Wrindu’s Complete Guide to CT and PT Testing

Current transformers (CTs) and potential transformers (PTs), also known as voltage transformers (VTs), connect high-voltage power systems with measuring, protection, and control equipment. These transformers must provide accurate and reliable electrical signals. Therefore, regular CT and PT testing helps engineers confirm that the transformers work as required.

A CT and PT analyzer combines several testing functions in one instrument. It can check important transformer characteristics such as ratio, polarity, excitation performance, knee point, winding resistance, burden, ratio error, and phase displacement.

For power utilities, substations, switchgear manufacturers, testing companies, and maintenance teams, a suitable CT and PT analyzer can make field testing faster and more convenient. More importantly, accurate test results can help identify problems before they affect protection or measurement systems.

Click the image to know more about Wrindu CT and PT Analyzer.

Why Are CT and PT Tests Important?

CTs reduce high current to a measurable secondary current. PTs or VTs reduce high voltage to a lower voltage signal. Protection relays, meters, and control systems then use these signals to monitor the power system.

If a transformer has an incorrect ratio, polarity, or accuracy, the connected equipment may receive an incorrect signal. As a result, protection and measurement functions may not work as expected.

CT and PT testing can help engineers check:

• Transformation ratio
• Terminal polarity
• Excitation characteristics
• Knee point voltage and current
• Secondary winding resistance
• Secondary burden
• Ratio error
• Phase displacement
• Overall measurement performance

For this reason, CT and PT testing is useful during equipment commissioning, maintenance, troubleshooting, and quality inspection.

What Is a CT and PT Analyzer?

A CT and PT analyzer is a multifunctional test instrument for checking current transformers and potential or voltage transformers.

A CT analyzer focuses mainly on current transformer testing. In comparison, a PT analyzer focuses on voltage transformer testing. A combined analyzer can perform both types of tests with one device.

Transformer	Test Item	Main Purpose
CT	Excitation characteristics	Evaluates CT core excitation performance
CT	Knee point voltage and current	Identifies the point where the CT core enters strong saturation
CT	Ratio and polarity	Checks transformation ratio and terminal polarity
CT	Secondary winding resistance	Checks the resistance of the secondary winding
CT	Secondary burden	Measures the load connected to the secondary circuit
CT	Ratio error and phase displacement	Evaluates measurement accuracy
PT/VT	Ratio and polarity	Checks transformation ratio and terminal polarity
PT/VT	Excitation characteristics	Evaluates excitation performance
PT/VT	Secondary winding resistance	Measures secondary winding resistance
PT/VT	Ratio error and phase displacement	Evaluates transformer accuracy

Modern analyzers can also calculate test results automatically. In addition, many models can store data and create reports for later review.

What Are the Main CT Testing Functions?

How Does CT Excitation Testing Work?

CT excitation testing checks how the transformer core responds as the secondary voltage increases.

During the test, the analyzer applies the required test signal and records the relationship between voltage and excitation current. Engineers can then evaluate the excitation curve and identify abnormal core behavior.

The test is especially important for protection-class CTs because core saturation can affect the current signal sent to protection equipment.

What Is a CT Knee Point Test?

The CT knee point represents an important point on the excitation curve. Around this point, a small increase in voltage can cause a much larger increase in excitation current.

Engineers use knee point testing to evaluate CT core performance and identify whether the transformer meets the required characteristics.

The Wrindu RDHG-E Variable Frequency Transformer Comprehensive Tester can perform CT knee point testing up to 30 kV.

How Does CT Ratio Testing Help?

CT ratio testing compares the actual transformation ratio with the required ratio.

For example, if a CT has a specified ratio of 1000/5 A, the test checks whether the transformer produces the expected secondary current under the test conditions.

An abnormal ratio result may indicate a connection problem, transformer issue, or incorrect transformer information.

The RDHG-E supports a CT ratio range of 1–10000 with an accuracy of ±0.2%.

Why Is CT Polarity Testing Necessary?

