25 Nov 2016 |
Research article |
Sustainable Development, the Circular Economy and Environmental Issues
Wind Turbines: How to Prevent Breakdown!
Wind Turbines – Editor’s Note
Many electrical problems could cause a complete shutdown of an onshore and offshore wind turbines farms: the most common are defective wind turbine sensors and three-phase faults. When one of these defaults happen, power is shutdown by the system! An emergency repair team need to be sent onsite or offsite, sometimes at great expenses, to find and correct the problem. For an offshore wind farm composed of 150 wind turbines of 2 mega watts (MW), it could mean 300 MW lost during several days. All this precious energy could be loosed because of a tiny defective sensor… Researchers from the Power Electronics and Industrial Control Research Group (GRÉPCI) at École de technologie supérieure (ÉTS) of Montreal found an innovative solution to these problems to keep wind turbines producing and running!
The world is heading towards the production of green electric energy from renewable energy sources, especially, wind power [1-3]. The world production is growing with the installation of offshore and onshore wind farms. The international economic perspective encourages investment in offshore where the wind is stronger and more consistent than onshore . Technically, the interconnection of these wind farms with electric grids provides a better quality of power, an improvement in the transit of power and stability of the network.
Wind turbines based on wound rotor asynchronous generators have the major drawback due to slip rings, brushes and multiplier that significantly increases maintenance costs, especially for wind turbines in the sea.
To reduce these drawbacks, some manufacturers have developed wind turbines based on synchronous machines where the slip rings and brushes are eliminated such as with permanent magnets (PMSG) .
Transport of the energy produced by the wind farm towards the principal terrestrial grid system is performed with high voltage direct current (HVDC) systems or high voltage alternating current (HVAC) .
The HVDC transmission system may be an appropriate and feasible choice than high voltage alternating current (HVAC) transmission when the distance between the converter stations is far. It can improve the effect of grid faults on the wind farm and give the wind farm better voltage and frequency stability , .
Studies already done in the field of HVDC system do not take into account the problem cases of measurement uncertainty, uncertainty in modeling, malfunction or failure occurring as in the sensor measures concerning wind energy systems installed on land and / or especially wind turbines in the sea. The usefulness of the non- linear estimators for outputs estimates has become necessary. The method proposed resolves this problem by using non-linear estimators Extended Kalman Filter (EKF) for output estimates.
Kalman Filter was co-invented by Rudolf Emil Kálmán, an electrical engineer. It is “a mathematical technique widely used in control systems and avionics to extract a signal from a series of incomplete and noisy measurements”. They are widely used in signal processing, control systems, and guidance, navigation and control .
The Kalman filter has two distinct phases: Predict and Update. The prediction phase uses the estimated state from the previous time to produce an estimate of the current state. In the step of updating the observations of the current time are used to correct the predicted in order to obtain a more accurate estimated state.
We have created a simulation of an offshore wind farm composed of 150 wind turbines (2 MW each) equipped with Permanent Magnet Synchronous Generators (PMSGs) and sensors to estimate the rotor position and speed (figure 6). The wind turbines DC voltage produced goes through a DC/AC inverter. The three-phase AC power generated would go to an offshore station (identified as station 1 in figure 6), to convert the high voltage from AC to DC (voltage source converter of a high voltage direct current – VSC-HVDC). This +/- 150 KV DC 100 km submarine cable (dc-bus) would connect to an onshore station (identified as station 2 in figure 6), converting the DC voltage in three-phase AC voltage, connected to a main grid (315 kv / 60 Hz identified as system 2 in figure 6).
Three extended Kalman filters (EKF) were added to this simulation: the first offshore for the wind farm, the second at station one and the third at station 2. Three common problems were simulated:
- A three-phase fault on the PCC (Point of Common Coupling) at offshore AC grid (default 1);
- Upper DC cable fault in the 100 km DC cable between station 1 and station 2 (default 2);
- A three-phase fault on the PCC (Point of Common Coupling) at onshore AC main grid (default 3).
Simulation results demonstrate:
- The control developed with EKF algorithm for integration of both stations shows its effectiveness in the presence of three-phase fault by minimizing its impact on the system (default 1);
- The impact of the fault on the main grid side current amplitude has been clearly reduced and the system response time is fast (default 2);
- The proposed system can overcome the severe three-phase fault. The entire system stabilizes under the developed control strategies with EKF algorithm (default 3).
The actual time standard of an electrical power network response, as suggested in , can go up to 150 ms. This requirement is difficult to meet, particularly, in VSC-HVDC systems interconnected with wind farms. In the absence of an appropriate control strategy, this fault may lead the electrical installations to a significant material damage in. For this, several research studies have begun in this area to resolve the problematic of the AC and DC faults [15-18]. The majority of these studies have shown an improvement in the quality of energy in the presence of a fault while mitigating the impact on the system, but the response time and amplitude of the measured variables remain higher. The comparison with the proposed research shows the improvement of the power quality and the reduction of time and amplitude response.
Simulation results show that the estimated rotor speeds follow the reference speeds obtained by the algorithm that corresponds to the MPPT, and the estimated total dc voltage exactly follows the reference voltage. Also, the offshore 3L-NPC VSC station is able to transfer the real power of offshore wind farm to the onshore grid successfully and efficiently. Similarly, other simulation results indicate that the onshore 3L-NPC VSC station is capable to ensure the stability of the dc voltage of the HVDC line.
Finally, we conclude that the control strategy proposed with the Extended Kalman Filter estimator gives excellent dynamic and transient performance before, during and after the occurrence of the fault on the PCC at ac main grid side, on the PCC at offshore ac grid side and at DC side.
To get information on this subject, we invite you to read the following research article :
Mounir Benadja and Ambrish Chandra (2015). Adaptive Sensorless Control of PMSGs based Offshore Wind Farm and VSC-HVDC Stations. IEEE Journal of Emerging and Selected Topics in Power Electronics. (PDF)
Mounir Benadja is a Ph.D. candidate in the department of Electrical Engineering at the ÉTS. His current research interests include power electronics for renewable energy sources and high-voltage direct-current transmission systems,
Research laboratories : GREPCI – Power Electronics and Industrial Control Research Group
Ambrish Chandra is a professor in the Electrical Engineering Department at ÉTS. His research interests focus on the quality of voltage, power factor correction, control and integration of renewable energy sources.
Program : Electrical Engineering