IMTC 2005 – Instrumentation and Measurement Technology Conference Ottawa, Canada, 17–19 May, 2005
Modular Architecture for Remote Management of Power Generation Plants G. Brugnoni, M. Gagliarducci, D.A. Lampasi, L. Podest`a Department of Electrical Engineering, University of Rome “La Sapienza” via Eudossiana, 18 – 00184 Rome, Italy Phone: +39 06 44585543, Fax: +39 06 4883235, E–mail:
[email protected] Abstract – This paper describes a network architecture to remotely monitor and control complex power generation plants. Such architecture was designed as a sequence of independent blocks, in order to allow the interchangeability among elements belonging to the same block type. The resulting systems can be adapted to the particular plant, the available communication media and the sustainable costs. The architecture and the proposed solutions were applied to two types of generation plants, based on photovoltaic and wind micro– generators. Keywords – Communication systems, power generation control, power generation maintenance, power system monitoring, remote sensing, solar power generation, wind power generation.
I. INTRODUCTION Nowadays the production of electrical power is moving from the big power plants to micro–generators spread along the territory (distributed generation). In particular, due to both economical and ethical considerations, the renewable sources provide a more and more relevant contribution to the available power and were introduced also for domestic power supply. There are situations where the connection to the national distribution network is unpractical: for example, photovoltaic (PV) panels are often used to feed some isolated systems as radio stations, street signs, water pumps, and so on. In these cases, the energy is locally produced and managed (stand– alone plants). The places where this kind of plants are usually located (like, in our experience, mountains or woods) are difficult to reach, and neither telephone lines nor other communication systems are available. Analogously the presence of maintenance operators should be as low as possible. For these reasons, such plants need to be managed by remote systems and the control should imply the possibility for users to remotely operate on the plant. In preceding works, some systems for the remote monitoring and control (MC) of photovoltaic plants [1] and of wind generators [2] were developed. These systems were more and more improved in order to include a wide set of phenomena and to reduce the global costs. The final target of these efforts became the unification and the compliance of all the available facilities in a more general network, independently of the specific conditions of the generation. This paper presents such structured architecture.
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All the plants are locally managed by a control unit (CU) and are supervised by a global server (GS), practically a standard PC running LabVIEWT M . In normal conditions, the CUs register and report the overall performances, while the GS periodically interrogates them (for example, once a day). Therefore, provided that a suitable alarm (AL) system exists in case of incorrect behavior, the required transmission rate is pretty low and the interval between transmission may be very long. In critical situations, the GS is replaced by external operators directly informed by the CU and able to operate on the CU itself. The presence of smart acquisition units close to the plant suggests to use it for a sort of local control (LC). In fact, since the employed hardware allows a real–time (RT), cost–effective and flexible LC, it can partially or completely substitute the existing (analog) control devices. Finally, two implementations, developed according to the considered principles, are presented. They consists in the MC of a PV plant and of a wind plant, respectively, thus representing the most diffused families of micro–generators. These examples are easily reconfigurable to cover a wide set of common configurations. II. ARCHITECTURE OF THE NETWORK Fig. 1 shows the basic principles of the proposed architecture. The division in many blocks was chosen to provide complete modularity: each block can be substituted and adapted to the particular plant, environment and specifications (especially, type of connection and LC). For example, the expensive hardware in section IV may be replaced with a cheap general– purpose microcontroller. The sensors–transducers (ST) and signal conditioning (SC) sections are specific for each application. It is valid also for the LC, even if generally common functions can be found (switch– driving and load regulation). The data acquisition (DAQ) system is quite interchangeable, even if the software must be adapted to the data processing. The performed measurements may be relative to electrical parameters of the plants (as the produced power) or to environmental quantities (as temperature and wind speed). The transmission (TR) system (intermediate and to the GS) is totally independent of the plant. Its
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Plant
ST
SC
DAQ
Buffer
TR
Channel
GS
TRAL
Channel
OP
LC AL
Fig. 1. General architecture of the network (relative to one CU). The function of each block is explained in the text. The two examples proposed in nest sections indicates some alternatives that have been tested to implement such blocks.
TABLE I S IGNIFICANCE OF THE SYMBOLS USED FOR THE ELEMENTS AND THE FUNCTIONS OF THE NETWORK .
