UPS Fast Transfer Control for Out-of-Phase Undervoltage Failures
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Solution Overview
Problem
Existing uninterruptible power supply (UPS) systems with static transfer switches using thyristors face challenges in minimizing transfer time between power sources during undervoltage events, especially when load currents are not in phase with the power source voltage, leading to uncontrolled current and potential persistence of disturbances due to the need for high inverter current capacity and voltage oscillations.
Innovation Solution
A UPS system switchable between higher and lower efficiency modes, utilizing an inductor in series with thyristors and a controller that synchronizes gate commands with reserve voltage and introduces leading phase compensation, estimating when out-of-phase currents will reach zero to minimize transfer time and provide counter voltages if necessary, thereby reducing parallel time between power sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If thyristors are used in static transfer switch to switch load between two power sources, then the transfer time is reduced to less than half the period of power sources, but uncontrolled current passes between the two power sources and output waveform depends on output voltage and output impedance
Solution Approach 1:
The controller performs preliminary synchronization of the two power sources before switching, ensuring their output voltages are aligned in phase and magnitude. This preliminary action prevents uncontrolled current flow during the transfer by establishing proper voltage conditions beforehand, allowing the thyristors to switch without causing harmful circulating currents.
Solution Approach 2:
The controller continuously monitors the output voltages and phase angles of both power sources, using feedback to adjust the timing and phase of the transfer switch operation. This feedback mechanism ensures that the transfer occurs at the optimal moment when voltage differences are minimal, preventing uncontrolled current while maintaining fast transfer time.
2Adaptability or versatility
If inverter current capacity is increased to handle out-of-phase currents during transfer, then transfer can occur with non-synchronized currents, but the inverter requires very high current capacity and causes strong voltage oscillations on the load
Solution Approach 1:
The controller acts as an intermediary that manages the transfer process by introducing a brief parallel operation period where both power sources operate simultaneously with controlled current sharing. This intermediary approach allows the system to handle out-of-phase conditions without requiring excessive inverter current capacity, as the controller orchestrates the current distribution to prevent harmful oscillations.
Solution Approach 2:
The system uses periodic switching of the thyristors in a controlled sequence, with each thyristor pair switching at specific intervals synchronized to the power source frequencies. This periodic action allows the inverter to manage current flow in manageable pulses rather than continuous high current, reducing the required current capacity while maintaining transfer capability with out-of-phase conditions.
3Object-generated harmful factors
If the transfer between power sources is delayed to allow current to reach zero, then uncontrolled current is avoided, but the transfer time increases by up to half the period of power sources
Solution Approach 1:
The controller performs preliminary synchronization of the two power sources before switching, ensuring their output voltages are aligned in phase and magnitude. This preliminary action prevents uncontrolled current flow during the transfer by establishing proper voltage conditions beforehand, allowing the thyristors to switch without causing harmful circulating currents.
Solution Approach 2:
The system changes the operating parameters of the thyristors by using forced commutation techniques and adjusted gating signals that allow the thyristors to turn off even when current is not at zero. This parameter change enables faster transfer while preventing uncontrolled current by modifying the thyristor switching characteristics beyond conventional operation.
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
The solution enables fast transfer from higher efficiency modes (VFD or VI) to a lower efficiency mode (VFI) within 5 milliseconds, minimizing the duration of uncontrolled current and disturbance persistence, while maintaining load voltage quality.
Implementation Method 1
An inductor is connected in series between the thyristors and a secondary power source
Implementation Method 2
the controller synchronizes thyristor gate commands for the thyristors with the reserve voltage
Implementation Method 3
the controller controls the applicable phase of the inverter for the second time period to provide a counter voltage in order to attempt to turn off the applicable thyristor
Data Source
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AI summary
A UPS system may be operated selectively in either a high efficiency mode or a low efficiency mode. An inductor is connected in series between the transfer switch and a secondary power source. The transfer switch includes thyristors arranged in a reverse connected configuration. A controller synchronizes gate commands to the thyristors in order to synchronize reserve voltage introduced by the inverter. If the load currents are not in phase with a fundamental voltage of the secondary power source, the controller waits up to a first time period in order to estimate when current through the switches will reach zero. If the out of phase current will reach zero by the end of a second time period, the inverter is switched on at the end of the second time period.