Impact of Isolation Resistance Faults in Photovoltaics

A photovoltaic (PV) array is an investment that is not subject to wear. This hypothesis might have persisted for years, however, this does not make it tenable: even carefully planned and executed arrays need monitoring, an occasional inspection, and, at times, repairs. Jochen Siemer, PHOTON International 2016.

Adding to the statement of Jochen Siemer we (emazys) can confirm that not all PV assets are made up of ”carefully planned and executed arrays”.

Indeed market analysis from WoodMackenzie (Global solar PV operations & maintenance 2020 Report) confirms that the annual PV plant operations and maintenance costs will grow to just over 9 billion USD in 2024. These costs are complex in nature and vary from system to system, but one driver is ground faults on the DC side of the PV array.

Isolation resistance (Riso) faults are the most common DC fault, and it is estimated that 50 % of all PV Riso faults go undetected. Riso is undesirable because they lead to financial loss while also being a safety hazard. Normally Riso faults do not occur spontaneously, but rather they manifest over time as the electrical insulation of the PV array degrades. It is however possible to locate and mitigate the faults before they manifest in the PV system and impact the return on investment. 

In general terms, we can understand Photovoltaic Riso faults as short circuit faults, that lead to electrical current flow in the grounding gear connecting the DC power generation and the System Power Components, see Figure 1. 

The electrical isolation resistance Riso can vary over time when insulation materials degrade. When this happens the collective of the DC power generation and System Power Components reaches different operation states characterized by safety hazards, production losses, and increased service costs.

Photovoltaic Riso schematic setup

Figure 1: The main setup for a DC power generator with isolation resistance Riso to the ground, while also connected to the System Power Components. The System Power Components are tied to the electricity grid symbolized by the electrical tower to the right. The System Power Components are equipped with a ground (GND) leakage current monitor.

Isolation resistance in different intervals

The key term for discussing ground faults in PV arrays is Riso. Please note that sometimes insulation resistance is also used in this context. Strictly speaking, insulation refers to the mechanical and dielectric properties of insulation material itself. Riso is the electrical isolation resistance of the system. Normally Riso falls into different categories as follows: 

  • Riso > 20 MΩ (healthy PV array)
    In this state, you do not need to worry about ground faults. Riso is very high and there are no leakages
  • Riso is between 20 and 3 MΩ (potentially degraded insulation)
    In this state, you need to pay attention when e.g. buying or selling PV assets or before taking on operations and maintenance (O&M) responsibilities. Should Riso be on a downward path there will be considerable expenses associated with monitoring and maintaining the PV asset going forward.
  • Riso is between 3 and 1 MΩ (intermittent ground faults)
    In this state, expenses start to show up since ground faults appear, but they do so as intermittent events. This state often leads to frustration and service team truck rolls in vain. Simply put, the ground faults are gone once the technicians reach the PV array in the field, and the result is an increase in both lost revenue and service costs. 
  • Riso < 1MΩ (permanent power loss and risk of fire).
    In this state, we have manifested ground faults and permanent power loss. One fault can lead to more faults and not rarely, this state will turn into electrical arcing or heat dissipation in the system components. In this state, we see fires and irreversible damages.

PV circuit grounding essentials

A DC ground fault is the condition where electrical current is flowing through the equipment grounding conductor. For this reason, it is essential to understand all components of the PV array, and not just focus on the circuit found inside the individual solar panels.

First, we have the Equipment Grounding Conductor (EGC) which is the conductive path, that provides a ground-fault current path and connects metal parts of the PV equipment to the grounded conductor. Then we have the Grounding Electrode Conductor (GEC) which is connecting system equipment to the grounding electrode. Last we have the Grounding Electrode. This is a Grounded Conductor which is a metal spear driven into the soil to create actual grounding, see Figure 2.

When the PV system is installed Riso is typically more than 40 MΩ and it means that there is a very high barrier for current leakages. But over time Riso can go down substantially and cause current to flow in the Equipment Grounding Conductor.

