Electrical Power Conversion of a River and Tidal Power Generator Preprint Eduard Muljadi, Vahan Gevorgian, and Alan Wright National Renewable Energy Laboratory James Donegan, Cian Marnagh, and Jarlath McEntee Ocean Renewable Power Company To be presented at the 2016 IEEE North American Power Symposium Denver, Colorado September 18–20, 2016 © 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy Operated by the Alliance for Sustainable Energy, LLC This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Conference Paper NREL/CP-5D00-66866 September 2016 Contract No. DE-AC36-08GO28308 NOTICE The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC (Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Available electronically at SciTech Connect http:/www.osti.gov/scitech Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 OSTI http://www.osti.gov Phone: 865.576.8401 Fax: 865.576.5728 Email: [email protected] Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5301 Shawnee Road Alexandria, VA 22312 NTIS http://www.ntis.gov Phone: 800.553.6847 or 703.605.6000 Fax: 703.605.6900 Email: [email protected] Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795. NREL prints on paper that contains recycled content. Electrical Power Conversion of a River and Tidal Power Generator E. Muljadi, V. Gevorgian, A. Wright, J. Donegan, C. Marnagh, J. McEntee Abstract—As renewable generation has become less expensive energy investments in the power sector outpacing net during recent decades, and it becomes more accepted by the investments in fossil-fueled power plants. The most rapid global population, the focus on renewable generation has growth and the largest increase in renewable capacity occurred expanded to include new types with promising future in the power sector and was dominated by three technologies: applications, such as river and tidal generation. Although the utilization of power electronics and electric wind, solar photovoltaic (PV), and hydropower. machines in industry is phenomenal, the emphasis on system This paper describes the electrical power conversion design is different for various sectors of industry. In precision aspects of river and tidal generation. Although modern power control, robotics, and weaponry, the design emphasis is on converter control is available to control the generation side, accuracy and reliability with less concern for the cost of the final we choose the design on the basis of minimizing the capital product. In energy generation, the cost of energy is the prime expenditures and operations and maintenance expenditures; concern; thus, capital expenditures (CAPEX) and operations and maintenance expenditures (OPEX) are the major design thus, the architecture is relatively simple and modular for ease objectives. of replacement and maintenance. The power conversion is This paper describes the electrical power conversion aspects simplified by considering a simple diode bridge and a DC-DC of river and tidal generation. Although modern power converter power converter to take advantage of abundant and low-cost control is available to control the generation side, the design was PV inverters that have well-proven grid integration chosen on the bases of minimizing the CAPEX and OPEX; thus, characteristics (i.e., the capability to produce energy with good the architecture is simple and modular for ease of replacement power quality and control real power and voltage at the grid and maintenance. The power conversion is simplified by considering a simple diode bridge and a DC-DC power converter side). to take advantage of abundant and low-cost photovoltaic inverters that have well-proven grid integration characteristics (i.e., the capability to produce energy with good power quality and control real power and voltage on the grid side). Index Terms—ocean power, marine, hydrokinetic, tidal, river, renewable, water power, variable generation, distributed generation. I. INTRODUCTION HIS work is based on a hydrokinetic power generating T system for river and tidal generators. The analysis is based on a tentative tidal generator with the power rating of 40 kW, although the same generator can be operated as a river Fig. 1. Typical bottom-mounted tidal generator (source Wikipedia) generator. From the resource perspective, tidal generation is dependent on the resource availability; thus, the resource must A typical tidal generator can be mounted on the bottom of be identified by preliminary assessment and survey [1]. One an estuary, and its shape and structure are similar to that of the difference between a tidal generator and a river generator is wind turbine generator as shown in the Figure 1. The Savonius the water flow driving each turbine. In river generation, the turbine type is also used as the river and tidal generators. It water flows in one direction and is more or less constant. In may be placed vertically or horizontally depending on the tidal generation, the water flows in two directions: during the characteristics of the resource and the site. flood, the water flows from the ocean toward the land or river; This paper is arranged as follows. Section II discusses the during the ebb, the water flows back toward the ocean. There electrical power conversion. The permanent magnet generator are various ways to harness tidal generation, either from a is presented in Section III, followed by the power converters single turbine [2–3], a combination of wind generators and in Section IV. Finally, Section V presents the conclusion and tidal generators [4], or in a multiple-turbine or tidal power summary. plant environment [5]. Each site has its own characteristics; thus, control and optimization may be unique for different II. SYSTEM-LEVEL DESCRIPTION installations [6–8]. The electrical power conversion in tidal generation is very In 2014, renewable energy expanded significantly in terms similar to the generating system of wind power generation. It of capacity installed and energy produced, with renewable consists of major components: a rectifier (variable AC to DC 1 This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. converter) and an inverter (DC to AC 60-Hz converter). III. PERMANENT MAGNET SYNCHRONOUS GENERATOR Depending on the generator type, the rectifier can be a passive The PMSG is very popular in variable generation rectifier [9] or an active rectifier [10]. For this discussion, we applications. Many of these applications are based on direct- assume that the system described in this paper is based on a drive generators that use rare-earth permanent magnets (PM). passive rectifier. Thus, a DC-DC converter is used to match The advantage of rare-earth PMs is that the residual flux and maintain the DC bus voltage constant. With an active density is very high, commonly B = 1.4 Tesla or higher. In rectifier, it is possible to control the DC bus voltage by comparison, the commonly used PM in the past has been the utilizing the active rectifier alone. This section presents the ferrite magnet, which has a low flux density, B = 0.4 Tesla or electrical power conversion. The maximum power point lower. tracker (MPPT) is similar to the one used in wind power A high flux density magnet is very important because, for generation, however, the implementation is determined by the the same power rating and rotational speeds, the higher the shape of the performance coefficient (Cp) characteristic (Cp flux density, the smaller and lighter the generator will be. It is versus tip-speed-ratio). Detail discussion can be found in [11]. assumed that the PM generator used is surface mounted. The A river and tidal generator using a permanent magnet equivalent circuit of a PM generator is shown in Figure 4. synchronous generator (PMSG) is investigated. A PMSG has the advantage of higher efficiency and reliability. Because of A. PMSG Connected to a Constant DC Bus via a Rectifier its lack of a field winding and the nature of variable In this section, the PMSG is connected to a constant DC generation, the PMSG is typically controlled by a series of bus voltage via a three-phase rectifier.