energy sources into the power systems of isolated communities. Previous studies ... resources can be incorporated into remote area power systems, in order to ...
Renewable Energy Micro-grid Power System for Isolated Communities Samuel Tucker and Michael Negnevitsky, Senior Member, IEEE Centre for Renewable Energy and Power Systems, University of Tasmania Abstract— Technological advancement in the renewable energy industry is increasing the feasibility of incorporating renewable energy sources into the power systems of isolated communities. Previous studies have demonstrated how renewable energy resources can be incorporated into remote area power systems, in order to reduce operating costs and harmful emissions. The variable nature of renewable resources, however, is still an obstacle that needs be overcome in order to provide a robust power system using solely renewable energy technology. Research on optimising power systems reliability, has led to multiple methods for improving the design of robust renewable energy systems. This paper proposes a stand-alone power system that uses wind and solar energy, as well as a hydrogen fuel cell back up supply. The energy sources are combined with specific control techniques in order to totally eliminate the need for a diesel generator. A requirement of this proposal is that all major elements be commercially available. Keywords— Isolated Community, Renewable Energy, Hybrid Power System, Wind, Photovoltaic, Fuel Cell, Hierarchical Control Systems
I. INTRODUCTION Isolated communities require a robust, cost effective power supply. Diesel generators are commonly used in isolated communities as they have high reliability, low capital requirements and are easy to deploy [1]. On the other hand, they produce harmful emissions and the economic costs are increasing rapidly. The operating expense of such a machine is escalating as a result of the increasing fuel and fuel delivery prices. This can result in the running costs reaching as much as 10 times its initial capital outlay in the first year of operation [2]. Another possibility for supplying power to an isolated community is the construction of transmission lines. However, the costs associated with the assembly and maintenance of these lines can be prohibitively high. Due to the increasing operational expenditure and harmful emissions of diesel generators, there has been a trend towards more environmentally friendly energy sources with lower operating costs. Renewable energy sources (RES), like those of wind and solar power, have high capital outlay but comparatively low operating costs. The capital requirements of a RES system are constantly being reduced with the advent of technological developments. As the price of renewable energy systems decreases, they become increasingly competitive, thus making them a more viable option [3]. This paper proposes a robust power supply system, built from commercially available elements, for an isolated community using solely RES.
Reliability is a major issue when developing a RES system [4]. Past experiences provide many different techniques for improving a RES system in this respect [3-14]. Typical considerations for reliability are: unit sizing, types and combinations of sources, energy storage techniques, control procedures and the systems internal connections. The viability and unit sizing of a RES system is directly dependent on the quality of the resource in the area under consideration [15]. Therefore, the design for any RES system is largely site specific. An analysis of data specific to various sites, in terms of the relevant power sources, has been applied to RES systems [6, 12]. The focus of this paper, however, will be the general design specific to the robust performance of a hybrid RES system, under a variety of environmental conditions. A hybrid system encompasses multiple types of energy sources. Combining elements into a hybrid arrangement is one solution for increasing the reliability of a system [3, 9-14]. Hybrid schemes can be especially beneficial when complementary RES are used [13]. An example of complementary power sources is the energy harnessed from wind and solar irradiation. Wind turbine generators (WTG) generally have higher generation rates during night hours, due to the better wind conditions (stronger and more constant winds). Photovoltaic (PV) cells, on the other hand, can only produce power during daylight hours. Combining sources like WTGs and PV cells, however, may not be sufficient to satisfy reliability requirements. In order to ensure a constant power supply to all customers, a backup power source is also required. Energy storage technologies are a good alternative to backup generators. The quality of renewable resources is unpredictable at best and, as such, energy storage is seen to be vital to a RES system [4, 7, 8, 13]. Hybrid power systems often incorporate a variety of energy storage techniques, as part of energy storage systems (ESS), for loss of supply events. Numerous different energy storage schemes are available commercially. A comparison of various energy storage techniques has been applied in order to attain the most suitable ESS for a specific RES system [7, 8]. A major limitation of conventional energy storage techniques is the finite energy capacity. To achieve maximum power production over a required duration, an ESS can be chosen to use a fuel, hence allowing constant power production as long as fuel is available. One of many available ESSs is a proton exchange membrane (PEM) fuel cell (FC). A PEM FC stores energy in the form of hydrogen using electrolysis, and also uses hydrogen as a fuel for power production. FC’s are able to operate at multiple power levels depending on load requirements. Commercially available FCs have a greater than 20 year expected life time, which is
