Integration of sustainable energy sources through power electronic converters in small distributed electricity generation systems

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Integration of sustainable energy sources through power

electronic converters in small distributed electricity generation

systems

Citation for published version (APA):

Tao, H. (2008). Integration of sustainable energy sources through power electronic converters in small

distributed electricity generation systems. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR632347

DOI:

10.6100/IR632347

Document status and date: Published: 01/01/2008

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Integration of sustainable energy sources

through power electronic converters

in small distributed electricity

generation systems

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magniﬁcus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 21 januari 2008 om 16.00 uur

door

Haimin Tao

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prof.dr.ir. A.J.A. Vandenput en

prof.dr. X. He

Copromotor: ir. M.A.M. Hendrix

This work was supported by the Dutch funding agency for university research, Technologiestichting STW

Copyright c2008 H. Tao

Printed by Eindhoven University Press, The Netherlands Cover design by Haimin Tao

CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Tao, Haimin

Integration of sustainable energy sources through power electronic converters in small distributed electricity generation systems / by Haimin Tao. - Eindhoven : Technische Universiteit Eindhoven, 2008.

Proefschrift. - ISBN 978-90-386-1734-3 NUR 959

Trefw.: statische omzetters / vermogenselektronica / elektrische energie ; opwekking / invertoren.

Subject headings: power convertors / power electronics / distributed power generation / invertors.

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Integration of sustainable energy sources

through power electronic converters

in small distributed electricity

generation systems

by

Haimin Tao

Members of the doctoral defense committee: prof.dr.ir. A.C.P.M. Backx (Chairman)

prof.dr.ir. A.J.A. Vandenput (Eindhoven University of Technology, 1st_supervisor)

prof.dr. X. He (Zhejiang University, China, 2nd supervisor)

ir. M.A.M. Hendrix (Eindhoven University of Technology, Co-supervisor) prof.dr.ir. R.W. De Doncker (RWTH Aachen, Germany)

prof.dr.ir. A. Van den Bossche (Ghent University, Belgium) prof.dr.ir. J.H. Blom (Eindhoven University of Technology) dr. J.L. Duarte (Eindhoven University of Technology, Advisor)

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Summary

Integration of sustainable energy sources

through power electronic converters

in small distributed electricity

generation systems

This thesis aims to investigate how sustainable electricity generators such as fuel cells and photovoltaics and appropriate storage elements like batteries and supercapacitors are best integrated in energy systems suitable for domestic appli-cation. Research topics in this context include bidirectional and multiport dc-dc converter topologies, modeling and control of power converters, means for storing energy, system power ﬂow management, public utility interconnection system, and power quality control.

For integrating primary sources and energy storage, a multiport system struc-ture is proposed. Compared with the conventional strucstruc-ture that uses multiple converters, a multiport converter promises integrated power conversion by utiliz-ing only a sutiliz-ingle power processutiliz-ing stage.

An extensive topology study resulted in a family of multiport bidirectional dc-dc converters based on several basic bidirectional switching cells and a general topology that combines a dc-link with magnetic-coupling. A multiport bidirec-tional converter can be constructed from the proposed basic bidirecbidirec-tional switch-ing cells. The presented converter concept provides a method to integrate power sources with widely diﬀering characteristics. Furthermore, based on the interleav-ing technology, solutions for high-power applications are provided. The proposed basic bidirectional switching cells are extended to polyphase interleaved versions. The implementation has been focused on three-port energy management sys-tems. Three converter topologies were implemented, namely, the three-port triple-active-bridge (TAB) converter, a two-input bidirectional converter that combines a dc-link with magnetic-coupling, and the triple-half-bridge (THB) converter, all taking a fuel cell and supercapacitor generation/storage system as an example. The three-port system is modeled using an averaged circuit model, and control strategies based on a multiple-feedback-loop scheme were developed, aiming at

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tight regulation of the load voltage and prevention of load transients from aﬀect-ing the operation of the primary source.

In order to accommodate speciﬁc operating characteristics of the sources and storage elements (for instance, wide operating voltages), several improvements for the proposed converter topologies were made, aiming at soft-switching, reduced current stress, and higher eﬃciency. The developed control methods include duty ratio (volt-seconds balance) control for the three-port TAB converter, variable hysteresis band control, and asymmetrical wave control.

The three converter topologies were verified with laboratory prototypes. The performance of the converters was investigated for a closed-loop control imple-mented with different digital signal processors (DSPs). The power flow in the system is proved to be controllable. A substantial improvement in the efficiency when using the soft-switching control method is observed. Practical issues like soft start-up and generation of high-resolution digital phase shift were discussed.

The second part of the work is the PWM inverter control and grid interconnec-tion of small energy generainterconnec-tion systems, taking power quality control into account. Small distributed generation (DG) systems provide standby service during grid outages and, when operated during peak load hours, potentially reduce energy costs. A high-performance PLL for a single-phase inverter is realized by means of a transport delay which generates a virtual quadrature signal and an orthogonal ﬁlter is used to enhance the PLL performance when the grid voltage is distorted. To achieve zero steady-state error for both the voltage and current regulations, and to implement selective harmonic compensation, resonant controllers are used. For controlling single-phase inverters, proportional resonant (PR) controllers can eliminate the steady-state error and are more stable than a proportional-integral (PI) controller.

At the system level, a line-interactive fuel cell UPS/DG system was proposed, designed, and tested. The power processing unit comprises a TAB converter and a grid-interfacing inverter. The system can flexibly operate in stand-alone or grid-connected mode. An automatic and smooth transition between the two oper-ating modes can be achieved by using a static transfer switch and ramping up the reference signal in a few consecutive grid cycles during the transition. A genera-tion system can simultaneously be operated as an active filter to deal with local harmonic-producing loads. The active filtering function is integrated into the sys-tem and realized solely by the control software. It is shown that a supercapacitor in the system compensates for the instantaneous power fluctuations, overcomes the slow dynamics of the fuel cell, and handles the periodical low-frequency ripple in the power drawn by the inverter. This advantage eliminates otherwise needed energy buffers in the rest of the system as long as a sufficient control bandwidth of the TAB converter is guaranteed.

