An Update on the Electronic Connection Issues of Low Power SWTs in AC-Coupled Systems: A Review and Case Study
Configuration of the System under Review and Introduction to the Case Study
2. Requirements for Applying Grid Connection in Each Component of a SWT
2.1. The Inverter for the Grid Connection
2.1.1. Inverters Designed Specifically for SWT Applications
2.1.2. General Purpose Photovoltaic (PV) Inverters
- Frequency drop—power regulation in AC-coupled microgrids: in the case of AC-coupled microgrids, it is common to use frequency drop in order to regulate the power generated by the inverter . Based on the frequency variation, the generator-side inverter will move the working point of the PV generator, changing the working voltage so that the current is adjusted to the desired value. However, as this characteristic is different for a SWT, this regulation might not work as expected.
- Maximum Power Point Tracking (MPPT): in the same way, the inverter will move the working point of the PV generator to track for the maximum power point, changing the working voltage according to the PV’s I–V curve. However, as this characteristic is different for a SWT, this regulation might not work as expected either. The variability in wind speeds creates a challenge for turbine control, because the aerodynamic efficiency of the turbine rotor is related to maintaining an optimal relationship between current wind speed and rotational speed . Sudden gusts of wind can also cause an increase in the fatigue load on turbine components that might reduce the lifetime of the turbine.
- Time response: PV generation has no inertia, whereas SWT generation does. This means that the two methods will have different time responses, which might affect the correct functioning of the converter.
- Waiting time: the time until the inverter starts producing, usually in the tens of seconds (during which the generator is unloaded). The inverter control routines may cause periods of disconnection from the grid, which are allowable for PV applications, but which might result in conditions that compromise the integrity of the SWT due to situations where there might be high voltage and overspeed in the wind turbine, requiring an overvoltage device.
- High DC voltages: in Figure 5 it is shown that inverters for grid-connected power generation usually work with voltages higher than 100 V, up to hundreds of volts . SWTs designed for battery charging applications usually use 12, 24 or 48 V as nominal voltages, which are far from the voltage range used for grid connection. This effect means that some adaptation will be needed for voltage matching, either at the SWT’s generator or at the power converter, or both.
- Outages: in grid-connected systems, outages have to be considered. In IEC 61400-2 standard , it is stated that “Electrical network outages shall be assumed to occur 20 times per year. An outage of up to 24 h shall be considered a normal condition”. This fact equally necessitates overvoltage protection for the SWT, and adds the need for it to be able to work in continuous mode.
2.1.3. The Inverter in the Case Study
2.2. The Power Controller
2.2.1. Overvoltage Protection
- It is mandatory: as derived from analysis of the effects on the PV inverter (2.1.2), it is mandatory to have an overvoltage protection device for the times when there is no load connected to the SWT, such as an outage in a grid-connected SWT.
- It has to be able to work in a continuous mode: also derived from the referred analysis; grid power outages may last up to 24 h in normal conditions, which can be considered continuous mode operation for the overvoltage device.
- These electronic overvoltage protection devices are common for SWT battery-charging applications, where there is a stable voltage reference from the battery and the voltage variations are slow and small, as the battery maintains voltage stability. This is not the case in grid-connected turbines: in the case of an inverter waiting time, for example, the SWT will be open-loaded and the DC voltage might change abruptly if the wind is high enough at that moment. In that case, it is possible that the control algorithm of the electronic overvoltage protection device will try to diminish the rotational speed sharply, delivering a (relatively) large amount of power to the resistive load. This action will produce a sudden brake in the SWT, with corresponding fatigue loads. This might be thought of as an emergency system, but will certainly highly decrease the lifetime of the SWT if it is used as a normal condition.
2.2.2. Controllers for Wind Turbines: Rectifiers
2.2.3. The Power Control in the Case Study
2.3. The Generator
The Generator in the Case Study
2.4. A Short List of SWTs Below 5 kW and Their Main Characteristics for Grid Connection Capacity
- Generator: PMSG technology dominates. The number of pole pairs is quite high to avoid the need for a gearbox. The output voltage of the generator is higher than 200 V.
- Overvoltage protection device: the chopper plus dump load is always present; other devices (either mechanical or electronic) are also present in some models.
- Braking system: usually, the overvoltage protection device is also used as a braking system.
- Controller: some of the models include a MPPT controller.
3. A Review of Existing Options for Overvoltage Protection
- Proprietary products, i.e., manufactured in-house and used exclusively by manufacturers of SWTs;
- Off-the-shelf products existing at a commercial level, i.e., equipment manufactured by electronic equipment manufacturers, and which can be used in different wind turbine models;
- Free or open source designs, i.e., designs that are available in the literature and/or on the Internet, and that allow anyone to manufacture them.
3.1. Proprietary Devices
3.2. Commercially Available Devices
3.2.1. PowerOne Turbine Controller
3.2.2. Morningstar TriStar Turbine Controller
3.2.3. Voltsys Wind Turbine Controller
3.2.4. Midnite Controller
3.3. Freely Available Designs
3.3.1. Wind Empowerment
3.3.2. Overvoltage Relay Protection
4. An Open Source Proposal for Overvoltage Protection for a 1 kW SWT: A Case Study and Preliminary Field Results
4.1. Proposed Solution for Overvoltage
4.2. Experimental Prototype and Field Evaluation
4.2.1. Analysis of Energy Performance
4.2.2. Proposed Energy Improvement When Using a Solar Inverter
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|Phase Inductance||0.0407 H||Phase Voltage||75 Vrms|
|Phase Resistance||11.6 ohm||Torque||23.21 Nm|
|PM Flux||1.2 T (neodymium)||Angular speed||400 rpm|
|Turns per coil||355||Efficiency||73%|
|Manufacturer||Model||Rated Power (W)||Number of Pole Pairs||Type||Voltage (Nominal, V)||Furling||Centrifugal||Chopper + Dump Load||MPPT (+ grid)||Electronic||Mechanic||Electro magnetic Brake||MPPT||Passive Mechanical Pitch Control||Stall|
|SD wind energy||SD3||3000||PMG||x||x|||
|Potencia Industrial||Hummingbird 5 kW (Colibrí 5 kW)||5000||PMG||0–450 VDC||x||x|||
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Bufanio, R.; Arribas, L.; de la Cruz, J.; Karlsson, T.; Amadío, M.; Zappa, A.E.; Marasco, D. An Update on the Electronic Connection Issues of Low Power SWTs in AC-Coupled Systems: A Review and Case Study. Energies 2022, 15, 2082. https://doi.org/10.3390/en15062082
Bufanio R, Arribas L, de la Cruz J, Karlsson T, Amadío M, Zappa AE, Marasco D. An Update on the Electronic Connection Issues of Low Power SWTs in AC-Coupled Systems: A Review and Case Study. Energies. 2022; 15(6):2082. https://doi.org/10.3390/en15062082Chicago/Turabian Style
Bufanio, Rubén, Luis Arribas, Javier de la Cruz, Timo Karlsson, Mariano Amadío, Andrés Enrique Zappa, and Damián Marasco. 2022. "An Update on the Electronic Connection Issues of Low Power SWTs in AC-Coupled Systems: A Review and Case Study" Energies 15, no. 6: 2082. https://doi.org/10.3390/en15062082