Polarity testing confirms that the CT terminals have the correct polarity relationship.

Correct polarity matters because CTs connect to protection relays, meters, and other secondary circuits. An incorrect connection can cause unwanted measurement or protection behavior.

Therefore, engineers should include polarity verification when commissioning or checking CT installations.

Why Measure CT Secondary Winding Resistance?

Secondary winding resistance provides useful information about the CT secondary circuit.

An unusual resistance value may point to a winding problem, connection issue, or other abnormal condition. The result can also support other CT test results.

The RDHG-E provides a resistance measurement range of 0–300 Ω.

What Is CT Burden Testing?

CT burden refers to the load connected to the CT secondary circuit.

The connected instruments, relays, cables, and other components contribute to the total burden. If the burden becomes too high, the CT may not maintain the required performance.

Therefore, burden testing helps engineers understand the actual load on the CT secondary circuit.

The RDHG-E supports an AC burden measurement range of 0–300 VA.

How Are CT Ratio Error and Phase Displacement Tested?

Measurement CTs require accurate current transformation. For this reason, engineers may also check ratio error and phase displacement.

These results help evaluate how closely the CT follows its specified measurement characteristics.

The RDHG-E provides phase measurement accuracy of ±3 minutes and a resolution of 0.3 minutes.

What Tests Can a PT or VT Analyzer Perform?

PTs and VTs require several tests to confirm their electrical performance.

PT Ratio Testing

PT ratio testing checks whether the actual voltage transformation matches the specified ratio.

A correct ratio helps meters, protection relays, and control systems receive the expected voltage signal.

The RDHG-E supports a PT ratio range of 1–10000 with an accuracy of ±0.2%.

PT Polarity Testing

PT polarity testing verifies the relationship between the transformer terminals.

This test helps confirm that the PT connects correctly to the secondary measurement and protection circuit.

PT Excitation Testing

Excitation testing evaluates the PT core response under the required test conditions.

The results can help engineers identify abnormal excitation characteristics and support transformer condition evaluation.

PT Secondary Resistance Testing

Secondary resistance testing measures the resistance of the PT secondary winding.

Engineers can use the result together with other test data to check the condition of the transformer and its secondary circuit.

PT Ratio Error and Phase Displacement

A PT analyzer can also evaluate ratio error and phase displacement.

These measurements are useful when engineers need to check the accuracy of voltage measurement and protection signals.

How Does a Modern CT and PT Analyzer Work?

Modern CT and PT analyzers use digital measurement and automatic control to simplify transformer testing.

The Wrindu RDHG-E uses DSP and FPGA technology to control the test process and process measurement data.

During testing, the analyzer can:

1. Set the required test parameters.
2. Apply the test signal.
3. Measure electrical values.
4. Calculate key transformer parameters.
5. Display the results.
6. Store test data.
7. Transfer data through USB.
8. Generate test reports.

The RDHG-E also uses a variable-frequency and low-frequency testing method. This design can reduce the need for some traditional test equipment combinations that rely on voltage regulators, voltage boosters, and current boosters.

As a result, one multifunctional analyzer can support several common CT and PT tests.

What Types of CTs Can a CT Analyzer Test?

CT applications vary across power equipment and substations. A suitable CT analyzer can support different transformer structures and installation conditions.

Typical applications include:

• Toroidal or ring-type current transformers
• Bar-type current transformers
• CTs installed in power transformers
• CTs installed in oil-filled circuit breakers
• CTs installed in switchgear
• Protection CTs
• Measurement CTs

The exact test capability depends on the analyzer model and the transformer under test. Therefore, engineers should check the required test range and functions before selecting equipment.

What Is an MVCT Tester?

MVCT tester is a term used for multifunctional current transformer testing equipment.

Some MVCT testers combine several functions, such as ratio testing, polarity testing, excitation testing, knee point testing, resistance measurement, and burden testing.

However, product names can vary between manufacturers. Therefore, users should compare the actual test functions, measurement ranges, accuracy, and application instead of choosing equipment based only on the term “MVCT.”

