Abstract: This paper mainly studies the control strategy of a direct-drive permanent magnet synchronous wind power converter system to improve its low-voltage ride-through capability. The mathematical model of the PWM converter is studied, and the energy transfer of the converter is analyzed. The grid-side converter adopts a grid voltage-oriented vector control strategy, while the machine-side converter adopts a rotor field-oriented id=0 vector control strategy, establishing a solid theoretical foundation for the study of low-voltage ride-through technology in direct-drive permanent magnet wind power generation systems.
This paper details the reactive power demand during energy storage crowbar and grid voltage dips. Based on this, a novel reactive power control strategy is proposed for grid-side converter faults, and simulations verify the effectiveness of the control strategy. Next, the control logic for low-voltage ride-through of a direct-drive permanent magnet wind power system is presented. Using Matlab/Simulink on a direct-drive permanent magnet wind power system simulation platform, the low-voltage ride-through of the system is achieved by employing strategies such as energy storage crowbar, reactive power control during faults, and tip speed ratio control.
Keywords: Wind power converter; Permanent magnet generator; Low voltage ride-through;
Research on the Low Voltage Ride through Capability of Directly-driven PM Wind Generation System
Abstract: The control strategy for directly driven wind turbine with permanent magnet synchronous generator was investigated to enhance its capacity of low voltage ride through (LVRT). The mathematical model of PWM converter is studied, and the power's transformation is analyzed. The controller of grid-side converter uses voltage-oriented vector control strategy and the controller of motor-side converter uses the rotor flux-oriented vector control strategy. That established a good theoretical basis for the direct drive permanent magnet wind power system low voltage ride through technology.
This paper describes power storage crowbar and the demand of reactive power when grid voltage dips. The paper presents a new power control strategy, when the grid-side converter is failure. Then simulation results verify the effectiveness of the control strategy. Then, the study puts forward a set of control logic for the based low voltage ride through. power control strategy during the breakdown time.
Key words : Wind Power Converter; Permanent Magnet Synchronous Motor; Low Voltage Ride Through
introduction
With the rapid development of wind power generation in my country, the impact of wind power systems on the national power grid is also increasing. Countries with leading wind power technology have successively issued quantitative standards for grid fault ride-through, requiring wind turbines to achieve low-voltage ride-through operation and provide reactive power support to the grid during grid faults, so as to help the grid quickly restore normal operation.
This paper proposes a coordinated control scheme for converters during low-voltage ride-through (LVRT) operation of permanent magnet direct-drive (PMD) wind turbines, based on the operating characteristics of PMD wind turbines with dual PWM converters connected to the grid. During grid faults, the generator-side converter is controlled according to changes in the electromagnetic power input to the grid to limit the generator's electromagnetic power, thereby balancing the power across the DC-side capacitor and stabilizing its voltage. The grid-side converter is controlled according to the grid voltage sag depth to provide a certain amount of reactive current, which is beneficial for grid voltage stabilization and recovery, maintaining stable operation of the wind turbine, and improving the PMD wind turbine's LVRT capability.
Simulation results show that the control strategy can effectively support the recovery of grid voltage and improve the low voltage ride-through capability of direct-drive permanent magnet wind turbines.
1. Overview of Low Voltage Ride-Through Technology and Analysis of Low Voltage Characteristics
1.1 Overview of Low Voltage Ride-Through Technology
Low-voltage ride-through (LVRT) refers to the ability of a wind turbine to remain connected to the grid and even provide some reactive power to support grid recovery when the voltage at its grid connection point drops, until the grid returns to normal, thus "riding through" this low-voltage period (region). LVRT technology is one of the most critical technologies in wind power systems, crucial for the large-scale application of wind power generation.
Europe started early in wind power generation and has developed rapidly. In some Nordic countries, such as Denmark and Germany, wind power has become a major form of energy. This includes provisions for wind turbine fault operation and provides detailed regulations on the grid-connected operation characteristics of wind turbine generators. Figure 1.1 shows the low-voltage ride-through capability curve in the European E.ON standard. The area above the solid line in the figure indicates that wind turbines are not allowed to disconnect from the grid; disconnection is only permitted below the solid line.
