Proper Application and Evaluation of the Surge Test for PV Panels

The recent energy cost increases, gains in PV Panel efficiency, and increased financial assistance from local and federal governments have launched a new frenzy for the purchase of PV Panels for homes and businesses. Since these panels are connected to power company lines and equipment, and are permanently mounted to occupied structures, the testing that panels must undergo are quite extensive.

One of the tests included in the comprehensive type test program is the Impulse Voltage Test, described in IEC 61730-2 “Photovoltaic (PV) module safety qualification--Part 2: Requirements for Testing,” Paragraph 10.5. This paragraph describes specific testing that is designed to ensure the panels can withstand lightning and transient voltages in the field. In this article, we will examine the surge test requirement of the PV Panel Standard, IEC 61730-2, Para. 10.5, its origins, and the problems associated with applying the pulse and determining the test results.

IEC 61730-2 integrates other IEC Standards to include as much knowledge and structure to the PV Panel test program as possible. Specifically referenced is IEC 60060-1, High-Voltage test techniques - Part 1: General definitions and test requirements, and guidance from that Standard is essential to understand how to apply the pulse to the PV Panel. The Impulse Voltage Test was designed “To verify the capability of the solid insulation of the module to withstand over-voltages of atmospheric origin. It also covers over-voltages due to switching of low-voltage equipment.” Testing can be performed with equipment such as the MegaPulse Impulse Tester (see sidebar “IEC 61730-2 Impulse Tester”).

The Impulse Voltage Test for over-voltages of atmospheric origin (presumably lightning) are taken directly from IEC 60060-1, Section 6: “Tests with Lightning Impulse Voltage”. However, the PV Technical Committee departed from IEC 60060-1 in allowing the Tests with Lightning Impulse Voltage to suffice as the test for ìover-voltages due to switching of low-voltage equipment”, i.e. power line surges. IEC 60060-1 Section 7 describes a different pulse for these transients, but by combining the lightning and transient tests and applying only the Lightning Impulse Voltage, the PV Technical Committee has waived the transient pulse test recommended by IEC 60060-1.

PV Surge Test

IEC 60060-1, Section 6 requires waveform tolerances be met while connected to the device under test. The capacitance of the PV panel does present challenges in meeting this requirement. Delivering waveforms into a capacitive load is a well-documented problem; IEC 60060-1 Para. 19.2 Note 2 states “In specific cases, it may be impossible to adjust the shape of the impulse within the tolerances recommended, to keep the oscillations and/or the overshoot within the specified limits or to avoid a polarity reversal. Such cases should be dealt with by the relevant Technical Committee.”

Until guidance from the PV Technical Committee is received on this point, IEC 60060-1 (Fig. 1) offers suggestions for the evaluation of waveforms that contain oscillations or overshoot due to capacitive loads. Fig. 1 a through d show how to evaluate some waveforms which exhibit overshoot or oscillations.

The pulse waveform delivered to the panel needs to be in tolerance during the test, not just into an open circuit. This means that the panel’s capacitance must be compensated for by the test equipment used to perform the test. No matter which test equipment is being used, the following procedure must be followed:

• Prepare the panel in accordance with the instructions in IEC 61730-2, Para. 10.5.3 a and b.
• Using a calibrated capacitance meter, measure the capacitance of the PV Panel.
• Connect the waveform generator as recommended by the manufacturer, using the appropriate tap for the measured panel capacitance.
• Connect an appropriate high voltage probe across the outputs of the waveform generator while it is connected to the panel. Adjust the oscilloscope to allow capture of a single waveform with a rise time of 1.2uSec and a duration of 50u. (The time scale of 1uSec per division is recommended to allow comparisons to IEC 60060-1 Fig. 1, a through d.
• Arm the oscilloscope, charge the waveform generator to the value required by IEC 61730-2 Table 8, and deliver a pulse to the PV Panel.
• Evaluate the waveform per the guidelines in the next Section of this article.
• If the results of the waveform evaluation are acceptable, continue pulse applications until three pulses of each polarity have been delivered.

