Data Acquisition in Photovoltaic Systems

The Solar Amphitheatre at Valahia University of Targoviste is the first integrated PV building in Romania and one of the few in Eastern Europe. The European Union is aiming to increase the use of renewable energy which can replace fossil fuel, diversify energy resources and reduce carbon dioxide emissions. Technologic developments, increasing energy request, reduction of fuel supplies, environment pollution and a much computerized society are factors that intensified research on new energy production solutions. Photovoltaic systems convert sunlight energy into electric energy and are characterized by modularity, operating autonomy and long operation period. The paper presents the authors’ experimental results regarding the operation of the “Solar Amphitheatre” photovoltaic system and aspects on monitoring the electric energy supplied by a photovoltaic generator.

inputs of multiple channels and drives a single ADC can substantially reduce the cost of a system (Szekely, 1997). Specialized data acquisition systems for PV installations require a study of sample rates and an optimal configuration of the measuring chain. This chapter brings informations regarding the structure of data acquisition systems used in the monitoring of photovoltaic installations. It shows the operating principles of building blocks and the operation is performed by simulations using LabVIEW™ -Laboratory Virtual Instrumentation Engineering Workbench. An important part of the presentation is dedicated to current measurement and data acquisition systems dedicated for monitoring PV systems. Applied solutions and experimental results are discussed in terms of accuracy and optimization needs of the operation.

PV system and data acquisition
The chapter presents the authors activity and results regarding the operation of a PV system and aspects on monitoring the electric energy supplied by a PV system built in University "Valahia" of Targoviste, Romania. Part of the ICOP DEMO 4080-90 European research program, this PV system has been realized by the staff of the Electrical Engineering Faculty, Targoviste, Romania (Andrei et al., 2007).
The PV system integrated into the roof of the building has been designed using 66 Optisol SFM 72Bx glass roof integrated multi crystalline Si modules produced by Pilkington Solar and 24 ST40 thin film modules produced by Siemens which generates a total amount of 10 kWp. Position of panels on the southern front is shown in Figure 2. These modules can be serial or parallel connected. The dc voltage produced by the PV system is converted by the Sunny Boy inverters (SWR 700, SWR 1100, SWR 2000 and SWR 2500) and supplied directly into the public electricity system - Figure 3. The use of a controller to monitor the operating parameters ensures the sine-wave form of the voltage and current, with a low amount of harmonics. The control of operations serves to totally automated functioning and to adjustement of the MPP (maximum power point).
The connection diagram of the PV system has been designed after a series of shading effects analyse and buiding placement restrictions (Dogaru Ulieru et al., 2009). www.intechopen.com PV system monitoring was done by two methods: -Sunny Data Control equipment - Figure 4 -automatically reads the selected measuring data from the memory of Sunny Boy Control and stores it on the PC. Sampling interval has been preset to 10 minutes (day and night). -The data acquisition system -represented by an association between the hardware equipment (AT-MIO 16XE50 acquisition board, a signal conditioning device) and the application software (LabVIEW™) which implements the required functions, playing the part of an interface between the human operator and the measurement system . To ensure the accuracy of the measurement, the operating parameters of the PV system and the configuration of the acquisition system are taken into account and have imposed the signal conditioning and the setting of the signal source, of the field and of the channels. Analog inputs can be differently configured, with a voltage level of ±2.5V, ±5V, ±10V (bipolar/single polar) which can be selected through the configuration program of the acquisition board. The block diagram of the acquisition system is presented in Figure 5. Figure 6 shows the LabVIEW™ block diagram of the virtual instrument attached to the data acquisition system (Cepisca et al., 2004).

