Charis S. Demoulias

Modeling and Field Measurements of Photovoltaic Units Connected to LV Grid. Study of Penetration Scenarios

The operation of photovoltaic (PV) units connected to the grid is characterized by several uncertainties due to the number of currently operating units, the points where these units are sited, the exported power, and the injection of harmonic currents. The objective of this paper is to investigate the impact of the penetration of PV units in the low-voltage (LV) network. Thus, a model has been developed for the computation of load flows, harmonics, and voltages in the feeders. A PV panel is modeled as an irradiance-driven current generator, with an embodied maximum power point (MPP) tracking algorithm; it is connected to the network via electronic switches that represent the PV inverter. Simulations are performed with PSIM and Harmoniques simulator packages. Results from the simulated model are validated with data acquired from field measurements and bibliography. Finally, the acceptable penetration level of PV systems is investigated for several scenarios, depending on the topology of the LV network, the number, size, and locations of loads and PV units.

Harmonic impact of small photovoltaic systems connected to the LV distribution network

Since the penetration of photovoltaic (PV) systems in the low voltage (LV) distribution network is increasing, the need to register and model the contribution of these systems to the harmonic distortion of current and voltage waveforms is becoming an up-to-date issue. As PV systems incorporate power conditioning units, which are harmonic generating devices, they will have an influence on quality of supply, reliable operation of system equipment as well as component life expectancy. This paper investigates the harmonic impact of a 20 kWp PV system connected to the LV distribution network in Greece. The harmonic behavior of the PV plant as a function of the solar radiation under several weather conditions is analyzed. Measurements results are compared to those obtained from the power simulator package PSIMcopy. The level of penetration of PV systems in the LV distribution network without harmonic limits been exceeded is investigated.

Ampacity of Low-Voltage Power Cables Under Nonsinusoidal Currents

This paper investigates the ac resistance in the presence of harmonics and proposes ampacity derating factors for cables made according to CENELEC Standard HD603. These cables are widely used in low-voltage industrial and building installations. Four-conductor cables of small, medium, and large conductor cross sections are considered. The fourth conductor is used as the neutral conductor. The cables are modeled using finite-element analysis software. The ac/dc resistance ratio is shown to increase with the frequency of the current and the cross section of the conductor, the increase being much larger when zero-sequence harmonics are present. A derating factor is defined and calculated for five typical nonsinusoidal current loads, for example, computer equipment. The derating of the cables ampacity is shown to be very large when zero-sequence harmonics are present. The cross section of the neutral conductor is shown to be significant only when zero-sequence harmonics are present. The validity of the method is verified by comparison with data given in IEEE Standard 519-1992 and with measurements conducted on a cable feeding a large nonlinear load

A Fault Clearing Method in Converter-Dominated Microgrids With Conventional Protection Means

The integration of converter-interfaced distributed generation in microgrids has raised several technical issues, including the successful operation of protective devices during faults. The protection issue is associated with the lack of large current injection during a fault, due to limits imposed by the semiconductor switches. This paper proposes a fault-detection and clearing control strategy method for symmetrical and asymmetrical line faults in a looped microgrid. The protection devices are simple overcurrent devices with the same settings, due to the looped microgrid topology. The proposed method is applied without using any kind of physical communication. The fault is detected by measuring indirectly the microgrid impedance. After the fault identification, the distributed energy resources (DERs) adjust their control in order to inject a current proportional to the measured microgrid impedance, according to a droop curve. This means that the DER closer to the fault injects a relatively larger current, achieving by this way a selective coordination of the protection means. The effectiveness of the proposed control strategy is evaluated through detailed simulation and experimental tests.

Microgrid wireless energy management with energy storage system

The penetration of renewable energy sources in small-scale power production gives the opportunity parts of the grid to work as microgrids. The microgrid should be able to work both in grid-connected and island mode, while its voltage and frequency deviations follow the EN 50160 standard. The use of energy storage system is generally recommended in order to absorb the mismatches between the demand and the generation side and to preserve the quality of the microgrid voltage. While the up to day research is mainly concentrated on energy management based on communication, this paper proposes a wireless method for keeping the voltage and the frequency within the limits, using a battery as an energy storage system (ESS). An analytical expression for calculating the battery capacity is also proposed. The active and reactive power sharing among the parallel resources is achieved using the droop control method and an algorithm proportional to droop characteristic and the rated apparent power of each resource. According to the values of frequency and voltage and the State of Charge (SoC), the battery is connected in the microgrid, working in charging or discharging mode. A microgrid consisting of two inverter-interfaced power resources, a battery and a constant power load is investigated. Simulation results demonstrate that the proposed wireless control method provides the load with a high quality voltage in both grid-connected and islanded mode under several load scenarios.

