An Overview of Power Topologies for Micro-hydro ... - IEEE Xplore

3rd IEEE International Symposium on Power Electronics for Distributed Generation Systems (PEDG) 2012

An Overview of Power Topologies for Micro-hydro Turbines Sabar Nababan1), E. Muljadi2), Fellow, IEEE, F. Blaabjerg3), Fellow, IEEE 1)

Department of Electrical Engineering, Faculty or Engineering, University of Mataram, Indonesia, E-mail: [email protected] 2) National Renewable Energy Laboratory, Golden CO, 80401 USA, Email: [email protected] 3) Department of Energy Technology, Aalborg University, Denmark, E-mail: [email protected]

Abstract— This paper is an overview of different power topologies of micro-hydro turbines. The size of micro-hydro turbine is typically under 100kW. Conventional topologies of micro-hydro power are stand-alone operation used in rural electrical network in developing countries. Recently, many of micro-hydro power generations are connected to the distribution network through power electronics (PE). This turbines are operated in variable frequency operation to improve efficiency of micro-hydro power generation, improve the power quality, and ride through capability of the generation. In this paper our discussion is limited to the distributed generation. Like many other renewable energy sources, the objectives of micro-hydro power generation are to reduce the use of fossil fuel, to improve the reliability of the distribution system (grid), and to reduce the transmission losses. The overview described in this paper includes micro-hydro power generation, stand-alone topologies, fixed speed generation (FSG), variable speed generation (VSG), direct-connected grid integration topology, and PE grid integration topologies.

Keywords-component: power topologies, micro-hydro, generator, power electronics, fixed speed generation, and variable speed generation. I. INTRODUCTION Hydropower is considered to be one of the most important renewable energy sources (RES). RES are the second largest contributor to the global electricity production after fossil fuel. In 2010, they are accounted for 19.4% of total electricity generation, ahead of the nuclear power generation (13%).

Figure 1. Renewable energy share of global electricity production 2010 [1].

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Most of the electricity generated from renewables comes from hydro power (83%) and the other comes from wind, solar, tidal geothermal energy, etc as shown in the Figure 1 [1]. Based on the Kyoto protocol and Bali Climate Change Conference [2], each country in the world should support the use of RES for electricity, not only for rural electrification but also for the grid integration as part of the distributed generation (DG) and bulk power generation connected to the transmission system [3]. Based on capacity, hydropower can be classified as: pico (< 5kW), micro (5kW – 100 kW), small (101 kW – 2000 kW), mini (2001kW – 25000 kW), and large (> 25000 kW). But this classification may differ from country to country. Based on the types of installation, hydropower can be categorized as: impoundment, diversion, and pump storage, and based on the turbine types can it be classified as reaction or impulse systems [3]. The classification of hydropower by type of installation are typically divided into five categories, which are: impoundment type (large system, use dam to store water), diversion type (river diversion is used), run of river (uses natural flow of river, does not require impoundment), and pump storage (when the demand is low, water is pumped back to reservoir; when high demand, water is released)[3]. The overview in this paper is devoted to the micro-hydro power topology, usually “run of river”. There are two ways of operation of micro-hydro: stand-alone mode and grid integration mode. Stand-alone mode is usually used in rural electrification to serve small load and applications where frequency regulation is not very critical. In the grid integration mode, it is typically used to reduce the consumption of fossil fuel, to improve the reliability of the distribution system (grid), and to reduce the transmission losses. Modern micro-hydro power generation is usually integrated into the distribution network through power electronics (PE) at the interface with the grid. The application of PE in various kinds of configuration have shown that PE can improve the behavior/performance of RES. PE as interface between generator and distribution system of the grid can improve the power quality of the customer by minimizing the harmonics produced, providing a better ride through capability of the power generation, providing reactive power control or voltage regulation at the distribution system connection point [4-7,16].

