Catalin Gavriluta

Hierarchical Control of HV-MTDC Systems With Droop-Based Primary and OPF-Based Secondary

This paper proposes a hierarchical control architecture designed for an arbitrary high voltage multiterminal dc (MTDC) network. In the proposed architecture, the primary control of the MTDC system is decentralized and implemented using a generalized droop strategy. Design criteria for dimensioning the primary control parameters, including voltage limits, are offered by analyzing the transients appearing in the system. The proposed secondary control is centralized and regulates the operating point (OP) of the network so that optimal power flow (OPF) is achieved. Compared to previous works, this paper further elaborates, both analytically and through simulations, on the coordination between the primary and secondary control layers. This includes how local primary controllers have to be driven by the centralized controller in order to ensure a smooth transition to the optimal OP.

Decentralized Primary Control of MTDC Networks With Energy Storage and Distributed Generation

Multiterminal dc networks are drawing a lot of interest lately in applications related to distributed generation, particularly in those that also integrate energy storage (ES). A few approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on custom droop characteristics obtained by combining concepts of droop and dc-bus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac grids, as well as various droop characteristics for different elements connected to the dc network, are defined. For the ES, the state of charge is taken into account at the primary control level and included in the droop characteristic, creating a two-variable droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a photovoltaic generator, a lead-acid battery, and a connection point to the ac grid.

Adaptive Droop for Control of Multiterminal DC Bus Integrating Energy Storage

Multiterminal dc (MTDC) systems are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate wind or photovoltaic (PV) generation with energy storage (ES). Several approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures are mainly application specific and, thus, can be still improved in order to provide a more generic approach. This paper proposes an improved primary control layer for an MTDC system. The concept is based on the combination of a droop control method and dc bus signaling in order to provide a more generic and flexible solution. In this paper, different droop characteristics are proposed for the various elements connected to the dc bus. All of them are specifically tailored around five operation bands, which depend on the dc bus voltage level. Special attention is paid to the integration of ES: the state of charge (SoC) is considered at the primary control level, yielding a surface characteristic that depends on the SoC and the dc bus voltage. The scaling of the system has been analyzed together with the proposed control strategy and the overall operation has been validated through simulations by considering a 100 kW PV system with energy storage. Experimental results were obtained on a scaled laboratory prototype rated at 10 kW.

Centralized protection strategy for medium voltage DC microgrids

This paper presents a centralized protection strategy for medium voltage dc microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers. The proposed method is also supported by a backup protection that is activated if communication fails. This paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators. The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop approach.

Resonance analysis of a wind power plant with modal approach

A detailed harmonic resonance analysis of power systems is more and more important due to the increasing number of power electronic devices. The topic is especially well-grounded in wind power plants that besides having several possible sources of harmonics have many inductive and capacitive elements. This combination forms a severe concern over harmonic resonance that composes a threat to the power quality and the function of wind power plants. This paper presents and applies the harmonic resonance mode analysis (HRMA) in a model of an off shore wind power plant with and without a passive filtering. The results of the harmonic resonance mode analysis are verified and compared with the results calculated with the frequency scan.

Storage system requirements for grid supporting PV-power plants

Large penetration of renewable energy is currently attenuated by concerns regarding their impact on the controllability and reliability of the electrical system. As the inclusion of energy storage is to a great extent the solution to these issues, this paper proposes a methodology for approaching the calculation of the size of the energy storage to be connected to a PV power plant for providing inertia emulation, primary control, and the reduction of power fluctuation. A complete control strategy, developed for the inclusion of these services in the operation of a dc-ac grid connected converter has been implemented and validated through simulation results. The obtained results are validated by experimental tests performed on a scaled 10 kW prototype.

Adaptive droop for primary control in MTDC networks with energy storage

Multi-terminal DC (MTDC) systems are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate wind or PV systems together with energy storage. This paper proposes an improved strategy for the primary control of MTDC integrating energy storage (ES) units. The concept is based on the combination of a droop control method and dc-bus signaling in order to provide a generic approach to be used as basis for a hierarchical control structure in a multi-point DC-bus system. The proposed method considers the State of Charge (SoC) of the energy storage system at the primary control level, yielding a surface characteristic for a droop control that depends both on the SoC and the dc bus voltage. Finally, a method for designing and tuning the system is proposed and analyzed theoretically and through simulations and experiments on a 10kVA system with energy storage.

Decentralized control of MTDC networks with energy storage and distributed generation

Multi-terminal dc (MTDC) networks are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate energy storage. Several approaches for controlling the operation of such systems have been proposed in the literature; however the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on droop and dc-bus signaling to serve as a base framework for implementing a hierarchical control structure in a MTDC system. As it will be further discussed in this work, five operating bands as well as various droop characteristics for different elements connected to the dc-bus were defined. For the energy storage the state of charge (SoC) was taken into account at the primary control level and it was included in the droop characteristic, creating a two variables droop surface. The proposed control strategy was validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a PV generator, a lead-acid battery and one connection point to the ac grid.

A communication-assisted protection for MVDC distribution systems with distributed generation

In this paper, a communication-assisted protection scheme is proposed for medium voltage dc (MVDC) distribution networks. MVDC grids are applicable for connection between microgrids, upgrading the transmission capacity of ac lines by the conversion to dc, and integration of renewable energy systems to the distribution grids. However, protection issues are one of the main challenges in the development of the VSC-based dc networks. Due to the special behavior of the dc fault currents, it is almost impossible to coordinate the overcurrent relays based on the time inverse grading. Hence, in the proposed scheme, to provide a fast and selective protection, each proposed relay communicates with two other relays to operate as the main protection for a dc feeder and the backup protection of the adjacent feeder. Hardware in the loop simulation approach by use of the OPAL-RT real time simulator is used to verify the performance of the proposed method.

A simple approach for fast controller prototyping for a three phase interleaved DC-DC converter

Initial control prototyping in power electronics is often a matter of trial and error because of the complicated mathematics involved in modeling systems based on their internal structure. Moreover, in practice, small changes as the introduction of a measurement filter or the change of the sensing system appear all the time. If the controller design method was based on the internal structure of the system, then these small changes would mean a re-iteration through all the cumbersome equations. This paper presents a simple methodology for fast control prototyping based on the approximation of the Bode plots. As prerequisites, a power electronics simulation software as PSIM is needed. The method is presented by taking as a study case the design of a current control loop of a 30kW three phase interleaved DC-DC converter. Simulations are presented for the different considered controllers, and finally the method is validated by experimental results.