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Introduction

The global average air temperature at the surface of the Earth has increased by about 0.74o_{C over the past century, and is likely to continue rising [1]. Human}

activities such as burning fossil fuels cause emission of the greenhouse gases (mainly carbon dioxide) that contribute to global warming. Electricity generation is one of the major contributors to environmental problems. Thus, development of clean energy sources becomes increasingly important to the global environment.

Furthermore, we human beings are challenged by the depletion of fossil fuel reserves. Green energy sources that allow for sustainable development are therefore becoming more interesting. Our present standard of living can only be maintained by tapping sustainable sources of energy such as solar power, wind power, hydro power, wave power, geothermal power, tidal power, biomass, and others. The way energy is generated and supplied will undergo a fundamental change.

As most sustainable energy is harvested as electricity, innovations in electric power conversion technology are crucial for the economic feasibility of the use of sustainable energy. This work investigates how sustainable electricity generators such as fuel cells and photovoltaics and appropriate storage elements like batteries and supercapacitors (also named ultracapacitors) are best integrated in energy systems suitable for domestic application. Power electronic converters provide the electrical interface between the sources, storage, and loads, and the availability of reliable and low-cost converters will accelerate the deployment of sustainable energy systems. From a power electronic point of view, fundamental research topics in the above context are

• novel converter topologies, • converter control and modeling, • means for energy storage, • system power ﬂow management, • power quality control,

• public utility interconnection system, • generator control and protection, etc. 1

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1.1 Alternative power generation systems

Alternative generation systems that utilize renewable energy sources are gaining popularity due to their high operation efficiencies and low CO₂emission levels. To-day, in the fields of electric power systems and power electronics a lot of research effort is being put into the development of alternative electricity generation sys-tems. We are turning to clean and safe sustainable energy sources such as wind, photovoltaic, and fuel cells that, as believed, will contribute to a secure energy future. Introduced in the following are some widely developed clean generation systems, with a special attention paid to small scale ones.

1.1.1 Solar energy

Solar energy, regarded as being inexhaustible in a time frame relevant to the human race, is a truly renewable energy source. Sunlight can be directly converted into electricity by a photovoltaic (PV) system, which uses the photovoltaic eﬀect of semiconductors. The output power of PV systems ranges from a few watts for portable applications such as calculators, to megawatt power stations. Solar arrays have been used to power satellites and spacecraft, and in remote areas as a source of energy for applications such as roadside emergency telephones, remote sensing, and oﬀ-grid home power [2].

Except high initial installation cost, which is expected to decline considerably in the coming years, PV systems are very promising in the alternative energy generation market, particularly for powering private buildings.

The easiest way to use electricity generated by PV systems is to connect the solar panels to the utility grid through an inverter. PV arrays produce power only when illuminated. The production of electricity from solar sources depends on the amount of light energy. For stand-alone or grid-interactive PV systems, a large energy storage mechanism, commonly being batteries, is often used to store the captured electrical energy so that the energy can be made available for use when the sun is not shining. Furthermore, as a backup source of power the storage can also level transient power in the system and provide for excessive load power demand peaks.

1.1.2 Wind energy

The amount of wind energy around the world is enormous. Wind energy is clean, renewable, and widely distributed. In small scale individual turbines wind power is used to supply electricity to rural residences or locations where the power grid is not accessible. In large scale wind farms it is used to generate energy for electric grids.

To convert the kinetic wind energy into electricity, wind turbines are used. These range from a few hundred watt generators for residential use to several megawatt machines for wind farms and oﬀshore. In fact, in remote areas small wind turbines in combination with battery storage have been used for household electricity generation over many decades [3]. Furthermore, in the areas where the

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1.1. Alternative power generation systems 3

power grid is accessible, rooftop mounted small scale wind turbines can generate power for household use to alleviate power distribution problems and can provide emergency power when utility power fails.

However, like solar energy, the availability of wind energy is uncertain, heavily relying on the weather. Furthermore, the output power of a wind turbine is not easily controllable because of the large inertia of the wind turbine blades. It therefore needs to be backed by storage, when stand-alone operation is desired. To compensate for the varying power output, grid-connected wind turbines may utilize the power grid as a virtual energy buﬀer.

1.1.3 Micro combined heat and power

Cogeneration is already well established in industry, but for small scale private use, micro combined heat and power (microCHP) systems are still in development. CHP systems provide a source of heat by utilizing the waste heat of the electrical generating process, thus promising a high utilization of the primary energy source. MicroCHP systems can replace existing hot water boilers and operate in single homes, apartment complexes, or small commercial buildings, providing both heat and power.

Most microCHP systems use natural gas for fuel because it is the cleanest fossil fuel, is widely available, and easily transported through pipelines. In the future biomass and hydrogen based fuels may be considered. Currently, microCHP systems are based on several diﬀerent technologies including internal combustion engines, Stirling engines, steam engines, microturbines and fuel cells [4].

Unlike industrial CHP systems, microCHP systems are usually driven by heat-demand, delivering electricity as a byproduct. Due to the ﬂuctuating electric power demand of the facilities, some kind of electrical energy storage mechanism can therefore improve the system’s performance and overall energy eﬃciency. When the system is expected to deliver backup power during utility outage, storage becomes necessary.

In the Netherlands, microCHP projects are being ﬁeld-tested and demonstrated in several cities [5]. As a transition technology toward a truly sustainable energy system, microCHPs promise higher eﬃciency and lower CO₂ emission than con-ventional coal-burning power plants.

1.1.4 Fuel cell generator

Fuel cells are electrochemical energy conversion devices that convert hydrogen-rich fuel and oxygen into water, generating electricity and heat. As an environmentally friendly energy conversion technology, fuel cells have the potential to revolutionize power generation. Fuel cells have many advantages. By direct energy conver-sion, fuel cells enable higher eﬃciency [6], thereby making them a promising clean power solution for applications as small as cell phones to as large as utility power generation.

A typical fuel cell system consists of a fuel processor, fuel cell stack, and power electronic interface. Usually, a fuel cell produces a dc voltage from hydrogen-rich

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fuel gas and air that ﬂow over two cell electrodes. The principal by-products are water, carbon dioxide and heat1. Among various kinds of fuel cells, polymer electrolyte membrane (PEM) (also named proton exchange membrane) fuel cells provide a high output power density at room temperature, and relative ease of start-up and shut down [6].

