Thomas Bruckner

Optimal pulse-width modulation for three-level inverters

The three-level neutral-point-clamped voltage source inverter is widely used in high power, medium voltage applications. This paper studies continuous and discontinuous pulse-width modulation for this inverter. Detailed analysis of discontinuous modulation shows that the average switching frequency is not directly proportional to the carrier or sampling frequency, since additional switching transitions occur between different regions of discontinuity. At typical switching frequencies for high power applications (up to 2 kHz) these additional transitions contribute significantly to the inverters total losses, so that a proper comparison of the harmonic performance can only be carried out under constant loss conditions with varying carrier frequency. This comparison is performed for a typical industrial medium voltage inverter. The paper then considers the major issues of neutral-point voltage balancing and loss distribution within the inverter, for the identified optimal modulation schemes.

The Tolerable Windows Approach: Theoretical and Methodological Foundations

The tolerable windows (TW) approach is presented as a novel scheme for integrated assessment of climate change. The TW approach is based on the specification of a set of guardrails for climate evolution which refer to various climate-related attributes. These constraints, which define what we call tolerable windows, can be purely systemic in nature – like critical thresholds for the North Atlantic Deep Water formation – or of a normative type – like minimum standards for per-capita food production worldwide. Starting from this catalogue of knock-out criteria and using appropriate modeling techniques, those policy strategies which are compatible with all the constraints specified are sought to be identified. In addition to the discussion of the basic elements and the general theory of the TW approach, a modeling exercise is carried out, based on simple models and assumptions adopted from the German Advisory Council on Global Change (WBGU). The analysis shows that if the global mean temperature is restricted to 2°C beyond the preindustrial level, the cumulative emissions of CO2 are asymptotically limited to about 1550 Gt C. Yet the temporal distribution of these emissions is also determined by the climate and socio-economic constraints: using, for example, a maximal tolerable rate of temperature change of 0.2°C/dec and a smoothly varying emissions profile, we obtain the maximal cumulative emissions, amounting to 370 Gt C in 2050 and 585 Gt C in 2100.

Optimization of solar district heating systems: seasonal storage, heat pumps, and cogeneration☆

The dynamic energy, emission, and cost optimization model deeco is further developed and applied to the analysis of solar district heating systems with seasonal storage in a pilot project of the Bavarian Research Foundation. The optimum integration of condensing boilers, compression and absorption heat pumps, and cogeneration of heat and power is computed for 100 well insulated housing units with an annual total heat demand of 616 MWh. Collector areas between 1 and 2.5 m2 per MWh heat demand and water storage volumes between 1.2 and 4.2 m3 per m2 collector area satisfy between 32 and 95 per cent of the total heat demand by solar thermal heat. Compared with a reference case with individual condensing boilers and electricity taken from the public grid, selected scenarios achieve (non-renewable primary) energy savings between 15 and 35% associated with cost increases between −20% and 140%; cogeneration turns out to be quite attractive from an economical point of view. With cogeneration and a solar contribution to the heat supply of 80%, emission reductions of CO2-equivalents by 33%, SO2 by 20%, and NOx by 22% can be achieved at cost increases of 120%. Fossil fuel savings of more than 40% are possible if electricity is produced from non-fossil energy sources.

How to Select the System Voltage of MV Drives—A Comparison of Semiconductor Expenses

The application of a particular medium-voltage converter in a certain application depends on numerous criteria. However, in completely new installations, the choice of the system voltage is very often uninhibited of external constraints. Then, the voltage level shall be selected to enable the most efficient power conversion at the lowest cost. This paper is dedicated to help in finding the best voltage level for three-level neutral-point-clamped voltage source converters (3L-NPC VSCs) with respect to the power semiconductor devices. Three insulated-gate-bipolar-transistor-based 3L-NPC VSCs of different voltage levels (2.3, 3.3, and 4.16 kV) are investigated and compared regarding their maximum output power, semiconductor efficiency, and semiconductor cost per MVA output power. The effects of thermal cycling, the loss distribution within the converter, and switching frequencies from 300 to 1050 Hz are considered in the evaluation.

Climate change decision-support and the tolerable windows approach

The Tolerable Windows Approach (TWA) to Integrated Assessments (IA) of global warming is based on external normative specifications of tolerable sets of climate impacts as well as proposed emission quotas and policy instruments for implementation. In a subsequent step, the complete set of admissible climate protection strategies which are compatible with these normative inputs is determined by scientific analysis. In doing so, minimum requirements concerning global and national greenhouse gas emission paths can be determined. In this paper we present the basic methodological elements of TWA, discuss its relation to more conventional approaches to IA like cost–benefit analyses, and present some preliminary results obtained by a reduced-form climate model.

