Transformerless inverters trade a barrier for efficiency
PV inverter design has shifted toward transformerless topologies because dropping the line-frequency transformer raises conversion efficiency and cuts size, weight and cost. The same transformer also provided galvanic isolation between the PV array and the AC grid, and removing it removes that passive barrier — once it is gone, internal insulation degradation can become a shock hazard rather than a contained fault.
Start-up isolation resistance testing fills the gap
Without a fixed isolation barrier, a transformerless inverter has to verify isolation actively. Before it connects and begins feeding the grid, the controller measures insulation resistance between the DC side and earth; it is allowed to energize only if that reading clears the threshold, and it must withhold start and raise a fault if insulation is degraded.
The switch inside this sensing loop has a narrow job description: it has to stand off the full DC bus, add essentially no leakage of its own, and disconnect the measurement circuitry completely while idle.
Where a sealed high-insulation reed relay fits
SHR AUTOSENSOR’s sealed high-voltage, high-insulation reed relays match this duty on three points:
- Contact-to-contact and contact-to-coil withstand to 4000 VDC, which covers residential and commercial PV bus voltages with headroom.
- Insulation resistance to 10 TΩ (1013 Ω), so relay leakage does not corrupt the very insulation reading the inverter is taking.
- Hermetically sealed contacts, so humidity and contamination cannot build a surface-leakage path across an open contact over years outdoors.
Isolating the MOSFET sensing array
Isolation monitoring is commonly built around a MOSFET sensing array, and for the measurement to be valid that array has to be fully detached from ground until the instant of test. With an open-contact resistance of 100 GΩ, the reed relay holds the array floating before enable; on self-test it closes for the measurement and reopens, returning the sensing circuit to its isolated state.
Two parameters set the achievable accuracy
The monitoring front end is a small analog chain: a divider or injection circuit senses the bus, the path runs through a relay matrix, and the controller compares the result against the threshold in the applicable grid and safety standards. Two relay numbers dominate. Insulation resistance in the teraohm range means the relay adds negligible parallel conductance, so the reading reflects the array’s true insulation rather than the switch. Open-contact resistance of 100 GΩ means an idle sensing array is genuinely disconnected, so residual bus voltage cannot bias later readings or stress the sensing electronics. Because both properties come from a hermetic seal, they hold across humidity, altitude and outdoor age — the conditions under which an open-frame part grows surface leakage. A front end built this way can be qualified once rather than re-margined as the product ages.
What it changes in the field
The inverter keeps the efficiency and cost advantages of a transformerless design while restoring an equivalent safety level by verifying isolation at every start. A damaged cable, moisture ingress or a cracked panel backsheet is caught before energization, which protects installers and owners. Using a sealed mechanical contact rather than a semiconductor in the isolation path also keeps that path linear and leakage-stable across the full temperature range. For the designer, the relevant relay figures are the 4000 VDC withstand and the 10 TΩ insulation; everything else in the monitoring loop is built around those two.
About SHR AUTOSENSOR
SHR AUTOSENSOR TECH LIMITED (SHR AUTOSENSOR) manufactures high-voltage reed relays, mercury wetted relays and high-voltage DC contactors. Contact: [email protected] | +86 13761571029 | www.reed-relay.com