Ulrich Boeke

A simple method for parallel operation of inverters

Telecom applications require uninterruptible power supplies for different power levels. If the number of different converter systems should be kept low, parallel operation of inverters is of interest. Thus, a master/slave architecture of an inverter system is presented which allows parallel operation of an arbitrary number of inverter modules by a simple control method. All inverter modules can be equal and no extra overall control unit is needed. The decision, which of the modules becomes master, is made via a single common status line automatically. The output current of the master is used as a command reference current for all slaves. It is shown that the master inverter module is able to stabilise the common output voltage independent of the number of slaves. Furthermore it is investigated how the load current and the reactive filter current is shared by the modules. Apart from results of a simulation, experimental work is presented which confirms the theory.

Energy efficient DC-grids for commercial buildings

Modern power electronics enable the efficient supply of almost all kinds of electric applications in commercial buildings, e. g. lighting, IT- and telecommunication equipment, as well as speed-controlled drives in heating, cooling and ventilation applications. This potential can be used more effectively using a 380 VDC Grid compared to a traditional AC Grid in commercial buildings. The European ENIAC R&D project consortium DC Components and Grid (DCC+G) is developing suitable, highly efficient components and sub systems for 380 VDC grid to show the benefits of DC grid concept on test site in an office environment. The newly developed DC Grid components and their integration into a generic system are presented in this paper. The targeted overall efficiency saving compared to AC grid is 5% and the energy conversion from PV is calculated to be 7% higher compared to traditional PV inverter conversion. This paper also shows the layout of the office demonstrator at Fraunhofer Institute in Erlangen, Germany, and describes general benefits of a DC Grid system.

Transformer-Less Converter Concept for a Grid-Connection of Thin-Film Photovoltaic Modules

A transformer-less converter concept for grid- connected photovoltaic systems is proposed that combines a DC/DC converter front-end with a DC/AC inverter. The converter system has an earth-connected DC input, as required from many thin-film photovoltaic modules. The DC/DC converter increases the positive photovoltaic DC-bus voltage by its negative DC output voltage to supply a grid-connected 3-phase inverter. This architecture extends todays power electronic converter topologies for thin-film photovoltaic modules considering their special requirements with the ambition to realize higher power conversion efficiencies at lower cost.

Energy efficient low-voltage DC-grids for commercial buildings

The European ENIAC R&D project consortium DC Components and Grid (DCC+G) is developing suitable, highly efficient components and sub systems for 380 VDC grid to show the benefits of DC grid concept on test site in an office environment. The newly developed DC grid components and their integration into a generic system are presented in this paper. The targeted overall efficiency saving compared to AC grid is 5% and the energy conversion from PV (photo voltaic) is calculated to be 7% more cost effective compared to traditional PV installations. This paper also shows the realized DC grid prototypesupplying an office building of the Fraunhofer Institute in Erlangen, Germany, and describes general benefits of a DC grid system. The DC grid prototype consists of a DC lighting system, a DC low power supply for IT infrastructure, DC electric vehicle charger, a DC μCHP unit, DC photovoltaic MPPT units, a central rectifier and grid controller unit as well as a mixed AC/DC power monitoring unit. It is shown that less conversion losses and higher distribution efficiency can be achieved with a 380 VDC grid compared to conventional AC grids.

Scalable fluorescent lamp driver using magnetic amplifiers

A linear variable inductor is proposed as means to control the operation of a fluorescent discharge lamp in a load resonant circuit. Thus, multiple lamps can be operated with independent control supplied from a single DC/AC voltage inverter that operates with a constant switching frequency in e.g. backlights of LCD-TVs

DC power grids for buildings

Direct current (DC) power grids are an interesting option for buildings to connect natural DC power sources such as photovoltaic power systems with DC loads like lighting, IT systems as well as speed-controlled electric motors of heating, ventilation and air-conditioning systems. The paper documents learnings, measurements and efficiency differences of a test bed installation with both a 230 V AC and a 380 V DC grid. Both subsystems supply LED luminaires with electricity from utility mains and AC respectively DC grid connected photovoltaic solar power systems. An efficiency advantage of 2 % has been measured with the 2 kW DC grid test bed. 5 % energy savings are described as potential.

Economic viability improvement of solar powered Indian rural banks through DC grids

Power shortages result in power outages for period of 8 to 10 Hrs a day in rural areas due to significant gap between electricity demand and supply. Rural banking is one of the sectors severely affected by power. Majority of population in emerging markets like India live in rural areas. Therefore, reliability and operational efficiency of rural banking is of utmost importance to include rural population into national main stream. To overcome the problem, solar hybrid power system with battery backup is gaining popularity. However, expensive components like solar array, battery in the system hindering its wide adaptation. In this paper, efficient DC electrical distribution is proposed to reduce the sizes of expensive solar and battery components. Economic analysis is performed for a typical core banking rural bank. It is shown that by adopting DC grid, Rs. 1.22 lakh can be saved per rural bank. The saving will be Rs. 1.83 billion, if DC grid is implemented across all the 15,000 branches of rural banks.