Continuous String Current Measurement in Photovoltaic Power Plants
Higher Yield at a Lower Cost
The Spanish company Grenergy Renovables runs a number of large PV power plants
operating in the megawatt range. In order to pay out the guaranteed yearly proceeds to the
investors, the individual string currents in the plant in Murcia are monitored using the
Solarcheck system from Phoenix Contact.
Large photovoltaic (PV) power plants typically deliver several megawatts (MW) of power. The
power generated by these plants is environmentally friendly. Furthermore, PV power plants
do not adversely affect the environment by using hazardous fuels or moving parts, and do
not produce noise or odours. The processes performed at these plants are not complicated.
The PV panels are simply positioned to receive the sun’s radiation, which they convert into
electricity. The simplicity of this type of installation might lead one to think that PV power
plants could be set up and then simply left to operate on their own. This impression is
reinforced by the fact that these plants are typically situated and operated in remote areas.
However, this apparent simplicity proves on closer inspection to be more complex in practice,
as these plants require extensive expertise and sophisticated equipment.
Headquartered in Madrid, Grenergy Renovables has recognised that solar power is a clean
and inexhaustible energy source. This well-known systems integrator has been successfully
developing, planning, installing, and operating large PV plants in the megawatt range for
many years. The company currently has 25 employees and is working with two external
companies in terms of services leasing. Grenergy Renovables has at present 45 MWp
connected to the grid and in addition projects on hand with a total amount of 100 MWp. The
company has formed partnerships with selected manufacturers in the photovoltaic industry,
thereby ensuring first-class applications with long-term reliability. To achieve this, close
cooperation between the PV plant owners, the investors, and the local authorities is
essential. Transparent relationships help ensure an effective business model. Grenergy
Renovables has also gleaned this insight on the technical side of photovoltaic plants.
Trouble Sources Reduce Yield
A PV power plant with an output of 2 MWp comprises up to 10,000 PV panels, several
kilometers of cable of various sizes, and a variety of connection and switching points. In
order to optimise the plant design, the panels are first connected in series into so-called
strings that are combined in parallel at a suitable point and fed to the inverter via collector
lines. This structure is also used in a project by Grenergy Renovables in the Murcia region.
The rated power at this PV power plant is 2 MWp, with a guaranteed annual yield of 3.25
million kWh. The PV panels at the Murcia plant are connected as 512 strings, with groups of
16 strings combined in a string box to form a collector line. A total of 32 string boxes have
been installed, each with a collector line connecting to the main inverter.
This Spanish PV power plant has been implemented optimally from both a technical and an
economic point of view. Nonetheless, the individual components, such as PV panels, cables,
and their connection points are always potential trouble sources. Aging panels, cables
damaged by animal gnawing, and other failures all act to decrease yield. For this reason,
despite their simplicity of operation, PV plants should not be left to operate unsupervised. If
the plant’s operating company has provided a guarantee of yield, it has an especially justified
interest in ensuring that the solar plant continues to function optimally.
Web Page Displays Current Data
When monitoring PV plants, the effort required to make measurements must be balanced
against cost. Measuring the individual string currents is an ideal way to monitor the plants,
since the behaviour of these currents provides a good indication of how various factors affect
the yield. Grenergy Renovables is using the Solarcheck system from Phoenix Contact to
carry out this task. The Solarcheck system’s measuring modules each contain eight
individual current measurement channels that perform non-contact string current detection
via a Hall sensor. Two measuring modules are installed in
each string box so that 16 strings are measured per box. In
addition, the Solarcheck modules detect the voltage of the
16-string bundle. One digital input of the measuring module is
connected to the door contact in each box in order to detect
when the box is opened (Figure 1).
The data acquired in the Solarcheck modules is passed
directly to the associated communication module via a twowire lead. Each communication module collects and feeds
eight measuring modules and transfers the data to the higherlevel ILC 170 ETH 2TX Inline controller from Phoenix Contact
Figure 1 String box design for 16
strings with two Solarcheck
SCK-M-8S-20A string current
measurement devices and the
SCK-C-Modbus communication
module for eight measuring
modules.
via an RS485 interface (Figure 2). The entire installation can
thus be covered using just eight communication modules
coupled to measuring modules. The communication modules
are installed in every fourth string box, where they are
supplied by a Step-PS-1AC-24DC-0.75 power supply from Phoenix Contact. The Inline
controller collects all measured signals at the Murcia PV power plant via the Solarcheck
system. The ILC 170 ETH 2TX then prepares the data for user-defined visualisation and
makes it available via a Web page to allow the
plant operator to access information about the
status of the plant at any time (Figure 3).
Correlating Trouble Descriptions to Events to
Reduce Maintenance Effort
The current generated by PV panels is directly
related to solar radiation. A radiation sensor is Figure 2 The ILC 170 ETH 2TX controller
processes the data from the Solarcheck
used in Murcia as a reference to provide a reliable system, which is coupled to the Modbus
way to differentiate between trouble-related losses
RTU network via an RS485 connection.
and natural fluctuations in solar radiation intensity. The two most important types of
information provided about the current electrical generation are the current operational
performance and an evaluation of the total yield. Here, as well, monitoring the string currents
and voltages provides a sound basis for checking the fed-in energy and thus the
effectiveness of the inverter. The string current indicates a variety of troubles from the PV
panel all the way through to the inverter, since these problems often affect the conductivity of
the string or of the contact points. Since the strings
are connected in parallel, it is not possible to detect
individual string current fluctuations at the inverter.
Measuring the individual string currents therefore
provides a suitable way to monitor the proper
operation of each string (Figure 4).
Since many factors affect the operation of the PV
Figure 3 Networking principle of the Phoenix
Contact Solarcheck string monitoring
system.
power plant, correlating trouble descriptions to
events
and
external
factors
is
of
particular
importance. Phoenix Contact’s string monitoring systems have allowed Grenergy
Renovables to gain a great deal of experience in this area and thereby optimise their
maintenance strategy. For example, the company was able to identify typical trouble
descriptions and determine the required repair effort.
This knowledge has allowed the company to improve
reaction times and minimise effort when handling
various events, therefore lowering maintenance costs
and reducing spare parts inventories.
Summary
The
Solarcheck
string
monitoring
system
from
Phoenix Contact has satisfied the requirements of the
4 Example of raw data
Spanish firm Grenergy Renovables thanks to its Figure
evaluation: reduced output in a string.
flexible operational capabilities. This system allows PV
power plants that have been customised to local conditions to be equipped with the
appropriate measurement equipment. Monitoring in this way increases the operator’s
experience with regard to external factors and internal troubles, thereby in turn optimising the
maintenance scheme, materials planning, and manpower. String current monitoring is
therefore a robust type of operational control that significantly supports the transparency of
environmentally friendly power generation.
If you are interested in publishing this article, please contact Becky Smith:
[email protected] or telephone 0845 881 2222.