The white paper, Solar Installations Control and Protection, provides an international overview of solar power capacity growth, system types and costs, including a description of inverter types and connection topologies. After detailing why accurate current measurements are necessary for efficient control, equipment protection and safety, authors Bernard Richard, Claude Gudel and Stéphane Rollier at LEMdescribe how attributes of the company’s current transducers make them suitable for solar power installations.
Connection of the solar array through an inverter to the grid can be made either by using a transformer or directly without transformer. Transformer-less installations have no galvanic isolation, with a consequent risk of leakage to earth: both configurations may also be used with or without energy storage in a battery. Depending on the purpose (size, efficiency, weight, range, galvanic insulation), today’s market offers a choice of different inverters, each with their own advantages and their drawbacks.
Four main inverter designs are commonly encountered. Two designs use a transformer (at low or high frequency) and two designs are transformer-less — with or without a DC chopper or step-up converter.
For each different topology, the current and voltage measurement is carried out on the DC output of the solar panel in order to determine current and voltage generated, and to define the MPP (Maximum Power Point) where the maximum output wattage can be extracted from the solar panel.
Current measurement is also needed as an input to the control loop of the inverter, and to ensure protection against short circuit or overload. Open-loop and closed-loop Hall-effect technologies are used for the current and voltage transducers.
DC Current Injection Measurement
In transformer-less designs and in high-frequency transformer configurations, the DC current injected into the grid must be limited to a maximum value of between 10 mA and 1 A, according to different standards that apply in different countries (relevant standards include IEC 61727, IEEE 1547, UL 1741, and VDE 0126-1). This necessitates use of transducers with very high accuracy (better than 1%) and very low offset and gain drifts; an ideal technology is the closed-loop Fluxgate transducer.
Leakage Current Measurement
Transformer-less inverters without galvanic isolation have a potential for leakage currents to occur. For safety reasons, therefore, it is a requirement to monitor leakage current, and as a safety device, it is desirable that the measurement be non-contact and non-intrusive. Any AC, 50/60 Hz, leakage currents will be small – up to 300 mA (as a typical value, or more, depending on the capacitance due to the solar panel-roof configuration) and is measured as the residual component remaining from a differential measurement of currents in several conductors. Special care is also brought to the sudden variations of 30 mA in leakage currents, attesting a contact of solar panels by a person. The transducer used must measure these fluctuations. Once again, requirements include accuracy and, especially, low offset and gain drifts, to ensure resolution of these small measured currents.
Earth Fault Current Measurement
For safety reasons, the earth fault current, arising from an insulation defect has to be monitored in transformer-less designs. The transducer used to measure the earth fault current must be able to measure AC and DC signals as the earth fault current could be AC or DC, depending where the fault (for example, a short circuit) occurs; and depending on whether the PV panel is grounded or not. Similar requirements to those for the leakage current measurement apply. Accuracy, while still important, is less of a consideration in this case, as short-circuit currents are higher than the leakage currents.
All of these residual current measurement needs in PV transformer-less inverter designs are a fundamental safety requirement; they must conform to all relevant standards, and there can be no compromise on security or reliability of the leakage current measurement function.