Symbol
Significance
CU
Local control unit
GS
Global server
ST
Sensors–transducers
SC
Signal conditioning
DAQ
Data acquisition
LC
Local control
TR
Transmission system
AL
Alarm message
TRAL
Alarm transmission system
MC
Monitoring and control
Fig. 2. General scheme of the PV plant and of the MC system.
choice is regulated only by logistic (available means of communication and required alarm time) and economical considerations. The described units are summarized in table I. In sections IV and V, two applications of above guidelines are presented, along with some useful practical considerations. In both cases, different alternatives were tested for the same block.
the GSM PPP connection, the SMS service was favored for the communications from and to the operators. The GPRS extension allows the implementation of mobile web–servers and mail–servers. Obviously, the costs for solutions based on ground PPP and Internet are lower; and they are even lower when a LAN exists. The wireless version (WLAN) avoids cumbersome cables, but operates on small distances; the possibility to remarkably extend the dimension of the WLAN, offered by directional antennas, was examined; however the GSM offers better performances at lower initial costs. Due to economical and legal concerns, the wireless communication allows only short–distance links, so they were used just for intermediate paths before the actual communication system (for example, among groups of plants or for separated transducers).
III. AVAILABLE MEANS OF COMMUNICATIONS IV. MANAGEMENT OF A PHOTOVOLTAIC PLANT The network manages several kinds of communication systems and media: • point–to–point phone (PPP); • GSM (Global System for Mobile communication), including SMS (Short Message Service) and GPRS (General Packet Radio Service) standards; • Local Area Network (LAN); • Internet; • radio–frequency (RF) wireless. Since the GSM system is able to communicate practically in every place, provided that such place is included in a GSM cell, it is the most general solution. In particular, along with
Many configurations are possible for a generic PV plant. To include a meaningful amount of conditions, a basic configuration, representative of all the stand–alone and not guarded plants, was established. It consists in (Fig. 2): • a PV generator; • accumulators; • a charge regulator incorporating a MPPT (Maximum Power Point Tracker); • an inverter. The correct behavior of the plant depends on the correct functioning of each single element. The developed structure
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Fig. 4. Two sections of the front panel of the configuration VI: the former for the selection of the existing components, the latter for the parameters.
Fig. 3. The roof of our Department with some PV modules.
is an extension of [1] (Fig. 3) to functionally reproduce the architecture of Fig. 1. The TR is based on a GSM modem (Pocket GSM by Digicom). The particular working principle of the PV cells allows to concentrate the PPP data exchange during the night. Anyway the SMS service integrates it with a cost–effective communication to the operators. The ST and SC parts are implemented by means of a circuit appositively designed, featuring by a platinum thermoresistor PT100 [3] and a calibrated PV sensor cell (PVSC). The PVSC was introduced to obtain an estimation of the incident radiation G on the PV generator and thus of the theoretical available electrical power, according to [4] [5]. The short–circuit current ISC of the PVSC (measured through the voltage across a 5 mΩ precision resistor [6]) can be approximated to the photogenarated current IP [1]: ISC IP .
(1)
IP can be viewed as the intensity of the current generator contained in the equivalent circuit modeling a generic lighted cell. The value of IP depends on the intensity of the incident solar radiation G [W · m−2 ], on the cell area S [m2 ] and on the responsivity Rph [A · W−1 ] [5]. Thus: G=
ISC IP . Rph S Rph S
(2)
G is also useful to evaluate the theoretical power W for a PV generator having efficiency η and total area Stot : W = η · G · Stot .
(3)
The high–cost modules (FP–2000 controller and FP–AI– 100 FieldPointT M by National Instruments) implement both the DAQ and the TR on LAN. They are managed by a virtual instrument (VI) developed in LabVIEWT M RT (real–time). The VI checks if the PV plant components are working well by measuring: voltage, current and supplied power; current and power actually received by dc and ac loads; accumulators’ voltage. The produced power is compared, with a sampling time Ts = 0.1 s [4], to the theoretical value, obtained exploiting the PVSC and the thermoresistor. The power adsorbed by the