Waveform Evaluation

First, the waveform should be checked to make sure it fits within the tolerance envelope of IEC 61730-2, or identically IEC 60060-1 (Fig. 2). The rise time must be within 30% of 1.2µsec, and the duration must be within 20% of 50µsec, using the definitions for rise time and duration from the Standard waveform. You may find some variance from the tolerances when this waveform is delivered. When using a surge generator with output taps for testing PV Panels with various capacitances, using the tap corresponding to the measured capacitance of the PV Panel will in almost all cases result in an in-tolerance waveform. In the event that the waveform shape is out of tolerance, changing taps can sometimes improve the waveform and bring it into tolerance. Here are some suggestions for waveform improvement if the waveform is out of spec:

• If the rise time is too fast: Use a tap designed for a lower capacitance PV Panel.
• If the rise time is too slow: Use a tap designed for a higher capacitance PV Panel.
• If the peak value is showing overshoot, try using a tap designed for a lower capacitance PV Panel.

Overshoot/Oscillation Measurements

Reference to IEC 60060-1 Para. 19.6.1 and Fig. 1a to d will theoretically allow a peak value to be established when oscillation or overshoot is evident in the resulting waveforms. If the oscillation has a frequency of greater than 0.5 MHz or the overshoot has equal to or less than 1µsec, the peak value can be established by drawing a mean curve through the anomaly. Calculation of the rise and duration of the waveform should use the peak value of the mean curve.

If the oscillation has a frequency of less than 0.5MHz, or if the overshoot has a duration of greater than 1 us, then the crest value of the waveform is used as the peak voltage, and calculations of the proper waveform tolerances are done using the crest value.

Determination of the waveforms requires evaluation at a fixed time base. Evaluation of acceptable waveforms, as shown in Fig. 1a through d, are to be compared to the actual waveform over the first 4 uSec. Evaluation of undetermined waveforms, as shown by Fig. 1e through h, are compared to actual waveforms over the first 8 uSec.

Waveforms generated with the correct tap but with a capacitance of greater than 77 nF result in an overshoot greater than 1nS. Reference to IEC 60060-1 Fig. 1d shows that the waveform is evaluated using the waveform crest as the peak value.

At some point, as the capacitance rises, the duration of the waveform will shorten when the crest value is used as the peak value; or the waveform shape will be characterized by IEC 60060-1 Fig. 1f , instead of Fig. 1d. In these cases, the duration is too short because too much energy is consumed by the overshoot, leaving too little energy to sustain a proper duration. The inductance of the leads and circuit components limit the abilities of an analog tester to deliver an in-tolerance waveform to PV panels with capacitance over 77nF.

IEC 61730-2 Para. 10.5.5 states pass criteria as showing no evidence of major visual defects; nor surface tracking or dielectric breakdown during the test. Dielectric breakdown can be gauged by evaluating the delivered pulse’s waveform. If the panel breaks down, all of the waveform’s energy will be delivered to the short circuit quickly, producing a waveform shape much like IEC 60060-1 (Fig. 3 or Fig. 4), which show lightning pulses that are chopped on the front and tail of the waveform, respectively.

IEC 61730-2 Impulse Tester

The Megapulse Impulse Tester (photo) provides the pulsed waveform needed to conduct the input test on PV panels in accordance with IEC 61730-2 Para. 10.5.3. The waveform is in tolerance with the requirements from 800V to 9kV. Capacitance compensation presents an IEC1180 tolerance 1.2X50 waveform to panels having 10 to 80nF capacitance, and taps from 80 to 180 nF are provided for reference testing only. The 0 nF tap provides a 1.2X50 waveform to an open circuit for evaluation. The 1.2x50 is just a shorthand for the waveform per IEC 60060. It describes a waveform with a 1.2 µsec rise time and a 50-µsec duration.