Medium-Voltage Impedance Measurement Unit for Assessing the System Stability of Electric Ships
The requirements for improved reliability of shipboard power systems are steering the development of medium-voltage ac (MVAC), and medium-voltage dc (MVDC) systems as direct replacements of the conventional low-voltage generation and distribution practice. At the same time, the increased use of power electronics is fundamentally changing the nature of the shipboard power system sources and loads, inflicting low- and high-frequency dynamic interactions that did not exist in the system before. In order to design a robust power system and to guarantee the stable operation for a wide operating range, impedance interactions at various interfaces can be analyzed with an in-situ impedance measurement unit (IMU).
This paper presents the first hardware implementation of measurement equipment capable of identifying small-signal dq impedances of MVAC and voltage direct current systems using a single-phase wide-bandwidth injection algorithm. To extract impedances from a system, the IMU is connected at the desired interface point as shown in Fig. 1. The core of the IMU is the perturbation injection unit (PIU), which comprises three power electronic building blocks (PEBBs) connected in series or parallel, depending on the desired mode of operation. In the shunt current injection mode, the three PEBBs are configured in series and operate as a single-phase cascaded multilevel converter as shown in Fig. 2 (a). In the series voltage injection mode, three PEBBs are connected in parallel, forming a single-phase modular interleaved converter as shown in Fig. 2 (b).
The proposed solution is capable of injecting sinusoidal, chirp, or multi-tone signals, providing flexibility in terms of measurement speed, signal-to-noise ratio, and identification precision. The injection of wide-bandwidth signals reduces the measurement time significantly, while yielding accurate dq impedance results. Finally, the IMU is experimentally validated on medium-voltage variable voltage source supplying a resistive load with the injection of single-phase wide-bandwidth signals. The medium voltage IMU developed in this work can undoubtedly aid in the design of advanced Navy shipboard platforms with contemporary all-electric architecture. The IMU is a necessary technology for the development of cutting-edge hybrid MVAC and MVDC power systems for future electric ships that feature high reliability and survivability without compromising the stability of the system.