INTEGRATED ASSESSMENT OF LONG-TERM CLIMATE POLICIES: PART 1 - MODEL PRESENTATION

An integrated assessment model (IAM) conceived in the vein ofthe inverse approach is introduced. The model is designed tohelp social actors in making informed judgments about climatechange impact targets, mitigation costs, and implementation mechanisms.Based on these normative decisions, the model verifies whetherthere exist long-term future emission paths that satisfy theuser-defined constraints. If they do, the model determines anemission corridor containing all permissible emission trajectories.An overview of the IAM is provided and short descriptions ofthe model components are presented. Forward and inverse modesof application are explained. Examples based on impacts of climatechange on aggregated potential crop production in Western Europeand South Asia illustrate how the model can be applied in differentmodes. The examples demonstrate how the inverse approach separatessocial judgments shaping climate policy from the model-basedanalysis of their implications. The examples also show the differencein climate change tolerance between developed regions in temperatezones and less developed regions in already warm climate zones.

Methodological aspects of the tolerable windows approach

The tolerable windows approach (TWA) allows the climate policyformulation process to be safeguarded in the following way. First,guardrails are defined in order to exclude intolerable climatechange impacts, on the one hand, and unacceptable socioeconomicconsequences of climate change mitigation measures, on the other.Second, a scientific analysis is conducted to investigate thefeatures of those emission paths that are compatible with theguardrail constraints. The fundamental methodology of the TWA isbest described in terms of the theory of differentialinclusions. This emerging mathematical theory already providesnumerical methods applicable as long as the underlying integratedassessment models are of limited complexity. In order to identifyemissions corridors, we propose a novel calculation schemeapplicable also for large-scale integrated assessment models.

SENSITIVITY ANALYSIS OF EMISSIONS CORRIDORS FOR THE 21ST CENTURY

We investigate the sensitivity of emissions corridors for the 21st century to various factors that are currently under debate in the climate change arena. Emissions corridors represent the range of admissible emissions futures that observe some predefined guardrails on the future development of the human-climate system. They are calculated on the conceptual and methodological basis of the tolerable windows approach. We assess the sensitivity of the corridors to the choice of time-resolved as well as intertemporally aggregated guardrails that exclude an intolerable amount of climate change on the one hand and unbearable mitigation burdens on the other. In addition, we investigate the influence of climate sensitivity on the corridors.Results show a large dependence of emissions corridors on the choice of guardrails and the value of climate sensitivity T2CO2. If the guardrail on climate change is specified in terms of a maximum admissible global mean temperature increase Tmax to be observed at any time, the size of the corridors is predominantly determined by a climate impact resilience parameter κ=Tmax/T2CO2. As κ is varied from values below 0.5 to values above 1.5, we move from cases where no emissions profile whatsoever can observe all guardrails, to cases where no significant emissions reduction seems necessary given the range of emissions scenarios for the 21st century. The limits on admissible mitigation efforts influence predominantly the timing and the economic viability of emissions reductions. A large mitigation flexibility allows for ‘wait then run’ emissions paths, while low flexibility asks for a significantly more prudent approach.

Climate system modeling in the framework of the tolerable windows approach: The ICLIPS climate model

The computational burden associated with applications of theTolerable Windows Approach (TWA) considerably exceeds that oftraditional integrated assessments of global climate change. Aspart of the ICLIPS (Integrated Assessment of Climate ProtectionStrategies) project, a computationally efficient climate model hasbeen developed that can be included in integrated assessmentmodels of any kind. The ICLIPS climate model (ICM) is implementedin GAMS. It is driven by anthropogenic emissions of CO2,CH4, N2O, halocarbons, SF6, andSO2. Theoutput includes transient patterns of near-surface airtemperature, total column-integrated cloud cover fraction,precipitation, humidity, and global mean sea-level rise. Thecarbon cycle module explicitly treats the nonlinear sea watercarbon chemistry and the nonlinear CO2 fertilized biosphereuptake. Patterns of the impact-relevant climate variables arederived form empirical orthogonal function (EOF) analysis andscaled by the principal component of temperature change. Theevolution of the latter is derived from a box-model-typedifferential analogue to its impulse response function convolutionintegral. We present a description of the ICM components and someresults to demonstrate the models applicability in the TWA setting.

The active NPC converter for medium-voltage applications

The three-level neutral point clamped voltage source converter (NPC VSC) is widely used in high-power, medium-voltage applications. To overcome its major drawback-the unequal loss distribution among the semiconductors-the use of active NPC switches (ANPC) was proposed previously. In this paper, a simple feedforward loss-control scheme for the ANPC converter is presented. The principle of operation of the ANPC VSC is verified experimentally for the first time. Electrical measurements on a low-voltage test bench prove the proper function of the ANPC converter. Infrared junction temperature measurements confirm the improved loss distribution. Finally, the authors demonstrate the superior performance of the ANPC converter in various medium-voltage applications.