Midpoint voltage stabilization of split dc bus in three-phase four-wire shunt active power filters: Prototyping of a midpoint voltage balancer for the ECSEL JU funded project CONNECT

The increasing adoption of small-scale photovoltaic systems and electrical vehicles, which are commonly distributed unequally across three-phase feeders, aggravates voltage imbalance in electrical power distribution networks (EPDNs). Voltage imbalance in an EPDN poses a great threat to the normal operation of connected electric devices, causing overloading and overheating of distribution transformers, power loss in neutral wires, efficiency degradation of induction motors, and more.

Three-phase four-wire (3P4W) shunt active power filters (APFs) can be employed to deal with issues associated with voltage imbalance by injecting or sinking current to or from the point of common coupling (PCC) of an EPDN. In order to fully compensate the voltage imbalance in a PCC, a dc bus midpoint, to which the neutral current flows, must be provided.

In this thesis, a midpoint voltage balancer (MVB), which consists of a dual-switching-leg, two split capacitors, and two neutral inductors, is proposed. The MVB is designed to deliver a voltage-stable midpoint for a 20 kVA 3P4W shunt APF, which is used to enhance the voltage quality of the PCC by providing voltage imbalance compensation, voltage harmonic mitigation, and power factor correction.

Due to the utilization of a dual-switching-leg, the neutral current handling capacity of the MVB is doubled compared to some of existing neutral point configurations. An interleaved control is implemented in the dual-switching-leg, which minimizes the propagation of high-frequency current into the midpoint. In addition, the neutral inductors are designed in a way to enable ZVS operation across all IGBT switches of the MVB, when the neutral current is smaller than 29 Arms.

The operation of the MVB is governed by a dual-loop control, that includes an inner cur-rent loop and an outer voltage loop. The current loop facilitates the tracking of the total inductor current to the neutral current, preventing current injection into the midpoint. In addition, an active damping scheme is embedded in the current loop to damp the resonance introduced by the LC network. The voltage loop safeguards the midpoint against excessive voltage variations by addressing voltage imbalance between the two split capacitors. Furthermore, the voltage loop compensates any regulation error from the current loop by adjusting the current reference of the neutral inductors accordingly.

MATLAB simulation is carried out, which verifies the effectiveness of the proposed MVB. Moreover, a 20 kVA prototype has been built, and the experimental results show that the voltage ripple of the midpoint is maintained within around 1.3% of the total dc bus voltage under a neutral current transient of 58 Arms at 50 Hz.

Therefore, the prototyping of the proposed MVB is a promising step towards the success of the ECSEL JU funded project CONNECT. The MVB provides a voltage-stable midpoint to the 20 kVA 3P4W shunt APF, which will be a final deliverable from TU/e to the CONNECT project. In addition, the proposed MVB also has a great potential in bipolar dc micro grid applications.