significantly longer than battery packs in similar system designs [16]. In industry, it is common to have separate units for the storage of energy (Electrolyser) and release of energy (PEM FC). Once an ESS has been selected for connection to a power system, effective control techniques must be developed in order to control the systems voltage levels, frequency, and excess energy usage. Specific control techniques have been developed in [5, 13] to effectively manage the voltage, frequency and excess power usage throughout the respective systems. This paper will use a hierarchical command scheme for system management. Individual PID controllers will manage the voltage and frequency of each component in the system and an overseeing controller will be implemented for excess power management. Another consideration for a power system is the interconnection of each component. A DC bus has been previously used as the point of common coupling for all power sources and the load [13, 14, 17]. By coupling all the generating components to the same point, the number of control elements used in the system can be reduced, increasing the system efficiency. In particular, the power sources can share an inverter, which allows simple control of the load side voltage and frequency. The proposed system topology (see Figure 1) uses a DC-bus as the point of common coupling. All the above mentioned publications define robust power supply systems that use techniques including: Maximum Point Power Tracking (MPPT), maximisation of system efficiency by minimising the number of system elements, the use of multiple power sources, optimised ESS, as well as specific converters and control mechanisms. Hybrid systems, with effective battery backup schemes, have been developed in [10, 13] with MPPT and optimised control techniques in order to negate the need for a diesel generator. However, the limitations associated with the battery backup systems may cause undesired system consequences, especially in the case of prolonged low resource availability or single unit outages. The system proposed in this paper is based solely on renewable energy sources. WTGs, PV cells and PEM FCs make up the main structure of the proposed scheme (see Fig. 1). The type and capacity of each source is selected to suit synthesised load and resource profiles. A hierarchical control system is developed and optimised with separate controllers used for each renewable source. An overseeing controller is implemented to allow maximum usage of the energy available. The system is tested over a range of different wind, solar and loading conditions. II. SYSTEM DESCRIPTION The fully renewable energy sourced system was designed to meet the following requirements: • Maximum power capture from the PV panels • All excess power generated is to be stored as hydrogen • Sufficient power supply from PEM FC when demand exceeds available generation • All major components must be commercially available The proposed system consists of the following major components (as seen in Fig. 1):
1. 2. 3. 4. 5.
WTG system PVA system with MPPT Energy Storage System: PEM FC and Electrolyser Load side inverter Overseeing Control System for Power Flow Management
Figure 1 System Topology
This paper optimises the type and capacity of each source. First, the load must be analysed. A. AC Load An example of synthesized load data, derived from the data given in [2], is presented in Fig. 2. This is the collation of hourly data over an annual period. The load characteristics are defined in Fig. 2 including the off peak and on peak load variations (21 to 5 hours and 9 to 16 hours respectively), as well as the maximum peak load requirements (16 to 21 hours). The generation parameters of each element may be chosen to fit the load profile data. However, future load requirements may not be covered. The growth of an isolated community will be taken into account in the next section. B. Power Source Component Selection In order to select an appropriate WTG, or PV panel for an array, the site resource characteristics must be taken into account. In this case study, the winds speed and the solar irradiation (see Fig. 3). The site resource profiles is synthesised to reflect typical data from [5, 10]. Further considerations for specific WTGs and the photovoltaic array (PVA) include: capital cost, maintenance costs, lifetime, the availability of device, and power characteristics. The cost per kWh is also an important factor, but this can be determined by comparing the resource profile and power production to the total elements costs. C. Energy Storage Selection An ESS ideally encompasses the following key properties: a quick response time, ability to store all spilled energy and to release energy for a required amount of time. It must also be able to fully supplement the power generators in order to meet the maximum load demand. The costs, lifetime and power characteristics all play an important part of the selection of ESS. III. DESIGN PROCESS A systems load characteristics can be highly variable but still reasonably predictable. In order to understand the current and future load requirements, several years of data may need to be thoroughly analysed.
Figure 2 - Load Profile
Figure 3 - WTG Power Characteristics
Figure 4 - Maximum Power Points for the PVA
The initial considerations will be reviewed at a later stage in this paper. As in the WTG selection, the ESS accounts for the excess power requirements.
