1 Introduction
Grid-connected wind power is the fastest-growing renewable energy technology internationally over the past decade. The biggest difference between grid-connected wind turbines and traditional grid-connected power generation equipment is that grid-connected wind turbines cannot maintain grid voltage and frequency during grid faults, which is highly detrimental to the stability of the power system. Grid faults are abnormal operating conditions of the power grid, mainly including short circuits or open circuits in transmission lines, such as three-phase to ground, single-phase to ground, and line-to-line short circuits or open circuits, which cause drastic changes in grid voltage amplitude.
Doubly fed induction generators (DFIGs) are currently the mainstream wind turbine model both domestically and internationally. Their generating equipment is a DFIG induction generator. When a grid fault occurs, the existing protection principle is to immediately disconnect the DFIG induction generator from the grid to ensure the unit's safety. However, with the continuous increase in the capacity of individual wind turbines and the expansion of wind farms, the mutual influence between wind turbines and the grid has become increasingly serious. There is growing concern that if a grid fault forces a large number of wind turbines to disconnect due to their own protection mechanisms, it will severely affect the operational stability of the power system. Therefore, as the capacity of DFIG induction generators connected to the grid continues to increase, the grid's requirements for them are becoming increasingly stringent. Typically, the generators are required to remain connected to the grid even when voltage drops occur due to grid faults (fault ride-through) and to quickly help the power system restore stable operation after the fault is cleared. In other words, wind turbines are required to have a certain low-voltage ride-through capability. To address this, some new international grid operation rules have been proposed. For example, the power company in northern Germany (e.onnetz) requires wind farms to operate without disconnecting from the grid within the voltage range shown in Figure 1 (i.e., the shaded area in the figure) [1][33]. The wind turbines must remain connected to the grid for at least 300 ms after the grid voltage drops to 15%, and only disconnect from the grid when the grid voltage drops below the curve. Here, voltage refers to the voltage at the connection point of the wind farm. Nationalgrid, which supplies power to parts of the UK, requires all grid-connected power plants or wind farms to remain connected to the grid for 140 ms when a transmission line above 200 kV fails [2]. Scottish Hydro-Electric also has similar requirements regarding the operation of power plants or wind farms without disconnecting from the grid during grid faults [3].
Figure 1e. Onnetz's voltage range requirements for wind farms to operate without disconnecting from the grid during grid faults [33]
To improve the low-voltage ride-through capability of wind turbines, it is essential to study the operating characteristics of doubly-fed induction generators (DFIGs) in current mainstream wind turbines, investigating their transient behavior during grid faults and recovery processes to eliminate or mitigate potential damage to the units under on-grid control. Many studies [4-7] have reported that under grid voltage dips, DFIGs in wind turbines can cause rotor-side overcurrent. Simultaneously, the rapid increase in rotor-side current leads to a rise in the DC-side voltage of the rotor excitation converter, causing oscillations in both active and reactive power currents. This is because, under instantaneous grid voltage dips, the stator flux linkage of the DFIG cannot keep pace with the sudden change in stator terminal voltage, resulting in a DC component. Due to the decrease in integral, the stator flux linkage remains almost unchanged, while the rotor continues to rotate, generating significant slippage. This leads to overvoltage and overcurrent in the rotor windings. If the grid experiences an asymmetrical fault, the rotor overvoltage and overcurrent phenomena will be even more severe because the stator voltage contains a negative-sequence component, which can generate a very high slippage. Overcurrent can damage the rotor excitation converter, while overvoltage can cause the rotor winding insulation of the generator to break down. In order to protect the generator excitation converter, it is imperative to adopt overvoltage and overcurrent protection measures.
To ensure the safe, grid-connected operation of doubly-fed induction generators and their excitation converters during grid faults and to adapt to the requirements of new grid operation rules, the academic and engineering communities both domestically and internationally have conducted extensive research on the protection principles and control strategies of doubly-fed induction generators during grid faults. According to literature reports, current low-voltage ride-through technologies generally employ three schemes: one uses rotor short-circuit protection technology (crowbar protection), the second introduces a new topology, and the third uses a reasonable excitation control algorithm. These will be analyzed and introduced in detail below.
2. Rotor short-circuit protection technology
This is a method commonly used by some wind power manufacturers. It involves installing a crowbar circuit on the rotor side of the generator to provide a bypass for the rotor-side circuit. When a voltage drop is detected due to a fault in the grid system, the excitation converter of the doubly-fed induction generator is locked out, and the bypass (energy release resistor) protection device of the rotor circuit is activated at the same time. This limits the current passing through the excitation converter and the overvoltage of the rotor winding, thereby maintaining the generator from disconnecting from the grid (at this time, the doubly-fed induction generator operates as an induction motor).
The following are some of the more typical crowbar circuits:
(1) Hybrid bridge crowbar circuit [9], as shown in Figure 2, each bridge arm is composed of a controller and a diode connected in series.
Figure 2 Hybrid bridge-type crowbar
(2) IGBT type crowbar circuit [9] As shown in Figure 3, each bridge arm consists of two diodes connected in series, and an IGBT device and an absorption resistor are connected in series on the DC side.
