The first part of this series introduces the grid commutated inverter (LCC). This article will discuss voltage source converters (VSC) and compare the two topologies.
VSC is now the preferred implementation target for the following reasons: VSCs have lower system costs because of their simpler allocation. VSC achieves bidirectional flow of current, making it easier to reverse the direction of power flow. The VSC can control the active and reactive power on the AC side. VSCs do not rely on AC networks like LCCs, so they can supply passive loads and have black-start capabilities. The use of insulated gate bipolar transistor (IGBT) valves eliminates the need for commutation operations required for thyristors and enables bidirectional current flow.
Table 1 compares LCC and VSC. The voltage level of VSC is typically in the range of 150kV-320kV, but some voltage levels can be as high as 500kV. There are several different types of VSC. Let's take a look at two-level, three-level, and modular multilevel.
* See the 16th International Conference on Environmental and Electrical Engineering of the Institute of Electrical and Electronics Engineers (IEEE) "A review of LCC-HVDC and VSC-HVDC technologies and applications."
Table 1: Comparison of inverters
Two-level voltage source converterAs shown in FIG. 1, the two-level VSC has IGBTs, and each IGBT has a reverse diode connected in parallel therewith. Each valve includes a plurality of IGBT/diode assemblies in series. The IGBT is controlled using pulse width modulation (PWM) to help form the waveform. Since the IGBT turns on and off several times during PWM implementation, switching loss occurs, and harmonics are a factor.
Figure 1: Two-level VSC (HVDC Inverter Image courtesy of Wikipedia)
Three-level voltage source converter
As shown in Figure 2, the three-level VSC improves the harmonics problem. The three-level inverter has four IGBT valves per phase. Two of the diode valves are used for clamping voltage, but you can replace them with IGBTs for better controllability. Turn on the top two IGBTs to get a higher voltage level, turn on the middle two IGBTs to get the intermediate (or zero) voltage level, and open the bottom two valves to get a lower voltage level.
Figure 2: Three-level VSC (HVDC Inverter Image courtesy of Wikipedia)
Modular multilevel converterThe MMC differs from the other two converters in that each valve is an inverter module with a built-in smoothing capacitor. The MMC replaces a valve with multiple IGBTs with multiple cascaded inverter modules. Each of these modules represents a specific voltage level. The inverter module in the MMC is a half bridge or full bridge converter.
Figure 3: Modular Inverter Type (HVDC Inverter Image courtesy of Wikipedia)
The MMC method significantly improves harmonic performance so that filtering is usually not required. It is also more efficient than two- and three-level VSCs because it does not have the same switching losses as IGBT valves.
Figure 4: Waveform output (image courtesy of SVC PLUS VSC technology)
To monitor power factor, voltage and current levels, the signal can be measured on the measurable side of the AC and DC of the station. Upon receiving this information, the inverter control device can make the required adjustments to maintain a stable power level and an appropriate power factor. A protective relay system or intelligent electronic device (IED) collects signal information. See Figure 5.
Figure 5: Signal interpretation
Isolated current and voltage measurements using fully differential isolation amplifiers are one of TI's reference designs for measuring both AC and DC signals. The design guide explains how to use an isolated op amp to adjust the signal to increase amplitude and reject any common-mode voltage and noise. The MCU with the onboard ADC will analyze and interpret this signal. Information determined from the waveform is fed back to the control of the inverter, which adjusts the changing phase and voltage levels to maintain stability.
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