Virtual Inertia Control for DFIG Wind Turbines
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Solution Overview
Problem
Doubly-fed asynchronous machines (DFIG) in wind turbines face significant thermal loading and performance limitations when operating in virtual mass inertia mode due to high junction temperatures and increased cyclic heat loads on power semiconductor components, particularly near synchronous speed, which restricts dynamic grid frequency support and increases the risk of thermal overload.
Innovation Solution
A method to dynamically adjust the maximum rotor current limit based on the rotor speed and its change rate, reducing the current limit as the rotor approaches synchronous speed, thereby reducing thermal stress on power semiconductor components and allowing for safe operation without exceeding maximum junction temperatures, allowing for continuous dynamic grid support.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the rotor current is increased to provide dynamic grid frequency support in virtual mass inertia mode, then the active power output is increased beyond nominal system output, but the junction temperature of power semiconductor components increases significantly, particularly near synchronous speed
Solution Approach 1:
The patent applies dynamics by making the maximum rotor current limit adaptive rather than fixed. The control device dynamically adjusts the current limit based on real-time rotor speed and its rate of change, allowing the system to optimize power output while preventing thermal overload. This is achieved through continuous monitoring and adjustment of the current limit parameter as the rotor passes through different speed ranges, particularly around synchronous speed where thermal stress is highest.
Solution Approach 2:
The patent changes the parameter of maximum rotor current limit as a function of rotor speed and speed change rate. By modifying this critical parameter adaptively, the system can increase active power output when needed while maintaining junction temperatures within safe operating limits. The parameter change is particularly significant near synchronous speed, where the current limit is reduced to prevent excessive thermal loading on power semiconductor components.
2Reliability
If the system operates continuously in virtual mass inertia mode to provide dynamic grid support, then grid frequency support is maintained, but the cyclic heat load on power semiconductor components increases the risk of thermal overload
Solution Approach 1:
The patent applies beforehand cushioning by implementing preventive thermal protection measures. The control device continuously monitors rotor speed and speed change rate, and proactively adjusts the maximum rotor current limit before excessive thermal conditions can develop. This anticipatory approach prevents thermal overload by reducing current limits in advance when the rotor approaches speed ranges that generate high cyclic heat loads, particularly near synchronous speed, thereby ensuring continuous reliable operation.
3Temperature
If the maximum rotor current limit is reduced near synchronous speed to prevent thermal overload, then junction temperature is controlled, but the active power output is limited in the synchronous speed range
Solution Approach 1:
The patent applies local quality by implementing spatially and operationally differentiated current limits. Rather than applying a uniform current limit across all operating conditions, the control device applies specific current limit values tailored to particular rotor speed ranges and speed change rates. This allows the system to maintain high power output in speed ranges where thermal conditions are acceptable, while applying restrictive current limits only in specific local conditions (near synchronous speed) where thermal overload risk is highest, thus optimizing the local thermal-electrical trade-off.
Data Source
Figure 1
Figure 2a~4b
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AI summary
A method for operating a power plant (1) with a doubly fed asynchronous machine (11) and an inverter (17) in a virtual inertia operating mode for grid frequency support is disclosed. The method features that when a current mechanical speed (nr) of the rotor (13) is within a predetermined speed range ([nVM-; nVM+]) around a synchronous speed (nsyn) of the rotor (13) or a current rotor frequency (fr) is within a predetermined frequency range ([fVM-; fVM+]) around a zero frequency of the rotor (13), a current maximum current limit (Ir,max) for a rotor current to be supplied by the inverter (17) is reduced depending on the current values of the speed (nr) or rotor frequency (fr) and the speed change rate (dnr/dt) or frequency change rate (dfr/dt) of the rotor (13).The method further features that a current setpoint (Ir) for the rotor current is determined based on a power demand (P*, Q*) from the grid (2) as a function of measured current operating parameters. The method further features that the converter (17) is controlled to switch pulsed sinusoidal voltages and currents with the current rotor current setpoint (Ir), limited to the current maximum rotor current limit (Ir,max), to the rotor (13). A converter device (26) and a power plant (1) incorporating this device are also disclosed.