High-Voltage Dielectric Loss Tester Selection Guide for Power Plants: Don't Be Fooled by Manufacturer Specifications

Last month, while reviewing the procurement of preventative testing equipment for a municipal power supply company, we encountered three bidders whose dielectric loss testers completely failed in the on-site simulation test: under the strong interference environment of a 220kV busbar being energized, the test data fluctuated by more than 8%, completely failing to meet the industry standard requirements of DL/T962-2005, even though the brochures for these products all stated "accuracy ±0.5%".

Many power maintenance teams only look at the price and accuracy parameters when buying dielectric loss testers, only to find on-site that either the anti-interference capability is too poor to use, or the wiring method is incompatible and cannot test grounding equipment, rendering the tens of thousands of yuan worth of instruments useless. Today, we'll thoroughly explain the selection experience our team has gained from 10 years of on-site testing.

In short: It is an integrated testing instrument specifically designed for use in power plants and substations to measure the tangent (tgδ) of the dielectric loss of electrical equipment such as transformers, instrument transformers, bushings, capacitors, and surge arresters under 1-10kV high voltage conditions.

• Stable measurement of insulation loss data in environments with strong electromagnetic interference, enabling early detection of latent defects such as insulation aging and moisture.
• Supports both positive and negative wiring, allowing on-site testing without removing the equipment's grounding wire.
• Comes with its own high-voltage power supply and standard capacitors, requiring no additional equipment; ready to use on-site.

• Cannot measure the high-frequency dielectric constant of non-metallic materials such as ceramics, plastics, and mica (that's the job of a laboratory high-frequency dielectric loss meter).
• Cannot replace an insulation resistance meter for continuity testing.
• Cannot replace a withstand voltage tester for withstand voltage testing.
• Cannot test equipment exceeding its rated voltage level; forcibly increasing the voltage will directly burn out the instrument.

• Power supply companies and power plant maintenance teams requiring on-site preventative testing.
• Units holding licenses for installing (repairing, testing) power facilities for handover testing and fault detection.
• Power equipment manufacturers performing on-site commissioning tests before shipment.

• For universities and research institutes conducting research on the dielectric properties of materials, buy the laboratory model using the high-frequency resonance method; don't waste money.
• For those only testing low-voltage electrical appliances, high-voltage output dielectric loss function is unnecessary.
• Products with a budget below 8000 yuan are generally substandard, with inadequate anti-interference capabilities, making them unusable in the field.

Many manufacturers exaggerate unimportant parameters, but only these truly affect field performance:

1. Anti-interference capability is the top priority. In field testing, the surrounding area is filled with live high-voltage equipment, and power frequency interference is common. Instruments without frequency conversion anti-interference capabilities will produce data that will make you question your existence. We have conducted comparative tests: under 200% power frequency interference, ordinary single-frequency instruments have an error exceeding 5%, while dual-frequency conversion models can maintain an accuracy within ±1%, fully meeting standard requirements.
2. CVT Self-Excitation Function: Essential Recommendation
   Currently, capacitive voltage transformers (CVTs) account for over 60% of substations. Without the self-excitation function, CVT testing requires disconnecting the busbar for rewiring, tripling the workload and increasing the risk of working at heights. Most mainstream models now include this function, adding less than 2000 RMB to the cost, making it a worthwhile investment.
3. Sufficient Accuracy is Enough; Blindly Pursuing High Accuracy is Useless
   Field testing is affected by humidity, temperature, and grounding conditions, inherently introducing some error. The national standard requires an accuracy of ±(1% reading + 0.1%). The mainstream ±1% + 0.04% accuracy on the market is perfectly sufficient. Spending twice the money for ±0.5% accuracy yields less than a 0.2% difference in actual field use, making it completely unnecessary.
4. Lighter Weight is Better
   Maintenance personnel often need to carry instruments up substation stairs. Older models, weighing 30kg, were difficult for two people to lift. Newer integrated designs, weighing around 16kg, can be carried by one person, saving considerable effort.

• The upper limit for capacitance measurement at 10kV output is generally 30000pF. When measuring large capacitors, the voltage must be reduced to 5kV, otherwise overcurrent protection will be triggered.
• In environments below -10℃, the LCD screen may stutter or display incompletely. In northeastern and northwestern winters, proper insulation is necessary for outdoor use.
• When performing reverse wiring tests, the instrument grounding must be separate from the test sample grounding; otherwise, the data will have a systematic error of approximately 0.05%.

1. 220kV Substation Main Transformer Bushing Dielectric Loss Testing: Choose a mainstream model with reverse wiring low-voltage shielding. No wire disconnection is required; one phase can be tested in 10 minutes, twice as fast as older models.
2. CVT Non-Wire Disconnection Testing: A model with CVT self-excitation function must be selected. No high-voltage lead disconnection is needed; the capacitance and dielectric loss of C1 and C2 can be measured in one test, improving on-site efficiency by more than 3 times.
3. 500kV Substation Field Testing
   Choose a high-end model with multi-frequency conversion and strong anti-interference capabilities, capable of stable data output even with the entire busbar energized.

1. Misconception: Heavier is better. Older instruments used power frequency transformers, making them heavy. Newer models use switching power supplies, are half the weight, and offer better performance. Don't be misled by outdated ideas.
2. Misconception: More features are better. Many manufacturers add a bunch of useless features, such as Bluetooth transmission and cloud storage, which are completely unnecessary in field testing and increase the probability of failure. Simpler functions are often more stable.
3. Misconception: Imported brands are always better than domestic ones. Domestic mainstream manufacturers, such as Wuhan Guodian Zhongxing JSY-03 series and Wuhan Guobai Electric GB2322, have mature dielectric loss meters technology with anti-interference capabilities even better than imported brands. They are only one-third the price of imported ones, and after-sales service is more convenient. There is absolutely no need to buy imported ones.

1. First, check if it has dual-frequency conversion anti-interference function. If not, pass.
2. Next, check if it has reverse wiring + CVT self-excitation method function. If not, don't consider it.
3. If your budget allows, choose a lightweight one; it's really convenient for on-site use.
4. Finally, check the accuracy. As long as it meets national standards, it's fine. Don't pursue the highest parameters.

A final reminder: Before buying, always ask the manufacturer to conduct on-site interference testing. Take the instrument to the substation to actually test it. Only pay after the data is stable. Don't just look at the parameters in the brochure.

1. What causes large fluctuations in data during dielectric loss testing?

In most cases, it's due to poor grounding. Either the instrument grounding and the sample grounding are connected together, or the grounding stake has too high a contact resistance. Check the grounding first, then check for interference from nearby high-power frequency converters.

2. Is a 0.1% difference in test data between different brands of dielectric loss meters normal?

Yes, it's normal. Many factors influence on-site testing. As long as the error is within the national standard's allowable range, it's fine. Don't worry about minor differences.

3. Does a dielectric loss meter need annual calibration?

According to metrological requirements, it must be calibrated annually by a legally authorized metrology institution; otherwise, the test data will not have legal validity.