The major problem associated with fuel cell applications is that fuel cells have a long time constant because of the slow fuel supply regulation and hydration control. Thus, eﬀective implementation of a fuel cell system requires energy storage. It should also be noted that a fuel cell is a weak power source and its operating output dc voltage is widely variable depending on the fuel ﬂow rate and the power it supplies [6].

As the demand for various applications such as remote power, backup sys-tems, and distributed generation increases, fuel cell systems are anticipated to be widespread. So far, the use of fuel cells in residential power generation has been limited by cost considerations, but prices are decreasing. The use of fuel cells for electricity and heat generation for home applications is generating interest [7].

In summary, in terms of power availability and system dynamics, the aforemen-tioned generation systems have in common that the generated power is not ideal for immediate use. The primary power needs to be conditioned. In most cases, an incorporated storage mechanism would increase the system performance or realize added functionality, for example, the system can operate in both stand-alone and grid-connected modes.

In this study, the main focus is on fuel cell systems. Medium-/low-power fuel cell systems ﬁnd application mainly in electric vehicles and residential power generation. For both applications power electronics is a key element that interfaces the primary source and the storage to the rest of the system. The design of the power conditioning system, which concerns the choice of a suitable converter topology and control strategy, is a challenging task. The energy management in a storage-backed fuel cell system should consider the optimum energy usage control, start-up control, load transient control, and charging and discharging controls for the storage. The design of the power conditioning system should take the following items into account:

• maximum utilization of the primary source; • easy power ﬂow management;

• simplest possible converter topology; • system voltage ratio requirements; • isolation requirements;

• energy storage requirements;

• easy implementation and low cost, etc.

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1.2. System structure 5 Isolated Isolation? Nonisolated Transformer frequency? HF transformer LF transformer (e.g., 50Hz) Battery voltage rating? HV ac-link HV dc-link Fuel cell voltage rating? HV fuel cell HV battery LV battery Cycloconverter Battery voltage rating? In parallel with fuel cell Via dc-dc converter LV: low-voltage HV: high-voltage LF: low-frequency HF: high-frequency LV fuel cell HV battery In parallel with HV dc-bus LV battery Battery position? Via dc-dc converter On the main

power flow path In parallel with

fuel cell Isolation?

Nonisolated Isolated

Figure 1.1: Various system structures for a fuel cell and battery generation system.

1.2 System structure

Power electronics for generation systems usually contains a dc-dc and a dc-ac stage. In the following discussion only the dc-dc stage is considered. Basically, two system structures have been reported by researchers, namely the conventional structure based on separate converter stages and the multiport structure based on a single power conversion stage.

1.2.1 Conventional structure

The conventional structure of a fuel cell system reported in the open literature is illustrated roughly in Fig. 1.1 [8]. The main considerations are the isolation requirements and the voltage rating of the fuel cell and the storage. Because of the diversity of fuel cells and storage devices it is not possible to choose just one topology considered as the best.

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Storage Fuel cell Storage Fuel cell dc-dc (a) (b) Fuel cell (c) Storage Bidirectional dc-dc converter Inverter dc-dc Inverter Inverter

Figure 1.2: Diﬀerent battery positions in a fuel cell system, showing (a) in parallel with

the fuel cell, (b) on the main power ﬂow path, and (c) connected to the dc bus through a bidirectional dc-dc converter.

position of the storage (e.g., batteries). As illustrated in Fig. 1.2(a), batteries may be connected in parallel with the fuel cell. With this configuration, the fuel cell is effectively a battery charger. The fuel cell current, however, is not controlled directly. The mismatch between fuel cell and battery impedance also presents a problem [8]. As shown in Fig. 1.2(b), batteries can also be on the main power flow path to define a bus voltage, but high-voltage batteries are not a good choice because of their cost and reliability. A dc-dc converter (e.g., boost converter) can be placed between the fuel cell and the battery. The converter controls the current taken from the fuel cell. In the scheme shown in Fig. 1.2(c), batteries are placed outside the main power flow path and connected to the dc bus through a bidirectional dc-dc converter. The converter acts as an active filter to improve the dynamic response and to level the power difference between the generator and the load [9]. An advantage of this configuration is that it is possible to choose an optimal battery voltage. Note that in some applications the dc-dc converter that connects the fuel cell is not used (see e.g., Fig. 1.3).

Traditionally, individual converters are used to provide interfaces for power inputs of the system. In principle, any basic power electronics topology can be used to design a power converter for a fuel cell system.

For diverse applications, diﬀerent system conﬁgurations were reported. Fig. 1.3 illustrates a typical structure for electric vehicles, where a high-voltage fuel cell is directly connected to the high-voltage bus (around 300 V) [10]. A bidirectional dc-dc converter is used, providing an interface between the low-voltage batteries (normally 12 V) and the high-voltage bus. The batteries are charged or discharged during the transients (e.g., acceleration and regenerative braking) through the bidirectional converter. The batteries also provide energy for a cold start.

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1.2. System structure 7 Fuel cell Batteries Inverter Inverter Fuel cell compressor motor Traction motor drive High-voltage bus (300 V) Low-voltage battery (12 V) Bidirectional dc-dc converter

Figure 1.3: System structure of a fuel cell vehicle [10].

Source 1 Multiport bidirectional dc-dc converter Source 2 Storage 1 Regulated dc outputs Storage 2

Figure 1.4: Multiport system structure.

1.2.2 Multiport structure

The multiport structure is emerging as an alternative for small generation systems, where there is often more than one power input. The whole power processing unit may be viewed as a single power stage. In a “black box” fashion, a multiport dc-dc converter (shown in Fig. 1.4) can be used to interface multiple power sources and storage devices. It regulates the system voltages and manages the power ﬂow between the sources and the storage elements. The control of the entire system can be centralized in a single processor. A multiport converter may best satisfy integrated power conversion, eﬃcient thermal management, compact packaging, and centralized control requirements.

