Technical Brief_Voltage and Current Source Inverters_Rev0_26 Dec 2016
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This document summarizes and compares two types of variable frequency drive topologies: current source inverters and voltage source inverters. Current source inverters use large inductors to maintain a constant current supply, but have poor dynamic response times and produce significant harmonics. Voltage source inverters use capacitors to maintain a constant voltage supply, provide faster response times, produce low harmonics, and do not transfer common mode voltages to motors. The document defines various acronyms and technical terms and outlines key differences in performance characteristics between the two inverter types.
Technical Brief_Voltage and Current Source Inverters_Rev0_26 Dec 2016
- 1. GP Technologies Ltd. © 2016 Daniel Lang, P.Eng. dlang@gptechnologies.com GP Technologies Ltd. 200 9016 51 Ave. Edmonton, AB GP TECH Power System Experts Acronyms: 1. SCR – Silicon Controlled Rectifier 2. GCT – Gate Commutated Thyristor 3. SGCT – Symmetrical Gate Commutated Thyristor 4. GTO – Gate Turn-Off Thyristor 5. IGBT – Insulated Gate Bipolar Transistor 6. IGCT – Insulated Gate Commutated Thyristor 7. IEGT – Injection Enhanced Gate Transistor Current Source Inverter (CSI) A current source inverter uses SCRs, GCTs, or SGCTs to rectify the input voltage (known as an active front end). A large inductor forms the DC link, which stores energy and maintains constant current for the inverter section. The inverter section uses GTOs or SGCTs to create the pulse width modulated output signal. Voltage Source Inverter (VSI) A voltage source inverter uses diodes to rectify the input voltage. A large capacitor forms the DC link, which stores energy and maintains constant voltage for the inverter section. The inverter section uses IGBTs, IGCTs, or IEGTs to create the pulse width modulated output signal. Dynamic Response Def.: system response to change in operating point from one steady state condition to another, following a change in system variables (load torque, voltage, current, etc.) Dynamic response of a drive system is limited by mechanical inertia and electrical inductance. Time rate of change of current proportional to size of inductor CSI design includes large inductors for energy storage and signal conditioning, resulting in poor dynamic response. VSI utilize a DC bus capacitor and are able to provide instantaneous current, vastly improving drive dynamic response. Power Quality VSI utilizes a 24 or 36 pulse phase shifting transformer, which produces very little harmonics and operates at high power factor. The active front end of a CSI produces significant source harmonics and results in a large displacement power factor when operating at low torque or speed. Common Mode Voltage Def.: voltage signal offset that is “common” to multiple output nodes. Common mode voltage can occur from ungrounded delta or wye sources, resulting in the neutral voltage drifting away from zero. This results in higher line-neutral voltages and thus stress on insulation. CSI transfers common mode voltage to the motor due to lack of ground reference in inverter stack. VSI design utilizes isolation transformers and a clamped neutral point, resulting in no common mode voltages. CSI VSI Inverter is fed from a constant DC current source Inverter is fed from a constant DC voltage source Output voltage is dependent on load impedance Output current is dependent on load impedance Output current is independent of load impedance Output voltage is independent of load impedance Produces large input and output harmonics Produces low input and output harmonics Variable Frequency Drive Topologies: Current Source vs. Voltage Source