Medium Voltage Inverter Frequency Control for Power Failure
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Solution Overview
Problem
Conventional methods for compensating instantaneous power failure in medium voltage inverters, such as the cascaded H-bridge (CHB) type, face issues with overvoltage and overcurrent trips due to regenerative energy and non-linear motor speed variations, leading to reduced reliability and operation time.
Innovation Solution
A method that dynamically adjusts the output frequency of the inverter by reducing it when input voltage falls below a threshold, then increasing it when the DC link voltage exceeds a threshold to prevent overvoltage and overcurrent trips, using a controller to manage the power cells and motor load effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the output frequency is reduced to obtain regenerative energy during power failure, then the power failure compensation capability is improved, but the voltage of the DC link increases causing overvoltage trip
Solution Approach 1:
The control method monitors the DC link voltage in real-time and uses this feedback to dynamically adjust the output frequency. When DC link voltage exceeds a threshold, the system increases the output frequency to reduce regenerative energy and prevent overvoltage trip, thereby resolving the contradiction between maintaining power failure compensation and avoiding overvoltage.
Solution Approach 2:
The system dynamically adjusts the output frequency based on operating conditions rather than maintaining a fixed reduced frequency. This dynamic control allows the system to optimize between obtaining regenerative energy during power failure and preventing DC link overvoltage, transforming a static contradiction into a manageable dynamic balance.
2Speed
If the slip frequency increases beyond a certain value, then the motor speed control is achieved, but the output current increases causing overcurrent trip
Solution Approach 1:
The system monitors output current and uses this feedback to limit the slip frequency. When the output current approaches a threshold, the control method adjusts the frequency reduction rate or minimum frequency to prevent excessive slip, thereby achieving motor speed control while avoiding overcurrent trip.
Solution Approach 2:
The control method changes the frequency parameter dynamically based on motor load and speed requirements. By adjusting the frequency reduction rate and minimum frequency according to actual operating conditions, the system maintains effective speed control while keeping output current within safe limits.
3Reliability
If the inverter interrupts PWM output during input power failure, then the power failure protection is achieved, but the load restoration time increases causing operational loss
Solution Approach 1:
The system prepares regenerative energy during the power failure period by controlling the output frequency in advance. This preliminary energy storage allows the inverter to quickly restore power to the load without prolonged interruption, reducing the operational loss while maintaining power failure protection.
Solution Approach 2:
The system converts the harmful effect of power failure into beneficial regenerative energy by operating the motor as a generator during the failure period. This regenerative energy is stored and then used to quickly restore the load, transforming the power failure event from a purely harmful interruption into an opportunity for energy recovery and rapid restoration.
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
This approach enhances the reliability of medium voltage inverters by preventing overvoltage and overcurrent trips, ensuring continuous operation and extending the operation time compared to conventional methods.
Implementation Method 1
regenerative energy for controlling a power failure period at the initial stage of power failure may be obtained
Data Source
AI summary
According to an embodiment, a method for compensating instantaneous power failure includes determining whether an input voltage of a plurality of power cells is less than or equal to a first threshold voltage, decreasing, when the input voltage is less than or equal to the first threshold voltage, an output frequency of the inverter, determining whether a voltage of a direct current (DC) link is greater than or equal to a second threshold voltage, and increasing, when the voltage of the DC link is greater than or equal to the second threshold voltage, the output frequency of the inverter. Overvoltage trip may be prevented at the power restoration time and overcurrent trip caused by increase in the slip frequency may be prevented. Thereby, reliability of a medium inverter may be enhanced, and a continuous operation time increased compared to the conventional cases may be ensured.