In small generation systems a power electronic converter is needed to provide an interface between power sources and storage, to supply local ac loads and possibly dc loads with regulated outputs, as well as to connect to the utility grid. For instance, Fig. 1.5 shows a possible fuel cell system for domestic application based on the multiport structure. A bidirectional converter manages the power ﬂow between the fuel cell generator, storage, and load. The whole system is able to operate in both stand-alone and grid-connected modes. In case of stand-alone

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Power flow Inverter Local dc loads Fuel cell Local ac loads Grid Multiport bidirectional dc-dc converter Storage

Figure 1.5: Small fuel cell generation system based on the multiport structure.

operation, the storage is used to match load transients. In case of grid-connected operation, the auxiliary energy source is needed for correct start-up and other functionalities.

1.3 Literature overview

Power electronics is one of the key factors enabling sustainable energy technolo-gies [11]. Various power circuits have recently been investigated in an attempt to explore reliable, highly eﬃcient, high power density, and low-cost power process-ing systems for alternative energy generation. The converter topologies reported in the literature can be roughly classiﬁed into bidirectional converters and unidi-rectional converters. For interfacing storage devices bidiunidi-rectional converters are needed, whereas for interfacing primary sources unidirectional converter topologies should be investigated. A new class of converters – multiport converters – which enable integrated multisource power conversion is emerging from recent research work. Although this study will focus on the multiport converter topologies, an overview of typical two-port (i.e., single-input single-output) converters developed for fuel cell systems and suchlike will be given as well.

1.3.1 Bidirectional dc-dc converters

Bidirectional dc-dc converters can transfer power between two dc sources in ei-ther direction. They are essential in high-performance storage-backed generation systems.

Two-quadrant buck/boost

A bidirectional dc-dc converter can be as simple as a two-quadrant buck/boost converter (shown in Fig. 1.6(a)), which may fulfill the requirements for interfacing energy storage with a fuel cell system [12], [13], [14], [15]. The converter allows bidirectional power flow and can achieve high efficiency because of low parts count and lack of a transformer. Furthermore, low parts count allows for compact packag-ing and competitive pricpackag-ing. This type of circuit can be extended to an interleaved

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1.3. Literature overview 9 (a) (b) + + + V2 V1 _V 1 + V2

Power flow Power flow

Figure 1.6: Basic bidirectional topology: (a) the two-quadrant buck/boost converter

(b) polyphase interleaved topology.

structure [16], as shown in Fig. 1.6(b), in order to reduce the ripple current. This is beneﬁcial for both fuel cells and batteries. The interleaved converter has the advantage of interleaved operation for both boost and buck modes, small passive components and less ripple current. Moreover, it is also possible to integrate the inductors on one magnetic core [17].

Boost full-bridge

In many situations, a large voltage transfer ratio and electrical isolation are re-quired when incorporating storage into a generation system. This often leads to a converter topology with a high-frequency transformer. In the category of isolated bidirectional dc-dc converters, several full-bridge derived converter topologies have been proposed in the literature, with the aim to reduce switching loss, minimize electro-magnetic interference (EMI), and increase eﬃciency [18], [19], [20], [21]. The full-bridge boost converter (shown in Fig. 1.7) is widely investigated and con-sidered as one of the best choices. The properties of this topology are current-fed from the low voltage side, simple voltage clamp circuit implementation, a simple transformer winding structure and low turns ratio, and high choke ripple frequency (twice the switching frequency).

The full-bridge boost converter can achieve high efficiency because the active clamp circuit provides lossless snubbing, soft-switching operation, and synchro-nous rectification when in charging mode. Furthermore, switching devices can be paralleled at the low-voltage side. On the other hand, in order to avoid large voltage spikes on the input bridge the converter needs an active clamping circuit. This circuit also has a start-up problem. This can be overcome by adding a flyback winding [19].

Half-bridge and push-pull

By replacing the diodes of the secondary rectiﬁer stage in some unidirectional topologies, bidirectional power ﬂow can be achieved. Fig. 1.8 shows an exam-ple [22]. The converter is a combination of a half-bridge (on the primary side) and

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LS + LB Acti ve clamp LS: Leakage inductance LB: Boost inductor Power flow + V1 V2 Full-bridge Full-bridge Low-voltage _High-voltage

Figure 1.7: Full-bridge boost bidirectional converter [18].

LS + Power flow Lo + NP1 NP NS NS V1 V2 Balancing winding and catching diodes Half-bridge Push-pull

Figure 1.8: Half-bridge and push-pull bidirectional converter [22].

a current-fed push-pull topology (on the secondary side). It enables charging and discharging by utilizing the bidirectional power transfer property of MOSFETs. The topology’s advantages are low stresses on the switches, galvanic isolation, low ripple in the battery charging/discharging current, and a minimal number of active switches [22]. However, the converter is intended for low-power applications.

Dual-active-bridge

The dual-active-bridge (DAB) dc-dc converter (shown in Fig. 1.9) is a promising topology for bidirectional applications. The DAB converter was proposed in [23], followed by a detailed investigation in [24] and a series of improvements such as improving the switching conditions [25], [26], [27] and reducing the circulating reactive power [28], [29]. It is also possible to use other switching bridge cells different from a full-bridge [30]. The DAB converter uses phase shifting to control the power flow through a high-frequency transformer. It has been proposed for high power density and high-efficiency dc-dc converting applications. The DAB converter has a number of attractive features such as bidirectional power flow, low device stresses, small filter components, low switching losses, buck-boost operation, and the utilization of the transformer leakage inductance.

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1.3. Literature overview 11 L Power flow + V2 + V1 Phase-shifted Full-bridge Full-bridge

Figure 1.9: Dual-active-bridge (DAB) bidirectional converter [23].

L + 400V battery backup Phase A Phase B Neutral 120/240V, 60Hz Fuel cell + Full-bridge Half-bridge (voltage doubler) PWM inverter High-voltage bus

Figure 1.10: Low-cost power conditioning system for a fuel cell system based on the

DAB topology [31].

