• Strong insolation: on a tilted plane, approximately 1.9 kWh/m² (latitude 40º N).
• Existing 15 kV grid.
• Size of available lot (area of approx. 30,000 m²) and location at short distance from a hydroelectric power station.
• Good infrastructure and ready access via national and regional roads.

Other advantages to the site chosen were that the required land was immediately available, close to Madrid, apt from the standpoints of public accessibility, operation and maintenance, and ideal for a 1-MW demo plant. The environment prevailing at the photovoltaic plant site (impacted by the existence of a nearby reservoir used for irrigating crops in inland Spain's hot dry climate) is home to unique forms of fauna and flora. Migratory birds use the reservoir for hibernating and as a seasonal habitat. Other bird and mammal species that depend on the water supply reside there all year round. Plants such as thyme, broom, and lavender grow alongside the man-made canals. When initially acquired, the site was strewn with concrete and foundation blocks left over from the early sixties, when the canal was built; this debris was removed during plant construction.

The third field, a single axis tracker with a ± 200 V DC bus array, uses Saturn LGBC high efficiency cells in its solar generators. Its orientation shifts from east in the morning to west in the evening. This field is equipped with a 100-kVA self-commutated IGBT inverter and the tracker consists of four horizontal rows oriented north-south, measuring 2.5 m wide by 80 m long each. A proximity detector accurately identifies the position of each row. If the tracker is found to deviate by more than 1º from the correct orientation, the system is re-set by a PC to the optimum adjustment angle and the tracker position is rectified by the four electric motors, one per row. The system consumes 700 Wh per day, i.e., less than 0.2% of the electricity it generates.

The tracker's three operating modes - ideal tracking, backtracking and fixed positioning - are software-controlled. It usually operates in backtracking mode. To accommodate shading, the tracking angle is restricted to ±60° over the horizontal, which reduces the energy gain by less than 3% compared to complete tracking. An additional security switch limits movement beyond this limit..

Connection to the existing 15-kV grid constituted a problem because of the latter's weak short-circuit power - only 11 MVA. An efficient low cost solution was sought that would at the same time guarantee high quality output current. Each of the three fields is coupled to the AC system by an inverter.

A 450 kVA nominal capacity, dual 6-pulse line commutated inverter with thyristors is used in the two main - 456 and 423 kWp - fields.

Twelve-pulse performance is achieved for each field by connecting each of the two inverters to the two LV winding converters on the field booster transformer, with phase difference deriving from a Y- or V-connection in each case. This suppresses essentially the 5th and 7th harmonics of the AC power generated.

A 100 kVA IGBT inverter converts tracking system DC to AC power. The conversion unit consists of three bridges connected in parallel and commutated at a frequency of 2,500 Hz. This relatively low frequency was chosen because it ensures low switching losses, high efficiency and low harmonics. The power factor and harmonics content are slightly better than in line-commutated inverters, although performance is somewhat lower.

The data acquisition system installed in May 1994, designed to operate automatically, is fully operational and equipped with a 32-bit completely pre-emptive, multitasking, real time operating system. It gathers actual measurements from 128 channels and calculated figures from 14. The parameters obtained are classified as follows:

• Meteorological data
• Electrical data (AC and DC characteristics)
• Energy flow (AC and DC energy values)
• Incident recording (ground fault, overheating of inverter, status etc.)
  

In addition to data acquisition, the monitoring system entails independent measuring to capture data on irradiance, the DC power in each array field and the 15-kV AC energy output. This data is displayed on 8-digit electromechanical counters.

The main monitoring program was developed in Microsoft Visual C++ and LabVIEW for Windows NT. The computer is equipped with a DMA (Direct Memory Access) controller for real-time performance. The main monitoring program acquires the data from 128 channels using double buffering, converts the data to physical units, analyses the data, processes 14 calculated channels and finally the data is graphically displayed in real time and recorded. The data are acquired and averaged at 10-second intervals and stored in a file generated hourly. Four host computers communicate via modem and can access the generated files or they can view the real time data. The program has six screens for viewing the real time data. The monitored digital signals indicate system operation processes. These digital signals are used for diagnostic purposes. The energy flow is also visualised on the following screens.