This condition is a ground fault by definition. The material damage causing the ground fault is normally invisible to the naked eye when the PV system is observed in the field. The cost of troubleshooting ground faults can therefore become substantial if the technician is relying on voltage-based testing principles. Such testing methods do not allow early-stage detection and localization of faults.

Figure 2: Illustration of a PV array connected to an inverter (right side) and various conductors that makes up the full PV circuit.

The cost of photovoltaic Riso faults and service

PV Riso faults lead to system shutdown. Not just power loss from the solar panel or conductor where a fault is present! To better understand the economic impact of these faults, we can look at an example and calculate a baseline cost per fault. 

In this example 1 combiner box has 20 strings with 24 panels in each string, which gives us a total of: 

20 x 24 = 480 panels 

The electrical energy output power from 1 solar panel, is the peak power x the average hours of sunlight x 0.75 %.
This calculation gives us the ”daily number of Watt-hours”. 

If we insert 250 W as a standard value of peak power we get the following: 

Energy = 250 Wp · 5 hours · 0.75 = 937.5 daily Watt − hours = 0.94 kWh per solar panel.

The daily combiner box production is thus: 0.94 kW h · 480 panels = 451.2 kWh 

We can set the energy price at a fixed average value of 0.1 USD per kW h. 

With a ground fault in the PV array connected the combiner box, the financial loss per day is therefore: 

0.1 USD · 451.2 kWh = 45 USD 

And lastly we can calculate the loss of cash over one month of downtime: 

45 USD · 30 days = 1350 USD 

At emazys we are of course heavily biased, but we readily conclude that: a solution that will quickly pinpoint the location ground faults, well before Riso < 1 MΩ (permanent power loss and risk of fire)” should be sought after by those who wish to increase the PV array return on investment. 

The challenge is that most known methods for analyzing PV Riso faults are less than optimal. In fact, equipment used to assess the safety of PV arrays by measuring Riso is often relied on also for troubleshooting. But simple voltage measurements and ”voltage pulse” testing are not adequate for pinpointing faults in the early stages. 

The ” real-life” fluctuating values of Riso, the intermittent nature of faults, and the internal resistance of voltage testers are just some of the factors that make it complex to troubleshoot faults in a consistently accurate way. 

Also note that ”voltage pulse” testing, in some cases cause damage to the PV equipment by ionising metal parts and thin conductors in the system.

A simple solution for troubleshooting

Since 2011 emazys has been developing technologies for troubleshooting solar PV power plants and arrays, and our core products are ”easy-to-use” portable analyzers that can be brought into the field and applied for troubleshooting panel strings.

The strength of the technology is speed and accuracy. The actual testing principle is based on impedance measurements, which are very gentle to the PV array. Among the many troubleshooting and testing features that are built into the Z200 PV Analyzer, we find the Ground Fault Finder.

This testing mode can within 60 seconds determine the string voltage Voc, the isolation resistance Riso and the position of a ground fault. The unique feature is that the ground fault position is found using an impedance test circuitry, so a fault can accurately be pinpointed at Riso < 3MΩ i.e. well before the inverter shuts down production to avoid even larger leakage currents.

All in all, this allows for preventive rather than reactive maintenance. With an average fault cost of 1350 USD per event, the PV Analyzer cost of about 7500 USD is a safe bet for just about any PV power plant operators who deal with ground faults on a regular basis.

Electrical Isolation Resistance in Photovoltaic Arrays – it matters

A new maintenance philosophy

People will do much more to avoid pain than they will to gain pleasure – so the saying goes. And it seems that we can recognize this tendency when looking at the complexity of modern solar power plants.

It is indeed a ”pain” to understand it all and be able to operate it perfectly. On the other hand, we have found clear evidence of an economic upside associated with more advanced troubleshooting.

With this article, it is our hope is to inspire a common goal of producing as many kWh of energy as the global fleet of solar panels can! Instead of reducing downtime, why not avoid downtime all together?

There are substantial economic benefits involved when progressing from reactive to preventive testing. Moreover, impedance testing works well in low sunlight irradiation conditions, so when technical teams are in the field, the emazys tester will almost always be applicable. This makes it possible to exploit every minute.

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