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Causes for Insulation faults in PV systems and detection methods
Preprint · November 2018
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Causes for Insulation faults in PV systems and detection methods
Ramesh Suryanarayana
Introduction: In photovoltaic systems with a transformer-less inverter, the DC is isolated from ground.
Modules with defective module isolation, unshielded wires, defective power optimizers, or an inverter internal
fault can cause DC current leakage to ground (PE - protective earth). Such a fault is also called an isolation fault.
This document describes how to measure the nominal insulation resistance of PV system, identify and
troubleshoot an insulation fault in a PV system.
Every PV operator knows the following error synchronizing with the grid. If the measured resistance
situation: one or more inverters start late, especially on value is below a pre-set limit, the inverter must not be
rainy days. The message is “Fault – Insulation”. connected to the grid.
Technical background
Because the individual PV modules are
connected in series to form strings, very high system
voltages, in the range of approx. 300 – 1,000 V, can
occur in the case of PV systems. To ensure that there is
no earth potential equalization, all system components
must have sufficient insulation. The ideal current to
earth, which is very small, is called residual or earth
leakage current. The residual current consists of the
partial fault currents from the individual components
(PV modules, DC cabling and inverters). The
insulation resistance of the components can be
considered as a parallel connection to earth.
Here one can easily see that the “Inverter 5, 15
𝑹𝒊𝒓
and 17” starts up much later. 𝟏
=
𝟏 𝟏 𝟏
The background is that each inverter in a PV + +
𝑹𝒊𝒓_𝒎𝒐𝒅𝒖𝒍𝒆 𝑹𝒊𝒓_𝑫𝑪_𝒄𝒂𝒃𝒍𝒆 𝑹𝒊𝒓_𝑰𝒏𝒗𝒆𝒓𝒕𝒆𝒓
system carries out an insulation measurement before
A PV module must have an insulation 6. You will see that one or more readings are not
resistance of at least 40 Mohm/m² (DIN IEC 61215, showing 0 Volt; these strings have an isolation fault.
DIN EN 61646).The insulation resistance of a module The voltages measured can help trace the problem.
can be calculated as follows (minimum value):
𝟒𝟎𝑴𝒐𝒉𝒎𝒔/𝒎𝟐
𝑹𝒊𝒓_𝒎𝒐𝒅𝒖𝒍𝒆 =
𝑴𝒐𝒅𝒖𝒍𝒆 𝒂𝒓𝒆𝒂
For a module array with identical module types: For example:
String with 9 solar panels Voc = 300 V
𝑹𝒊𝒓_𝒎𝒐𝒅𝒖𝒍𝒆
𝑹𝒊𝒓_𝒇𝒊𝒆𝒍𝒅 = PE and +DC (V1) = 200V (= modules 1, 2, 3, 4, 5, 6,)
𝑷𝑽 𝒂𝒓𝒆𝒂
Inverters measure and monitor the total PE and –DC (V2) = 100V (= modules 7, 8, 9,)
insulation resistance value of all connected This fault will be located between module 6 and 7
components. The threshold value is at Rir> 1 KOhm/V,
but must be at least 500 KOhm (according to DIN VDE General solution:
0126-1-1). Exceptions are PV systems with galvanic Check the earth wiring on AC side, check the
isolation from the mains (using transformers). Due to isolation on DC side (PV side).
the conflict of standards in plants with large generator
areas, determining the threshold value was recently 1. Check if the inverter is well grounded,
developed by SMA Solar Technology AG together 2. Then check whether DC wires are pinched between
with the German professional association. module frames and mounting structures, whether there
is mounting screw through the back of a module,
Possible cause: conductors rubbing against the roof, mouse chewing on
conductors, broken PV modules, incorrect wiring of
1. Damaged PV panels or DC wires, such as mounting PV modules.
screw through the back of a module or a conducting
wire pinched against a mounting rail; 3. Check whether the enclosure of junction box or DC
isolator is properly sealed to against water ingress or
2. Poor connection between PV panels caused by poor condensation.
quality or aging of cable junction;
Reference:
3. Water ingress or damp condensation in junction box
due to not properly sealed junction box or DC isolator 1. S. Achitpon, R. Wattanapong, K. Nipon and Y.
enclosure, which will lower the insulation resistance Suchart, Renewable Energy, 32 (2007) 31-43.
and cause a “PV Isolation low” fault.
Detection of Insulation Fault: 2. S. Pingel, O.Frank, M.Winkler, S. Oaryan, T.
Geipel, H. Hoehne and J. Berghold, 35 th, IEEE
CAUTION! Touching non-insulated parts of the Photovoltaic Specialists Conference (PVSC), (2010),
string or frame could cause severe injury. Use pp. 2817–2822.
appropriate safety gear and safe measuring
instruments. 3. International Electro technical Commission,
Standard IEC 61215. Crystalline silicon terrestrial
1. Switch off the AC connection. photovoltaic (PV) modules, Design qualification and
2. Measure and make a note of the open-circuit voltage type approval, Edition 2.0 (2005)
of all strings.
3. Disconnect the PE (AC earth) and any earthing from
the inverter. Leave the DC connected.
– Red LED lights up to signal an error
– Isolation fault message is no longer displayed
because the inverter can no longer take a reading
between the DC and AC.
4. Disconnect all DC wiring but keep the DC+ and DC-
from each string together.
5. Use a DC voltmeter to measure the voltage between
(AC) PE and DC (+) and between (AC) PE and DC –
and make a note of both voltages.
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