Abstract
With the high proportional renewable energy integration and rapid increase in the DC loads, such as the electric vehicle and distributed energy storage, the DC distribution system becomes a prospective solution for the urban power grid enhancement for its high-efficiency and eco-friendly nature. In most DC distribution systems, power interfaces are applied to connect low-voltage DC (LVDC) distribution systems with multiple medium-voltage (MV) systems in order to improve the operating reliability and economy. Compared to other types of multiports power interfaces, the three-port-isolated modular multilevel converter (I-M2C) has shown many advantages, including low cost, high power density, and low control complexity. However, the I-M2C cannot handle the power imbalance at the bipolar LVDC port like the other MMC-based three-port power interfaces, which limits the operating range and decreases the stability of the I-M2C in bipolar LVDC application. In order to solve the bipolar imbalance problems, a novel balance control method is proposed in this article. The proposed balance control method is based on symmetrical decomposition. By decoupling the MV power control and the LV bipolar power compensation control, the proposed method can eliminate the bipolar voltage deviation under different working conditions. The simulation results prove the validity and good control performance of the proposed method.
Highlights
With the global-scale urbanization in the past few decades, the urban power grid has met many operating challenges (Sun et al, 2021a; Zhang et al, 2021)
In order to achieve the symmetry power balance at the bipolar low-voltage DC (LVDC) port, half of the ISMs are parallelly connected to the positive LVDC link capacitor and the other half are parallelly connected to the negative LVDC link capacitor
Where uarmju and uarmjl are the upper and lower voltages of the phase unit j, N is the submodule number per arm, k is the ratio of high-frequency ratio, UdcL are the pole-to-pole voltage of the LVDC bipolar port, D is the DC modulation freedom, dj is the AC modulation freedom of phase j, UdcM is the MVDC voltage, and uj is the MVAC voltage of phase unit j
Summary
With the global-scale urbanization in the past few decades, the urban power grid has met many operating challenges (Sun et al, 2021a; Zhang et al, 2021). The voltage balance method eliminates the bus voltage deviation by regulating the bipolar power inputs from the outer systems with the assist of power interfaces (Cui et al, 2019). A power interface cannot balance the bipolar voltage unless special topology improvements and corresponding control frames are implemented. Since the MMC-based three-port SSTs have multiple power submodules, the voltage balance of the LVDC port can be achieved by controlling the LVDC power. 2) An LVDC bipolar balance control method based on symmetrical decomposition is proposed to control the bipolar differential power without changing the power distribution on the MV side. 3) With the assist of the proposed control method, the I-M2C with the bipolar LVDC port can maintain bus voltage balance under large bipolar power imbalance.
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