The doubly-fed wind turbine is currently the most widely used model in the wind power industry, with over 70% of installed wind turbines being of this type. One of the key advantages of the doubly-fed system is that only a portion of the power passes through the converter, allowing for separate control of active and reactive power. However, due to the relatively small capacity of the converter, it is highly sensitive to grid faults, which can cause damage to the semiconductor devices within the converter. Therefore, reliable protection mechanisms are essential to ensure the safe operation of these systems.
During voltage dips in the grid, transient DC components appear in the stator flux of the doubly-fed machine, and negative sequence components may also occur during asymmetric faults. These components create a significant slip condition for the rotor, especially when operating at high speeds, leading to increased rotor voltage and current. The resulting transient voltages and currents pose a serious threat to the fragile semiconductor devices in the rotor converter.
Traditionally, wind turbines have used passive Crowbar circuits for self-protection during grid faults. These circuits typically consist of thyristors that short-circuit the rotor circuit, causing the machine to behave like an induction motor. Once the grid returns to normal, the thyristor turns off, and the system re-connects to the grid. However, passive Crowbars require a large amount of reactive power from the grid, making them unsuitable for modern grid requirements.
To address these limitations, an "active Crowbar" circuit was introduced. This type of circuit uses IGBTs to allow for more precise control, enabling the Crowbar to be disconnected at the right time to allow the rotor converter to restart without the turbine going off-grid. The active Crowbar design provides better performance and compliance with current grid codes.
Based on the analysis of the doubly-fed wind turbine's behavior during voltage drops, the main circuit and control circuit of the Crowbar were designed using a 1.5MW doubly-fed system as a reference. The design was tested on a 2MW experimental platform to verify its effectiveness.
Common active Crowbar configurations are illustrated in Figure 3-1. The circuit in Figure 3-1(a) includes thyristors and diodes connected in series, while Figure 3-1(b) uses anti-parallel thyristors. Figures 3-1(c) and 3-1(d) use IGBTs as switching devices. Among these, Figure 3-1(d) is more cost-effective, utilizing a single IGBT and a rectifier diode, which allows for smaller and more efficient resistor selection.
For a 1.5MW system, the resistance value is calculated based on worst-case conditions, such as full load during a complete voltage drop. The resistor size is determined to be approximately 0.49Ω after considering temperature drift. The IGBT selected must handle the maximum current and heat generated during the Crowbar operation, with a rating of 1700V and 2400A.
To reduce parasitic inductance and voltage spikes, the IGBT switching circuit is carefully laid out, with nearby placement of freewheeling diodes and the shunt resistor. A detailed diagram of the Crowbar main circuit is shown in Figure 3-2.
In terms of control, the Crowbar is activated when rotor voltage exceeds a threshold, and deactivated when the rotor current falls below a certain level. The control circuit includes voltage detection, current sensing, and IGBT triggering, as shown in Figure 3-3.
The IGBT drive circuit, as depicted in Figure 3-3(b), uses a 2SD300C17 driver chip, along with optical fiber signal transmission for improved noise immunity. Fault signals are monitored and processed to ensure safe operation, as shown in Figure 3-6.
Testing of the drive circuit included overvoltage and short-circuit protection tests, with results confirming the effectiveness of the soft-shutdown and active clamp features. Experimental verification on a 1.5MW test bench demonstrated the reliability and performance of the Crowbar during low-voltage ride-through events.
In summary, this chapter analyzed the behavior of doubly-fed wind turbines during voltage dips and designed an effective Crowbar circuit to protect the converter. The implementation and testing confirmed that the system meets modern grid requirements, ensuring stable and safe operation under fault conditions.
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