Kingrun Transformer Tester, Baoding (2026)

14/07/2025

Assessing Transformer Reliability Through Short-Circuit Voltage(Usc) Analysis
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Definition and Role of Short-Circuit Voltage
Short-circuit voltage (Usc:short-circuit impedance voltage of the transformer in %) is a key parameter indicated on a power transformer’s nameplate. It represents the percentage of rated voltage that must be applied to the primary winding to circulate rated current when the secondary winding is short-circuited. Usc is crucial for evaluating the transformer’s electrical characteristics, such as short-circuit impedance, reactance, and the prospective short-circuit current and electrodynamic forces. Accurate knowledge of this value is essential for safe and reliable transformer operation.
Standards and Testing Recommendations
International standards such as IEC 60076-1 and IEEE guidelines specify that the short-circuit voltage should typically fall within 4% to 12% of the rated voltage. CIGRE recommends verifying this value during transformer acceptance and routine maintenance testing to ensure conformity with design specifications. A deviation greater than 2%–3% from the nameplate value may indicate structural or design issues.
For example, during an on-site acceptance test, a 1000 kVA, 10 kV/0.4 kV power transformer with a nameplate Usc of 6% was found to have a measured short-circuit voltage of 8.3%, deviating by 2.3 percentage points. This exceeds the standard tolerance and suggests potential internal issues such as winding displacement or mechanical deformation due to transportation impact. Although the transformer remained operational, such a discrepancy raises concerns about increased short-circuit forces, elevated dielectric stress, and long-term reliability.
Abnormalities and Operational Implications
An abnormally high Usc often points to structural changes inside the transformer—such as increased winding separation or elevated leakage flux—possibly caused by mechanical shock or deformation. Conversely, an abnormally low Usc may suggest core saturation or shorted turns in the winding. Short-circuit voltage measurements, when combined with diagnostic techniques like Sweep Frequency Response Analysis (SFRA), can help identify mechanical or electrical faults. If Usc values fall outside acceptable limits, detailed inspection and corrective maintenance are recommended. Establishing a regular Usc monitoring protocol enhances early fault detection, extends transformer life, and ensures stable grid operation.
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21/03/2025

What is the relationship between the phase displacement vector group and the insulation level of the transformer?
There is no direct and necessary relationship between the phase displacement vector group and the level of insulation of the transformer.

The phase shift vector group is mainly determined by the connection mode of the transformer winding, which reflects the phase relationship between the voltage of the primary side and the secondary side of the transformer, and is used to represent the connection group of the transformer winding, which has an important impact on the parallel operation, voltage regulation, harmonic suppression and other aspects of the transformer3.

The insulation level of the transformer refers to the voltage level that its insulation structure can withstand and the ability to withstand various overvoltages, which mainly depends on the design voltage level of the transformer, the performance of the insulation material, the design of the insulation structure and the manufacturing process. For example, transformers with high voltage levels need to use thicker insulation layers, better quality insulation materials, and more reasonable insulation structures to ensure their insulation performance to withstand higher operating voltages and possible overvoltages.

However, in some special cases, the choice of a phase shift vector group may have an indirect effect on the insulation level. For example, in some power systems with a large number of harmonics, if a phase displacement vector group with strong harmonic suppression ability (such as Dyn11) is selected, the damage of harmonic current and voltage to the transformer insulation can be reduced, which is conducive to maintaining the insulation level of the transformer to a certain extent. However, this effect is achieved indirectly by improving the operating conditions, and it is not the phase displacement vector set that acts directly on the insulation level.

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06/11/2024

Do all Transformers Need Temperature Rise (Heat-run) Testing?
Not all transformers require temperature rise testing. Generally, temperature rise testing is primarily for the following types of transformers:
Large or High-Power Transformers: These transformers generate significant heat under load and must monitor their temperature rise to ensure safety.
Oil-Immersed Transformers: Due to the characteristics of the oil cooling system, regular temperature rise testing is necessary.
Dry-Type Transformers: Temperature rise must be monitored, especially under high load conditions.
Newly Installed or Significantly Maintained Transformers: Testing is needed to confirm performance under actual operating conditions.
For small, low-power, or non-critical application transformers, temperature rise testing is often unnecessary. However, in specific cases, such as prolonged high loads or harsh environmental conditions, temperature monitoring may be considered.

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06/11/2024

What Is the Temperature Rise (Heat-run) Test of a Transformer?