Based on the DAB topology, a low-cost 10 kW converter system was presented in [31]. However, the bidirectional property of the topology is not utilized because the converter is used to connect a fuel cell. As shown in Fig. 1.10, the proposed system consists of a DAB converter to boost the fuel cell voltage to 400 V dc and a pulse-width modulated inverter to convert the dc voltage to two split-phase 120 V ac lines (the US standard). The converter has a full-bridge at the low-voltage side and a voltage doubler at the high-voltage side. High-voltage batteries are directly connected to the high-voltage bus. Compared with the existing fuel cell converter systems, this circuit promises low cost, lower component count, smaller size, and reduced dc-dc converter peak current. An interesting feature of this topology is the dual function (voltage doubler and neutral phase leg) provided by the ﬁrst phase leg at the inverter side.

Boost dual-half-bridge

A newly developed zero-voltage switching (ZVS) bidirectional dc-dc converter was proposed in [10] and [32] for interfacing batteries to a fuel cell generator. The de-scribed topology (shown in Fig. 1.11) may be named boost dual-half-bridge (DHB).

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Power flow + V2 + V1 Phase-shifted

Boost half-bridge Half-bridge

Low-voltage High-voltage

Figure 1.11: Boost dual-half-bridge (DHB) bidirectional converter [32].

Similar to the DAB converter, in this topology the power ﬂow through the trans-former is controlled by phase-shifting the primary and secondary bridges. The advantages of this converter are simple circuit topology and soft-switching im-plementation without additional devices. These advantages make the converter promising for applications such as auxiliary power supply in fuel cell vehicles. However, the half-bridge structure (the capacitors) at the low-voltage side needs special design considerations because of high current.

1.3.2 Unidirectional dc-dc converters

Because most primary sources like fuel cells cannot sink power, the interfacing converter does not need to be bidirectional. Several unidirectional converters that are suitable for fuel cells were reported in the literature.

Push-pull

For some low-cost applications, the push-pull topology is adopted to connect a fuel cell [33], [34], [35] or to provide an interface between a low-voltage bus and a high-voltage bus [36]. Storage devices like batteries and supercapacitors can be connected to the high-voltage bus directly. This configuration significantly simplifies the converter design.

The power processing system illustrated in Fig. 1.12 consists of a push-pull converter boosting the fuel cell voltage (48 V) to the high bus voltage (200 V) and an inverter to produce 120 V / 240 V, 60 Hz ac outputs. Two sets of lead-acid batteries are connected to the high-voltage dc bus to supply transient load demands. Efficient and smooth control of the power drawn from the fuel cell and the batteries is achieved by controlling the front-end dc-dc converter in current mode. However, this topology requires high-voltage batteries. For a push-pull converter it is difficult to equalize or symmetrically wind the two halves of a center-tapped winding. Furthermore, the power switch on/off times as well as their forward voltage drops are never exactly equal. These irregularities could contribute to transformer saturation and may result in converter failures, and should therefore be taken care of when using the push-pull topology.

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1.3. Literature overview 13 Fuel cell + Phase A Phase B Nuetral 120/240V, 60Hz PWM inverter + High-voltage battery backup Voltage-fed push-pull +

Figure 1.12: Voltage-fed push-pull converter for a fuel cell system [33].

Fuel cell + Current-fed push-pull PWM inverter High-voltage bus Snubber circuit

Figure 1.13: Current-fed push-pull converter for a fuel cell system [37].

The dual of the voltage-fed topology, the current-fed push-pull topology shown in Fig. 1.13 was also reported [37]. In principle, the current-fed push-pull converter is basically a boost converter with electrical isolation. This topology suﬀers from high voltage spikes at the switching instants. A voltage clamp snubber circuit (as shown in Fig. 1.13) is therefore needed to protect the MOSFETs from over-voltage due to the leakage inductance of the transformer. In addition, a smooth startup procedure should be provided in order to limit the inrush current during startup.

Phase-shifted full-bridge

The well-known ZVS phase-shifted full-bridge (FB) dc-dc converter (Fig. 1.14(a)) can certainly be used to interface to a fuel cell, especially for high-power appli-cations [38]. However, full ZVS operation can only be achieved in a limited load and input-voltage range. The duty cycle loss resulting from the leakage induc-tance is also a drawback of the circuit. Several techniques have been proposed to extend the ZVS operating range of the converter. An interesting improvement was proposed in [39], as shown in Fig. 1.14(b). The ZVS of the primary switches is achieved by employing two magnetic components whose volt-second products change in opposite directions with a change of phase shift between the two bridge

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+ Vin Full-bridge LS NP NS NS Vo CB + Vin Full-bridge NP NS NS Vo CB1 CB2 NC NC (a) (b) Coupled inductor

Figure 1.14: Phase-shifted full-bridge converter, showing (a) the topology [38] and (b)

its improvement [39]. High-voltage bus Active clamp LS + Vin Vo

Figure 1.15: Active clamp current-fed half-bridge dc-dc converter [40].

legs. One magnetic component is the transformer while the other magnetic compo-nent is either a coupled inductor (shown in Fig. 1.14) or a single-winding inductor. The transformer is used to provide isolated output(s) and the inductor is used to store energy for ZVS. With this technique, ZVS of all the switches over a wide range of the input voltage and output load can be achieved with a minimal duty cycle loss and minimal circulating current.

An active-clamping current-fed half-bridge topology designed for fuel cell sys-tems (as shown in Fig. 1.15) was presented in [40]. This topology is also an isolated boost converter. It realizes a current source function and therefore is friendly to fuel cells. However, like other isolated boost topologies, the switching conditions for the power switches are poor and active-clamp circuits are necessary.

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1.3. Literature overview 15

+ Vin

Vo

Figure 1.16: High-power three-phase six-leg converter [42].

Polyphase interleaving

For high-power applications, polyphase interleaving technique has been applied to the phase-shifted full-bridge topology [41], [42]. The resulting converter shows a three-phase six-leg circuit topology as illustrated in Fig. 1.16. Owing to the interleaving, this converter signiﬁcantly reduces the ripple current. A disadvantage is that it suﬀers from high parts count.

Multicell topology

The multicell technique has been used in [43] to improve the efficiency and reduce the filter size. As shown in Fig. 1.17, several dc-dc isolation stages are connected in parallel at the dc input of the system and operate in high-frequency resonant mode. Each dc-dc isolation stage supplies a full-bridge inverter stage. Series connection of the full-bridge inverter stages forms the overall ac output of the system [43]. Because of the cascaded connection, low-voltage high-current power MOSFETs can be used for all switching cells. This results in a higher efficiency compared with conventional isolated dc-dc converters. Similar to cascaded multilevel inverters, the full-bridge cells are operated in an interleaved pulse-width modulation (PWM) mode. Such operation can significantly reduce the size of the ac output filter [43]. However, this multicell topology requires complicated control and a large number of fully isolated gate drivers.

