Current Source Inverter Vs. Voltage Source Inverter Topology
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Effective June 2014 White Paper WP020001EN Supersedes August 2010 Current source inverter vs. Voltage source inverter topology Written by: Abstract Aaron VanderMeulen and John Maurin, In the medium voltage adjustable speed drive The converter section converts utility/line Application Engineers market, the various topologies have evolved with AC voltage (50/60 Hz) to DC. The DC link components, design, and reliability. The two major transmits the DC voltage to the inverter, provides types of drives are known as voltage source ride-through capability by storing energy, and inverter (VSI) and current source inverter (CSI). provides some isolation from the utility/line. In industrial markets, the VSI design has proven to The inverter converts the DC voltage and be more efficient, have higher reliability and faster transmits a variable voltage or current and dynamic response, and be capable of running frequency to the motor. By independently motors without de-rating. changing the current and frequency, the VSI fully integrated design saves money with drive can adjust the torque produced by the higher efficiencies, minimizing install time, motor as well as the speed at which eliminating interconnect power cabling costs, it operates, respectively. and reducing building floor space. Efficiencies There are more components typically required for are 97% with high power factor through all a fully integrated system, which includes an input load and speed ranges. Fast dynamic response isolation, a transformer (or reactor), and an output for rapid changes in motor torque and speed filter (option), shown in Figure 2. allow a wide range of applications. Minimum component count increases the mean time to failure (MTTF), an important number in critical DC Filter Isolation Converter Inverter uptime applications. Also, new replacement Link (Optional) M motors are not required for retrofit applications. All of these factors produce a high-quality, robust, industrial design. Figure 2. Introduction Current source inverter The way each of the drive building blocks Adjustable frequency drives (AFDs) are designed operates defines the type of drive topology. to allow full torque and speed control of the The first topology that will be investigated is operating motor; how this is accomplished the current source inverter (CSI). The converter varies among manufacturers and the various section uses silicon-controlled rectifiers design topologies. All medium voltage industrial (SCRs), gate commutated thyristors (GCTs), AFDs consist of a converter section, a DC link, or symmetrical gate commutated thyristors and an inverter section (see Figure 1). (SGCTs). This converter is known as an active rectifier or active front end (AFE). The DC link uses inductors to regulate current ripple and to store energy for the motor. The Converter DC Link Inverter M inverter section comprises gate turn-off thyristor (GTO) or symmetrical gate commutated thyristor (SGCT) semiconductor switches. These switches Figure 1. are turned on and off to create a pulse width modulated (PWM) output regulating the output frequency. White Paper WP020001EN Current source inverter Effective June 2014 vs. Voltage source inverter topology The design in Figure 3 implements cascaded SGCT devices to Size consideration achieve a 4160V system rating. A gate driver is required for each of the switching devices to control the device switch timing. Different drive topologies require many integrated components to work. The SC9000E EP VSI drive is fully integrated, while many CSI drives are non-integrated. Fully integrated design incorporates all necessary components for a “cable-in, cable-out” installation. In the following figures, the different integrated designs are illustrated. The dashed lines indicate components incorporated in a single assembly design. Non-Integrated Drive DC Isolation Converter Inverter Filter Figure 3. Input Link M The CSI design requires input and output filters due to high harmonic content. The input (Figure 3) is similar to a low voltage (LV) drive six-pulse input. At higher horsepower, a six-pulse active front end Figure 5. (AFE) input creates harmonics in the power system and poor power factor. To mitigate this issue, drive manufacturers combine either Non-integrated designs require an external isolation means. In a input transformers or reactors and harmonic filters to reduce the CSI design, this would be a transformer or a harmonic filter/reactor. detrimental effects of the drive on the power system at the point Poor power factor and harmonics generated by the CSI input of common coupling (PCC). require very large K-rated transformers or reactor/filter banks. Incorporating all these components within a single assembly Voltage source inverter significantly increases the footprint of the non-integrated drive. The voltage source inverter topology uses a diode rectifier that In