A closer look at PV ground-fault testing tools
Over the past few weeks, we have been discussing the switch from reactive to proactive operations and maintenance (O&M) at solar photovoltaic projects of all sizes. When you’re monitoring trends to anticipate failures before they occur, you can improve system uptime, minimize O&M costs, and improve site safety.
Our latest post explained how a small selection of robust test tools can deliver better results for photovoltaic (PV) technicians working across a range of O&M activities. Today, we will hone in on the most critical job technicians have to perform, ground-fault testing.
There’s little margin for error where ground-fault testing is concerned. The methods you use to identify faults may not only safeguard your systems from fire and shock hazards. They can spare you from carrying more equipment than you need, spending excess time in the field, and potentially replacing equipment you don’t need to replace.
Before we talk about some tools that can help raise your ground-fault testing game, let’s look at a couple common ground faults we see in the field.
A ground fault can occur during the installation process as modules are being put down onto the racking system. A conductor gets wrapped around a piece of metal. Installers are torquing down clamps. A fault can happen right there.
Maybe the conductor is rubbing on that piece of metal, and the fault occurs many months or years down the road.
This is an example where the current carrying conductor is touching metal, putting the system at risk of a ground fault.
While testing older systems, you might see an outdated method of grounding with lugs fastened to the module frames and bare copper wire running between the lugs to create an electrical bond.
Installers used lugs because this was the standard in the industry and often specified by module manufacturers. But they didn’t always use the right outdoor-rated lug or make a proper connection. Even if the installer got everything right, contact between dissimilar metals may have caused corrosion. There might no longer be an actual connection to ground.
Here is an example of equipment grounding before the arrival of UL 2703 racking.
Tools to identify faults
Whether performing proactive or reactive O&M, use the following Megger tools to check for faults confidently and systematically.
Starting big at the combiner box and going smaller through the conductors, use the 2.5 kV Insulation Resistance Tester to segment out different sections of the array to identify locations for ground faults.
If you haven’t used the guard terminal, give it a try. It can really save the day.
Let’s say you’re reading 200 Gigaohms everywhere you go and then suddenly the measurement drops. What’s the cause?
Wrap a piece of #6 ground wire around the whole bundle and you can sense the surface contamination using that blue guard test lead. You might have been gearing up to replace a bad conductor when the culprit was surface contamination from salt or mud.
When you’re carrying enough tools to fill the back of a golf cart, it’s nice to have a handheld device with a guard terminal on it.
After isolating the strings to rule out segments of the array with an adequate connection to ground, the DCM is your go-to device for locating ground faults. You can check three points from line to line: positive to negative, positive to ground, and negative to ground. Knowing string length and voltage, you can use those three different points to locate your ground fault.
The DCM can read up to 2,000 volts and 1500 amps DC and 1,500 V and 1,500 A AC. It also has quick-connect adapters for connecting directly to modules or strings.
IEC 62446-1, the international standard for PV system testing, documentation, and maintenance, specifies the need for a continuity test to make sure you’ve got an electrical connection between two points. But there’s no standard to measure continuity.
While many technicians use a multimeter to verify resistance, the multimeter has less than 0.10 microamps of current to make the test. It’s also common to use a buzz feature on the multimeter to verify continuity. If you don’t track numerical readings, you won’t know when degradation is occurring in the electrical elements. The DLRO gives you a baseline value so you can monitor for changes over time.
For continuity tests, the DLRO gives more accurate, actionable results.
The TDR can find faults very quickly on three-phase cables on the AC side of the inverter. You enter cable properties into the tester. The device sends a low-voltage pulse into the cable under test, looks for any impedance change, and reflects it back on a display screen. The result can show you the distance from the test lead to the fault.
The TDR takes a high-end test method used in telecommunications, cable television, and the electrical industry, and adapts it at a lower price point for PV testing.
The DET is another device that was created for one set of specialized industry needs and then applied to PV testing for more efficient and accurate O&M.
In the electrical industry, the DET is used to filter noise during substation testing. For PV testing, the DET enables technicians to perform a fall of potential test to measure ground resistance.
It is important for large scale systems to verify the resistance to ground is within safety specifications. The ground system can limit the voltage induced on conductors by lightning strikes, line surges or unintentional contact by high voltage lines.
Learn more at NABCEP 2023
To see Megger’s PV test tools up close and speak with our team about using Megger tools for ground-fault testing, join us at the NABCEP 2023 Continuing Education Conference in St. Charles, Missouri, March 27 to 30.
If you’re registered to attend, be sure to see Megger’s product development leader Felix Lesmes and our partner Ryan Mayfield, founder of Mayfield Renewables, March 28 at 9 a.m., as they present “Continuity, Bonding, and Ground-Fault Locating in Solar PV Systems.”