What Is the Temperature Rise (Heat-run) Test of a Transformer?
Transformer temperature rise testing is a procedure to evaluate the temperature increase of a transformer during operation. This testing determines the temperature rise of the transformer under different load conditions, ensuring it operates within a safe range to prevent failures or damage due to overheating. The temperature rise test typically involves the following steps:
Preparation: Connect the transformer to the testing equipment, ensuring all instruments are functioning properly.
Applying Load: Operate the transformer under various load levels and record the running time.
Measuring Temperature: Use temperature sensors to measure the temperature in the transformer windings and oil (if it's an oil-immersed transformer).
Data Analysis: Analyze the test data to calculate the temperature rise and compare it with standard values to assess the transformer's performance.

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29/10/2024

How to Measure DC Resistance More Accurately？
Methods of DC Resistance Measurement
- According to IEC 60076-1. standards, there are two methods for measuring transformer DC resistance:
- **Bridge Method**
- **Volt-Ampere Method** (using a transformer DC resistance tester).
- The testing instrument should have an accuracy of at least 0.2 grade. Depending on the transformer capacity and resistance value, appropriate current ranges should be selected to measure the DC resistance of all windings.
- For transformers with tap windings, the DC resistance of all taps should be measured. For windings with a neutral point, the phase resistance should be measured, while for those without a neutral point, the line resistance should be measured.
Principles and Methods for Rapid DC Resistance Measurement
- Transformer windings have significant inductance and minimal resistance; as the capacity increases, so does the inductance, while the resistance decreases, resulting in a larger time constant.
- When measuring winding resistance, the charging current must undergo a transient process to reach a stable value after the DC power supply is turned on, making it crucial to measure the DC resistance quickly and accurately.
Considerations During Measurement
1. **Charging Time**
- It is essential to allow sufficient charging time for large-capacity transformers, as the charging duration is longer.

2. **Clamp Contact**
- Ensure that the clamp connections are secure to eliminate errors caused by poor contact.

3. **Temperature Deviation Impact**
- A 1°C temperature deviation in the three-phase coil will increase the measurement error to nearly 0.4% at room temperature.

4. **Tap Switch Operation**
- The no-load tap switch should ensure the positioning device is in the specified position; the on-load tap switch should be operated electrically.

5. **Environmental Records**
- During testing, record the ambient temperature and the temperature of the transformer oil.

6. **High Voltage Risk**
- When there is current \( I \) in the DC circuit, energy in the transformer core magnetic field can produce high voltage when disconnected. Therefore, a discharge circuit should be used to gradually reduce the current.

7. **Avoid Changing Taps**
- During resistance measurement, do not switch the no-load tap switch to change taps to avoid arcing and degradation of oil quality.

In summary, accurately measuring the DC resistance of transformer windings is essential for maintaining their reliability and efficiency. By following established standards and employing appropriate measurement techniques, engineers can ensure that all connections are secure, resistances are balanced, and temperature rises are correctly calculated. Understanding the various methods of measurement and considering factors such as charging time, contact quality, and environmental conditions will significantly enhance the precision of results. Ultimately, these practices contribute to the safe and effective operation of transformers, reducing the risk of failures and prolonging their operational lifespan.
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18/10/2024

The most important indicators to pay attention to when choosing a suitable DC resistance tester.
When customers buy a transformer DC resistance tester, they usually choose a DC resistance tester with a high current, and they are not very clear about why they choose a DC resistance tester with a high current. In many cases, the DC resistance tester chosen is not reasonable.
Output current of the tester:
The output current of the transformer DC resistance tester is usually between 10mA-50A, and different test currents should be selected according to different test objects during the actual measurement. When the selection current is too small, the measurement accuracy will be affected.
Output voltage of the tester:
The output voltage of the transformer DC resistance tester is 5V/8V/15/V/18V/25V/50V, etc. In fact, the output voltage of the instrument determines the speed of measuring the DC resistance value of the transformer, and the output power of the instrument with high output voltage is large, and the measurement speed is fast, usually when the output voltage is twice as high, the measurement speed will be four times higher. When measuring a larger transformer, the output voltage is much slower than the output voltage is higher because the test product will generate a large energy consumption during the measurement. At present, many low-end products on the market have an output voltage of less than 10V, and some manufacturers produce products with an output voltage of less than 5V in order to reduce product costs. Therefore, when choosing an instrument, we should not only look at the size of the output current, but also understand the output voltage of the instrument.

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11/10/2024

How to measure the contact resistance contained in the loop without changing the circuit?
A new approach will solve it. This method is useful for measuring contact resistance in complex mechanical assemblies. Contact resistance is defined as the ratio of the voltage across a contact to the current flowing through a pair of closed contacts. It conforms to Ohm's law. There is an interface between Metal 1 and Metal 2. The current I from the current source flows through this interface and can be read from the ammeter. The voltage drop on the interface can then be read from the voltmeter to U. The contact resistance value Rx can then be calculated.