Voltage regulation in low-voltage rural feeders with distributed PV systems

The increasing penetration of photovoltaic (PV) systems in low-voltage rural feeders causes voltage rise and increased harmonic pollution. It is shown that under certain conditions violation of the requirements of the EN 50160 standard can occur. A control method that determines the required reactive power of each PV inverter so that the voltage profile is within the standards requirements is shown. No need for communication between the inverters is required. For this purpose, a detailed inverter simulation model is developed which is validated against measured data from existing PV inverters. The control method presented here requires oversizing of the inverters at the beginning and end of the feeder.

Fault ride-through capability of a microgrid with wtgs and supercapacitor storage during balanced and unbalanced utility voltage sags

The concept of microgrids is recently attracting considerable interest. However, in order to widely integrate microgrids within the distribution networks, a shift in the philosophy of interconnecting them with the utility grid seems necessary. A grid-connected microgrid is required to possess Faults Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. In this paper, a control strategy for improving the ability of an inverter-based microgrid to ride though symmetrical and asymmetrical grid faults is proposed. The microgrid is formed of several Distributed Energy Resources (DERs), which utilize Wind Turbine Generators (WTGs) as primary renewable energy source, each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, aim of the proposed control strategy is to keep the microgrid connected to the grid, according to the FRT requirements, while maintaining an acceptable voltage profile within the common ac bus. Each DER is controlled to support the voltage within the microgrid by injecting reactive power, without any physical communication. During unbalanced utility voltage conditions, the DERs operate collectively in order to compensate the undesirable negative and zero sequence voltage components. Thus, a set of balanced three-phase voltages is provided within the common ac bus. Simulation results demonstrate that the microgrid can ride through heavily balanced and unbalanced utility voltage sags, while supplying its loads with a high quality voltage profile.

Modeling and measurement of small Photovoltaic systems and penetration scenarios

The operation of Photovoltaic (PV) units connected to the system is characterized by several uncertainties due to the number of running units, the points where these units are sited, the exported power and the injection of harmonic currents. The objective of this paper is to investigate the penetration level of PV units in the low voltage (LV) network. Starting from field measurements a model has been created for the computation of harmonics and voltages in the grid. The PV unit is modeled as a dc voltage source connected to the network via electronic switches, representing the PV inverter. Simulations were performed in the simulator packages PSIM© and Harmoniques©. Results from the simulated model agree with the results obtained by the measurement campaign. Following this the acceptable penetration level of PV systems in the LV distribution network is investigated for several scenarios, referring the number, the locations and the operation of the units.

A simulation tool for extended distribution grids with controlled distributed generation

In this paper, a new software tool is presented for the simulation of electrical networks under steady-state conditions. Its distinct advantage is the robust integration of distributed generation droop controls, while offering the ability to simulate extended networks fast and reliably. The proposed simulation tool is based on the combination of two well-known software products, namely MATLAB and OpenDSS. The latter is employed as an unbalanced power flow solver, whereas the former implements the droop control of DG units. Simulation results for a simple and extended low-voltage network show the effectiveness of the proposed tool and mainly the reduction in the execution times over other conventional time-domain-based software products.

Analytical Calculation of the Electrical Energy Losses on Fixed-Mounted PV Plants

This paper presents analytical expressions for the annual energy losses on dc and ac cables, the annual energy losses on the step-up transformer, and the optimum transformer size for fixed-mounted photovoltaic (PV) plants of any size. These expressions are based on some unknown parameters, which can be easily estimated from data provided by the equipment (inverters or transformer) manufacturer and from available data in any meteorological database. The analytical expressions are meant to be used by the design engineers for comparing-from the energetic point of view-different cable cross sections and transformer sizes with various power loss classes or cooling medium, without having to perform multiple detailed simulations, as the current practice is. The validity of the derived expressions was tested against measurements taken in two existing 1003.52- and 491.4 kWp PV installations during the past 2 years. The proper use of the derived expressions during the design phase of a PV system is demonstrated through a detailed working example.