General principles of data acquisition systems
Data acquisition systems are products and/or processes used to collect informations that can be processed or stored by a computer to document or analyze some phenomenon (Judd, 2008). Measurement systems are used in the data acquisition industry since 1981 and have supplied several million data acquisition channels on an international basis. Data acquisition systems come in many different PC technology forms to offer flexibility when choosing a measurement system. The organization of information flow in the system is an important problem in designing and operating the measurement. Two aspects are essential for this organization: the kind of transmission in the system (serial, bit-by bit, parallel) and the mode of information exchange between system devices (Nawrocki, 2005).
The functional components of a basic data acquisition are: -Transducers and sensors; -Conditioners or circuits standardizing the level of transducer signal to the range of the input voltage of the analog-to digital converter (ADC); -Analog-to digital converters (ADCs) to convert analog into digital signals; -Devices for visual display of measurement results (display of digital measurement instruments, oscilloscopes or a computer monitor); -Computer with dedicated software and memory resources. More complicated data acquisition systems can be constructed in the hierarchical structure. On the lowest level are subsystems to collect data from physical quantities. The main controller of the system receives processed measuring data and sends commands relating to the execution of a measuring procedure or a set of commands to subsystems. Analog-to-Digital (A/D) conversion of an analog signal involves two processes:  Sampling -after this operation signal is represented by a set of values {x(kT s )}, drawn at time periods T s - Figure 7. Sampling frequency should be high enough to provide a number of samples sufficient to reproduce the signal in the analog form. According to the Shannon theorem, the sampling frequency f s should not be lower than twice the upper frequency f u of the sampled signal spectrum: Otherwice, the reproduction of the discrete signal recorded yields a distorted analog signal, caused by a too low sampling frequency, phenomenon called aliasing. In order to eliminate aliasing, is utilised a lowpass input filter or an antialiasing filter.  Quantizing: assigning to every sample a value from a set of N values into which the measurement range is divided. Figure 8 shows a LabVIEW™ application for the quantizing of voltage into an n=3 bit digital signal, the number of quants is 2 3 =8. The A/D converter is connected to an analog input signal; it measures the analog input and then provides the measurement in digital form suitable for use by a computer. The resolution of an A/D input channel describes the number or range of different possible measurements the system is capable of providing. This specification is almost universally provided in term of "bits". For example, 8-bit resolution corresponds to a resolution of one part in 2 8 -1 or 255, 12bit corresponds to one part in 2 16 -1 or 65,536. To determine the resolution in engineering units, simply divide the range of the input by the resolution. A16-bit input with a 0-10 Volt input range provides 10 V / 2 16 or 152.6 microvolts. Table 1 provides a comparison of the resolutions for the most commonly used converters in data acquisition systems. E ro a re c . From the point of view of the method of conversion there is a variety of different types of A/D converters used in data acquisition. The most commonly used A/D converters in today's data acquisition products are divided into: -Dual Slope/Integrating -with a good attenuation of interferences during the integration process - Successive approximation -provides resolutions in the 10 to18-bit range, and depending on the resolution, offers sample rates up to tens of Megasamples per second. -Flash -characterized by the shortest conversion time -Sigma Delta provides very high resolution, especially in converting continuous signals. Most multi-channel data acquisition systems are based upon a single A/D converter. A multiplexer is then used between the input channels and the A/D converter. The multiplexer connects a particular input to the A/D, allowing it to sample that channel. Figure 9 depicts a typical, multiplexed input configuration. The primary disadvantage of this system is that even if the switching and sampling are very fast, the samples are actually taken at different times. The ability to sample inputs at the same instant in time is typically referred to as simultaneous sampling. There are two ways to achieve simultaneous sampling. The first is to place a separate A/D converter on each channel. They may all be triggered by the same signal and will thus sample the channels simultaneously. The second is to place a device called a sample & hold (S/H) on each input. When commanded to "hold", the S/H effectively freezes its output at that instant and maintains that output voltage until released back into sample mode. Once the inputs have been placed into hold mode, the multiplexed A/D system samples the desired channels. The signal to be sampled will all have been "held" at the same time and so the A/D readings will be of simultaneous samples. The simultaneous sampling configurations are shown in Figure 10. www.intechopen.com A criterion applied to classify data acquisition systems is the transmission type of digital announcements (data, addresses, commands): serial transmission or parallel transmission. The interface system assures equipment and adjustments of devices attached to the bus. The original PC-bus, ISA-bus, is a simple, robust, and inexpensive interface that has certainly stood the test of time. However, most of today's plug-in board business is based on the PCI bus (or variants such as cPCI and PXI). The external box vendors now have Ethernet and USB standards to work with as well as some less used, but very viable, interfaces such as Firewire, CAN, and perhaps the oldest standard in computing, RS-232. Software has progressed from the original DOS-based version to hyghly complex applications such as MATLAB and LabVIEW™ that are both easy to use and able to take advantage of today's powerful computers.

