Pablo Garcia-Gonzalez

Application of a Repetitive Controller for a Three-Phase Active Power Filter

This paper presents the detailed design, analysis, and application of the controller for a shunt active power filter based on a pulsewidth modulation dc-to-ac voltage source converter. The controller is mainly tailored to compensate harmonic currents of nonlinear loads connected to the mains. However, it can also achieve reactive-power compensation and mains-current balancing when required. The controller has a two-layer structure. The outer layer generates the current references for the inner layer. The former uses a plug-in discrete-time repetitive algorithm for current-harmonic compensation, a proportional-integral algorithm to maintain the dc-capacitor voltage in spite of unmodeled losses and a reactive-power-reference generator. The inner layer uses state-feedback with integral action for current control. The repetitive controller is justified to improve the tracking of the periodic current references required by the active filters. The stability of the resulting closed-loop system is studied and some indication of the system robustness is given. The proposed controller has been tested in a prototype with balanced and unbalanced nonlinear loads. A discrete-time model of the filter has been used from the beginning. The microcomputer delay when calculating the controller output and the delay due to the anti-aliasing filters have been included in the inner system state-variable model

Comparison of thyristor-controlled reactors and voltage-source inverters for compensation of flicker caused by arc furnaces

The objective of this paper is to compare the performance of thyristor-controlled reactors (TCR) and shunt-connected PWM voltage source inverters (PWM-VSI) for compensation of flicker caused by arc furnaces. First of all, arc-furnace principles are presented in order to explain the main characteristics of the problem. Secondly, traditional TCR control are analyzed. An improved measuring procedure is suggested to enhance TCR performance showing that it achieves faster compensation than more traditional methods. Thirdly, PWM-VSI control for flicker compensation is described in detail using Parks transformation. The analysis shows how real and reactive power control can be decoupled. Continuous-time and discrete-time models are considered. Finally, a TCR control and a PWM-VSI control are compared by simulation using data and measurements from a real arc-furnace installation. The analysis considers three different periods of the production cycle: (a) bore-down, (b) fusion, and (c) refining. It is clear from the results obtained that a shunt-connected PWM-VSI is better than a TCR for flicker compensation. This can be easily justified noting that the bandwidth of the PWM-VSI control system is far better than that of the TCR control. However, the control system for a PWM-VSI inverter is more complicated than that of a TCR. Besides, the latter uses a better-established technology than the former.

An Aggregate Model of Plug-In Electric Vehicles for Primary Frequency Control

Summary form only given. The penetration level of plug-in electric vehicles (PEVs) has the potential to be notably increased in the near future, and as a consequence, power systems face new challenges and opportunities. In particular, PEVs are able to provide different types of power system ancillary services. The capability of storing energy and the instantaneous active power control of the fast-switching converters of PEVs are two attractive features that enable PEVs to provide various ancillary services, e.g., primary frequency control (PFC). However, concurrently, PEVs are obliged to be operated and controlled within limits, which curbs the grid support from PEVs. This paper proposes a new model for PEV using a participation factor, which facilitates the incorporation of several PEV fleets characteristics such as minimum desired state of charge (SOC) of the PEV owners, drive train power limitations, constant current and constant voltage charging modes of PEVs. In order to reduce computational complexity, an aggregate model of PEVs is provided using statistical data. In the end, the performance of PEVs for the provision of PFC is evaluated in a power system. Results show that PEV fleets can successfully improve frequency response, once all the operating constraints are respected.

Control system for a PWM-based STATCOM

The ever-increasing switching frequency of modern solid-state power switches, together with the application of multi-converter topologies, make it possible to use pulse width modulation (PWM) in high power applications of STATCOMs (static synchronous compensators). This paper proposes and details a control system for a PWM-based STATCOM. First of all, a discrete-time model of the STATCOM is derived to take into account the discrete-time implementation of the controller. Secondly, the control algorithm is detailed. It ensures decoupled control of the real and reactive power exchanged between the power converter and the electric-energy system. This is necessary to control the DC capacitor voltage during transients of the exchanged reactive power. Finally, the control of the capacitor voltage is explained in detail. The controller is tailored to keep the capacitor voltage almost constant in spite of the fast control of the reactive power. This helps to reduce the capacitor size significantly. The main contributions are illustrated using a 15 kVA laboratory prototype.

An Aggregate Model of Plug-in Electric Vehicles Including Distribution Network Characteristics for Primary Frequency Control

Summary form only given. In the future, the number of plug-in electric vehicles (PEVs) that will participate in the primary frequency control (PFC) is likely to increase. In our previous research, the computational complexity of the PFC problem for a large number of PEVs was reduced using aggregate models of PEVs. However, in the literature on the PFC, the distribution network characteristics have not been included in the aggregate models of PEVs for the PFC, despite the fact that PEVs will be dispersedly connected to the distribution network. This paper proposes an aggregate model of PEVs for the PFC that further incorporates distribution network characteristics, i.e., the distribution network power loss (DNPL) and the maximum allowed current (MAC) of the lines and transformers. The DNPL variation is formulated according to the line and transformer impedance, spatial distribution of PEVs and loads, and active power variation of PEVs. Then, DNPL variation together with the MAC of the lines and transformers are incorporated in the proposed model of PEVs. Finally, the simulation results show an excellent agreement of 98% between the detailed model and the proposed aggregate model of PEVs.

