VFD Pre-charge Circuit Overvoltage Protection
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Solution Overview
Problem
Variable frequency drive (VFD) circuits in drilling rigs face performance issues and component failures due to deep voltage notches from weak power sources, leading to overvoltage ringing and DC link capacitor failures, with existing solutions being either expensive or providing limited protection.
Innovation Solution
Incorporating an overvoltage relay into the pre-charge circuit of VFD circuits, which selectively controls current flow to prevent overvoltage conditions on the DC link capacitor bank, using a controller to monitor and compare DC link voltage to defined conditions to manage the operation of pre-charge and overvoltage relays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If notch reactors are added at VFD circuit inputs for overvoltage protection, then the protection effectiveness is improved, but the device size and cost increase
Solution Approach 1:
The patent changes the operating parameters of existing components (resistors and relays in the pre-charge circuit) to provide overvoltage protection. By adjusting the resistance values and relay timing characteristics, the circuit can detect and respond to overvoltage conditions without requiring additional bulky notch reactors, thus maintaining protection effectiveness while minimizing device size.
Solution Approach 2:
The pre-charge circuit is made multi-functional by enabling it to perform both its original pre-charge function and the new overvoltage detection/protection function. The same resistors and relays are utilized for dual purposes: limiting inrush current during startup and detecting overvoltage conditions on the DC link, eliminating the need for separate dedicated overvoltage protection components.
2Device complexity
If EMI filters are removed to reduce cost and complexity, then the device complexity is reduced, but the overvoltage protection capability is weakened
Solution Approach 1:
The pre-charge circuit is made multi-functional by enabling it to perform both its original pre-charge function and the new overvoltage detection/protection function. By utilizing the same existing components for dual purposes, the patent reduces device complexity while maintaining overvoltage protection capability, avoiding the need for separate dedicated protection circuits.
Solution Approach 2:
The pre-charge circuit serves itself by using its own components (resistors and relays) to detect and respond to overvoltage conditions. The circuit monitors its own operating state and automatically takes protective action when overvoltage is detected, eliminating the need for additional external protection devices and reducing overall system complexity.
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 provides effective overvoltage protection, preventing failures and offering a smaller, more cost-effective alternative to existing solutions, ensuring reliable operation of VFD circuits despite voltage notches from weak power sources.
Implementation Method 1
a rectifier connected to the input to convert an AC power input to a DC power
Implementation Method 2
a DC link capacitor bank comprising one or more capacitors connected to the DC link to smooth the DC link voltage
Implementation Method 3
one or more pre-charge relays each operable in an on and off state to selectively control a current flow through the one or more resistors
Implementation Method 4
an overvoltage relay operable in an on and off state to selectively cut-off a current flow to the DC link capacitor bank, so as to prevent an overvoltage condition
Data Source
AI summary
A variable frequency drive (VFD) circuit includes an input connectable to an AC source, a rectifier to convert an AC power input to a DC power, a DC link to receive DC power from the rectifier and having a DC link voltage thereon, a DC link capacitor bank with one or more capacitors connected to the DC link, and a pre-charge circuit coupled to the DC link capacitor. The pre-charge circuit further includes one or more resistors, one or more pre-charge relays each operable in on and off states to selectively control a current flow through the resistor(s) so as to control an initial pre-charge of the DC link capacitor, and an overvoltage relay operable in on and off states to selectively cut-off a current flow to the DC link capacitor bank, so as to prevent an overvoltage condition in the DC link capacitor bank.