Fuzzy Logic Controller for Phase Controlled Rectifier
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Solution Overview
Problem
In uninterruptible power supplies (UPS) with phase controlled rectifiers, existing firing angle control methods using PI controllers result in large over and under shoots during load changes, leading to potential damage due to inaccurate DC voltage measurement, and faults like SCR short or drive board failures are difficult to detect with limited measurements.
Innovation Solution
An open-loop fuzzy logic controller is used to select a firing angle for the rectifier based on instantaneous output power and input line voltage, eliminating the need for direct DC bus voltage measurement, and compares the generated firing angle with a PI controller to detect faults.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a PI controller is used for firing angle control of the rectifier, then the control system is simple and reliable, but large over and under shoots occur during load changes causing DC bus voltage instability
Solution Approach 1:
The patent changes the control parameters by using instantaneous measurements of output power and input line voltage as inputs to the fuzzy logic controller, instead of using error signals as in traditional PI controllers. This allows the firing angle to be adjusted based on actual system state, reducing voltage overshoot and undershoot during load changes while maintaining control reliability
2Measurement precision
If direct DC bus voltage measurement is used for feedback control, then accurate voltage control is achieved, but the system becomes vulnerable to measurement failures and requires additional protection mechanisms
Solution Approach 1:
The patent extracts the dependency on direct DC bus voltage measurement by using an open-loop fuzzy logic controller that determines firing angle based on output power and input line voltage measurements only. This eliminates the feedback path that could fail due to DC bus voltage measurement issues, making the system more reliable while still achieving accurate voltage control through the intelligent control algorithm
3Device complexity
If traditional control methods are used with limited measurements, then the control system is simple, but rectifier faults such as SCR short or drive board failures are difficult to detect
Solution Approach 1:
The patent implements a comparison mechanism where the firing angle generated by the fuzzy logic controller is compared with the firing angle generated by the PI controller. This feedback-like comparison provides fault detection capability without significantly increasing system complexity, as it uses the existing controller outputs to detect anomalies indicating SCR short or drive board failures
Data Source
AI summary
An uninterruptible power supply (“UPS”) has a phase-controlled rectifier coupled to a source of AC power and having an output providing a DC bus, the output of the phase-controlled rectifier coupled to an inverter. A first controller generates a firing angle for the rectifier and a fuzzy logic controller generates a firing angle for the rectifier. In an aspect, the rectifier is controlled by the firing angle generated by the first controller during normal operating conditions of the UPS and the rectifier is controlled by the firing angle generated by the fuzzy logic controller during abnormal operating conditions of the UPS. The abnormal operating conditions can include loss of a direct DC bus voltage measurement and or a period of time after the UPS experiences a large load change. In an aspect, the firing angle generated by the first controller is compared to the firing angle generated by the fuzzy logic controller and a rectifier fault condition determined to exist when the two firing angles differ by at least a threshold amount. In an aspect, the first controller is a PI controller.