Figure 5 - Typical Wind Speed & Solar Irradiation Profiles
In this case the load data given in Fig. 2 will be used as a base and future requirements will be predicted using a simple multiplication factor. The future load data has the following characteristics: Maximum peak load of 2MW (16 to 21 hours), maximum off peak load of 500kW (21 to 5 hours) and maximum on peak load of 1.4MW (8 to 16 hours). From this data, the power source capacity can be selected. A. Wind Turbine Generator Selection Commercially, there are many choices for WTGs that fit the wind profile conditions outlined in Fig. 3. Comparing Fig. 2 and Fig. 3, it can be seen that, the maximum peak load occurs around the same time as the minimum resource availability. In particular, it can be noted that the irradiation level reaches zero at the time of maximum peak load. At 19 hours, the WTGs and the PEM FC must be able to supply 2MW at low wind speeds. Initially, two 850kW low wind speed turbines are considered, and the remainder of the power is contributed by the ESS. Multiple turbines should allow for the possibility of one turbine outage without greatly affecting the system. The associated power characteristic for the WTGs is seen in Fig. 4. B. Photovoltaic Array During times in which the PVA is operational, the wind speed is almost half the nominal speed of the selected WTGs. At 16 hours the combined WTGs may only produce 600kW, where the maximum load requirement is 1.4MW. It would be preferential for the PVA to supply the remainder of the power (800kW). The irradiation level at 16 hours is only 400W/m2 (Fig. 3). An array with a capacity of 800kW at 400W/m2 may have a nominal capacity of 3.5MW, which is not feasible or necessary. As the capacity of the PVA must be limited to be viable, a nominal 1MW PVA is initially considered for the system, composing nearly 4000 panels. A 1MW PVA is able to fully complement the WTGs during times when the irradiation level is at its peak. The PVA system may supply 200kW at 400W/m2. See Fig. 5 for the maximum power output at any given irradiation level.
C. Energy Storage System Electrolysers can be connected to either an AC or DC source and can operate over a wide range of power inputs (from 0% to 100%). The electrolyser is connected to the DC bus and used during times of surplus generation to create hydrogen fuel for the PEM fuel cell. A converter is used to control the DC input to the module. The electrolyser must be able to convert up to 1.2MW of power into hydrogen (see Fig. 6). An electrolyser with a 1.2MW capacity may consist of multiple smaller units (207 5.8kW units), or may be custom designed to only three or four (300-350kW) units with multiple stacks. As mentioned above, the FC will have to contribute a significant portion of power to the load during peak times. The FC must supply, at most, an extra 1.2MW of power. The required power can be achieved with four FC stacks totalling a 1.3MW capacity. D. Revisiting Component Considerations The initial system configuration consists of: 2x850kW WTGs, 1MW PVA and 1.3MW FCs. The current section will look at optimising the initial choices with reference to single unit outages and hydrogen production efficiencies. The hydrogen requirement of the PEM FC is an important factor in optimising the systems generation capacity. Taking into account the combined efficiencies of the FC and electrolyser (48%), it can be seen that the power required to generate hydrogen is much greater than what can be achieve from the FC operation. The FC efficiency factor has been incorporated into the design calculations. In the situation that all generation units are online (system is in normal operation), the total excess energy will be very small. In the case when any single unit is offline (be it one WTG the PVA or one FC Stack) the power production cannot match the load requirements. The lack of power can be noted to occur particularly during times of maximum power demand (see Fig. 6). Fig. 6 demonstrates the lack of power in any single unit generation outage and the FC requirements. In order to overcome the lack of generation during single unit outages, it is proposed that three 850kW turbines are used. An increase in wind capacity also allows the PVA capacity to be lowered to 750kW, resulting in the number of required panels being reduced by nearly 1000. An increase in the electrolyser capacity (to 2MW) must also be noted.