In my country, there are the following requirements for low voltage ride-through. Figure 1.2 shows the low voltage ride-through capability curve in the Chinese standard. When the voltage (three phases) at the grid connection point of the wind farm is in the area above the voltage profile line in the figure, the wind turbines in the farm must ensure uninterrupted grid-connected operation; when the voltage (any phase) at the grid connection point is below the voltage profile line in the figure, the wind turbines in the farm are allowed to disconnect from the grid. The low voltage ride-through requirements for wind farms stipulated in this regulation are: (1) The wind turbines in the wind farm have the low voltage ride-through capability to maintain grid-connected operation for 625ms when the voltage at the grid connection point drops to 20% of the rated voltage; (2) When the voltage at the grid connection point of the wind farm can recover to 90% of the rated voltage within 2s after the voltage drop, the wind turbines in the wind farm maintain grid-connected operation.
This indicates that in the event of a momentary drop in grid voltage, wind turbines cannot disconnect from the grid as easily as before, and need to provide support to the grid during grid failures, just like traditional thermal power generating units.
1.2 Impact of Grid Voltage Dips on Wind Power Systems
The direct-drive permanent magnet synchronous wind power generation system used in this paper is shown in Figure 1.3. The wind turbine directly drives the permanent magnet synchronous generator and connects it to the grid via a back-to-back dual PWM converter.
The power Pe injected into the grid by the wind power generation system can be expressed by Equation 1-1:
When the voltage drops, assuming the inverter is operating at unity power factor and has reached its rated current, iq is limited and cannot increase. At this time, the active power injected into the grid decreases, and the decrease is consistent with the voltage drop, expressed as:
2 Control Strategy for Permanent Magnet Direct-Drive Wind Power Converter System
2.1 Grid-side converter control strategy
The mathematical model of the converter in the three-phase stationary coordinate system is transformed into the two-phase synchronous rotating dq coordinate system by the transformation matrix. After the transformation, the mathematical model of the three-phase PWM converter in the two-phase synchronous rotating coordinate system is obtained.
Assume the AC output voltage of the converter is:
In equation (2-7), P > 0 indicates that the converter operates in rectification mode, absorbing energy from the grid; P < 0 indicates that the converter operates in energy feedback mode, where energy is transmitted from the DC side to the AC grid. Q > 0 indicates that the converter is inductive relative to the grid, absorbing lagging reactive current; Q < 0 indicates that the converter is capacitive relative to the grid, absorbing leading reactive current. Therefore, the d-axis and q-axis components id and iq of the current vector actually represent the active current component and reactive current component of the converter, respectively.
As can be seen from the circuit topology, when the AC input power is greater than the load power, the excess power will cause the DC voltage to rise; conversely, the capacitor voltage will decrease. Furthermore, since the converter's d-axis current is proportional to the input power to the AC side, the capacitor voltage can be controlled. The output of the voltage regulator is used as the setpoint for the d-axis component current (active current) id, which reflects the magnitude of the converter's input active current.
2.3 Machine-Side PWM Converter Control Strategy
2.3.1 Mathematical Model of Permanent Magnet Synchronous Motor
Assuming the magnetic circuit is linear, without saturation, hysteresis, or eddy current effects; and assuming the magnetic field of the permanent magnet is sinusoidally distributed around the air gap, neglecting leakage inductance, the voltage equation, flux linkage equation, and torque equation in the synchronous rotating coordinate system of the permanent magnet synchronous motor can be obtained using coordinate transformation theory, as shown in equations (2-8) to (2-10):
2.3.2 Rotor Field Orientation Vector Control Principle of Permanent Magnet Synchronous Motor
A standard three-phase alternating current passing through symmetrical three-phase windings generates a rotating magnetic field. The frequency (rotation speed) of this rotating magnetic field is the same as the frequency of the alternating current, and its amplitude is 1.5 times that of a single-phase current. The magnetic field is altered by controlling the rotational speed, position, and direction of the coils. Therefore, the rotating magnetic field vector in a motor can be controlled by the three-phase alternating current that generates it. Extending this concept of a rotating vector, we obtain voltage vector, current vector, magnetic field vector, etc. Vector control achieves the control of the target vector space's position by controlling the alternating current, thus meeting our requirements.