The multicell (multilevel) technique was also reported in [44], where it is used to avoid derating of semiconductors by a voltage reduction control through the use of a multilevel dc-dc converter. Voltage reduction is done by inhibiting a certain number of single fuel cells in the stack when the load current decreases, thereby reducing the overall system operating voltage variation. However, a complicated control system must be designed.

1.3.3 Multiport dc-dc converters

Multiport converters, a promising concept for alternative energy systems, have attracted increasing research interest recently. The use of a single power

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process-+ Vin 230V ac output Resonant-mode dc-dc stage PWM inverter

Figure 1.17: Multicell dc-ac converter based on high-frequency resonant-mode dc-dc

isolation stages feeding interleaved PWM-mode MOSFET dc-ac cells con-nected in series on the ac output [43].

ing stage to interface and control multiple power ports implies centralized and integrated power conversion from a variety of power sources.

Thus far, limited work on multiport topologies has been reported. Multi-port converters have also been referred to as multiple-input (MI), multiple-output (MO), or multiple-input multiple-output (MIMO) converters depending on the input and output conﬁguration of the system.

This section gives an overview of typical multiport topologies reported in the literature. Some of them have the property of bidirectional power ﬂow for one or more ports, while others are unidirectional. A variety of methods have been used to extend a conventional two-port converter to a multiport converter.

Time-sharing concept

MI flyback converter The time-sharing concept can be used to develop multiport converters. As shown in Fig. 1.18(a), the two-input flyback converter proposed in [45], [46], and [47] uses the coupling of a magnetic component to enable multiple-input. For each input there is a separate winding. To some extent, the converter can be regarded as two flyback converters operating in parallel, except for the combined transformer on one core and the shared secondary output rectifier. It is also possible to have multiple outputs by using multiple secondary windings and rectifiers to provide multiple isolated output voltage levels for different loads, as shown in Fig. 1.18(b). This topology is capable of interfacing sources of different voltage-current characteristics to a common load, while achieving low parts count. Similarly, it is also possible to use the forward topology to develop a multiport converter [47].

In the application example discussed in [47], the two inputs are from a solar panel and a rectiﬁed utility power. With the two inputs, the output voltage can

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1.3. Literature overview 17 + Two-input flyback + Vo + Vin1 + Vo1 Vo2 + MIMO flyback S1 S2 Ton2 Ton1 Toff S2 S1

Gate control signals (a) (b) (c) T Switching cycle Vin2 VinN Vin1 Vin2

Figure 1.18: Multiport converter using the ﬂyback topology, showing (a) two-input

flyback converter, (b) MIMO flyback converter, and (c) typical gating signals for the two-input flyback converter [47].

be well-regulated, notwithstanding the erratic nature of the power input from the solar cells. It is possible to implement maximum power point tracking and power factor correction [46]. The control scheme for this converter is based on the time-sharing concept. The duty cycle within one switching cycle is split up for the multiple inputs, that is, each input is active for a certain period in a switching cycle. The typical gating signals for the two-input ﬂyback converter are shown in Fig. 1.18(c). During Ton1, source V1 transfers power to the load, whereas during

Ton2, V2does.

The idea behind the time-sharing concept is simple. However, this method does not allow for a simultaneous energy transfer from the multiple inputs. The flyback topology implies that it is only suitable for low-power applications because of high current stresses. The input and output currents are both pulsating. This increases the filtering effort. Furthermore, the converter is unidirectional.

The topology presented in [48] (shown in Fig. 1.19(a)) is based on a similar idea. The concept of time-sharing is implemented on a larger time scale, that is, each port operates in an intermittent mode. A typical time-sharing switching interval is shown in Fig. 1.19(b). The converter can realize bidirectional power ﬂow on one port through the use of an extra winding. A relay is used to switch between the two windings according to the direction of the power ﬂow. This, however, is not a truly bidirectional port.

MI buck-boost converter A multiple-input buck-boost (MIBB) converter topol-ogy was introduced in [49] and further investigated in [50]. The circuit is shown in Fig 1.20(a). The multiple inputs are interfaced through a forward-conducting, bidirectional-blocking switch. The switch can be equipped with a gate turn-oﬀ

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+ + Vo S1 S2 S2 S1

Gate control signals

(a)

(b)

Switching interval Vin1 Vin2 + Storage S3 Relay Vin1 delivers power Vin2 delivers power

Figure 1.19: Multiport converter based on the time-sharing concept, showing (a)

topol-ogy and (b) typical gating signals [48].

+ Two-input buck-boost + Vo + VinN Vo1 Vo2

MIMO flyback converter with single primary winding S2

S1

D2T S2

S1

Gate control signals

(a) (b) (c) T Vin2 Vin1 + Vin2 + Vin1 D1T S2eff S1eff

when Vin1 > Vin2 Effective duty cycles D1effT

D2effT

Switching cycle

Figure 1.20: (a) MI buck-boost converter, (b) MIMO ﬂyback converter, and (c) gate

control signals [49].

(GTO) thyristor, a series MOSFET and diode pair, or several other switch com-binations. The inputs share a common inductor and the output capacitor. It is possible to provide isolation by replacing the single-winding inductor in Fig 1.20(a) with a coupled double-winding inductor. Then, the circuit actually becomes a MI ﬂyback converter. It is possible to have multiple outputs by adding more secondary windings as shown in Fig 1.20(b).

The converter has low parts count of both passive and semiconductor compo-nents, and provides either a buck or boost mode. Contrary from the MI ﬂyback converter in Fig. 1.18, multiple primary windings are removed in this topology; it only needs one primary winding. An advantage of this scheme is that the inductor is shared by all the inputs. This is indeed a signiﬁcant improvement in cost, mass and converter size, however, at the cost of losing isolation among inputs and the

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1.3. Literature overview 19

ability of matching substantially diﬀerent input voltage magnitudes.

