The first part of this series introduced the line-commutated converter (LCC). In this article, we will explore voltage source converters (VSC) and compare them with LCC topologies. VSCs have become the preferred choice in modern power systems for several reasons. They offer lower system costs due to their simpler design and fewer components. Additionally, VSCs support bidirectional current flow, which makes it easier to reverse the direction of power transmission. Another advantage is that VSCs can independently control both active and reactive power on the AC side. Unlike LCCs, which depend on the AC grid for commutation, VSCs can supply power to passive loads and even perform black-start operations. This is made possible by using insulated gate bipolar transistors (IGBTs), which eliminate the need for complex commutation processes required by thyristors and allow for more flexible control.
Table 1 provides a comparison between LCC and VSC. While VSCs typically operate in the voltage range of 150kV to 320kV, some advanced systems can reach up to 500kV. There are several types of VSC, including two-level, three-level, and modular multilevel converters. Let’s take a closer look at each.
A two-level VSC, as shown in Figure 1, uses IGBTs with parallel-connected diodes. Each IGBT is connected in series with other IGBT/diode assemblies. The waveform is shaped using pulse width modulation (PWM), but this process causes switching losses and introduces harmonics into the system.
Figure 1: Two-level VSC (HVDC Inverter Image courtesy of Wikipedia)
Moving on to the three-level VSC, as illustrated in Figure 2, this configuration improves harmonic performance. It has four IGBT valves per phase, with two clamping diodes or IGBTs used to manage voltage levels. By turning on different combinations of IGBTs, the system can produce higher, intermediate, or lower voltage levels.
Figure 2: Three-level VSC (HVDC Inverter Image courtesy of Wikipedia)
The modular multilevel converter (MMC) stands out from the two previous types. Instead of individual valves, the MMC uses multiple cascaded inverter modules, each containing IGBTs and a built-in smoothing capacitor. These modules can be half-bridge or full-bridge configurations. This design significantly reduces harmonic distortion and eliminates the need for external filtering. Additionally, it offers better efficiency compared to two- and three-level VSCs due to reduced switching losses.
Figure 3: Modular Inverter Type (HVDC Inverter Image courtesy of Wikipedia)
Figure 4 shows the output waveform of an MMC, demonstrating its superior performance in terms of smoothness and quality.
Figure 4: Waveform output (image courtesy of SVC PLUS VSC technology)
To ensure stable operation, the inverter continuously monitors key parameters such as power factor, voltage, and current. These signals are measured on both the AC and DC sides of the station. Once collected, the data is sent to the inverter control system, which adjusts the phase and voltage levels accordingly. A protective relay system or intelligent electronic device (IED) helps gather and interpret these signals. Figure 5 illustrates how these signals are processed.
Figure 5: Signal interpretation
For accurate measurement of both AC and DC signals, isolated current and voltage measurements using fully differential isolation amplifiers are widely used. Texas Instruments (TI) has developed reference designs that demonstrate how to use isolated operational amplifiers to amplify the signal, reject common-mode noise, and improve accuracy. The microcontroller unit (MCU) then processes the signal using its onboard ADC. The information extracted from the waveform is fed back to the inverter control system, enabling real-time adjustments to maintain system stability.
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