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II. MICROHYDRO POWER GENERATION II.1 Turbine The four most common types of turbines used for microhydro are: Pelton Turbine, Francis Turbine, Propeller Turbine, and Kaplan Turbine. Turbines can be split into four main groupings. High head (> 100 m): Pelton, Turgo, Francis; Medium Head (20m - 100m): Francis, Cross Flow; Low Head (5m-20m): Cross Flow, Propeller, and Kaplan; and Ultra Low Head (< 5m): propeller, Kaplan, Water wheel [3,30]. Selection of turbine for hydropower plant depending on three factors, that are: head, flow, and desired running speed of the generator. Every turbine has specific speed that related to the output power of the turbine to its running speed and the head across it, as follows: ܰ‫ ݏ‬ൌ

݊ܲͳȀʹ ‫ܪ‬ͷȀͶ

A scheme of a micro-hydro power generation system is shown in Figure 4. The power of a micro-hydro power system is: P (in kW) § 7 x Head x Flow (2) where, Head is in meter, and Flow is in m3/s. Head is the difference in elevation. Penstock is the pipe that carries water from the reservoir [3].

(1)

where: n = turbine speed (r/min) P = shaft power (kW) H = pressure head across the turbine (m) Specific speed does not depend on the size of the turbine. Figure 2 summarize the more precise ranges of head, flow and power applicable to the different turbine types. Pelton turbine is depicted in Figure 3 [30].

Figure 2. Head vs flow ranges of micro-hydro turbines [30].

Figure 4. Schematic of a micro-hydro power generation system [3]. II.2 Generator Felix and Simoes [8] and E. Muljadi, et. all [21] state that typically small renewable energy power plants are using induction machines (asynchronous generator), because they are widely and commercially available in the market, simple, reliable, cheap, lightweight, rugged, brushless (in squirrel cage construction), and requires very little maintenance [23]. It is also easy to operate them in parallel with large power systems. Bansal [23] states that there are two types of induction generators; wound rotor induction generator and squirrel cage induction generator. Depending on prime mover used (constant speed or variable speed) generation the schemes can be further classified as: (i) Constant-speed constant-frequency (CSCF) (ii) Variable-speed constant-frequency (VSCF) (iii) Variable-speed variable-frequency (VSVF) II.3. Constant-speed constant-frequency (CSCF) The prime mover speed, in this topology, is held constant by continuously adjusting the blade pitch and/or maintained by the torque speed characteristics of the generator. Range of operation slip of an induction generator is 1% to 5% above the synchronous speed [23]. II.4. Variable-speed constant-frequency (VSCF) This topology is usually used in the variable-speed operation of wind electric system. It yields higher output for both low and high wind speeds because of improvements in aerodynamic efficiency. Both horizontal and vertical axis wind turbines exhibit this gain under variable-speed operation [23].

Figure 3. Pelton Turbine [30].

II.5. Variable-speed variable-frequency (VSVF) The performance of a synchronous generator operating in isolated operation can be affected with variable speed prime mover. Self excited induction generator (SEIG) can be

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conveniently connected to resistive heating loads for variable speed generation that is frequency insensitive [23]. As mentioned there are two modes of operation of a microhydro turbine system. They are stand-alone operation and grid connected operation. Figure 6. Topology off-grid battery-less micro-hydro electric systems. IV. GRID INTEGRATION TOPOLOGIES

III. STAND ALONE TOPOLOGY III.1. Battery-Based Micro-hydro Power System Roberto and Guillermo [17] state that traditionally, stand– alone topology of micro-hydro power system uses DC or synchronous generator, mainly for charging battery bank and/or to pump water. The drawback of this topology is that the generator needs a rotating wound field fed by an independent excitation system and brushes. To overcome this problem, brushless generators, such as induction or permanent magnet, have been proposed. In this topology, as shown in Figure 5, battery-based system has a great flexibility and can be combined with other RES, such as wind turbine generators and photovoltaic system. It is often possible to use a relatively small battery bank and battery charger, because the river flow is usually consistent. The output power is limited by the size of the inverter, the potential of water, and the size of the turbine [9,10]. Small difference between the output power of the generation and the load consumption will be stored into or restored from the battery. Occasionally, dump load is also included in the system in case the battery bank is fully charged and total load is too small for the level of generation.

