Field Case: Matching the Withstand Test Set to the Load — Grounding Transformer, SVG Transformer, and MV Cable
Choosing the right AC withstand test equipment isn't just about voltage class. Two pieces of equipment can sit at the same voltage level and still need completely different test setups, depending on how much capacitive or inductive load they present. This case, from a photovoltaic plant commissioning project, shows that decision being made in real time — on the same site, on the same day.
The Project
The site was a 20MW photovoltaic power station under construction in Huarong County, Hunan Province, with part of the plant scheduled for energization within days. Before that deadline, every transformer, switchgear unit, and cable feeding that section needed to clear its full commissioning test sequence — insulation resistance, DC resistance, turns ratio, and, as the final step, AC withstand voltage testing.
Test 1: Grounding Transformer — Standard Withstand Test Set
The first withstand test was run on a dry-type grounding transformer, 630/140kVA, rated 36.75kV/0.4kV. With both windings shorted and the low-voltage side grounded, the team applied 68kV for 1 minute using a standard power-frequency withstand test set.
A faint crackling was audible partway through the test — on inspection, attributed to dust on the equipment rather than any insulation issue, with no flashover or breakdown observed. The test passed. For a transformer at this capacity, a standard test set was well within its working range: compact enough to set up quickly and more than capable of reaching the required 68kV.
Test 2: SVG Transformer — Why a Standard Set Wasn't Enough
The plant's SVG transformer was a different story. Its capacity was large enough that the capacitive/inductive load it presented exceeded what the standard withstand test transformer could drive to the required voltage — a standard set of comparable portability simply isn't built to push that much load to the necessary test voltage.
For this transformer, the team switched to a series resonance test setup — a setup built from a frequency control cabinet, excitation transformer, voltage divider, two reactors, and compensation capacitors. Instead of driving the load directly, the system uses the transformer's own reactance as part of a resonant circuit: after the crew configured start/stop voltage, start/stop frequency, divider ratio, and test duration, the system automatically swept frequency to locate the resonance point, then ramped to full test voltage from there. No breakdown or abnormal discharge occurred, and the test passed.
This is the practical reason resonance testing exists: it lets a comparatively compact set of equipment reach voltages on large capacitive/inductive loads that a same-size conventional test transformer physically can't produce.
Test 3: Medium-Voltage Cable — Same Logic, Different Load
By evening, six 35kV cables feeding the same section of the plant needed AC withstand testing. Each phase had already passed insulation testing individually. Long cable runs present the same challenge as large transformers — significant capacitive load — so the team again used the same resonance system, with the non-tested phase grounded and safe clearance maintained between the three cores and surrounding equipment at the cable's far end.
All six cables were tested at 42kV and passed, wrapping up the site's full test program around 4:00 AM the following day.
The Decision Behind the Equipment Choice
What this case actually demonstrates isn't a single test — it's a selection process repeated three times in one day: assess the load, then choose the test set that can actually drive it to the required voltage without oversizing the equipment for a smaller load or undersizing it for a larger one. A standard withstand test transformer handled the grounding transformer cleanly; the SVG transformer and the cable runs both needed the series resonance system to reach test voltage at all.
Getting this choice wrong in either direction costs time on site — a standard set that can't reach voltage on a large load means starting over with different equipment mid-test, and an oversized resonance setup for a small load is more rigging than the job requires.
Need Professional Advice?
Not sure whether a standard withstand test transformer or a series resonance system fits your equipment? Our engineers can recommend the most suitable testing equipment based on:
- Equipment voltage level and capacity
- Load type (transformer, cable, switchgear)
- Applicable test standards
- Site portability requirements
- Project-specific requirements
Contact us for technical recommendations and product selection assistance — visit www.hkvtest.com or reach our team on WhatsApp at +86 18071531810.
About HKVTEST
HKVTEST (Wuhan Hongzhi Gaocetest Electrical Technology Co., Ltd.) is a National High-Tech Enterprise specializing in the design and manufacture of electrical test and diagnostic equipment for medium- and high-voltage power infrastructure. Founded in 2010 and headquartered in the Optics Valley / East Lake High-Tech Development Zone in Wuhan, China, HKVTEST holds more than 10 patents covering cable fault location, VLF withstand and tan delta testing, wireless high-voltage phase verification, and switchgear/transformer testing equipment.
Our test systems are used by utilities, EPC contractors, and independent testing organizations across a wide range of grid environments, and are engineered to meet applicable international and industry test standards.
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