The large-scale harvesting of electricity through photovoltaic (PV/solar) cells continues to reduce the world’s dependence on fossil fuels. However, a 2022 report by the International Energy Agency (IEA) states the cost of new renewable installations (PV and wind farms) has increased, reversing, in the organization’s words, ‘a decade-long cost-reduction trend’. Solar farm efficiency needs to be optimised to produce a good ROI, and the most common threat to that efficiency is electrical current leaking to ground. Leaks can be the result of poor installation but, more commonly, they develop over time in the PV panels and in the cables carrying the DC voltage to the inverters for conversion into AC for connection to the grid.
Leaks are essentially the result of a proportion of the power generated by the PV cells finding a path to ground when the isolation resistance (RISO) between equipment and earth has reduced to less than 40MΩ. Between 20 and 40MΩ, the RISO can still be considered healthy. Less than 20MΩ and there’s cause for concern, as the insulation is degrading. A RISO of less than 1MΩ will almost certainly result in power loss and possibly permanent damage, and even fire.
Another factor to consider is that some faults are intermittent. For example, the build-up of moisture/condensation in a panel overnight can cause the RISO to be low during the first few hours of operation in sunlight.
Current leaks to ground can be detected by temporarily placing a current sensing circuitry between the terminals of the PV cells and ground. The current sensing circuit includes a resistor of a known value (typically a high impedance). When the circuit is switched in, the known resistance will effectively be placed in parallel with the RISO. The current that flows through the sensing circuit can be used to calculate the RISO. To engage the current sensing circuit, it is necessary to connect to a high voltage. Relative to ground, the negative and positive outputs of the solar panel will typically be about 600V in direct sunlight.
Electromechanical relays (EMR), solid state relays (SSR) and reed relays can all be used to electronically switch a high voltage. EMRs are a popular and trusted technology but, because the contacts are not in a vacuum or inert gas, a large contact gap (and consequently large relay body) is required to achieve a high stand-off voltage. With SSRs, there are no physical contacts, as the switching element is a transistor. However, SSRs have a relatively high leakage current which can be an issue when used in a leakage detection circuit. Also, SSRs may fail such that there is crossover between the control and switching sides of the device. Reed relays have very low leakage currents (down to 1nA) and very high standoff voltages (up to a few kV), and can be delivered in small packages. Another benefit of reed relays is that the switch contacts (the reeds) are hermetically sealed, so the contamination/oxidation issues that occur with electromechanical relays do not happen with reed relays.
Because there is no need to handle a high current in a PV current sensing circuit, and because low current leakage and high standoff voltage are so important, reed relays are the logical solution.
The most important things that must be considered when switching a high voltage with a view to determining a current leak are:
• Maximum Switching Voltage. The highest DC or AC (peak) voltage that can be switched.
• Minimum Standoff Voltage. The devices will cope with a higher voltage as, in the case of Pickering Electronics’ high voltage relays, they are tested at 500V above the declared standoff. For design purposes it is recommended that the standoff voltage given on the datasheet is not exceeded.
• Insulation Resistance. This is the resistance between any of the device pins. This needs to be very high (ideally greater than 1TΩ (Tera ohms, so 1 x 1012 ohms). Accordingly, for 600V, the current would be just 0.6nA. Or, to put it another way, when measuring a low current, the switching circuit employed must not introduce a current leak, which is why solid-state relays are considered unsuitable.
• External Shield Clearance. Some devices (typically low cost) have an external metal shield to protect against EMI from neighbouring relays. If the screen extends to the relay base, or is too close to the base, this can cause problems when placed on a PCB carrying high voltages. However, this clearance is not always stated on the datasheet, so it may be necessary to refer to technical drawings or measure the clearance on a sample device.
Other information that must be considered when designing monitoring circuitry, includes:
• Coil Voltage. The DC voltage needed to energize the coil and close the normally open contacts in the reed.
• Coil Resistance. If energising the coil using a transistor, you need to know the coil resistance to calculate the current the transistor must handle.
• Operating Temperature. Solar farm monitoring equipment is likely to be exposed to temperature extremes, and regular thermal cycling.
• Service Life? This is open to interpretation. Pickering states 1 x 109 operations for most applications, but the figure could be higher or lower depending on the exact application.
Recommended products
Pickering Electronics has an extensive range of high-performance, high voltage isolation reed relays that are ideally suited to use in PV current leakage monitoring applications. Furthermore, with device footprints starting at just 46mm2 , many relays can be accommodated on a single PCB. The following series are particularly recommended: Series 104 relays feature switching voltages up to 1kV and minimum standoff voltages up to 4kV. Maximum switch current is 1A (up to 25W) & maximum carry current is 1.5A. Series 119 relays have switching voltages up to 1kV and minimum standoff voltages up to 3kV. Maximum switch current is 0.7A (up to 10W) & maximum carry current is 1.25A. Series 131 devices have switching voltages up to 1kV and minimum standoff voltages up to 1.5kV. Maximum switch current is 0.7A (up to 10W) and maximum carry current is 1.25A.
The relays recommended in this guide are all instrumentation grade and the reed contacts are plated with either rhodium or ruthenium to ensure a long life – typically up to 5x109 operations – necessary because, as mentioned, RISO faults can be intermittent, so current leak detection tests should be performed several times a day. Pickering reed relays are of a formerless coil construction, which increases the coil winding volume, maximizes the magnetic efficiency, and enables for the use of less sensitive reed switches, resulting in optimal switching action and extended lifetimes at operational extremes. Internal mu-metal magnetic screening enables very dense side-by-side PCB-mounting, saving significant cost and space. SoftCenter™ technology cushions and protects the reed switch, minimizing internal lifetime stresses and extending the working life and contact stability. A full customization service is available.
Sponsored Content by Kevin Mallet, relay product manager, Pickering Electronics.
