Jose Ignacio Candela

Decoupled Double Synchronous Reference Frame PLL for Power Converters Control

This paper deals with a crucial aspect in the control of grid-connected power converters, i.e., the detection of the fundamental-frequency positive-sequence component of the utility voltage under unbalanced and distorted conditions. Specifically, it proposes a positive-sequence detector based on a new decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL), which completely eliminates the detection errors of conventional synchronous reference frame PLLs (SRF-PLL). This is achieved by transforming both positive- and negative-sequence components of the utility voltage into the double SRF, from which a decoupling network is developed in order to cleanly extract and separate the positive- and negative-sequence components. The resultant DDSRF-PLL conducts then to a fast, precise, and robust positive-sequence voltage detection even under unbalanced and distorted grid conditions. The paper presents a detailed description and derivation of the proposed detection method, together with an extensive evaluation using simulation and experimental results from a digital signal processor-based laboratory prototype in order to verify and validate the excellent performance achieved by the DDSRF-PLL

Intelligent Connection Agent for Three-Phase Grid-Connected Microgrids

The high penetration of distributed generation power plants, based on renewable energy sources (RESs), is boosting the connection of power converters to the electrical network. This generation concept would permit to form local networks, microgrids, when the main grid falls due to any kind of contingency in the network. However, the connection and disconnection of these local networks may give rise to undesired transient overcurrents that should be avoided. In order to solve this drawback, this paper presents a method oriented to carry out a stable intentional disconnection/reconnection of local grids from the main electrical network under grid-fault conditions. This control method has been implemented in a grid-connected power converter that acts as an intelligent connection agent (ICA) and adapts its operation mode according to its connection state. The proposed control also manages the operation of a controlled switch, which is responsible of disconnecting/reconnecting the microgrid from the mains. In this paper, the behavior of the ICA under transient conditions will be discussed, and finally, its simulated and experimental performance will be shown.

Current Harmonics Cancellation in Three-Phase Four-Wire Systems by Using a Four-Branch Star Filtering Topology

This paper presents a new solution for filtering current harmonics in three-phase four-wire networks. The original four-branch star (FBS) filter topology presented in this paper is characterized by a particular layout of single-phase inductances and capacitors, without using any transformer or special electromagnetic device. Via this layout, a power filter, with two different and simultaneous resonance frequencies and sequences, is achieved-one frequency for positive-/negative-sequence and another one for zero-sequence components. This filter topology can work either as a passive filter, when only passive components are employed, or as a hybrid filter, when its behavior is improved by integrating a power converter into the filter structure. The paper analyzes the proposed topology, and derives fundamental concepts about the control of the resulting hybrid power filter. From this analysis, a specific implementation of a three-phase four-wire hybrid power filter is presented as an illustrative application of the filtering topology. An extensive evaluation using simulation and experimental results from a DSP-based laboratory prototype is conducted in order to verify and validate the good performance achieved by the proposed FBS passive/hybrid power filter.

A Generalized Voltage Droop Strategy for Control of Multiterminal DC Grids

This paper proposes a generalized voltage droop (GVD) control strategy for control and power sharing in voltage source converter (VSC)-based multi-terminal DC (MTDC) grids. In the proposed approach, the conventional voltage droop characteristics of voltage-regulating VSC stations are replaced by the GVD characteristics. The proposed GVD control strategy can be operated in three different control modes including conventional voltage droop control, fixed active power control and fixed DC voltage control by proper adjustment of the GVD characteristics of the voltage-regulating converters. The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability. Simulation results, obtained by detailed modeling in a four-terminal high voltage DC grid demonstrated the efficacy of the proposed approach and its flexibility in active power sharing. Moreover, based on the obtained results, switching between operating mode of the GVD control approach does not results in oscillations of the active powers flowing inside the MTDC grids.

A protection strategy for fault detection and location for multi-terminal MVDC distribution systems with renewable energy systems

A fault location method and a fault clearance strategy are presented in this paper for medium voltage dc (MVDC) distribution system. MVDC systems are applicable for connection between microgrids (MGs) and integration of renewable energy systems (RESs) to distribution systems. Due to the specifications of fault current in dc systems, it is difficult to coordinate the over current (O/C) relays based on the time inverse grading. Hence, in this paper, a communication link between O/C relays is used to diagnose the fault location. On the other hand, the fault clearance is done by the operation of dc circuit breakers (DCCB) and isolator switches. In this protection strategy, O/C relays detect the faulty part using communication links and after the fault extinguishing by DCCBs, the dc switches isolate the faulty part. Finally, the sound parts of the system re-energize when DCCB are re-closed. Moreover, data transmission by communication links is based on the standard messages of IEC61850 protocol.

