Analysis and Performance Comparison of Different Power Conditioning Systems for SMES-Based Energy Systems in Wind Turbines
Abstract
:1. Introduction
- Load-step (Figure 1a), in which a sudden change in the load power takes place. This may be due to critical loads, temporary connection or disconnection of loads, faults in conventional power stations, etc. The energy storage system must provide or absorb the energy needed to fill this gap and keep the frequency stable. Existing technology tries to maintain several conventional power stations connected to the grid, working at a low output voltage level, which wastes energy. Therefore, an energy storage system avoids this waste of energy;
- Load-sharing [3] (Figure 1b), in which the unpredictability of the wind power makes not possible to control the output power of a wind farm and, consequently, the power can suffer relatively large fluctuations within a short time span. The energy storage system performs the power stabilization by absorbing any fluctuation in the wind energy produced and ensures that these large variations do not reach the grid, in such a way that smooth power is delivered to consumers;
- Grid-support [4] (Figure 1c), wherein the voltage dips that can occur in the grid lead to a current increase to maintain the grid power constant. The energy storage system provides the extra power necessary so that the grid power and frequency are affected to a lesser extent. For instance, they can keep industrial processes operating for a given time to avoid production disturbances, which are caused by sudden transients in the power delivered by a national AC grid. Usually, the sizing of an energy storage system depends on the rated power of the wind or photovoltaic farm to which it is linked. It is chosen as a percent of this rated power to give support P–f (active power–frequency).
2. Description of the System
- A three-phase pure resistive load demanding a constant power of 1.5 MW connected to the grid;
- A 2 MW variable-speed Wind Turbine (WT) based on an Induction Generator (IG; the Induction Generator parameters are listed in Table 1) plus a capacitor bank connected to the Point of Common Coupling (PCC) through a 690/1100 V transformer;
- An SMES system composed by a superconducting coil and a power conditioning system. This power conditioning system consists of one of the power converters mentioned in the introduction plus a grid filter, which is an L-filter for the VSCs and a C-filter for the CSC;
- The line-to-line rms grid voltage is 1100 V and the grid frequency is 50 Hz.
Parameter | Value | Units |
---|---|---|
Stator resistance | 0.01379 | p.u. |
Stator inductance | 0.04775 | p.u. |
Rotor resistance | 0.007728 | p.u. |
Rotor inductance | 0.04775 | p.u. |
Mutual inductance | 2.416 | p.u. |
Inertia constant | 5 | s |
Friction factor | 0.008726 | p.u. |
Pole pairs | 2 | - |
2.1. Operation Modes of the System
- WT output power is higher than the reference power (mode 1): this reference power can be the load power, calculated by means of the measured current and voltage and applying the PQ-theory [14], or any other power level specified by the grid operator. In this mode, the current through the coil increases and so does the stored energy, since it is absorbing the extra power from the IG.
- WT output power is equal to the reference power (mode 2): power does not flow through the SMES coil. The current remains constant at the same level that it had before both powers became equal.
- WT output power is lower than the reference power (mode 3): the current through the coil decreases and so does the stored energy, since the system supplies the necessary power to the grid to equal the reference power.
2.2. Profile of the Wind Turbine Output Power
- In t = 2.5 s the WT power is bigger than the load (or reference) power (mode 1).
- In t = 3.5 s the WT power decreases to equal the load power (mode 2).
- In t = 4.5 s the WT power becomes lower than the load power (mode 3).
3. Power Conditioning System
3.1. System Based on a Two-Level VSC
3.2. System Based on a Three-Level VSC
3.3. System Based on a Two-Level CSC
4. Control and Modulation Strategies
4.1. System Based on a Two-Level VSC
4.1.1. Control System
4.1.1.1. DC-link Voltage Controller
4.1.1.2. Active Power Controller
4.1.1.3. Current Controller
4.1.2. Modulation Strategies
- : the coil is in the charge state (mode 1).
- : the coil is in steady-state (mode 2).
- : the coil is in the discharge state (mode 3).
4.2. System Based on a Three-Level VSC
4.2.1. Control System
4.2.2. Modulation Strategy
4.3. System Based on a Two-Level CSC
4.3.1. Control System
4.3.2. Modulation Strategy
5. Simulation Results and Discussion
Grid impedance | Grid filter | Sampling & switching | DC-link control | Current control | Power control | ||||
---|---|---|---|---|---|---|---|---|---|
LG | 0.01 mH | L | 0.675 mH | fS | 2.5 kHz | KP | 3.4494 | 0.7775 | 0.0002 |
RG | 1 nΩ | R | 1.781 mΩ | fSW | 5 kHz | KI | 775.46 | 0.2535 | 0.13 |
- | KAW | - | 0.6475 | - |
Grid impedance | Grid filter | Sampling & switching | |||
---|---|---|---|---|---|
LG | 0.01 mH | C | 10 mF | fS | 28.8 kHz |
RG | 1 nΩ | - | fSW | 3.6 kHz |
5.1. Basic Simulation to Test the Three Operation Modes of the System
Title | Two-level CSC | Two-level VSC | Three-level VSC | ||
---|---|---|---|---|---|
VSC | DC-DC | VSC | DC-DC | ||
IGBT | FZ1200R33KL2C | FZ800R33KL2C | FZ1200R33KL2C | FZ600R17KE3 | FZ1200R33KL2C |
Diode | D3501N | — | D3501N | D4201N | D3501N |
5.2. Load-Step
5.3. Load-Sharing
5.4. Grid-Support
Item | Two-level CSC | Two-level VSC + DC-DC | Three-level VSC + DC-DC |
---|---|---|---|
Active power compensation | Medium-good | Good | Good |
Reactive power usage (Figure 18) | Yes, due to the C-filter | No | No |
Grid-filter (Figure 4, Figure 5 and Figure 6) | C-filter | L-filter | L-filter |
Modulation strategy | MS-SVM | THPWM + PWM | THPWM + PWM |
Harmonic content (Figure 17) | High | Medium | Low |
Low-order harmonics | High | Low | Low |
High-order harmonics | Medium | Medium-high | Low |
Number of devices (Figure 4, Figure 5 and Figure 6) | 12 | 10 | 22 |
IGBTs | 6 | 8 | 14 |
Diodes | 6 | 2 | 8 |
Total power losses (Figure 22) | Low | High | Medium |
Switching losses | Low | High | Medium |
Conducting losses | Low | Very low | Very low |
Max./Min. DC-voltage | ±1555 V | ±1800 V | ±1800 V |
Complexity of control | Low | Medium | Medium |
Reaction time | No major differences |
6. Conclusions
Acknowledgments
Conflict of Interest
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Rodríguez, A.; Huerta, F.; Bueno, E.J.; Rodríguez, F.J. Analysis and Performance Comparison of Different Power Conditioning Systems for SMES-Based Energy Systems in Wind Turbines. Energies 2013, 6, 1527-1553. https://doi.org/10.3390/en6031527
Rodríguez A, Huerta F, Bueno EJ, Rodríguez FJ. Analysis and Performance Comparison of Different Power Conditioning Systems for SMES-Based Energy Systems in Wind Turbines. Energies. 2013; 6(3):1527-1553. https://doi.org/10.3390/en6031527
Chicago/Turabian StyleRodríguez, Ana, Francisco Huerta, Emilio J. Bueno, and Francisco J. Rodríguez. 2013. "Analysis and Performance Comparison of Different Power Conditioning Systems for SMES-Based Energy Systems in Wind Turbines" Energies 6, no. 3: 1527-1553. https://doi.org/10.3390/en6031527