Marco Cupelli

Multiconverter Medium Voltage DC Power Systems on Ships: Constant-Power Loads Instability Solution Using Linearization via State Feedback Control

Bus voltage stability is a key issue in future medium-voltage DC (MVDC) power systems on ships. The presence of high-bandwidth controlled load converters (Constant Power Load, CPL) may induce voltage instabilities. A control design procedure is presented which starts at the modeling level and comes to control implementation. A control method based on a Linearization via State Feedback (LSF), is proposed to face the CPL destabilizing effect and to ensure the MVDC bus voltage stability. A multiconverter shipboard DC grid is analyzed by means of a new comprehensive model, which is able to capture the overall behavior in a second-order nonlinear differential equation. Exploiting DC-DC converters that interface power sources to the bus, LSF technique is able to compensate for system nonlinearities, obtaining a linear system. Then, traditional linear control techniques can be applied to obtain a desired pole placement. With reference to system parameters mismatch, LSF control design is verified by means of a sensitivity analysis, evaluating the possibility of an over-linearization strategy. Time-domain numerical simulations are used to validate the proposed control, in presence of relevant perturbations by means of a two-way comparison (average value model and detailed switching model).

Linearizing control of shipboard multi-machine MVDC power systems feeding Constant Power Loads

Voltage stability in Medium-Voltage DC (MVDC) power systems on ships is a key design goal. MVDC bus voltage stability can be impaired due to the presence of high-bandwidth controlled Constant Power Load (CPL) converters, which can induce negative incremental resistance instabilities. A linearization via state-feedback control is presented in this paper, to stabilize a MVDC bus voltage in presence of destabilizing CPLs. Once a linear control system is obtained, a traditional stable pole placement is operated. The control is designed to be implemented using the controlled DC/DC converters that interface the system power sources to the MVDC bus. The proposed control is verified by means of time-domain numeric simulations based on both an average state space model and a detailed power electronics circuit model, thus providing a two-way comparison.

Power Flow Control and Network Stability in an All-Electric Ship

The concept of an all-electric ship, while offering unprecedented advantages from the point of view of efficiency and flexibility of operation, has introduced new challenges in terms of stability and power flow control. The advent of a full power electronics power system has raised new questions from the point of view of system dynamics, particularly when dealing with the new medium-voltage direct current distribution. The overall goal of guaranteeing a secure operation of the power system has brought researchers to consider two main approaches: reducing the dynamics of the large load to operate in a range of dynamics compatible with traditional generation systems, or making the generator set smarter through its power electronics interface. This paper compares these approaches to stable operation, focusing on the latter considered more in line with the progress of technology and in general more appealing.

Why Ideal Constant Power Loads Are Not the Worst Case Condition From a Control Standpoint

This paper investigates the influence of control bandwidth on the stability of loads, which are interfaced through power electronic converters and are fed from a dc power source. When tightly regulated, these loads exhibit a constant power load (CPL) behavior. It is shown here that the ideal CPL assumption, prevalent in literature, may not represent the worst case in real-life applications. If the control bandwidth of the load is sufficiently high, the load behaves like a CPL, and the system stability margin decreases with the increase in output power. However, in a practical range, with a lower control bandwidth, the system stability margin is influenced critically by the converters characteristic impedance, as well as its output power. Under these conditions, the minimum stability margin may occur at a low-power range.

Decentralized Linear Quadratic Gaussian control of multi-generator MVDC shipboard power system with Constant Power Loads