(3) Crowbar circuit with bypass resistor [10]
As shown in Figure 4, when the grid voltage drops, the bypass resistor is connected to the rotor circuit through the power switching device. This provides a bypass for the large current generated during the grid fault, thereby limiting the large current and protecting the excitation converter.
During grid faults, the excitation converter remains connected to the grid and rotor windings, allowing the doubly-fed induction generator to operate synchronously with the grid both during and after a fault. When the grid fault is cleared, turning off the power switch disconnects the bypass resistor, and the doubly-fed induction generator returns to normal operation.
The rotor short-circuit protection technology using a crowbar circuit has several drawbacks: First, it requires the addition of new protection devices, thus increasing system costs. Second, during grid faults, although the excitation converter and rotor windings are protected, the generator operating as an induction motor will absorb a large amount of reactive power from the system, which will further deteriorate the grid voltage stability. Moreover, the switching operation of the traditional crowbar protection circuit will cause transient shocks to the system. Reference [1] proposes an improved scheme, which differs from the traditional scheme in that: after the rotor short-circuit protection resistor is disconnected, the rotor current control command is set to the actual value of the rotor current at that moment, thereby preventing transient shocks caused by the discrepancy between the command current of the rotor current controller and the actual current. Then, by gradually changing the rotor current command, the soft start of the rotor current controller is achieved. Under the action of the rotor current controller, the generator will gradually return to normal operation. This alleviates the transient shocks caused by the switching operation of the crowbar protection circuit to the system and shortens the transition time of the generator's low voltage ride-through to a certain extent. However, this reference is limited to studying the operation of generators without disconnecting from the grid during symmetrical faults and does not discuss the impact of the initial conditions of grid fault operation on the effect of operation without disconnecting from the grid.
3. Introducing a novel topology
In addition to the typical application of crowbar technology mentioned above, some literature has also proposed some novel low-voltage bypass systems, as shown in Figures 5 and 6.
Figure 5 Novel Bypass System
Figure 6a) Parallel connection of grid-side converter
Figure 6b) Series connection of grid-side converter
3.1 Novel Bypass System [11-13]
As shown in Figure 5, this structure is similar to a traditional soft-start device, with a series anti-parallel thyristor circuit connected between the stator side of the doubly fed induction generator and the power grid.
During normal operation, all these thyristors are turned on. During grid voltage dips and recovery, the maximum current that may occur on the rotor side increases with the magnitude of the voltage dip. To withstand the large current surge on the rotor side caused by large grid voltage dips, high-power IGBT devices with high current ratings are selected for the rotor-side excitation converter. This ensures the safety of the converter when it does not disconnect from the rotor windings during grid faults. When the grid voltage dips and then recovers, the maximum current on the rotor side may reach several times that before the voltage dip. Therefore, when the grid voltage dip is severe, to avoid the large current generated on the rotor side when the voltage recovers, the doubly-fed induction generator is disconnected from the grid through the anti-parallel thyristor circuit before the voltage recovers. After disconnection, the rotor excitation converter re-excites the doubly-fed induction generator. Once the voltage recovers to within the allowable range, the doubly-fed induction generator can quickly synchronize with the grid. Then, the stator is reconnected to the grid by turning on the anti-parallel thyristor circuit. This reduces the requirements for the IGBT's withstand voltage and current. For IGBT modules that can accept large currents for short periods, the off-grid operation time of the doubly-fed induction generator can be reduced. High power feed from the rotor side to the DC side leads to an increase in the DC side capacitor voltage. Since the DC side's withstand voltage rating depends on the size of the DC side capacitor, a crowbar circuit is designed on the DC side, and a resistor is installed on the DC side as an absorption circuit to limit the DC side voltage within acceptable limits.
The drawbacks of this approach are that it increases system cost and control complexity. Considering the DC component in the stator fault current, the thyristor needs to be able to turn off through the gate, requiring a large gate negative drive current and making the drive circuit too complex. If a through-type IGBT is used in the thyristor series circuit, the IGBT must be connected in series with a diode. Using a non-through-type IGBT results in significant conduction losses. Theoretically, while a contactor could replace the thyristor switch with no conduction losses, the disconnection time is too long. Furthermore, because this approach disconnects the generator from the grid during transmission system faults, it does not actively support the restoration of normal grid operation.
3.2 Series connection of converter
Typically, the back-to-back excitation converter of a doubly-fed induction generator is connected in parallel with the grid as shown in Figure 6a) [13-16]. This means that the excitation converter can inject or absorb current into the grid. To improve the low-voltage ride-through capability of the system, reference [17] mentions a new connection method, namely, connecting the converter in series with the grid. For example, the converter is connected in series with the grid through a series transformer at the generator stator. Then, the voltage at the stator of the doubly-fed induction generator is the sum of the grid-side voltage and the voltage output by the converter. In this way, the stator flux linkage can be controlled by controlling the voltage of the converter, effectively suppressing flux linkage oscillations caused by grid voltage drops, thereby preventing the generation of large currents on the rotor side, reducing the impact of grid disturbances on the system, and achieving the purpose of strengthening the grid. However, this method will increase the system cost significantly, and the control is also more complex.