Figure 5. Topology of off grid battery-based micro-hydro electric systems. III.2. Battery-less Micro-hydro-Electric Systems This topology, as shown in Figure 6, consists of a turbine generator that produces AC output at 120 V or 240 V, which can be connected directly to the household loads. The excess energy in this system can be consumed by dump loads, such as water or air heating elements. This method makes the total load on the generator constant. The drawback of such systems is that the peak or surge loads cannot be exceed. As the output of the generator is determined by the stream available head and flow, the peak or surge loads should not exceeds the available output power. In order to generate enough power for all household needs, including water and space heating; this type of system needs to be sized relatively large [9,10].

In a grid-connected system, the generator may be operated at fixed speed operation (grid frequency) or in a variable frequency operation [19]. IV.1. Fixed Speed Generation (FSG) In order to keep in synchronization, the speed of the generator unit of hydroelectric power plants must remain constant. Generally, there are optimization of hydro-turbines for an operating point defined by speed, head, and discharge. Operation at a fixed speed permits the limited deviations only by the head and discharge. In this operation, the turbine operates at constant rotational speed, and it allows small deviations from the nominal head and flow. This operation works as a constant voltage-constant frequency (CVCF) generation controlled by the grid [3,18,19]. IV.2. Variable Speed Generation (VSG) Currently, VSG are very common in wind generation. Similiarly, VSG in a hydro power generation can improve the efficiency of power conversion in water turbines. Some topology of the wind turbine can be adopted for micro-hydro turbine generation. This operation ensures a higher efficiency when the water flow varies throughout the day because the rotational speed of the turbine can follow the changes in the water flow. VSG works with constant variable voltagevariable frequency (VVVF) operation [4,5,12,15,16,19,20,21]. IV.3. Direct Connection Topology Conventional grid connection of micro-hydro power using FSG is shown in Figure 7. For direct drive micro hydro, the generator used can be a permanent magnet generator (PMG) or reluctance generator. With a gearbox, the rotational speed of the generator can be sufficiently high and conventional off the shelf induction generator can be used. Magnetizing current (reactive power) for SCIG being provided from the grid [11,12].

Figure 7. Direct connection micro-hydro power generation with the grid

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IV.4. Power Electronics Grid Integration Topologies In recent development, micro-hydro uses VSG. In advance, the VSG have to use the power electronics based AC/DC/AC converters as an interface between the generator and the utility grid [15]. This topology is illustrated in Figure 8. The generator in this topology can be a squirrel cage induction generator (SCIG), permanent magnet generator (PMG), and synchronous generator [12-14].

Gear Box

Variable Speed Hydro Turbine

G 3~ Capacitor Bank

AC/DC

Grid

DC/AC

AC/DC/AC Power Converter

Transformer

Figure 8. Variable speed generator system connection through full scale power converter. Another possible configuration is variable speed doubly– fed induction generator (DFIG) system, as shown in Figure 9. The stator of the DFIG is connected directly to the grid, and its rotor connected to the grid through AC/DC/AC power converter.

bridge rectifier, DC/DC boost converter, and three-phase three level voltage source inverter. Marcelo and Mario [23] improved the MHPP proposed in Reference [4]. They use Z-Sources Inverter (ZSI) and line filter replacing the Boost DC/DC Converter and Three-Phase Three-Level Voltage Source Inverter. The new converter topology of MHPP is depicted on Figure 11. The ZSI performs better than the traditional V-Source and I-Source converters. Common drawbacks in traditional VSource and I-Source are limited output voltage range, that requires either buck or boost operation shoot-through or open circuit problem, no interchangeable main circuits, vulnerability to electromagnetic interference (EMI) noise, difficult to use IGBT module for I source, and start-up difficulty [22]. Unique features of ZSI includes the buck-boost function by one stage conversion, immunity to EMI noise and mis-firing the power switches (i.e., mis-firing the power switches due to EMI noise will not destroy the converter). It has the advantages of both traditional converters: voltage and current converters, solves the problems of the traditional converters, has low or no in-rush current compared with the V-converter, and has low common-mode noise [22].