2.3.3 id=0 control strategy
The id=0 control aims to reduce the d-axis current of a permanent magnet synchronous motor to zero, and is the most commonly used control strategy for permanent magnet synchronous motors. Substituting isd=0 into equation (2-12), the electromagnetic torque equation becomes as follows:
To ensure that the actual current follows the given value, a feedback control quantity should be added to the above equation. Taking a PI regulator as an example, the final control equation of the system is shown in equation (2-15), where Kp and Ki are the proportional and integral coefficients of the current draw, respectively. The overall control block diagram of the system is shown in Figure 2.7.
Figure 2.8 Control block diagram of the machine-side converter.
2.3.3 Simulation of Machine-Side Converter
Based on the aforementioned mathematical model and control strategy, the generator-side converter of the direct-drive wind power generation system was simulated using the Matlab/Simulink simulation tool. The permanent magnet synchronous generator parameters were: stator resistance 0.00405485 Ω, stator inductance 0.3 mH, number of pole pairs 160, and permanent magnet flux linkage 1.48 Wb. The initial active power was set at 2 MW, which was reduced to 1.5 MW at 0.2 seconds. Due to the large moment of inertia of the generator, the adjustment time was relatively long. The simulation waveforms are shown in Figures 2.9-2.11. The simulation results show that the generator-side converter control strategy is correct, effective, and stable.
Figure 2.11 Electromagnetic torque waveform of permanent magnet generator
3. Low Voltage Protection Strategy for Direct-Drive Wind Power Generation Systems
When a voltage drop occurs in the power grid, overcurrent will occur in the grid-side converter. When current limiting is applied to the converter, overvoltage will appear on the DC-side bus. If this DC-side overvoltage cannot be eliminated, it will inevitably affect the safety of the entire power generation system, and may even damage the power generation equipment, causing a larger accident. Therefore, certain measures must be taken to eliminate DC-side overvoltage and improve the low-voltage operation capability of direct-drive wind power generation systems. DC-side overvoltage is caused by an imbalance between the output energy of the generator-side converter and the grid-side converter; therefore, releasing this excess energy is the fundamental way to solve the DC-side overvoltage problem.
3.1 Overvoltage Protection Scheme Based on Energy Storage Crowbar
Figure 3.1 illustrates a low-voltage ride-through scheme based on energy storage crowbars. A current-reversible chopper circuit connects the DC bus and the energy storage device, which can be a battery or supercapacitor. In this circuit, V1 and VD2 form a buck chopper circuit, supplying power from the DC bus to the energy storage device. When the DC voltage is too high, excess energy is stored in the energy storage device. V2 and VD1 form a boost chopper circuit, feeding energy from the energy storage device back to the DC bus. When the DC voltage is insufficient, the stored energy in the energy storage device is released to charge the bus capacitor, increasing the DC voltage. However, it is important to note that if both V1 and V2 are conducting simultaneously, it will cause a short circuit in the DC bus, potentially damaging the entire system.
The trigger signals of V1 and V2 can be triggered using a hysteresis comparison method, and the trigger signals of V1 and V2 are interlocked to prevent them from conducting simultaneously. However, this method cannot effectively control the switching frequency and is prone to damaging the switching elements. This paper uses two PI controllers to control the upper and lower limits of the bus voltage respectively, and interlocks the trigger signals of the two elements, as shown in Figure 3.2. When a sufficiently large energy storage device is selected, the energy storage crowbar can effectively protect against overvoltage and undervoltage on the DC bus. Due to the energy feedback effect of the energy storage device, the DC bus voltage drop caused by the recovery of the grid voltage is effectively suppressed. Compared with the use of energy-consuming crowbar protection, the impact of grid voltage drop on the operation of permanent magnet generator is basically negligible, effectively protecting the safety of the system. Furthermore, due to the energy feedback effect, energy loss is reduced. However, the effective protection of the energy storage crowbar is based on the existence of energy storage elements with sufficient capacity. As the degree and duration of grid voltage drop increase, its economic efficiency will significantly decrease.
