Variable Speed Drive Inrush Current Limiting via Controlled Rectifiers
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Solution Overview
Problem
The sudden inrush of current to a discharged capacitor bank during restart of electric drive systems for oil production equipment can damage components like fuses, as existing systems lack effective control mechanisms to manage this current.
Innovation Solution
A variable speed drive system with a converter, capacitor bank, and inverter, where a controller provides control signals to controlled rectifiers and switches to limit the recharge current to the capacitor bank, preventing dangerous inrushes by determining when the capacitor bank voltage is low and selectively controlling the rectification process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the system is restarted after power interruption or maintenance, then the capacitor bank needs to be recharged to resume operation, but uncontrolled rectification causes dangerously high inrush currents that can damage components like fuses
Solution Approach 1:
The controller detects low capacitor bank voltage before uncontrolled rectification occurs and preemptively activates controlled rectifier components (SCRs) and switches to establish current limiting. This preliminary action prevents the harmful inrush current from developing while still allowing necessary recharge current to flow.
Solution Approach 2:
The controlled rectifier components (SCRs) and switches act as intermediary elements between the power source and capacitor bank. These intermediaries provide controlled resistance and regulation during the recharge process, mediating between the need for rapid capacitor recharge and the need to limit inrush current to safe levels.
2Productivity
If uncontrolled rectification is used to quickly recharge the capacitor bank, then the recharge speed is maximized, but component damage risk increases due to high inrush currents
Solution Approach 1:
The system dynamically adjusts the rectification mode based on real-time capacitor bank voltage conditions. When voltage is low, controlled rectification with activated SCRs and switches provides current limiting. As voltage rises and approaches normal operating levels, the control signals are adjusted to allow faster uncontrolled rectification, creating a dynamic balance between safety and recharge speed.
Solution Approach 2:
The controller monitors capacitor bank voltage as a key parameter and uses this information to change the operational state of the rectifier components. By detecting voltage thresholds, the controller transitions the system between controlled and uncontrolled rectification modes, optimizing the recharge process while preventing component damage.
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 effectively reduces the risk of component damage by controlling the recharge current, ensuring safe and reliable system restarts and operation by limiting potentially harmful inrushes of current.
Implementation Method 1
An inductor is electrically connected between the power source and an intermediate junction at which the pair of rectifier components are connected to each other
Implementation Method 2
Each of the first, second and third converter sections includes a pair of rectifier components (e.g., diodes or silicon controlled rectifiers) that are electrically connected in series between the upper junction and the lower junction
Data Source
AI summary
Systems and methods for controlling potentially damaging inrushes of current to the capacitor bank of an electric drive system when the voltage on the capacitor bank is low. In one embodiment, a variable speed drive has a converter that converts AC power to DC power, a capacitor bank that receives the DC power, and an inverter that converts the DC power stored by the capacitor bank to AC output power. The converter has three sections that rectify the phases of three-phase input power. Each section has at least one controlled rectifier component, and the rectifier components have switches connected to them in parallel. When the voltage on the capacitor bank is low, the controlled rectifiers and switches are controlled to prevent dangerously high inrushes of current to the capacitor bank.