The gate control signals for the switches have the same rising edge, but the falling edges do not coincide, as illustrated in Fig 1.20(c) [49]. Each switch has a diﬀerent duty cycle. In this topology, only one input switch or output diode is conducting at any time. If the number of the switches turned on is more than one, the source that has the highest voltage level will supply the power to the load. Hence, only one of these dc sources is allowed to transfer energy to the load at a time. Depending on the magnitudes of the input voltages, the eﬀective duty cycle of each switch can be calculated [49].

This circuit configuration, as shown, allows for only unidirectional power flow. For sources such as solar cells and fuel cells, this is sufficient. For bidirectional power flow, as suggested in [49], the output voltage may serve as the input to another converter, or it can be fed back to one of the sources. In that way, a bidirectional MIMO converter can be constructed.

In short, these time-sharing based multiport topologies promise low cost and easy implementation. However, a common drawback is that power from multiple inputs cannot be transferred simultaneously to the load.

Dc-link coupling

Using a dc bus to link several switching cells is another way to enable multiple-source power conversion [12]. The dc-link method here refers to connecting multi-ple converter cells at a dc bus buﬀer capacitor and controlling the switching cells centrally.

As shown in Fig. 1.21, several power sources can be linked together through individual buck/boost bidirectional switching cells and a dc-link capacitor. For a unidirectional input, one of the two power switches can be replaced by a diode.

In the described automotive application [12], the power inputs include a fuel cell, a supercapacitor, and batteries. The dc bus buffer capacitor is charged by the input sources while it supplies power to the inverter stages. This converter topology resembles the interleaved boost converter except that the inputs are connected to different sources instead of a single one. Current-mode or voltage-mode control may be applied to regulate input source currents and the dc-link voltage. The advantages of this topology include bidirectional power flow and possible use of standard inverter phase leg modules. The drawback, however, is that this topology cannot efficiently handle a wide variety of input voltages.

Magnetic-coupling

The use of magnetic-coupling method through a multiwinding transformer makes it possible to connect sources having substantially diﬀerent operating voltages. The magnetic-coupling method here refers to isolated high-frequency (HF) linking of multiple power inputs/outputs. Power ﬂow control can be achieved by phase-shifting the high-frequency voltages (commonly being square-waves) presented to the windings. With magnetic-coupling, all the sources and loads are galvanically isolated.

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+ Vo VinN Vin2 + + Vin1 dc-link

Figure 1.21: Dc-link coupling MI buck/boost bidirectional converter [12].

+ + + V2 V1 V3 Power flow Full-bridge Full-bridge Full-bridge

Figure 1.22: Magnetic-coupling three-port bidirectional converter [51].

Using this method, a three-port converter (shown in Fig. 1.22) was proposed in [51] for a fuel cell and battery system. It was also recommended in [52] for an un-interruptible power supply (UPS). Each bride generates a high-frequency voltage with a controlled phase-angle. The transformer leakage inductances are used as energy transfer elements. This topology is suitable for medium-power applications (a few kilowatts) and has attractive features such as simultaneous power transfer from any input to any output, possible soft-switched operation, galvanic isolation, capability of matching diﬀerent voltage levels, bidirectional power ﬂow, and cen-tralized control. Disadvantages are the high parts count and limited soft-switched region when operating with wide input voltage ranges.

In a recently published work [53], a three-port converter having two current-fed ports is used to interface with multiple energy storage elements (batteries plus supercapacitors). The converter, pictured in Fig. 1.23, shows a similar topology to that of Fig. 1.22. It features two current-fed ports by use of two boost half-bridges for the two inputs. The current-fed property of the input ports is suitable

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1.3. Literature overview 21 + + V3 V1 Power flow + V2 High-voltage Low-voltage Low-voltage Current-fed Current-fed Boost half-bridge Half-bridge Boost half-bridge

Figure 1.23: Magnetic-coupling three-port bidirectional converter with two current-fed

ports [53].

for interfacing to fuel cells and batteries. However, this converter has a limited soft-switching operating region when the port dc voltages vary widely.

Flux additivity

A MI converter based on flux additivity was proposed in [54]. Fig. 1.24 shows the converter topology. It has two power inputs and one output. Instead of combining input dc sources in electric form, the proposed converter combines in-puts in magnetic form by adding up the produced magnetic fluxes together in the magnetic core of the coupled transformer. With phase-shifted PWM control, the proposed converter can draw power from two different dc sources and deliver it to the load individually and simultaneously, and output voltage regulation and power flow control can be achieved. Due to the current-fed structure of the converter, the converter has the ability to accommodate voltage variations of the sources. However, this topology is not bidirectional. Although soft-switching is achievable, the current stress of the switches is high. Therefore, its application is limited to medium-/low-power applications.

Connecting in series

Diﬀerent dc sources can be connected in series through a converter stage to im-plement a MI converter, as shown in Fig. 1.25 [55]. This two-input converter is formed by a series connection of two ordinary boost converter stages. During normal operation, the output voltage can be regulated. However, if one of the dc sources drops away, it will be diﬃcult to obtain the regulated output. The

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+ Vin1

+ Vin2

Vo

Figure 1.24: MI dc-dc converter based on ﬂux additivity [54].

Vin1

Vin2

+

Vo

Figure 1.25: MI converter by connecting two converter stages in series [55].

converter was proposed for small integrated wind and photovoltaic electricity gen-eration systems. The implementation of the converter is straightforward and is demonstrated in [56]. However, there are obvious limitations in this converter topology. For example, it does not support bidirectional power ﬂow.

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1.3. Literature overview 23 Vo + Vin2 + Vin1

Figure 1.26: MI converter based on a modiﬁed boost converter using three switches [57].

Three-switch boost

New ideas for MI converters are being investigated recently. A novel three-switch boost converter topology has been proposed in [57]. As shown in Fig. 1.26, the topology only needs three power switches, while providing bidirectional interfacing of two dc voltage sources with a dc-link. For the operation of the converter, the sum of the two dc voltages must be lower (or equal) than the dc-link voltage. Although the topology has a low power semiconductor count, it does not provide electrical isolation.