• Overall plant performance data
• Meteorological data
• Nukem DC and AC data
• BP solar DC and AC data
• BP tracking DC and AC data
• User selected channel data (displayed graphically)

There are three host computers in Madrid (at Ciemat, Endesa and Unión Fenosa) and one in Essen at the RWE office. Such software makes it possible to evaluate plant performance and defines maintenance needs. Plant performance analysis is generated from a database in real-time.

COSTS

The initial estimated cost for the project was 10,342,000 ECU. The actual capital outlay for plant installation amounted to 9,880,029 ECU, i.e., 4% lower than initially budgeted. This difference was due primarily to module prices, lower than envisaged, and exchange rate fluctuations (peseta Vs ECU).

The cost breakdown for the project is shown in the chart below.

The thyristor inverter and plant relay protection and resetting systems had to be modified in the first three years of operation, to enhance operation with a weak, incident-prone grid. Thanks to these modifications, the effect of the grid was mitigated substantially, plant performance rose to over 94% and overall availability to around 90%.

The electric power that the Plant can produce yearly is now estimated to be 1,200 MWh. Environmental soiling of the photovoltaic modules occasions power losses of from 2 to 8%, depending on the season of the year, farming cycles and weather conditions.

Photovoltaic conversion efficiency itself, measured in the terms listed below, has also been taken into account:

• Reference yield for each field, measuring the irradiation received in each
• Array yield, on the grounds of the DC power generated.
• Final yield for each field and the plant itself, indicative of the AC current produced.

The figures for 1997 and 1998 are shown in the tables below:

All these figures are referred to the Plant's installed power peak pursuant to CIEMAT readings expressed in KWh/day/KWp.

• The Plant has been operating in automatic mode uninterruptedly since June '94. The total energy flowing to the grid between July '94 and February '98 (project termination date) from the three fields was 1,915, 1,803 and 447 MWh, respectively, for a total of 4,165 MWh.
  
  By 31 December 1998, the total power generated came to 5,244 MWh.
  
  
• Spanish-German co-operation was highly satisfactory.
  
  
• The large fixed field inverters, with a capacity of 450 kVA each, can also operate at weak points in the network if the control loops are carefully adjusted and the system correctly designed.
  
  
• It takes significantly less time to commission 100-kVA IGBT inverter than line-commutated systems.
  
  
• Large-scale photovoltaic plants presently entail proportionally lower investment costs than small-scale systems.
  
  
• High efficiency solar cells reduce structure size and area required.
  
  
• The inverter regulator and associated compensation equipment regulator had to be re-adjusted due to low grid quality.
  
  
• Ordering modules in large quantities affords substantial economies of scale.
  
  
• In large-scale plants the extent and cost of civil works are likewise substantially smaller.
  

Pursuant to fundamental project objectives, the long-term availability and stability of all components is being reviewed. Indications for the design of future photovoltaic systems are being gleaned from a detailed comparison between the fixed and the tracking systems and a precise determination of their respective costs.

Moreover, a diagnostic program is currently under development to aid the operator and reduce O&M expenses. This program is designed to use contrasted data and O&M reports to analyse losses and predict power production.

Cost Of PV-generated electricity

The cost of the power generated is ECU1.1 per kWh, rather expensive if compared to the cost of electricity generated by conventional systems. This COE figure is particularly impacted by the high price of installed kW (approx. 10,000 ECU/kW), which could be substantially reduced if higher performance modules were used and supervision, operation and maintenance system costs could be cut back to levels lower than required in experimental facilities such as this one.

Project success

The project constitutes a landmark with respect to both photovoltaic technology and European institutional co-operation in this field. All the various participants - electric utilities, manufacturers or research bodies - have taken the lead in their respective sectors as far as large-scale photovoltaic plants are concerned.

Technologically and operationally successful, this project has confirmed the viability of the technologies employed and reached high performance and availability levels.