Transformer Gear Box

DFIG 3~

Variable Speed Hydro Turbine

Grid

AC/DC

DC/AC

!"

AC/DC/AC Power Converter

Figure 9. Variable speed DFIG system connection to the grid. IV.5. Power Circuit Topology Power circuit topology used for connecting SCIG or PMG to the Grid usually using full scale power converter AC/DC/AC.

Figure 11. Full scale power converter MHPP proposed in Reference [23].

Reference [14] has pointed out a solution for optimal operation of the Francis Turbine, the most popular hydro power generation in Small Hydro Plants.

Figure 12. The speed control of asynchronous machine in Reference [14]

Figure 10. Full scale Power Converter MHPP proposed in Reference [4]. J.L. Marquez et all [4] present a novel control approach of a three-phase grid connected micro-hydro power plant (MHPP) incorporating a maximum power point tracker (MPPT) for dynamic active power generation jointly with reactive power compensations of distribution systems. Detailed model of MHPP is shown in Figure 10. They use permanent magnet synchronous generator, three-phase diode

The efficiency of this turbine is highly dependent on the water flow, which generally has a large variation daily and seasonally, thus, variable speed operation was chosen to maintain high efficiency of operation. Due to the fact that all hydro generators are AC machines, the connection to the 50 Hz power grid can be done only using as interface the AC/DC DC/AC static converters. For large generator higher than 1 MW, the front end three level inverters are recommended. In

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this paper, we assume a system using a box in connecting the turbine to the asynchronous generator. L. Belhadji, et. all [27], presents a grid integration of variable-speed micro-hydro power plant (MHPP), based on a micro Axial-flow turbine (semi-Kaplan) coupled to permanent magnet synchronous generator (PMSG). Two back-to-back Voltage Source Inverters (VSI) are interfacing the generator [27].

For current and speed regulation they use the integrator based controller and a PI controller respectively. Intersective PWM methods is used for both of power converter [29]. There are two main issues in islanded operation, as shown in Figure 15, there are: generation-consumption equilibrium and basic output parameters control. They use the Vf control strategy for voltage amplitude and frequency control.

Figure 13. Global configuration of the proposed scheme of MHPP-PMSG in Reference [27] In the grid connection mode of MHPP, as shown in Figure 13, power electronics (PE) is used as an interface for supplying power to transfer the maximum power available at the inlet of the hydraulic turbine for the power grid. Generator side converter controls the speed of rotation of the shaft, while the network side inverter controls the DC-link voltage and ensures the quality of the injection current [27]. Andreica et all in reference [29] present a study of gridconnected or islanding operation of a micro-hydro power generation. In this two operation modes a power electronics converter were employed. In grid-connected mode, as shown in Figure 14, a maximum power point tracker (MPPT) is devoted to maximize the water extracted power.

A resistive load is supplied by an inverter through a LC filter Controller for the DC-link is charged by the PMSG-side converter. Depending on the load demand, the PMSG speed will vary. The DC-link voltage controller and the currents controller are PI controller and PI generalized (d,q) controller respectively [29]. Marinescu et all [32] present a hybrid power system based on renewable energy sources that working in stand-alone mode. The system contain a micro hydro-power plant and a wind turbine with and induction generator as shown in Figure 16.

Figure 14. Control structure grid-connected operation in reference [29]

Figure 16. The basic system configuration in Reference [32]

This system using a permanent magnet synchronous generator (PMSG). In order to fix the operating point by controlling the current, the PMSG side converter must guarantee the torque and the speed control. This is acts for the PQ control mode.