Tri-modal operation

Very recently, a new three-port topology has been reported in [58]. A so-called tri-modal half-bridge converter based on an isolated half-bridge converter topology was presented. The idea comes from the similarity between the half-bridge and the active-clamp forward converter topologies. As shown in Fig. 1.27(a), the proposed converter is formed by adding a free-wheeling branch (a diode and a transistor) across the primary winding of an ordinary half-bridge topology. The two power inputs are connected to the half-bridge and the active-clamp input. The gating signals are illustrated in Fig. 1.27(b). With the free-wheeling stage, the converter can utilize three modes of operation within a ﬁxed-frequency switching cycle to provide two independent control variables, thereby allowing tight regulation of two of the three power ports. One of the design assumptions is that there is a dc current present in the primary winding of the transformer because of asymmet-ric operation [58]. The topology has low parts count and thus is cost eﬀective. However, the input source current is discontinuous, which implies that it may only be suited to low-power applications. Furthermore, the converter has only one bidirectional port and thus does not support a regenerative load.

To sum up, the methods used to provide a multiport interface include the time-sharing concept, dc-link coupling via a dc bus, magnetic-coupling through a high-frequency transformer, using ﬂux additivity by a multiwinding transformer, putting sources in series, based on a modiﬁed conventional topology, or tri-modal operation.

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+ Vin1 + Vin1 NS NS Vo Free-wheeling S3 S1

(b) Gate control signals T Switching cycle S2

(a) Tri-modal half-bridge converter S3

S1

S2

Figure 1.27: Three-port tri-modal half-bridge converter, showing (a) the topology and

(b) the gate control signals [58].

Although multiport converters are increasingly ﬁnding applications in various systems like alternative generation [58], [59], [60], [61], electric vehicles [12], [57], UPS systems [52], [62], and hybrid energy storage systems [53], limited work on these topologies has been reported.

The existing multiport converter topologies have one or more of the following drawbacks: (1) unidirectional, (2) diﬃcult to match diﬀerent dc voltage levels in the overall system, (3) no electrical isolation, (4) high current stress, (5) limited soft-switching region, (6) high component count, (7) complicated control, (8) only suitable for low-power applications, (9) unable to transfer the power from multiple inputs simultaneously to the load.

To date, there is a lack of bidirectional multiport converter topologies that are capable of providing soft-switching, low current stress, smooth input and output currents, and ﬂexibility in matching a variety of input voltages. Development of novel multiport bidirectional converters therefore becomes the main objective of this work.

1.4 Overview of the thesis

1.4.1 Motivation and objective

The subject of this research work is relevant because distributed generation is becoming the preferred method of modern power generation. Our future power systems will require interconnecting all kinds of energy sources and most power

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1.4. Overview of the thesis 25

will be generated at the point of use. We are now experiencing a gradual trans-formation from centralized to distributed generation. Because the electricity is generated very near where it is used, distributed generation reduces the loss in transmitting electricity. Furthermore, it provides additional cost-saving cogenera-tion capabilities, allowing the user to utilize the normally wasted exhaust heat.

This thesis addresses the power electronic interface for the integration of sus-tainable energy sources in small distributed generation systems. A fuel cell system is used as an example in this study. The main objectives of this thesis are

• to explore novel multiport bidirectional converter topologies that are suited to multisource/storage power conversion;

• to model multiport converters and develop adequate control strategies; • to improve the converter’s performance by means of novel control methods

to achieve, for example, soft-switching;

• to realize added functionality in small distributed generation (DG) systems and design a high-performance utility interconnection system;

• to digitally implement and test the small DG system based on the proposed topologies and control methods.

1.4.2 Outline of the thesis

Both the topological study of the dc-dc converters (Part I) and an investigation of control strategies for grid-interfacing inverters (Part II) are addressed in this thesis. The work on the dc-dc stage makes up the major part. Fig. 1.28 shows the relationship between the chapters. This thesis is organized as follows.

Chapter 1 outlines the background of the research work by an introduction of typical alternative power generation systems and an overview of the system structures and converter topologies proposed in the literature.

Chapter 2 presents a triple-active-bridge (TAB) converter topology for energy management in a three-port system where a primary source is combined with an energy storage element. The topology consists of three active bridges coupled by a three-winding transformer. Both the single- and three-phase versions of the topology are analyzed. The small signal model of the TAB converter is investigated and diﬀerent power ﬂow control strategies are proposed.

Chapter 3 investigates a soft-switching method for the TAB converter. A simple and eﬀective duty ratio control method is proposed to extend the soft-switching operating range when input voltages vary widely. A dual-PI-loop control scheme is described to precisely control the power ﬂow. Practical issues such as the digital signal processor (DSP) control implementation and the design of the transformer are also addressed. The closed-loop simulation and experimental results of a laboratory prototype are included.

Chapter 4 provides an alternative way of integrating the two power inputs. A two-input bidirectional converter is presented that interfaces a primary source and storage device with a load by a combination of a dc-link and magnetic-coupling. The topology only needs six power switches while supporting bidirectional power ﬂow for all the power ports. Two control methods, namely variable hysteresis

Integration of sustainable energy sources through power electronic converters in small distributed electricity generation systems

Integration of sustainable energy sources through power

electronic converters in small distributed electricity generation

systems

Integration of sustainable energy sources

through power electronic converters

in small distributed electricity

generation systems

Integration of sustainable energy sources

through power electronic converters

in small distributed electricity

generation systems

Summary

Integration of sustainable energy sources

through power electronic converters

in small distributed electricity

generation systems

Contents

I

Multiport bidirectional dc-dc converters

29

II

Utility interconnection and system control

181

III

Conclusions

239

IV

Appendices

247

Chapter 1

Introduction

1.1

Alternative power generation systems

1.1.1

Solar energy

1.1.2

Wind energy

1.1.3

Micro combined heat and power

1.1.4

Fuel cell generator

1.2

System structure

1.2.1

Conventional structure

1.2.2

Multiport structure

1.3

Literature overview

1.3.1

Bidirectional dc-dc converters

1.3.2

Unidirectional dc-dc converters

1.3.3

Multiport dc-dc converters

(a)

(b)

1.4

Overview of the thesis

1.4.1

Motivation and objective

1.4.2

Outline of the thesis