A major challenge in MVDC shipboard power systems is maintaining Medium Voltage DC (MVDC) system stability, due to the presence of the negative incremental resistance behavior of Constant Power Loads (CPLs). This paper presents a decentralized Linear Quadratic Gaussian (LQG) control approach for individual local generator side converter to regulate and stabilize the MVDC system. In this control approach, we model the non-linear CPL, which the decentralized controller is responsible for, as a virtual disturbance and consider it as an additional state to be estimated by Extended Local Kalman Filter (ELKF). Using the estimated virtual disturbance, the set-point trajectory in steady state can be obtained and applied in Linear Quadratic Regulator (LQR) with the optimal state feedback gain. Accordingly, linearization of the constant power load is achieved via this accurate estimation of the virtual disturbance; stability is guaranteed via an optimal LQR, as well as the load is shared among the generation units via droop control. Due to the fact that the estimation of each individual virtual disturbance is based only on the local measurements and the local fix model implemented in ELKF, the MVDC system adaptive set-point is achieved in local controllers without communication. This assumption holds true not only in the case of a sudden large load connection or disconnection, but also in case of generator loss.

Trust infrastructures for future energy networks

Efficient use and distribution in future energy infrastructures largely depend on distributed control, metering and accounting functionalities. In such a Smart Grid essential components are distributed over the complete infrastructure, in particular parts of the infrastructure will be placed under possibly hostile end-users control. Thus, the dependability of the Smart Grid depends on the security of every component deployed. Considering the large variety of known attacks on IT infrastructures proper protection mechanisms need to be considered already in the early design of Smart Grid architecture and their components. The notion of Trusted Computing established in the PC area can also be used in Smart Grids to establish trust among all involved stakeholders and to ensure the proper functioning of devices. This paper discusses relevant security requirements and introduces a vision of a security infrastructure for energy networks built on hardware trust anchors.

Application of backsteppping to MVDC ship power systems with constant power loads

In future all-electrical ships with Medium Voltage DC distribution, the power system stability can be jeopardized by the presence of constant power loads, due to the negative impedance characteristic of their tightly regulated converters. In order to solve this problem we apply backstepping as a nonlinear control technique to the converter interfacing the generation side and which supplies the bus. In this work we demonstrate two versions of backstepping, which are compared to a centralized linearizing state feedback controller taken from literature.

Hardware in the Loop implementation of a disturbance based control in MVDC grids

This paper presents the Hardware in the Loop (HiL) validation of a control method to regulate the voltage of the DC Bus fed by Distributed Generation (DG) units in an islanded MVDC microgrid. The disturbance based control, employs a decentralized Linear Quadratic Gaussian (LQG) control of individual generator side converters to be executed locally, and which does not need remote measurements of loads and sources in its control scheme. This enables “plug-and-play” capabilities. The virtual disturbance model permits changes in the reference voltage and the variation in the load parameters based on local knowledge without the need of a communication infrastructure. The testing is performed via HiL implementation with Real Time Digital Simulator (RTDS). HiL Simulations show that this method can handle switching noise, ripple and other effects that are not present in averaged models.

Hardware in the loop implementation of linearizing state feedback on MVDC ship systems and the significance of longitudinal parameters

In future all-electric ships, which rely on a Medium Voltage DC distribution network, the power system stability can be jeopardized by the presence of constant power loads, due to their negative incremental resistance. In fact, this causes stability problems due to the negative impedance characteristic of tightly regulated converters. The Linearizing State Feedback (LSF) has shown the capability to stabilize these systems. The key modelling assumption for this control is the possibility to neglect cable parameters. In this paper we implement the LSF control in an FPGA hardware and simulate the shipboard power system in RTDS to test the validity of the cable assumption in Hardware in the Loop (HiL) tests.

GEYSER: Enabling Green Data Centres in Smart Cities

Information Technology is a dominant player of our modern societies; Data Centres, lying at the heart of the IT landscape, have attracted attention, with their increasing energy consumption being a constant topic of concern, especially when it comes to the negative impact on the quality of their surrounding environment. Nevertheless, recent technological and societal advances are paving the way for DCs to change their role from passive energy consumers into prosumers, thus, transforming themselves into leading players within their smart district surroundings. This paper describes the innovative GEYSER approach to enabling green networked DCs to monitor, control, reuse, and optimize both their energy consumption and production, and in particular from renewable resources, towards becoming active participants within Smart Grids and Smart Cities.