Megger tools for all O&M activities
One way to evaluate solar photovoltaic (PV) testing equipment is to categorize products, tool by tool, comparing their capabilities. Not many contractors follow this approach, and for good reasons.
First, the data collection and analysis would take an awful lot of time. The market has a large and growing variety of test tools, some aimed at smaller rooftop projects, others at large-scale solar farms.
What’s more, you run the risk of carrying more tools than you really need. A bloated toolkit increases your equipment costs and the costs associated with learning and operating the various tools. It also takes up more of your limited storage space.
Here is a better alternative.
Consider the essential tests that contractors perform at commissioning and on subsequent site visits for operations and maintenance (O&M) throughout a system's lifetime. Then look for a robust, feature-rich toolset that can effectively carry out core O&M activities on projects of any size.
This is the approach Megger has taken to develop the key products in our PV testing toolkit.
In a recent Megger blog post, we noted steps that contractors can take to switch from reactive to proactive O&M. Here we discuss how to handle a full range of O&M activities using a small selection of tools.
A powerful and versatile toolkit
With Megger’s flexible and robust set of tools, you can collect more accurate results while spending less time in the field on the following activities.
Measuring key output variables
The DCM1500S Digital Clamp Meter can help you spot check real-time power production up to 2,000 Vdc, meaning it can work on systems commissioned before or after the proliferation of ground-mount systems that reach a maximum of 1,500 V. The DCM1500S also has a temperature probe to capture a more complete picture of module performance and Bluetooth communications to easily transmit data to a phone or tablet.
Identifying faults in cables
You have three options in the Megger toolkit for fault testing. Use the DCM to systematically take voltage measurements on the DC side of the inverter when the array is energized. For intermittent faults or in cases where the DC voltage readings are inconclusive, use the MIT2500, a 2.5 kV Insulation Resistance Tester to segment and isolate different array sections until you can zero in on the faulted conductor. On the AC side of the inverter, the TDR, a time domain reflectometer, can help visualize faults, especially on inaccessible conductors like those buried underground.
Creating trend lines on tested conductors
The MIT works by trying to force current leakage out of a tested conductor. If the conductor is compromised and there is a path to ground, it will show up on the tool. If not, you can verify the integrity of the insulation. Starting on Day 1 and continuing on a regular schedule as systems age, use the MIT to test conductors and compare current readings to previous readings. Spot trends with the conductor insulation and you might identify a minor problem before it escalates to an emergency.
Documenting and interpreting test results
Once again you can take your pick from Megger tools depending on what you encounter in the field. Some companies do not allow technicians to work on live electrical equipment. If your company operates this way, you can use the MIT to perform insulation resistance tests on PV and inverter output circuits at nighttime. For added efficiency, the tester has adjustable pass/fail levels for quick field evaluation and internal data storage with Bluetooth connectivity so you can download results back in the office. In daytime, you can use the MIT to segment and isolate array sections, as previously noted, and then switch to the DCM to test individual conductors. The DCM can also store data and remotely transmit data to a connected device. For tests on the AC side of the inverter, the TDR’s onboard graphical display makes it especially easy to interpret test results.
Verifying metal components are the same potential
Use Megger’s DLRO digital low-resistance ohmmeters to check continuity within bolted or bonded connections. Attach one lead on any metallic surface and use the other lead to test adjacent metal components, including adjacent rows of modules in ground-mounted arrays. The DLRO’s high degree of accuracy, far more accurate than a digital voltmeter, is especially useful for measuring the resistance of bolted connections in electrical panels where conductors are terminated. The standard test leads allow for testing in ground-mount arrays and inter-row bonding. With a line extender, the DLRO can test system components up to 300 feet away.
Testing and verifying grounding system
For large-scale PV systems with complex grounding systems and difficult test environments, especially those with integrated substations, the DET2/3 earth tester offers high-resolution measurements of ground electrode resistance. The test compares the resistance on a tested electrode to the ground resistance on a current terminal and the ground resistance on a potential terminal, each connected to spikes driven into the ground at a distance from the installation. A good connection to ground can minimize the impact of lightning strikes or line surges that would otherwise put people and equipment at risk.
“Wet test” to find ground faults
There might be times when you wish to simulate rain or dew on a PV system, perhaps because test results in dry conditions are inclusive. In this case, IEC 62446-1, the international standard for PV system testing, documentation, and maintenance, specifies the procedure for a wet insulation resistance test. The only difference between a dry test and a wet test is that you thoroughly spray water or a surfactant onto the equipment that will be put to a wet test. Remember to follow safety precautions when testing electrical equipment in a wet environment, and use the MIT to check for faults.
In the weeks ahead, we will publish a third post in our tools for solar O&M series on the Megger blog. The next post will focus on ground-fault testing.