loads and the charge of the accumulators are compared with adjustable limits. When an anomaly occurs, the process sends an alarm reporting all the necessary diagnostic data. If the anomalous situation goes on, the SMS is sent again and again with adjustable rate. From the same measurements, the VI derives, every 30 s, the maximum and mean value of some quantities and efficiency indices (Average Efficiency, Array Yield, Reference Yield, Losses Array Capture) [4]. The MC system is also able to accept and execute remote orders from users. It is achieved by inserting suitable commands in a SMS, written on a normal (cellular) phone. These commands can change the configurations of the plant and the settings of the MC system itself. Obviously the commands must comply to a predefined syntax and a security policy is required: the SMS must contain a correct (modifiable) password; the access can be limited to a set of telephone numbers. V. MANAGEMENT OF A WIND PLANT The considered wind plant (Fig. 5) consists on an axial aeolic turbine coupled to a permanent–magnet generator [7]. A MOSFET step–down dc/dc converter ensures that the turbine is working in the maximum efficiency conditions. A PIC 16F876A [8], programmed in Assembler, monitors the rotation speed, the effective torque, the accumulators’ charge status; thus it represents the DAQ (Fig. 6). A RF wireless communication is used to receive data from an anemometer, by means of a couple of RF modems (Aurel WIZ903A8 at 900 MHz), intended as intermediate buffers. In this case, the LC is relevant: its main function consists in the generation of the Pulse Width Modulation (PWM) signal driving the MOSFET and therefore the converter output current ton Iin , δ= , (4) Iout = δ ton + tof f according to the desired control curve. The desired electromagnetic torque Td is proportional to the machine phase voltage, while the actual torque Ta is proportional to the rectifier output current. The microcontroller, using the equation corresponding to the control curve, derives the value of Td from
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The ordinary TS exploits a RS232–Ethernet converter (National Instruments) or a 28.8 kB/s Phone Modem (Trust). A GSM modem provides alarm SMSs for faults and battery discharge. Alternatively, the use of a compact GPRS modem (Telit GM862) connects the CU connection to Internet: in particular, the CU can be programmed as a mail–server. VI. CONCLUSION Several devices were tested to implement the remote monitoring and control of power generation plants and to replace the existing analog controls providing better flexibility. The adaptation of such devices to the concrete conditions of the plant suggested to put them in the context of a general network architecture, supervised by a central global server. In this way, a complete interchangeability among elements of the network was accomplished. Along with the normal management of the plant, the remote monitoring of the generators, of the energy converters and of the accumulating system are allowed, including facilities to operate in case of malfunctioning. The use of the GSM standard extends the effectiveness of the management to the plants far from electrical energy distribution network and from the usual telecommunication systems. Due to the low cost and diffusion of the GSM device the transmission system is fairly cheap and will become cheaper and cheaper. Moreover, considering the need to develop cost–effective micro–generators, we reduced the cost of the acquisition system by extensively introducing general-purpose microcontrollers. These characteristics were experimentally confirmed by two meaningful examples.
Fig. 5. Picture of the considered wind generator (our Department roof).
REFERENCES Fig. 6. Block diagram of the CU and schematic circuit of the energy conversion with emphasis to the LC.
the rotation speed of the generator. The PWM duty cycle δ is regulated to cancel the difference between Td and Ta [9]. If the batteries’ voltage exceeds 14 V, the PIC inserts in parallel an additional load. The condition of discharge of the batteries (voltage less than 10.5 V) can correspond to a normal long absence of wind, but it can also derive from a mechanical fault on the generator. An I2 C bus connects the buffer, a set of 8 external 64–kB EEPROM, and another PIC managing the TS. The double–PIC architecture allows an easy interchangeability and avoids to excessively burden the DAQ and the LC (the wind generator may work 24 hours per day). Moreover the EEPROMs store the data even in case of malfunctioning or absence of power supply. They can be written up to 106 times, that is sufficient for this application.
[1] M. Gagliarducci, D. A. Lampasi, and L. Podest`a, “Remote Monitoring and Control of Photovoltaic Plants by means of GSM System,” in Proc. 13th IMEKO TC–4 International Symposium, Sept. 2004, vol. 1, pp. 200– 205. [2] G. Brugnoni, D. A. Lampasi, and L. Podest`a, “Micrcontroller–Based System for the Management of a Wind Power Generation Plant (in Italian),” in Proc. Electrical and Electronic Measurement Group (GMEE) National Congress, Sept. 2004, pp. 1160–1164. [3] IEC Standard for Industrial Platinum Resistance Thermometer Sensors, IEC Std. 60751, 1983. [4] IEC Standard for Photovoltaic System Performance Monitoring – Guidelines for Measurement, Data Exchange and Analysis, IEC Std. 61724, 1988. [5] J. Singh, Semiconductor Optoelectronics, McGraw–Hill, 2000. [6] F. Lasnier and T. Gan Ang, Photovoltaic Engineering Handbook, Adam Hilger, 1990. [7] B. J. Chalmers and E. Spooner, “An Axial–Flux Permanent Magnet Generator for a Gearless Wind Energy System,” IEEE Trans. Energy Conversion, vol. 14, no. 2, pp. 251–257, June 1999. [8] PIC16F876A Data Sheet, Microchip. [9] F. Caricchi and al., “Testing of a New DC/DC Converter Topology for Integrated Wind–Photovoltaic Generating Systems,” in Proc. European Conference on Power Electronics and Applications, May 1993, vol. 2, pp. 1160–1164.
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