Transformer-Less Hybrid Series Static Compensator and its DC Capacitor Voltage Balance Technique Using Zero-Sequence Voltage

The hybrid series converter (HSC) has enhanced the operating modulation-index (MI) range compared to the parallel hybrid converter (PHC). However, the HSC is underutilized when employed for applications such as HVdc. The parallel chain links (PCLs) of the HSC generate rectified voltage of fixed amplitude, whereas the series chain links (SCLs) serve as voltage wave-shaping circuit. The maximum output voltage of the HSC is equal to the PCL rating. Moreover, HSC (and also PHC) demands three single-phase transformers for their operation and consists of series/director switches (DSs) that undergo high voltage and d
v
/d
t
stress. This article intends to extract all the advantages inherent with the HSC and utilize it to its maximum ability by restructuring and operating it as a transformer-less hybrid-series static compensator (THSSC). The SCLs of the THSSC act as a voltage-aiding circuit and reduce voltage stress across DSs. In addition, the THSSC has a lower energy storage requirement and requires fewer submodule capacitors and switching devices. During unbalanced grid conditions, capacitor voltages related to different phases of the THSSC tend to divert from reference. The required quantity of zero-sequence voltage (ZSV) is injected into the PCL and SCL of all three phases to govern capacitor voltages. The expression for reference ZSV is derived by using the feed-back and feed-forward methods. The converter's working principle and capacitor voltage control technique are authenticated using experimental results obtained under different grid voltage conditions.