Doubly-fed generator overcurrent protection via pulse switching
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Solution Overview
Problem
Doubly-fed electric machines face challenges in maintaining operation during power grid faults due to limited overcurrent capability of semiconductor power devices, leading to rapid torque changes and disturbances in the power grid, necessitating the activation of short-circuit units which can disrupt electricity supply.
Innovation Solution
A doubly-fed motor/generator system utilizing a wound-rotor induction machine with a power converter having IGBTs and MOSFETs, along with a DC capacitor and pulse-width-modulation control, to manage excitation currents and prevent overcurrents by switching between different pulse commands based on current levels, thereby minimizing the need for short-circuiting and maintaining grid stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a short-circuit unit is activated to bypass excitation overcurrent, then the excitation power converter is protected from damage, but the torque changes rapidly and wildly varies depending on rotation speed, causing significant disturbances in the power grid
Solution Approach 1:
The control method applies preliminary anti-action by detecting excitation overcurrent conditions and preemptively switching to a special control mode that limits current magnitude and adjusts phase angles. This prevents the harmful torque fluctuations that would otherwise occur when activating a short-circuit unit, while still protecting the power converter from damage.
Solution Approach 2:
The invention employs dynamic control by continuously adjusting the phase angle between excitation current and voltage based on the detected overcurrent condition. The control method dynamically modifies the excitation current characteristics to maintain converter protection while minimizing torque variations and grid disturbances.
2Ease of manufacture
If the current rating of semiconductor power devices is minimized to reduce cost, then the overcurrent capability is limited, but the ability to maintain operation during power grid faults is compromised
Solution Approach 1:
The control method changes operational parameters by detecting overcurrent conditions and switching to a special control mode that limits current magnitude and adjusts phase angles. This allows the system to maintain reliability during faults even with minimized power device ratings, as the control adapts to prevent exceeding device capabilities.
Solution Approach 2:
The invention applies preliminary action by implementing control measures before damage occurs. The control method detects potential overcurrent conditions and preemptively adjusts excitation current parameters, allowing the system to continue operating during faults without requiring oversized power devices.
3Reliability
If a resistor is inserted to reduce current through the short-circuit unit, then the overcurrent capacity is reduced, but the torque changes rapidly to that of the wound-rotor induction machine shorted with a secondary resistor
Solution Approach 1:
The invention replaces the mechanical/resistive current limiting approach with an electronic control system. Instead of inserting resistors to limit current, the control method uses power electronic switching and phase angle adjustment to achieve current limitation, thereby protecting the short-circuit unit while avoiding the rapid torque changes associated with resistive insertion.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables the resumption of secondary excitation control after minimal downtime during faults without short-circuiting, stabilizing torque and voltage, and maximizing the continuation of power supply by effectively managing overcurrents and torque fluctuations.
Implementation Method 1
the first power converter PWM-controls the self-turn-off power devices
Implementation Method 2
a DC capacitor connected between DC side terminals of the first power converter
Implementation Method 3
a wound-rotor induction machine having its stator side armature winding connected to an electric power grid
Data Source
AI summary
The excitation overcurrent detection unit for the doubly-fed electric machine is provided with a function to determine an excitation current magnitude relationship among three phases. The firing pulse is held to on-state or off-state to cause the largest-current phase and the second-largest-current phase to charge the DC capacitor by the operation of diodes. The conduction ratio of the third-largest-current phase or minimum current phase is controlled according to the detected current value to protect against a possible short-circuit across the DC capacitor. When the voltage of the DC capacitor exceeds a preset value, the voltage is suppressed by operating active or passive power devices.