Rx=I/I

Since the contact resistance varies with the environment and current passage, the measurement conditions should be close to the conditions of use. Accurate measurements must use four-terminal measurement technology and thermal EMF elimination technology. This indirect measurement method can be used to measure contact resistance or loop resistance. It requires three test points, three steps, and three formulas. This method has proven correct and can also be used to calibrate loop resistance standards.

Typical method of contact resistance testing

The four-wire (Kelvin) DC voltage drop is a typical method for contact resistance testing with microohmmeters, which ensures a more accurate measurement by eliminating its own contact resistance and the resistance of the test leads.

The contact resistance test uses two current connections for injection and two potential leads for voltage drop measurement; The voltage cable must be as close as possible to the connection to be tested and always within the circuit formed by the connected current leads.

Based on the measurement of the voltage drop, the microprocessor-controlled micro-ohmmeter calculates the contact resistance, while eliminating the error that can be generated by the thermal EMF effect in the connection (thermal EMF is the small thermocouple voltage generated when two different metals are joined together) They will be added to the total voltage drop measured, and if they are not subtracted from the measurement by different methods (polarity reversal and averaging, direct measurement of thermal EMF amplitude, etc.), errors will be introduced in the contact resistance test.
If a low resistance reading is obtained when testing the contact resistance of a circuit breaker using a low current, it is recommended to retest the contact at a higher current. Why would we benefit from using higher currents? Higher currents will be able to overcome connection problems and oxidation on the terminals, and under these conditions, lower currents may produce false (higher) readings.

It is important to maintain consistent measurement conditions in contact resistance testing to be able to compare with previous and future results for trend analysis. Therefore, when making periodic measurements, the contact resistance test must be carried out in the same location, with the same test leads (always using the calibration cable provided by the manufacturer) and under the same conditions to be able to know when the connection, connection, soldering or equipment will become unsafe.

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10/10/2024

Why Test Transformer DC Winding Resistance First?
When measuring the DC resistance and turns ratio of a transformer, it is generally recommended to measure the DC resistance first, followed by the turns ratio. This order is advised due to considerations related to transformer structure and electrical principles.

1. Transformer Structure and Electrical Principles
DC Resistance Measurement
DC resistance measurement is performed by applying a direct current (DC) voltage to the transformer windings and measuring the current to calculate the resistance. During this measurement, the windings carry a direct current, but since DC does not produce an alternating magnetic field, there is no magnetization or eddy current loss in the core.

Turns Ratio Measurement
Turns ratio measurement typically uses an alternating current (AC) signal (low voltage) to measure the voltage ratio between the primary and secondary windings, reflecting the turns ratio of the transformer. Because this measurement involves AC characteristics, the transformer's core will be magnetized, potentially causing short-term hysteresis effects.

2. Impact of Measurement Order
Measuring DC Resistance First
No Impact: Measuring DC resistance first will not magnetize the core since DC current does not create a magnetic field in the core. This ensures that the transformer remains in an unmagnetized state, which will not affect subsequent turns ratio measurements.
Accuracy: When the turns ratio is measured afterward, the core is in its normal state, ensuring accurate and stable turns ratio measurements.
Measuring Turns Ratio First
Magnetization Effect: During turns ratio measurement, the core will be magnetized due to the AC signal, possibly leading to residual magnetization, especially in cases where the core quality is suboptimal or the transformer has not been used for a long time. This residual magnetization may temporarily affect the DC resistance measurement.
Temperature Effect: If the turns ratio measurement takes a long time or the signal is relatively strong, the windings may heat up slightly, which could affect the accuracy of the DC resistance measurement.
3. Potential Consequences
Measurement Error: If the turns ratio is measured before the DC resistance, the residual magnetization effect or winding heating may cause the DC resistance measurement to deviate from its true value, potentially leading to incorrect assessment of the transformer's condition.
Temperature Dependency: Since resistance is proportional to temperature, measuring DC resistance first helps avoid errors caused by winding temperature rise during the turns ratio measurement.
4. Recommendation
Considering these factors, it is recommended to measure the DC resistance first, followed by the turns ratio when testing transformers. This sequence ensures the accuracy and reliability of the measurements.

By measuring in this order, you avoid the risk of DC resistance measurement errors due to core magnetization and ensure that the turns ratio measurement is unaffected by external conditions.
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23/09/2024

Do you know the influence of the ripple coefficient of the power supply on the accuracy of the DC resistance tester?
The ripple factor, in simple terms, is the peak-to-peak value of the AC component in the DC voltage.

