In her recent post, “Ground Fault Protection for Utility-Scale Solar Arrays,” my colleague Vanya Ignatova covered some of the basics to consider when designing ground-fault protection for utility-scale solar photovoltaic (PV) arrays. As she discussed, such protection is critical to prevent the system damage and fires that can result from overheated conductors. Insulation monitoring devices (IMDs) are important elements in such plans, and this post will look at some of the engineers need to know about when specifying and installing this equipment.
Why insulation monitoring matters
Maintaining insulation integrity on the direct current (DC) side of a large PV array is extremely important to fire prevention. The DC side includes the panels, junction boxes, conductors and other equipment leading up to the system’s inverter.
Damaged insulation can cause ground faults to occur in which an accidental contact occurs between an energized conductor and ground or equipment / array frame. Insulation faults can lead to conductor overheating and potential fire. IMDs are used to detect faulty insulation in ungrounded designs.
Specifiers need to consider the following factors when selecting an IMD for use in a PV array:
- Compatibility with the PV voltage on the DC side of the installation.
- Suitability for use in networks with a high leakage capacitance, for example up to 2000µF.
- Durability in harsh environmental conditions, including wide variability in temperature and humidity.
- Proper insulation fault detection around low insulation alarm thresholds (ie no false alarms), for example as low as 100 Ohms.
Once selected, IMDs also need to be configured for use in PV applications, considering the high leakage capacitance these systems can present. Depending on the product selected, this can require tuning the injection mode to PV use, or setting the maximum capacitance of the network.
Matching IMDs to meet unique PV demands
Proper selection of the IMD is necessary because insulation gets quite a workout in a PV system. Insulation resistance and earth leakage capacitance can vary widely between day and night, and during differing weather conditions, such as the change between clouds and bright sunshine. PV panels, themselves, account for 70 percent to 90 percent of total system insulation, and those panels are obviously on the front lines of weather exposure.
As a result of these varying conditions, field insulation levels can range from just a couple of kiloohms (kOhms) in the morning, up to 200 kOhms during a sunny afternoon’s peak production period. Figure 1 provides data from one PV installation over a two-day period that illustrates this wide variability in operating conditions.
Insulation resistance variations:
Earth leakage capacitance variations:
Given the spikes and troughs shown in these printouts, engineers need to be cautious in setting IMD alarm thresholds, otherwise false alarms can arise simply as a result of the normal variation in network resistance. The International Electrotechnical Commission’s Technical Specification 62548, “Design requirements for photovoltaic arrays,” specifies the following minimum alarm thresholds:
Schneider Electric suggests the best practice is to measure the lowest value of network resistance during normal conditions – that is to say without having an insulation fault. This lowest value may be reached in difficult environmental conditions, for example when humidity is at its highest level. Then the IMD alarm threshold can be set at 50% of this minimum R value. Note that there is no safety risk in having a very low alarm level.
If the alarm level is set at too high a value, the consequence is to have frequent false insulation faults (for example, in the morning, or during stormy weather), which in turn may result in loss of PV production and the generation of multiple alarm messages in the monitoring system. So, when selecting the IMD, it is important to ensure it has a user-configurable low alarm threshold (for example, as low as 100 Ohms) along with accurate measurement of low resistance values to help avoid false insulation alarms. The ability to record and track historical resistance and capacitance values can be useful for a better understanding of baseline operating conditions over a range of seasons and weather events. This data can aid preventive-maintenance efforts by providing early detection of potential problems.