Dimitar Bozalakov

Damping-Based Droop Control Strategy Allowing an Increased Penetration of Renewable Energy Resources in Low-Voltage Grids

The increased penetration of renewable distributed energy sources increases the challenges for distribution systems operators to keep the voltage variations within the prescribed limits. One of the voltage problems is caused by single-phase systems leading to voltage unbalance. Another problem is overvoltages caused by local injection of distributed energy sources. An effective solution for this problem is by active power curtailment. In this paper, a control strategy is proposed that mitigates voltage unbalance by using active power and it is equipped with a linear active power curtailment solution. The effect of the proposed control strategy and three state-of-the-art control strategies on the voltage profile and the curtailed power are studied. It can be concluded that the proposed control strategy has a beneficial effect on the voltage profile along a feeder and leads to less power curtailment compared to the state-of-the-art solutions. The proposed control strategy thus allows an increased penetration of distributed energy sources in the low-voltage grid.

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.

Two channel high voltage differential probe for power electronics applications

In this article, a high voltage two channel differential probe has been developed and tested in order to satisfy the requirements of power electronic measurements. The bandwidth of at least 10MHz is verified using a 50MHz sine wave generator. Voltage edges at 5V/ns like in real converters generated from a DC chopper are also applied and the probe is perfectly able to follow such signals. The probe is tested by using high voltage source up to 8kV DC and no coronas and flashovers were observed between the PCB layout and the components and between the components as well.

A New Synchronization Technique of a Three-Phase Grid Tied Inverter for Photovoltaic Applications

Three-phase grid synchronization is one of the main techniques of the three-phase grid connected power inverters used in photovoltaic systems. This technique was used to reach the fast and accurate three-phase grid tied inverter synchronization. In this paper a new synchronization method is presented on the basis of integrating the grid voltage two times (line-to-line or phase voltage). This method can be called “double integral synchronization method” (DISM) as it integrates the grid voltage signals two times to generate the reference signals of three-phase photovoltaic inverter currents. DISM is designed and simulated in this paper to operate in both analog and digital circuits of three-phase photovoltaic inverter system with the same topology. The digital circuit design and dsPIC33FJ256GP710A as a microcontroller (the dsPIC33FJ256GP710A with the Explorer 16 Development Board from microchip) was used practically in this paper to generate and control the sine pulse width modulation (SPWM) technique according to DISM for three-phase photovoltaic inverter system. The main advantage for this method (DISM) is learning how to eliminate the integration constant to generate the reference signals without needing any reference signals or truth table, just the line-to-line or phase voltage of grid.

New Pulse Width Modulation Technique to Reduce Losses for Three-Phase Photovoltaic Inverters

Nowadays, most three-phase, “off the shelf” inverters use electrolytic capacitors at the DC bus to provide short term energy storage. However, this has a direct impact on inverter lifetime and the total cost of the photovoltaic system. This article proposes a novel control strategy called a 120° bus clamped PWM (120BCM). The 120BCM modulates the DC bus and uses a smaller DC bus capacitor value, which is typical for film capacitors. Hence, the inverter lifetime can be increased up to the operational lifetime of the photovoltaic panels. Thus, the total cost of ownership of the PV system will decrease significantly. Furthermore, the proposed 120BCM control strategy modulates only one phase current at a time by using only one leg to perform the modulation. As a result, switching losses are significantly reduced. The full system setup is designed and presented in this paper with some practical results.

Long term assessment of a voltage unbalance mitigation scheme implemented by photovoltaic inverters

This paper applies a long term network observability analysis for investigating the potential contribution of photovoltaic (PV) inverters to the mitigation of voltage unbalance in low voltage (LV) feeders. For this purpose, a probabilistic offline state estimation algorithm is used, which simulates the time-varying action of local voltage (magnitude and unbalance) control schemes. The paper focuses on a control scheme that acts resistively towards the negative- and zero-sequence voltage components without modifying the total nodal injected power (three phase damping control scheme) For this long term evaluation, feeder- and user-specific smart metering (SM) data are used. The volatile character of PV generation and loads is modelled on a 15-min time scale. As a case study, a real LV feeder with distributed PV generation and long-term user-specific SM measurements is simulated. The three phase damping control results to be more advantageous compared with currently applied voltage control schemes.