Figure 15. Control structure in islanded operation in Reference [29]

The system contains two generators supplying the AC microgrid, the loads and elements used for the system’s control such as: an Electronic Load Controller (ELC) or dump load. Dump

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load is a variable resistance and a capacitor bank which supply the reactive power demanded by the induction generators and some reactive loads. A Banki (cross flow) hydraulic turbine used as prime mover for micro-hydro. Micro-hydro using synchronous generator (SG) and wind turbine using induction generator (IG). Power from SG is constant and power from IG is according to wind speed.

IV.6. Power Electronics (PE) Topologies There are four types of PE topologies commonly found: 1) AC to DC controlled and uncontrolled rectifiers; 2) DC to AC inverters; 3) DC to DC switch mode converters; and 4) AC to AC converters [25]. AC to DC Rectifier – Generally, rectifier circuits are used to generate a controlled DC voltage from either an uncontrolled AC source or the grid. When converting from the grid, the rectifier is usually connected to the system to provide a DC voltage to a certain load applications such as battery regulator and variable frequency drive (VFD) input [25,31]. DC to DC Converter – This converter converts DC voltage or current from one level to a higher or a lower level. It is found in many renewable applications and battery charging applications. They are commonly found in PV battery charging system. PV converter circuit is usually designed to maintain maximum peak power tracker (MPPT) of a PV array [25,31].

Figure 17. The proposed system structure in Reference [32] Voltage and frequency regulation for both generator, SG and IG, can be controlled through the excitation current of the SG’s field winding, combined with an adequate regulator. The electric tie constant is rather big and the regulating procedure is slow. This is caused by the excitation current flows through highly inductive circuits. Reactive power balance is needed for voltage control. Ion and Marinescu [33] present a control strategy that aims to improve the parallel operation of two micro-hydro power plants (MHPP) on an islanded micro-grid (MG). One of MHPP is using the synchronous generator (SG) and the other using induction generator (IG). The basic system is depicted in Figure 18. The proposed system guarantee the the voltage and frequency regulation. The SG voltage controller makes voltage constant and dump load makes frequency constant [33].

DC to AC Inverter – Inverter circuit generates AC voltage regulated from a DC input. They are commonly found in standalone AC power supply systems, utility connected DG systems, and on the motor side of the variable frequency drives (VFD) [25,31]. AC to AC converter – Converter change the AC voltage in one frequency to the AC voltage in another frequency for example in a cyclo-converter or a matrix converter [25,31]. IV.7. Components of Power Electronics Typically, PE devices found in power systems are divided of four basic categories of components: (1) Semiconductor switches; (2) Switch gating and controls systems; (3) Inductive components; and (4) Capacitive components. For more details on PE devices, see references [26,31]. 1) Diode Diodes is element of PE can stream flows in one direction only and block voltage in the reverse direction. These diodes are typically used to produce direct current and reverse voltage levels should be blocked. Diode has a negative temperature coefficient, which makes them difficult to parallel when higher level current is required [25,26,31].

Figure 18. Basic diagram of the studied system in Reference [33]. In order to reduce energy losses and optimize hydraulic resources utilization, the control system ensures active and reactive power management [33].

2) Thyristor Thyristor family of semiconductor switches including a number of similar devices with slightly different operational capabilities. They have the highest power handling capability of all semiconductor devices and circuits. It can be found in applications with rating reaches thousands of amps in almost all voltage levels, including the HV transmission. In general, such as thyristor is controlled with a gate control signal that initiates a change in the conduction state when the unit is forward biased. Once gated, the device will continue to

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operate in that mode until the voltage crosses zero or applied on their back, or gate turn-off signal is applied to a different (if any). Various configurations and packaging methods used to obtain a device that acts as a gate to turn on (SCRs), gate to turn off (GTOs), and MOS-controlled uni and bi-directional current flow (MCT) switch. Their main drawback is that their maximum switching frequency on the orders of magnitude slower than modern devices [25,26,31]. 3) MOSFET Metal-oxide-semiconductor field effect transistor (MOSFET) is a gate voltage controlled switch. MOSFET is usually found in low voltage range (