3.2 Overvoltage Protection Scheme Based on Auxiliary Grid-Side Converter
Figure 3.3 illustrates a low-voltage ride-through scheme that adds an auxiliary grid-side converter between the DC side and the grid. Grid-side converters in wind power systems typically use power devices such as IGBTs and IGCTs, which are relatively expensive. The added auxiliary converter can use relatively low-cost devices such as SCRs and GTOs to form an auxiliary converter, which is connected in parallel with the main converter. During grid faults, some current flows from the auxiliary converter into the grid, maintaining power balance on the DC side.
When the motor is under load, the common-mode voltage can still generate destructive current through the load bearing. Therefore, filters composed of passive components are being used, which are very effective in eliminating the effects of common-mode voltage. Active filtering technology and modulation-based methods are also being employed to reduce the inverter output common-mode voltage.
This approach requires current sharing control of the parallel master-slave converters during faults, and also needs to suppress circulating current between the parallel converters. When a voltage dip is detected on the grid side, the auxiliary converter is activated, issuing a current distribution command to perform low-voltage ride-through. Analysis shows that this approach necessitates determining the current rating of the auxiliary converter based on the allowable voltage dip depth of the grid. When the voltage dip is significant, a larger auxiliary converter capacity is required, resulting in poor economic efficiency. Furthermore, due to the low switching frequency of devices such as GTOs, certain harmonics will be injected into the grid during faults.
3.3 Reactive power support control strategy provided by grid-side converter
Significant voltage drops in the power grid generate a large amount of reactive power demand. The power grid also requires wind power systems to be able to control their output power factor as easily as traditional thermal power generation, operate safely within a certain power factor range, and quickly provide reactive power to the grid to regulate system voltage in the event of grid faults such as voltage drops.
As shown in Section 2.3 of this paper, by adopting grid voltage-oriented dq-axis current decoupling control of the grid-side converter, independent regulation of active and reactive power can be achieved. Therefore, when a grid voltage fault occurs, it is technically feasible to operate the grid-side converter of the direct-drive wind power system in static var compensating mode to provide stable reactive power support to the grid. When the original active reference current exceeds the limit value, the outer loop of the DC-side voltage can no longer effectively maintain the stability of the DC-side voltage. At this time, it is necessary to activate the DC-side unloading circuit to consume the excess energy accumulated on the DC side and keep the DC-side voltage within a safe range.
Simultaneously, considering the DC-side voltage being limited to the maximum bus voltage by Crowbar control during voltage dips (control strategy described in the previous section), the active current reference value is obtained through the grid voltage outer-loop PI regulator and limited to the maximum output current. Reactive current is obtained using iqref = i2max - i2dref. Since the deeper the grid voltage dip, the greater the power imbalance between the DC side and the grid side, and the greater the threat to the power generation system, this control strategy aims to keep the entire system within the protection range of the DC-side Crowbar while maximizing reactive power output to meet the grid's reactive power demand. The proposed control strategy is shown in Figure 3.4.
4. Low Voltage Ride-Through Achievement in Direct-Drive Permanent Magnet Wind Power Converter System
When using DC-side overvoltage protection circuits to protect the converter of a direct-drive wind power generation system from grid voltage drops, the switching between different operating states of the direct-drive permanent magnet wind power generation system is rarely addressed. To improve the operability of low-voltage ride-through control technology for direct-drive permanent magnet wind power generation systems based on DC-side Crowbar protection circuits, this paper proposes a complete control logic based on a large-capacitor energy storage Crowbar protection circuit, which also considers the grid-side converter's reactive power support to the grid and the wind turbine's speed regulation to adjust the tip speed ratio, as shown in Figure 4.1.
