like a closed switch. During the failure, it behaves as follows: With regard to the wind turbine, voltage is supplied as if it came from the network and the wind turbine is kept in normal operation. With regard to the network, an active and reactive pattern is generated in accordance with the needs of the network. During the failure, excess power is dissipated internally by SEPEC EÓLICO During Normal Operation with the SS closed the same of the MAINS. During the SAG, the nominal voltage has +/- 3% of amplitude. During the SS_OFF that migth last typically 1 miliseconds and no voltage drop as illustrated in the figure 10 plot: 3. Maximum OverCurrent Amplitude and Duration The order of magnitude of typical overcurrent values for a 0,2 pu sag is ZERO as compared to the nominal one as shown in figure Field Test Results Several test has been run (over 100) using a sag generator in the NM750 turbines under different load conditions. Following the Procedimiento Particular as especified by the PO12.3 and its corresponding verification protocol, the test has been successful. Below, some oscilloscope plot of the real voltage and current measured in the NM750 test, are shown (figure 10) Optionally, SET Q can be equipped to adapt reactive power both during the sag and in permanent regime. The system is based in a modular arquitecture that allows the configuration of different power for different turbines. The unitary module power of the TPVS is 700 KVA.

5 Unpredictible desincronization D. SEPEC EOLICO/Optimal/Distributed Better topology and independence from the machine Better efficiency and no unsettle of protections needed Better MTBF Better ROI + Operating Expenses Compatible with 0v PU up 2 seconds sags Grid Codes IV. SUMMARY Figure 10 Real Plot of Current (yellow), Mains Voltages During Sag (Blue) and Turbine Voltage (Green and Red). III. TOPOLOGIES COMPARISON In the following section the characteristcs of the four potential arqutectures are summarised: A. SET DVR LV/Distributed/Series MTBF equivalent to full converter. Higher wind farm availability against centralized one High permanent losses, 3% - 6%, transformer, filter & igbt s Technically suitable for those machines sensitive to fast disturbances Higher cost compared to the other topologies B. SET DVR MV/ralized/Series Potential environmental impact High cost for high yield & 0,2 p/u High over currents (> 2Inom. -> + cost or - efficiency) MTBF equivalent to full converter Lower availability in as compared to distributed one. High permanent losses, 3%- 6% tranformer, filter & igbt s C. SET Q/Parallel/Distributed Two operations MODES has been experienced, namely MODE A Rissing the Grid Voltage : High overcurrents: 13 times the nominal Very high power required to comply with 0,2 pu And MODE B: generating the reactive power consumed by machine during failure Mechanical and electrical transients Protections must be unsettle With regard to other solutions developed to cope voltage sags in wind turbines, the system presented in this paper, named SEPEC EOLICO, shows important innovations and advantages over the other ones. On one hand, the system brings into operation only when the failure occurs, avoiding unnecessary wind turbine disconnection. Moreover, SEPEC EOLICO is able to provide to the network what it needs during the failure so no electrical/mechanical transitories are produced, cutting down the consequent premature aging of the machine. The unit allows to operate with any kind of wind turbine, no depending on the technology and power. On the other hand, the SEPEC EOLICO system has been developed following the basis criteria of maximun efficiency and efficiency, therefore it provides minimun losses and highly robust behaviour. The system has been tested in real operation following the Procedimiento Particular of the Spanish Grid Code complying successfuly with a NM750, showing optimal results for voltage drops up to 0 Volts and for up to 2 seconds. Machine voltage remains stable in its normal operation level during failure and no need to unsettle the wind turbine protecction has been required. In conclusion, this wind turbine adaptation system represents probably the most effective topology regarding, efficiency, electrical dynamic estability, mechanical dynamic estability and investment. The mean time between failures (MTBF) is really high for SEPEC EOLICO system because it works only during the failure. Therefore the equipment cost is proportional to the yield what turns it into an optimal solution for wind turbine adaptation to face up to voltage sags. ACKNOWLEDGEMENTS We would like to thanks to the AEE for the opportunity to bring this solution into the market. REFERENCES [1] Delivering Energy and Climate Change Solutions_ EWEA 2007 Annual Report, Sarah Clifford Edition, March 2008.

6 [2] Yan Zhang y J.V. Milanovic,: Voltage Sag Cost Reduction With Optimally Placed FACTS Devices, 9th International Conference Electric Power, Quality and Utilisation, Barcelona [3] R. K. Varma y T.S. Sidhu : "Bibliographic Review of FACTS and HVDC Applications in Wind Power Systems," International Journal of Emerging Electric Power Systems: Vol. 7 : Iss. 3, Article 7, [4] GONZALEZ Fernando; MOLINA S.; CONTRERAS Angel; MENDOZA Javier; BUNEZ Julian; VISIERS Manuel; AGUDO Andres; AMARIS Hortensia: WINDFACT, a solution for the grid code compliance of the windfarms in operation, Proceeding of EPE 2007 Conference, 2007.