4. Adopt a new excitation control strategy
From a manufacturing cost perspective, the best approach is to achieve the same low-voltage ride-through effect without changing the system hardware structure, but by modifying the control strategy: enabling the generator to safely ride through the fault during a grid failure, while the converter continues to operate in a safe state.
Reference [18] used numerical simulation to study the excitation control of generators operating without disconnecting from the grid during three-phase symmetrical faults in the power grid. The results showed that by appropriately increasing the proportional and integral coefficients of the pi regulator in the existing doubly-fed induction generator excitation controller, it is possible to maintain generator operation without disconnecting from the grid during faults within a certain range. However, this reference did not conduct a detailed study and calculation of the range of generator operation without disconnection during faults. The method proposed in this reference is only applicable to maintaining generator operation without disconnection when the generator bus voltage drops slightly due to a symmetrical three-phase fault in the system. When the fault causes a severe drop in the generator bus voltage, the excitation converter will experience overvoltage and overcurrent.
Reference [19] uses hard negative feedback to compensate for the impact of generator stator voltage and flux changes on the active and reactive power decoupling control performance. This scheme can improve the operating characteristics of the doubly-fed induction generator during transmission system faults to a certain extent, and can limit the generator rotor current within a certain range, protecting the rotor excitation converter. However, the effective control of rotor current is achieved under the premise of increasing rotor voltage. Considering the limitation of the maximum output voltage of the rotor-side excitation converter, this scheme is only suitable for situations where the generator voltage drops slightly due to transmission system faults. For grid faults that cause a severe drop in generator stator voltage, this scheme will lose control of rotor current because the rotor-side excitation converter cannot provide a sufficiently high excitation voltage. In addition, reference [20] also suggests making full use of the grid-side converter's support for grid voltage during grid faults, and improving the control effect of generator not disconnecting from the grid during grid faults by coordinating the control of the rotor and grid-side converters.
References [27-32] propose a demagnetization protection principle. Based on an understanding of the transient physical processes of the generator during a grid short-circuit fault, an excitation control strategy is proposed to ensure that the doubly-fed induction generator does not disconnect from the grid during a grid short-circuit fault. To ensure the safe operation of the doubly-fed induction generator excitation inverter during the fault, the new excitation control strategy targets the transient characteristics of the generator's internal electromagnetic variables during the fault process. It controls the magnetic flux generated by the generator rotor current (during the fault transient, this flux only passes through the leakage flux path, which is a leakage flux) to counteract the influence of the "harmful" transient DC component in the stator flux on the rotor side.
The literature verifies the correctness of the control strategy under symmetrical grid faults through simulation and small-capacity tests, and analyzes the influence of various factors on the control effect. The in-depth analysis of the excitation strategy based on the demagnetization protection principle in the literature [32] shows that the initial conditions before the fault (stator voltage and slip) have a great influence on the fault effect of this control strategy. As the stator voltage before the fault increases, the rotor current may not be able to be controlled within the maximum transient current peak value that meets the safety requirements of the excitation converter. Only when the initial conditions before the fault are within the controllable operating range can the generator rotor fault current be controlled within the safe range of 2.0 pu under the action of fault excitation control.
5. Conclusion
This paper studies and analyzes the protection principles and control strategies of doubly-fed induction generators in the event of power grid faults by academic and engineering communities at home and abroad, and draws the following conclusions, which provide a reference for specific designs in practical applications.
(1) The power system requires that the doubly fed induction generator can remain connected to the grid during grid faults and provide support for grid stability. Therefore, based on the derivation of the basic electromagnetic relationship of the generator, the transient change process of the electromagnetic variables inside the generator during grid faults is analyzed, and a new excitation control strategy adapted to small-value grid faults is studied. That is, when a minor grid fault occurs and the voltage drop is not severe enough, a certain excitation control method can be used to achieve low-voltage faults when the safety factor of the generator and converter is short, without the need to trigger the crowbar circuit to protect the generator and converter.
(2) Under large transient faults, a crowbar short-circuit protection measure is generally required to protect the generator and converter. Because the rotor voltage and current will generally jump transiently after the crowbar circuit is triggered and when the grid fault is restored, and then decay. Using simulation tools, we can analyze and compare the advantages and disadvantages of various crowbar circuits, and compare and improve them in terms of cost, reliability, the best possible performance indicators, and adaptability to extreme working environments, so as to select the best solution and reduce the amplitude of the rotor transient current jump when the crowbar circuit is triggered under voltage drop conditions.
(3) Asymmetrical faults frequently occur during power grid operation. When an asymmetrical fault occurs, overvoltage and overcurrent phenomena become more severe because the stator voltage contains a negative sequence component, which can generate a high slip. However, most current research on severe faults focuses on symmetrical faults in the power grid, which cannot meet the requirements of actual power grid fault conditions and cannot be applied in practical engineering. Therefore, the control model and algorithm of the generator under asymmetrical faults in the power grid need further improvement and research.