Control system for a UPFC in a transmission line

This paper presents a detailed investigation of a UPFC control system. The dynamic model of the UPFC has been developed using the space-vector representation of the instantaneous three-phase variables using Parks transformation. The Parks transformation and the reference frame selected reduce the control of the real and reactive power flows to the control of the d- and q-axis currents, respectively. The control loops and the algorithms to ensure instantaneous power control decoupling are explained in detail. The controller also maintains constant the voltage at the DC-link of the back to back inverters. The problems derived from the discrete inverter output control are taken into account when using space-vector PWM. A simulator has been developed to illustrate the main results before building a prototype. A continuous-time model has been used for the power system and the control has been implemented in discrete time. The inverters have been simulated using ideal switches with 0.5 kHz switching frequency.

Aggregation of plug-in electric vehicles in distribution networks for primary frequency control

Plug-in electric vehicles (PEVs) have great potential in the near future to be connected in a large number to the power systems. This will lead to influence the overall dynamic behavior of power systems, specifically when PEVs participate in the primary frequency control (PFC). Modeling a large number of PEVs for PFC can be a complex and time consuming task. In order to reduce computational complexity, in [1], we proposed that a large number of PEVs was represented by an aggregate model which was connected to the transmission system. In [1], in the aggregate model of PEVs for the frequency stability analysis, the distribution network has been neglected, although in reality PEVs will be dispersedly connected to the distribution network. This paper proposes an aggregate model of PEVs that incorporates distribution network characteristics, i.e. the power losses. Thus, the proposed aggregate model represents the active power response of PEVs taking into consideration distribution network power losses (DNPL). To achieve this goal, the DNPL increment, which is caused by the participation of PEVs in the PFC, is mathematically formulated with respect to the active power variation of PEVs following a disturbance. Then, the proposed aggregate model of PEVs is obtained according to the PEV fleet behavior and also the calculated power loss variation. In order to compare the detailed model and the proposed aggregate model, the simulations are carried out in Matlab / Simulink. Finally, the simulation results show high level of correspondence between the detailed and the proposed aggregate model of PEVs following a disturbance in the power system.

Control System for a UPFC in a Transmission Line

Abstract This paper focuses on the control of the power flow through a transmission line using a PIM-based UPFC. The dynamic model of the UPFC has been developed using the space-vector representation of the instantaneous three-phase variables. The Pares transformation and the reference frame selected reduce the control of the real and reactive-power flows to the control of the d- and q-axis currents, respectively. The proposed control scheme produces fast and decoupled response of the real- and the reactive-power flow through a transmission line. The system performance has been simulated under normal operation condition and under fault conditions. A 15 k VA prototype with 750 Hz switching frequency has been built to illustrate the main contributions. Experimental results agree with the theoretical analysis and the simulation results. Finally, the bandwidth of the UPFC has been characterised. The theoretical closed-loop transfer function agrees with the experimental one. It is shown that the bandwidth of the UPFC is high enough to damp the main perturbations of a power system. Under fault conditions in a typical scenario, power system oscillations can be damped quickly with the UPFC.

Design of Plug-In Electric Vehicle's Frequency-Droop Controller for Primary Frequency Control and Performance Assessment

This paper describes a novel strategy to design the frequency-droop controller of plug in electric vehicles (PEVs) for primary frequency control (PFC). To be able to properly compare the frequency response of control system with and without PEVs, the design is done to guarantee the same stability margin for both systems in the worst case scenario. To identify the worst case, sensitivity analyses are conducted on a large set of system parameters performing eigenvalue analysis and bode plots. Three main contributions are included in this work: 1) we demonstrate that PEVs using the well-design droop controller significantly improve the PFC response while successfully preserving the frequency stability, 2) since the fast response of PEVs may cause to mask the governor-turbine response in conventional units, a novel control scheme is developed to replace some portion of PEVs reserve after a certain time by the reserve of conventional units during PFC, and 3) a method is proposed to evaluate the positive economic impact of PEVs participation in PFC. For the latter, the system PFC cost savings mainly through the avoidance of under frequency load shedding by PEVs are calculated. A large-scale power system and an islanded network are evaluated and compared through dynamic simulations, which illustrate the validity and effectiveness of the proposed methodologies.

Control of a Shunt Active Power Filter based on a three-leg four-wire electronic converter

This paper presents the analysis and the application of a current controller in an active power filter (APF) based on a PWM voltage-source electronic converter with three legs and four wires. The neutral wire is connected to the middle point of the DC-capacitor voltage. The controller proposed here is an extension of the one proposed for a three-wire Shunt Active Power Filter. The controller is a two-level nested controller. The outer-loop generates the reference current for the inner-loop. The latter, is a state-feedback current controller with integral action. The former consists of (i) a selective harmonic elimination technique and (ii) a DC-capacitor-voltage controller. This paper will focus on the neutral-wire current control and on the balance control of the DC-capacitor voltage. The performance of the control algorithm has been demonstrated using a test-rig with balanced and non-balanced non-linear loads.