Figure 4.1 illustrates the low-voltage ride-through control logic of a direct-drive permanent magnet wind power generation system. This logic can be described as follows: First, by monitoring the grid-side voltage, once a grid voltage dip is detected, the grid-side converter immediately implements a reactive power compensation control strategy for low-voltage faults. The DC bus voltage is limited by the DC-side Crowbar to the maximum allowable bus voltage, and simultaneously, the generator speed is adjusted to deviate from the optimal tip speed ratio, reducing the generator's output power. When the grid voltage recovers, the normal reactive power compensation control strategy is restored, and the bus voltage recovery is controlled by the grid-side converter's DC voltage loop. When the DC bus voltage recovers to the rated voltage, the generator speed is adjusted to return to the optimal tip speed ratio. When the grid voltage recovers, due to the lag in the grid-side converter's current loop control, the bus voltage will dip. When the DC bus voltage is detected to be below the lower limit of the bus voltage, the Crowbar is activated to release the energy from the energy storage components to support the bus voltage until the grid-side converter current recovers. Based on the above control logic and strategy, a complete low-voltage ride-through simulation model of the direct-drive permanent magnet wind power generation system is established, as shown in Figure 4.2.
The simulation results show that when the grid voltage drops as shown in Figure 4.3(a), the simulation model quickly detects the voltage drop fault and issues a command to reduce the speed of the generator on the turbine side as shown in Figure 4.3(c). By adjusting the speed of the wind turbine through the linkage shaft between the generator and the wind turbine, the wind turbine quickly deviates from the optimal tip speed ratio. The wind energy capture coefficient decreases rapidly in the time period of 0.2s-0.4s as shown in Figure 4.2(d). At the same time, as shown in Figure 4.2(e), the electromagnetic power generated by the generator also decreases from the rated power to about 0.5 times the rated power in the time period of 0.1s-0.3s. While the generator adjusts its speed, the grid-side converter activates a reactive power compensation control strategy for dip faults. The active current of the grid-side converter is given by the grid voltage loop and is approximately 0.6 pu. The reactive current is calculated to be approximately 1.4 pu. After being controlled by the grid-side converter current loop, the grid-side active and reactive currents quickly follow the given values as shown in Figures 4.3(i) and 4.3(j). The active and reactive power outputs of the grid-side converter are shown in Figures 4.3(f) and 4.3(g). During the period of grid voltage drop, the DC bus voltage is controlled by the Crowbar, and the waveform is shown in the 0.2s-0.4s segment of Figure 4.3(h). The DC bus voltage is 1.1pu. When the grid voltage recovers, the DC bus voltage recovery is controlled by the DC voltage loop of the grid-connected converter. The DC bus voltage is adjusted to the rated voltage value. At this time, due to the presence of the freewheeling diode in the Crowbar, when the energy storage capacitor voltage is higher than the bus voltage, the energy storage element feeds energy into the DC bus, causing the voltages of both to drop synchronously. When the bus voltage reaches the rated value, due to the presence of the inertial element and the lag of the current loop, the DC bus voltage continues to drop. When it drops to the lower limit of the DC bus voltage of 0.875pu, the Crowbar restarts and operates in boost chopper mode to release the energy in the energy storage element and support the DC bus voltage until all the energy is released. The above process allows Crowbar to release the energy stored during the grid voltage dip. As shown in Figure 4.3(f), the grid-side output power is higher than the rated value during this period (approximately 0.4s to 0.7s). Subsequently, under the regulation of the DC voltage loop of the grid-side converter, the bus voltage returns to the rated voltage, the entire system returns to normal operation, and the low-voltage ride-through process ends.
4. Conclusion
This paper first describes the technical characteristics of low-voltage ride-through (LVRT) and the impact of grid voltage dips on wind power systems. Then, it introduces the control methods for both turbine-side and grid-side converters. Based on the control characteristics of these converters, several methods for achieving LVRT are discussed. Finally, an overall control scheme is proposed, primarily based on energy storage crowbar and grid-side reactive power support control, supplemented by strategies such as tip speed ratio control. Simulation experiments verify the feasibility of this scheme, thus providing theoretical and experimental guidance for achieving LVRT in direct-drive permanent magnet wind power systems.
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