high-horsepower applications, transformers or filters/reactors converts utility/line AC voltage (60 Hz) to DC. The converter have to be placed outside of the control room. is not controlled through electronic firing like the CSI drive. The DC link is parallel capacitors, which regulate the DC bus voltage ripple and store energy for the system. Integrated Drive The inverter is composed of insulated gate bipolar transistor (IGBT) DC Isolation Converter Inverter Filter semiconductor switches. There are other alternatives to the IGBT: Input Link M insulated gate commutated thyristors (IGCTs) and injection enhanced gate transistors (IEGTs). This paper will focus on the IGBT as it is used extensively in the MV VSI drives market. The IGBT switches create a PWM voltage output that regulates the voltage and Figure 6. frequency to the motor. The design in Figure 4 shows a neutral point clamped (NPC) An integrated design goes one step further and places the isolation three-level inverter topology. The IGBT switching devices are within the drive assembly. However, an external contactor and an cascaded to achieve a 4160V system rating. isolation switch or a breaker are required to feed the drive lineup. Phase-Shifting Fully Integrated Drive Transformer DC Isolation Converter Inverter Filter Input Link M A Figure 7. N B Y C M Figure 4. 2 EATON www.eaton.com Current source inverter White Paper WP020001EN vs. Voltage source inverter topology Effective June 2014 The fully integrated design incorporates an input contactor, an One of the best selling points of a drive is the ability to control isolation switch, protective fuses, and a phase-shifting isolation motor speed. In a typical pump or fan application, pump and fan transformer in a complete drive assembly. The SC9000 EP VSI power curves can be applied to show the typical savings using a drive uses this approach in a very small footprint. By integrating all drive vs. a mechanical damping solution, where all excess power components into a single assembly, floor space can be reduced up is lost through heating. to 65% over the traditional non-integrated design. Table 1 shows All drives save money for fan and pump loads, but not all drives save the floor space required for a fully integrated 4160V, 1500 hp variable money equally. torque rated drive. Taking the numbers from the example above and using an average Table 1. Required Floor Space of $0.05 per kWh, the difference adds up quickly: more than $80,000 over five years. Inches (mm) Square Footage Because all drives operate as power conversion devices, how can Drive Switch- Trans- Addi- % Design Width Depth Area gear former tional Total Inc. one drive be more efficient than another? SC9000 EP, 95.00 50.00 33.0 — — 0.0 33.0 — Semiconductor switching devices have losses during turn-on and 24-pulse, (2413.0) (1270.0) turn-off times that result in inefficiencies. Presently, CSI designs VSI use MV SCR, GTO, SGCT devices in the inverter and converter. 6-pulse 146.00 40.00 40.6 16.3 — 16.3 56.9 72% A GTO is a thyristor (SCR) that can be turned on and off during w/reactor, (3708.4) (1016.0) conduction. Gating circuitry gives a small signal to turn on the CSI device, and a large reverse signal to turn off the device. An SGCT PWM 138.00 40.00 38.3 16.3 — 16.3 54.6 66% is similar to a GTO but can block voltages in both directions, and the w/reactor, (3505.2) (1016.0) gate drive circuitry is built around the device rather than mounted CSI separately. Modern VSI designs use MV IGBT devices and an 18-pulse 122.00 40.00 33.9 16.3 22.6 38.9 72.8 121% external low power gate driver. no reactor, (3098.8) (1016.0) CSI Table 3. SCR b 6-pulse 110.00 40.00 30.6 16.3 22.6 38.9 69.5 111% Maximum no reactor, (2794.0) (1016.0) Ratings VDRM VRRM ITAVM ITRMS — CSI 12,000V 12,000V 1500A 2360A — Switching Turn-on Turn-off diT / dt dvT / dt Qrr The SC9000 EP saves significant space over other non-integrated Characteristics time time CSI drive designs. The competition can be as much as 121% larger tgt = 14 µs tq = 1200 µs 100A / µs 2000V / µs 7000 µC for the same horsepower drive. Table 2 lists various output frequency and loading values for a NNot: Part number: FT1500AU-240 (Mitsubishi) 500 hp medium voltage applied to a test stand dynamometer. The efficiency values diverge significantly toward the lower Table 4. GTO b frequency and load spectrum of the test. Maximum Ratings VDRM VRRM ITGQM ITAVM ITRMS — Table 2. Drive and Motor System Efficiency 4500V 17V 4000A 1000A 1570A — Frequency (Hz) Load (%) CSI AFE SC9000 EP ∆ Efficiency Switching Turn-on Turn-off diT / dt dvT / dt dvG1 / dt dvG2 / dt 60 100 92 97.5 5.64% Characteristics switching switching 50 60 85 97 12.37% td = 2.5 µs ts = 25.0 µs 500A / µs 1000V / µs 40A / µs 40A / µs 40 30 77 96.6 20.29% tr = 5.0 µs tf = 5.0 µs — — — — 30 12.5 65 96 32.29% On-State Voltage VT (on-state) = 4.4V at IT = 4000A NNot: Part number: 5SGA 40L4501 (ABB) EATON www.eaton.com 3 White Paper WP020001EN Current source inverter Effective June 2014 vs.