Variable Speed Drive Ride-Through via DC Link Voltage Control
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Solution Overview
Problem
Variable speed drives (VSDs) in HVAC&R systems are susceptible to shutdowns during voltage sags due to their inability to maintain DC link voltage, leading to increased downtime and costs associated with additional hardware for ride-through capabilities.
Innovation Solution
The implementation of an Active Rectifier stage and inverter stage with a DC link stage, where the DC voltage is regulated by transferring control from the Active Rectifier to the inverter during voltage sags, allowing power flow reversal from the motor to the DC link to maintain voltage and energy storage, and resuming normal operation when input voltage recovers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional hardware (capacitors, DC boost converter, batteries, supercapacitors, motor-generator sets, flywheels, superconductive magnetic energy storage systems, fuel cells) is provided to improve ride-through capability, then the VSD can maintain DC link voltage during voltage sags, but the cost of the VSD significantly increases
Solution Approach 1:
The invention enables the motor-compressor system to serve itself as an energy storage device during voltage sags. By allowing the motor to operate as a generator and transfer energy back to the DC link, the system uses its own inertia and thermal storage capacity rather than requiring external energy storage hardware, thus improving ride-through capability without increasing device complexity or cost
Solution Approach 2:
The motor-compressor system performs multiple functions: it acts as both a motor during normal operation and as a generator/energy storage device during voltage sags. The compressor also serves dual purposes by providing mechanical loading during normal operation and acting as a thermal storage medium during sags. This multi-functionality eliminates the need for dedicated energy storage hardware
2Reliability
If motor speed and current sensors are added to enable load inertia-based ride-through, then the motor can act as a generator to maintain DC link voltage, but the cost of the VSD increases
Solution Approach 1:
The system uses existing VSD control capabilities to monitor motor current and infer operational state, eliminating the need for additional speed and current sensors. The control system leverages existing measurements and system model to achieve ride-through functionality without adding costly sensing hardware
3Reliability
If an Active Rectifier is used to compensate for input line voltage variations, then the DC link voltage can be maintained at rated value, but the input AC current increases as line voltage decreases
Solution Approach 1:
The invention converts the harmful effect of voltage sags into a beneficial opportunity by allowing the motor to operate as a generator during sags. The motor's back-EMF and inertia provide energy to the DC link, turning the voltage disturbance into a self-sustaining energy source that maintains DC link voltage without requiring increased input current
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 method enhances the ride-through capability of VSDs during voltage sags by minimizing DC link capacitor discharge, utilizing motor and compressor inertia, and thermal storage, thereby preventing system shutdowns and extending operational time.
Implementation Method 1
use the load inertia to generate power during a voltage sag
Implementation Method 2
An active rectifier includes power devices capable of switching on and off the line currents, together with specialized control methods
Implementation Method 3
The DC link voltage is then inverted to a variable magnitude, variable frequency AC voltage
Data Source
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AI summary
A method of providing ride-through capability in a chiller/refrigeration system employs a variable speed drive with an active converter stage, a DC link stage and an inverter stage for providing variable frequency and voltage to power at least one motor. An induction motor is coupled to the output of the inverter stage for driving a compressor in the chiller/refrigeration system. The ride-through method comprises operating the active converter to regulate the DC link voltage of the DC link stage to a predetermined voltage level until the current through the active converter equals a predetermined current limit, then transferring regulation of the DC link to the inverter upon reaching the current limit of the converter. The compressor is unloaded, and the power flow through the inverter is reversed to maintain the voltage level of the DC link stage. Pre-rotation vanes, slide valve, or check valve are used to unload the compressor.