current of diode; RS is the series resistance of cell; and, TC is the benchmark cell operating temperature The output voltage and current are modified to allow for changes in ambient temperature and irradiation levels. The PVA output voltage (VPVA) and current (IPVA) incorporates constants for the number of parallel and series panels used in the array. 2
Figure 6 - 1700kW WTG, 1000 kW PVA and 1300kW FC
Figure 7 - Proposed System with Single Unit Outage
The proposed system incorporates the following components: 3x850kW WTGs, 750kW PVA and 1.3MW FC. Fig. 7 demonstrates the effects of any single unit outage on the load matching ability of the system. It can be seen, in Fig. 7, that when a single WTG is offline, the FC must operate at maximum capacity. In all other situations only 3 of the 4 FC stacks are required. The excess power stored in any single day will range from 150kWhrs, when a WTG is offline, to 8000kWhrs when the system is operating normally. The excess energy is converted to hydrogen which may be used as fuel for vehicles, or sold to other communities. IV. SIMULATION MODEL A. The Wind Turbine Generator The proposed WTGs are modelled as multiple induction machines coupled mechanically to wind turbines. The generators are 4-pole 50Hz doubly-fed induction machines with an AC output voltage transformed to 1kV. The cut-in and cutout speeds of the units are 3.5 and 25m/s respectively and the WTGs are rated for 13m/s winds. In the proposed system the AC supply from the WTG is rectified using a diode bridge rectifier and the DC voltage is regulated to 1350VDC using a converter. The converter then feeds a DC bus to which all other sources are connected. B. The Photo-Voltaic Array A mathematical model of a PVA is used from [18] in order to simulate multiple PV panels. The model developed in [18] uses a simplified equivalent circuit as a basis for a solar cells output voltage (VC). This voltage is a function of the photocurrent (Iph): ln
, 1 where e is the charge on an electron; k is Boltzmann constant; IC is the cell output current [A]; VC is the cell output voltage [V]; Iph is the benchmark photocurrent; I0 is the reverse saturation
, 3 where nS is the number of series panels and nP is the number of parallel panels. The constants used for adjusting the array output for changes in insolation and temperature, CIT, CIS, CVT and CVS are given by: "# 1 4 $ $& $ 1 5 $ 1 ( # 6 *($# $ 1, 7 where α is the constant that represents the rate of change of the cell operating temperature due to changes in irradiation level; β and γ are the respective voltage and current temperature constants for the PV panels used; SC is the benchmark cell solar irradiation level; and, SX and TX are the variable Solar Irradiation and Ambient Temperature of the PVA cells respectively. The proposed PV panels have a 260W, 51.25V and 5.07A optimal output. Other characteristics used in the modelling include short circuit current (5.43A) and open circuit voltage (60.09V). The above specifications were recorded at an irradiation level of SC = 1000W/m2. The PVA consists of 2900 separate panels. 100 parallel groups of 29 series panels are required for the PVA to give an output of 750kW at 1450VDC at optimum operating point. The PVA voltage output allows for the proposed converters to be centred on the rectified DC output from the WTGs. The PVA is connected to two converters: one for MPPT, and one for voltage control purposes. The regulated voltage is then fed into the DC bus. C. Energy Storage System The ESS has two parts: the FC, and an Electrolyser. The FC is modelled as multiple fuel cell stacks with a voltage controller feeding the DC bus. The Electrolyser model is a DC load connected to the DC bus via a voltage regulator. D. Inverter The load is a 1000Vrms AC, switch controlled, variable load for simplicity of matching generation and load voltages. The load voltage is controlled by an IGBT inverter using PWM control signals and the inverted three phase rms voltage is simply: 2√2 2 ,-./0/ cos 6 7 8 , 8 2 6 where n is the harmonic number (n=1 for the fundamental component). The inverter is sourced from the DC bus.
E. Hierarchical Control System 1) Excess Power Management Control System: The overseeing control system has been designed to use voltage and current measurements to match the generation with the required load. It identifies with the following states: see Table 1. State 1 State 2 State 3
TABLE I STATE IDENTIFICATION Matched generation and demand (PVA and/or WTG only) Demand exceeds generation – PEM fuel cell starts generating Generation exceeds demand – Hydrogen production from Electrolyser
• • •
Table 2 describes the final values for individual operation of each source. Table 2 demonstrates the same operating parameters in states 1 and 3. Constant KP KI KD
State migration can be seen in Fig. 8:
State 2
State 1
State 3
Figure 8 - State Diagram
Voltages and currents from the WTG, PVA, FC and Electrolyser are all monitored as well as demand power in order to match the generation with load requirements. 1) Localised Component Controllers Each component has its own voltage regulator connected to its output. The PVA is controlled using dual Buck-Boost converters. One for maximum point power tracking (MPPT) and the other for voltage control of the DC bus. The WTG and the PEM FC, on the other hand, only have a single Buck-Boost converter for controlling the output voltage. The converters use IGBT switches controlled by variation of the duty cycle. The IGBT switching is controlled using Pulse-Width Modulated (PWM) signals. The PWM signals are generated using a combination of a sawtooth waveform and a factor proportional to the duty cycle required. The PID controllers use samples of the DC Bus voltage and make (limited) adjustments to the duty cycle in attempt to keep the voltage within the required operating range. The voltage set-point for the MPPT converter is dependent on the given irradiation level. The set-point for the voltage regulators is the DC bus voltage (1350V). The PID controllers have varying parameters dependent on the operating state as determined by the overseeing system. The gain values have the following limitations:
Stable over component output voltage in range 9001500V
