a. Analog Input Unit. Is constituted by two different types of devices. One of them consists of FP analogue input modules, which filter, digitalize, calibrate and scale raw sensor signals to engineering units.

The second one is a high-speed analogue-to-digital (ADC) conversion card. The FP modules are used for measuring signals at low sampling rate whereas the card is used for measuring signals which require to be captured at high rates. The analog input unit includes the following devices:

■ Module FP-TC-120: Filters the input signal by removing 60 Hz noise. A high-accuracy, 16 bit resolution ADC, with an ultra-stable voltage reference and built-in calibration circuit digitalizes input signals. It also provides cold-junction compensation.

Additionally, the device has the following technical features.

S Eight thermocouples or milivolt inputs.

S Four voltage ranges: ±25mV, ±50mV, ±100mV,-20 to +80mV S Update rate: 0.8 s.

у Offset error: 3|jV (in the minimum used range)

The FP-TC-120 module is used to measure the signals coming from the radiation/temperature sensor and from the DC current transducer.

■ Module FP-AI-100: This module is suitable to monitor milivolt and volt inputs from a variety of sensors and transducers. This module has the following technical features:

S Eight analog voltage input channels.

S 11 input ranges: ±15V, ±5V, ±1V, 0-15V, 0-5V, 0-1V.

S 12-bit resolution

S Update rate: 2.8 ms

S Offset error: 1.1mV (in the used range)

■ NI6024E Card: Is used to monitor the signals coming from the IAC and VAC transducer and the IDC and VDC signals coming from the I-V characteristic Unit. This board is also used for harmonic analysis of the AC signal generated by the DC/AC inverter. The board has the following technical features:

S 16 analog input channels.

S Voltage ranges: ±50mV, ±500mV, ±5V, ±10V.

S Sampling rate: 200KS/s S Update rate: 10 KHz. f Output voltage range: -10 to 10V.

b. Interface unit: Is constituted by the network communication FP-1000 module, which connects the Field Point I/O modules directly to the PC RS-232 port, and the NI-6024E Board which connects its analog input directly to the PCI port.

The FP-1000 module manages communication between the host PC and the I/O modules at a maximum rate of 115.2Kb/s via local bus formed by Field Point terminal bases.

The configuration of all hardware of the Field Point modular distributed I/O system and of the NI-6024E data acquisition board is made by software. The Field Point Explorer and NI — DAQ software are used to configure the FP-1000 system and the NI-6024E board respectively.

c. Unit for I-V Measurements. The monitoring system includes a unit by which the I-V characteristics of the PV-Generator can be obtained through a special design of the measuring circuits. The current and voltage measurements can be made at high sampling rates, allowing the achievement of the entire I-V curve in a short time (less than 5 sec); the fast achievement of the I-V characteristic is convenient to prevent cloud interference during outdoor measurements.

The PV current-voltage measurements are made varying the load of the PV generator, keeping it under illumination. During the time in which the load is varied, the operating point of the PV generator changes, allowing the current and voltage points to be captured along the I-V curve.

The principle of operation is an electronic load connected in parallel to the combination of the PV-generator in series with the battery bank. The electronic load is constituted by several transistors in cascade (Darlington combination), which are connected to the parallel combination of a resistor and a capacitor.

If the transistor conducts, the battery compensates the generator current, taking the generator close to the short circuit point. Reducing gradually the transistor base current, the generator moves from its short circuit point to the open circuit voltage status. It is required from the battery to supply current of several amps (sufficient to compensate the current generated by the PV-generator) at low voltages (<12V).

The variation rate of the base current is given by the time constant of the RC circuit. The scan of the I-V curve is automatically achieved through a relay which is driven through the virtual instrument, controlling the I-V measuring process. In order to achieve a uniform sweeping through the I-V curve, the base current was linearly varied. The linearization is

achieved by changing the time constant of the RC circuit during the I-V measurement, with the help of a tool included in the Virtual Instrument and implemented with LabVIEW. The entire measurement process is controlled by computer.