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Currently, most data acquisition companies provide hardware either at board level or external box and software. PC-based DAQ systems are available with a wide variety of interfaces. Ethernet, PCI, USB, PXI, PCI Express, Firewire, Compact Flash and even the venerable GPIB, RS-232/485, and ISA bus are all popular (Manea & Cepisca, 2007). RS-232 is by far the simplest and least expensive interface. Every PC shipped today can communicate over an RS-232 line. Only a compatible cable and a terminal program are required. RS-232 is limited to about 50 feet, but it can operate over huge distances with a little help from a modem. Only one device can be connected to a single RS-232 port. This interface is typically not electrically isolated from the host computer. For example, if a power line were inadvertently dropped across the RS-232 line, the host computer most likely would be destroyed. As a result, RS-232 may be appropriate for situations in which very few remote I/O systems are needed and where the likelihood of electrical transients is low. RS-485 allows multiple devices (up to 32) to communicate at half-duplex on a single pair of wires, plus a ground wire (more on that later), at distances up to 1200 meters (4000 feet). Both the length of the network and the number of nodes can easily be extended using a variety of repeater products on the market. Many RS-422/485 protocols were developed in the 1980s for such applications. Profibus, Interbus and CAN are a few. These were designed with proprietary interests in mind, and each subsequently received backing from several vendors in an attempt to develop an open international standard with widespread support. Ethernet has been around for more than 20 years and has become a commodity in modern business environments. Ethernet also has many characteristics that make it suitable for industrial networked and remote-sensor I/O applications. It combines the low cost of RS-232 with the multidrop capability of RS-422/485 and provides a clear standard for communications. Ethernet has become the medium of choice to communicate management data throughout the enterprise. Interoperability of Ethernet based data acquisition devices from multiple vendors has not always been stellar. However, most Ethernet based data acquisition systems are single vendor and this has not been a major issue in the acquisition space. The LXI Consortium has developed a specification that ensures simple and seamless multi-vendor interoperability. Gigabit Ethernet is a version of Ethernet that supports 1 Gigabit per second data transfer rates. Boasting the same speed capability as standard Ethernet, the fiber optic implementation extends the range of the system to 2 kilometers. Fiber also provides virtually absolute electrical isolation and has immunity to electrical and magnetic interference. Firewire is a high speed serial interface. However, at approximately the same time Firewire was being promoted, USB interface was coming on line. At this time, there are a wide variety of data acquisition vendors and products actively promoting USB devices, Firewire success has been confined to the original target market of audio and video. USB's simple plug-and-play installation, combined with its 480 Mbps data transfer rate, makes it an ideal interface for many data acquisition applications. Also, the popularity of USB in the consumer market has made USB components very inexpensive. USB's 5-meter range is perhaps its largest detraction as it limits the ability to implement remote and distributed I/O systems based on USB.
Today, board level solutions offering 24-bit resolution are now available as, e.g. 6.5 digit DMM boards. On the box side, USB 2.0 is capable of delivering 30 million 16-bit conversions per second and Gigabit Ethernet will handle more than twice that. The internal plug-in slot data transfer rates have increased 10 fold in recent years. The market of data acquisition equipment is very large, different companies propose new solutions. National Instruments offers several hardware platforms for data acquisition. The most readily available platform is the desktop computer, with PCI DAQ boards that plug into any desktop computer. For distributed measurements, the Compact FieldPoint platform delivers modular I/O, embedded operation, and Ethernet communication. For portable or handheld measurements, National Instruments DAQ devices for USB and PCMCIA work with laptops or Windows Mobile PDAs. In addition, National Instruments has launched DAQ devices for PCI Express, the next-generation PC I/O bus, and for PXI Express, the high-performance PXI bus.

Simulations and experimental acquisitions
To determine the operation characteristics of the photovoltaic panels and the panel arrays, the built data acquisition system allows to measure the values of current and voltage, to simultaneously trace characteristics (current-voltage, power-voltage, power-charge resistance) -see figures 11 and 12, to present the measured parameters (during the data acquisition) in tables, to save data into files for future processing. For alternative current systems it is necessary to obtain signals with volt-range amplitude to be applied to the input of the data acquisition board. For phase/line voltages there can be used voltage dividers (which do not ensure galvanic isolation) or voltage measurement transformers (ensure galvanic separation). Shunts (current-voltage converters) or current measurement transformers can be used for currents. The use of both voltage dividers and shunts must be done by taking into account the current through the voltage divider, the voltage drop on the shunt, the power dissipation, parasite resistances, self-heating effects and dynamic effects. The use of voltage-current measurement transformers guarantees galvanic isolation of the measuring system but it introduces ratio and angle errors and inadequate perturbation transfer. The solution adopted was to use current and voltage transducers based on the Hall Effect.

MODULES
To obtain a good magnetic sensor the magnetic field is concentrated around the transducer, by using a circular core (used as a flux concentrator and made of a material with high magnetic permeability), which ensures both an increase of the magnetic field in the area of the sensor and the independence from the position of the conductor inside the core. Nonlinear behavior of the flux concentrator can be obtained by using an operational amplifier, which injects a compensation current through the reaction loop. Figure 13 presents the front panel of the LabVIEW™ application which acquires and processes data in the AC circuit from the output of the phase B connected inverters. Figure 14 shows a LabVIEW™ application (Ertugrul, 2002) to determine the characteristics of PV panels, with the possibility of remote monitoring.

Experimental results in PV system
The following results are obtained with data acquisition system Sunny Data Control. Totally, around 90 channels are recorded permanently. All channels are stored in SBC+ memory for maximum 8 days; after exceeding this interval the data will be lost if not transferred to the computer. Depending on the connected sensors the Sunny Boy Control+ can monitor all inverters and PV panels to ensure an even higher performance and more sophisticated system diagnosis.   The application allows visualising the phase diagram - Figure 17 -and obtains information on the quality of electricity produced by PV system - Figure 18. After processing the data obtained with Sunny Data Control software following characteristics were obtained.

Conclusion
Even though the costs of installations producing electric energy with PV panels are high compared to the costs of conventional installations, the number of such systems is continuously increasing. It is very important to determine the output characteristics of the PV panels in order to achieve an accurate connection and operation of the device and reduce energy losses. Monitoring activities follow the operation analysis by periodical reports, papers, synthesis, with the precise aim to make the most accurate decisions to produce electric energy using unconventional sources.
To quantify the potential for performance improvement of a PV system, data acquisition systems has been installed. The importance of this chapter consists in the presentation of a dedicated DAQ used in PV system analysis and real data measurements. The operation is performed by simulations using LabVIEW™. The information obtained by monitoring parameters, such as voltage, current, power and energies are fed to the PC via the DAQ for analysis. The control interface has been developed by utilizing LabVIEW™ software. The system has been in operation during the last five years and all its units have functioned well.