The representation of the ripple can be expressed by an effective value or a peak value, and can be expressed in absolute or relative amount. For example, a power supply operates in a regulated state, its output is 100V5A, and the effective value of the measured ripple is 10mV, which is the absolute amount of ripple.

The relative amount is: ripple coefficient = ripple voltage / output voltage = 10mv / 100V = 0.01%, which is equal to one ten thousandth. The ripple factor, in simple terms, is the peak-to-peak value of the AC component in the DC voltage.

Ripple voltage can affect the operation of the system and bring noise. Therefore, there should be certain requirements for the ripple of this form, and the power supply should have sufficient filtering measures to limit the ripple to a certain range.

For the DC resistance test of the transformer, the narrow pulse reaches a certain height will interfere with the digital or logic control components, which will reduce the reliability and accuracy of the tester. Therefore, the amplitude of the narrow pulse should be limited. For a ripple similar to a sine wave, generally because of its low amplitude, it does not interfere with the control components.

The switching power supply of Kingrun's DC winding resistance tester series is developed by itself, with independent intellectual property rights, and its ripple coefficient accuracy has reached five ten thousandths, far higher than the precision level of the same industry, and has been the world's top DC resistance. The tester products are on the same level.

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Website:https://www.kritester.com

20/09/2024

How to perfrom test with turn ratio tester?
The turn ratio tester is a kind of alternating current low voltage test that helps to find out the ration of high voltage winding to all other windings with no load. It is a single and three phase transformer testing that can be used to measure the excitation current, turns ration and the polarity of windings in both single and three phase distribution. Actually, it can be tested phase by phase in the current, potential and also the tapped transformers as well. Primarily, the turn ratio tester is a device used to perform testing on every tap of each winding as well as measure the turns ration in between the windings.

Most probably, these ration measurements can be conducted in the entire tap positions that should be calculated by dividing induced reading voltage into applied voltage value. However, this ratio test is specifically made on three- phase transformers, but it can be taken up once at a time with a three phase turn ratio tester until all the three phase ratio measurements are completed.
Usually, the turn ratio tests can be made with turn ratio tester in the following ways that include:

1. Isolate the device to apply working on grounds to all the incoming and outgoing cables.
2. Connect the H designated three phase test lead at one end and ensure that it is properly notch the lines up.
3. Now, you connect the X designated three phase test at other end and make sure whether it lines up properly.
4. Connect all H1, H2, H3 test to the respective H1, H2, H3 transformer bushing or terminal. If H0 terminal or bushing is present, you can connect H0 test lead.
5. After that connect X1, X2, X3 test leads to respective X1, X2, X3 transformer bushings or terminal and also connect X0 as well.
6. Perform the measurement of turns ration for all tap positions.
7. Confirm the measured ratio is within 0.5% of calculated rations.
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02/09/2024

How to analysis three-phase unbalance in DC resistance test?
The three-phase resistance is unbalanced or the measured value is too different from the design value (factory test value). Generally, there are several reasons:

1. Poor contact between the conductive rod and the inner lead in the transformer bushing. For example, after the on-site transformer is overhauled, the bolts at the connection between the conductive rod in the bushing and the inner lead are not tightened tightly, causing the joint to heat up.

2. The tap changer is in poor contact. The resistance of individual taps is too large and the three-phase resistance is unbalanced due to the uncleanness of the inside of the tap changer, the peeling of the electroplating layer, and the insufficient spring pressure.

3. The low-voltage windings of large-capacity transformers are of double-helix or four-helix type. Due to the mutual displacement of the wires between the helices, the resistance of each phase winding is unbalanced.

4. Poor welding. Due to the poor welding quality of the lead and the winding, the resistance at the contact is too large, or one or several strands of the multi-strand parallel winding are not welded, resulting in the resistance being too large.

5. The phase-to-phase difference of the resistance has exceeded the specification when it leaves the factory.

6. Wrong measurement wiring and experimental methods.

Incorrect measurement wiring and test methods that cause resistance unbalance are generally:

1). The charging time is not enough, and the measured value is read when the current is not stable.

2). The connection position of the measurement wiring and the transformer connector is wrong, that is, the voltage lead is outside the current lead or the same position of the current lead during measurement, so that the resistance at the contact is also included in the measured value.

3). When measuring a certain winding, other windings are not disconnected from the grounding body, resulting in unstable charging.

Kingrun 3 phase winding resistance tester JYR 10S / 20S / 40S / 50S.

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21/08/2024

The satisfactory feedback from customers and users,showing more and more confidence in Kingrun testers.

JYL-200B contact resistance tester,Double-ended ground.
1..measure contact resistance of high voltage switch and circuit breaker;
2..test primar side resistance of transformer and grounding wire in groups.

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Website:http://www.kritester.com

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