Flexible Control of Power Flow in Multiterminal DC Grids Using DC–DC Converter

This paper proposes an efficient control framework that utilizes dc-dc converters to achieve flexible power flow control in multiterminal dc (MTDC) grids. The dc-dc converter employed in this paper is connected in cascade with the dc transmission line, and is therefore named cascaded power flow controller (CPFC). In this paper, a two-layer control strategy is developed for the operation and control of voltage source converter stations and CPFC station in MTDC grids. At the primary control layer, a novel differential voltage droop control is developed, while at the secondary control layer, a modified dc power flow algorithm-employing the new CPFC framework-is implemented. The overall control strategy enables the CPFC to regulate the power flow in the dc transmission line. The primary control guarantees the transient stability of the CPFC, and the secondary control system ensures the desired steady-state operation. The proposed voltage droop control framework helps the MTDC grid to remain stable in the event of a communication failure between the primary and secondary control layers. Static analysis and dynamic simulations are performed on the CIGRE B4 dc grid test system, in order to confirm the effectiveness of the proposed control framework for power flow regulation in MTDC grids.

Improving long line stability by integrating renewables using static synchronous generators

Daily increasing in renewable energy sources installed in dispersed location, has led to greater need for long transmission lines to use these green sources. Voltage and phase stability are major challenges for transmission system operators in the operation of these lines. In this paper, a Static Synchronous Generators based in Synchronous Power Controller (SSG-SPC) is presented and implemented in a long AC transmission system in order to reducing stability problems. Power control strategies, compatibility with the grid codes and improvement in voltage and phase stability by SSG-SPC are investigated. Simulations and Experimental test results confirms SSG-SPC capability for participation in reactive compensation, damping active power oscillations and modify stability margins.

Proposals for flexible operation of multi-terminal DC grids: Introducing flexible DC transmission system (FDCTS)

The current route of achieving the ultimate plan for flawless operation and control of the multi-terminal DC (MTDC) grids can be significantly accelerated by learning from the vast and valuable experiences gained from the operation of the AC power grids for more than a century. This paper introduces concept of flexible DC transmission system (FDCTS), inspired by the successful operation of flexible AC transmission systems (FACTS), to provide voltage regulation, power control and load flow control within MTDC grids. Considering the current advancements in the field of power electronics, this paper recognizes DC-DC converters as the first element of the FDTCS for providing voltage and power control in MTDC grids. By use of DC-DC converters, this paper developes two elements of the FDCTS, namely the cascaded power flow controller (CPFC) and hybrid power flow controller (HPFC). In this paper, to demonstrate the eligibility of the CPFC and HPFC to play the role of an FDCTS, they are included in the DC power flow formulation for DC voltage regulation and power flow control purposes.

Phase stability enhancement in big power networks using renewable generation units controlled by SPC

High penetration of Renewable Generation Units (RGU), has led to that the interaction of this newfound participators in power network with conventional units be more critical challenges for network operators. Effect on the phase stability in emplacement as well as on neighbor generation units is one of this essential challenges. In this paper, a RGU controlled by Synchronous Power Controller (SPC) which behaves as a big Static Synchronous Generation (SSG-SPC) units, are presented as solution to cope with this challenge. Small signal modelling of a power network in presence of SSG-SPC unit is used to analysis this solution. After illustrating SPC dynamic capabilities, study on IEEE-14B test system based on modal analysis, time domain investigation and real time test confirms that a SSG-SPC not only has not damaging impact on phase stability and dynamic of network, but on the contrary can improve it. Moreover, by having at least two dynamic freedom degrees in SPC, the SSG-SPC can adapt itself for keeping this improvement effect.

Synchronous power controller merits for dynamic stability improvement in long line by renewables

Growing universal demand for greater use of renewable energy resources (RES) installed in dispersed location, has caused to more urgent need for long transmission systems to use these green sources. Small signal stability is one of vital challenges for transmission system operators in presence of big RES. In this paper, a Static Synchronous Generators based in Synchronous Power Controller (SSG-SPC) is presented and its capabilities for dynamic stability enhancement in a long AC transmission system is investigated. Dynamic modelling of a long line and then study in Uncompensated and Compensated cases by using time domain simulation and modal analysis, and then experimental test confirms that SSG-SPC can be effective in active damping of power oscillations and reducing small signal stability challenges.