How to find photovoltaic ground faults

How to find photovoltaic ground faults

According to the Photovoltaic Systems textbook (published by NJATC), a solar PV ground fault is “the condition of current flowing through the grounding conductor.” This type of current flow, is an unintentional electrical connection. It flows between a current-carrying conductor in the PV array, and the equipment grounding conductor, see Figure 1 below.

When there is a ground fault present, the electric current that was supposed to flow to the inverter or the combiner box, is flowing directly to the ground terminal.

Above a certain current threshold, the inverter shuts down safety reasons. This shutdown is the essential problem, and there is no energy production until that fault is fully mitigated.

Grounding of PV arrays

To understand all components of the PV array, we can not just focus on individual solar panels. PV array faults may be more easily found and mitigated, once we understand how all the components of the PV array play together.

First, we have the Equipment Grounding Conductor (EGC). This 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. The direct earth contact creates the grounding, see Figure 1.

When the PV system is installed, the electrical isolation resistance (Riso) is typically more than 40 MΩ. At this point there is a high barrier for current leakages. Over time Riso can go down substantially, and this causes currents to flow in the Equipment Grounding Conductor i.e. ground faults.

The material damage causing the ground fault is normally invisible to the naked eye. The cost of troubleshooting ground faults, can therefore become significant, if the technician is relying on standard photovoltaic testing principles.

Such testing methods do not allow early-stage detection and localization of faults.

Solar system diagram
Figure 1: Illustration of a PV array connected to an inverter (right side) and various conductors that makes up the full PV circuit.

Intermittent ground faults

PV ground faults can periodic and intermittent. Typically moisture in the morning will induce an intermittent faults.

The energy production from a given string will be switched off until the equipment dries up, and the inverter goes back online.

The emazys Z200 has a build in ground fault detector. It can measure the position of a ground fault present in the PV array in a few minutes. The Z200 also has a timer function, which may be used for catching intermittent faults.

The user will set a time interval suitable for “catching” the ground fault. Hereafter the instrument is left in the field for e.g. 24 hours. When the user returns to the instrument, the results may be downloaded and the position of the fault can be read.

Troubleshooting costs on the rise

The safety requirements and details of most PV arrays can be studied in the IEC 62446-1:2016, Photovoltaic (PV) systems – Requirements for testing, documentation, and maintenance. Troubleshooting is however not yet an integral part of the “code of practice”.

PV ground faults have a clear consequence. The fault makes the solar inverter, or combiner box shut down completely. Production is only reestablished, when Riso becomes sufficiently high again.

For a residential PV array, a ground fault typically takes down 2 or 3 strings. The system owner has to pay a local service provider, for hours of troubleshooting, transportation, and scaffolds. Meanwhile, the faulty component is typically a cheap connector or a cable.

In some cases, PV ground faults are caused by modules with water intrusion, or by other more rare and exotic faults.

The cost associated with residential ground fault mitigation is often higher than the system owner appreciates. This is one of the reasons why some residential PV arrays are not properly maintained and serviced.

For large commercial and utility PV power plants, the ground fault problem is basically the same, but the scale is extended. More technicians have to be involved, and transport costs become essential. Entire PV arrays will be down until the faults are found. For utility-scale PV systems, a ground fault often means that 200-400 modules are not producing while the ground fault persists.

Another cost driver is observed when field technicians are looking for certain inverters, combiners, strings, or modules. While monitoring software can often pinpoint PV arrays on a circuit diagram, no efforts were made to label and map the rows of modules. And even when components are labeled, it still takes time to find the strings, because systems are huge these days – and they are only getting bigger. The Z200 PV Analyzer also has a solution for this problem.

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Impact of ground faults

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 faults in solar PV arrays. About 50 % of all PV Riso faults go undetected.

Riso faults are 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. 

Understanding solar PV array isolation resistance

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 characterised by safety hazards, production losses, and increased service costs.

Photovoltaic Riso schematic setup
Figure 2: 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.

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.

The financial cost of ground faults

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 

The fastest solution for troubleshooting PV

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 testing Riso faults are not 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 will often be tedious in operation.

The ” real-life” fluctuating values of Riso and the intermittent nature of faults makes the mitigation taks difficult. The internal resistance of voltage testers acts in parallel with Riso, and makes it even more complex to troubleshoot faults.

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

By using emazys PV test equipment, which is based on gentle impedance measurement, ground faults at almost any level of isolation resistance may quickly be located. The test requires very little system understanding, and may be carried out as soon as the instrument operator is familiar with the basic concerns of PV testing.