How Should You Choose a CT and PT Analyzer?

The right analyzer should match the transformers, test requirements, and working environment.

Which Test Functions Do You Need?

First, identify the tests required for your application.

For example, a project may require:

• CT ratio testing
• CT polarity testing
• CT excitation testing
• Knee point testing
• Secondary resistance testing
• Burden testing
• PT ratio testing
• PT polarity testing
• Ratio error testing
• Phase displacement testing

A multifunctional analyzer can reduce the need for several separate instruments.

Does the Measurement Range Match Your Transformer?

The analyzer must cover the actual transformer ratio, resistance, burden, and knee point requirements.

For example, the RDHG-E supports a CT and PT ratio range of 1–10000, a resistance range of 0–300 Ω, and knee point testing up to 30 kV.

Therefore, engineers can compare these ranges with the requirements of the transformers before purchase.

How Important Is Test Accuracy?

Accuracy directly affects the value of the test results.

When testing protection and measurement transformers, users should check voltage accuracy, ratio accuracy, resistance accuracy, and phase measurement accuracy.

The RDHG-E provides ±0.1% voltage accuracy and ±0.2% CT/PT ratio accuracy.

Does the Analyzer Support Automatic Reports?

Field testing can involve many transformers and large amounts of test data.

Automatic storage and report generation can reduce manual work. Therefore, data storage, USB transfer, printing, and report functions are useful features for testing teams.

The RDHG-E can store up to 1000 groups of test data. It also provides USB data transfer, a built-in printer, and automatic report generation.

Is the Analyzer Suitable for Field Testing?

Substation and maintenance teams often move testing equipment between different locations.

For this reason, portability, operating temperature, user interface, and simple test control can affect field efficiency.

A multifunctional portable analyzer can help reduce the amount of equipment that engineers need to carry to the test site.

What Are the Main Specifications of the Wrindu RDHG-E?

The Wrindu RDHG-E Variable Frequency Transformer Comprehensive Tester combines common CT and PT testing functions in one portable instrument.

Parameter	RDHG-E Specification
Test Purpose	CT and PT
Output	0–180 Vrms, 12 Arms, 36 A peak
Voltage Accuracy	±0.1%
CT Ratio Range	1–10000
CT Ratio Accuracy	±0.2%
PT Ratio Range	1–10000
PT Ratio Accuracy	±0.2%
Resistance Range	0–300 Ω
AC Burden Range	0–300 VA
Knee Point Test	Up to 30 kV
Phase Measurement Accuracy	±3 minutes
Phase Resolution	0.3 minutes
Data Storage	1000 groups
Power Supply	AC 220 V ±10%, 50 Hz
Working Temperature	-10°C to 50°C

The analyzer also includes a graphical LCD interface, built-in printer, USB data transfer, automatic report generation, and one-click testing functions.

Where Can CT and PT Analyzers Be Used?

CT and PT analyzers support testing work across different parts of the power industry.

Common applications include:

Substation Testing

Engineers can test CTs and PTs during commissioning, maintenance, and equipment inspection.

Switchgear Testing

CTs and PTs inside switchgear require correct ratio, polarity, and other electrical characteristics. Analyzer-based testing can help verify these parameters.

Protection System Testing

Protection systems depend on accurate transformer signals. CT excitation, knee point, ratio, polarity, and burden tests can provide useful information about CT performance.

Transformer and Breaker Maintenance

CTs installed around transformers and circuit breakers can also require periodic testing. A portable analyzer can simplify on-site inspection.

Manufacturing and Quality Control

Manufacturers can use CT and PT analyzers to check transformer characteristics before delivery.

What Problems Can CT and PT Testing Help Find?

CT and PT testing does not only produce numerical results. It can also help engineers identify abnormal conditions.

For example, testing may reveal:

• Incorrect transformation ratio
• Incorrect terminal polarity
• Abnormal excitation characteristics
• Unexpected CT saturation behavior
• High secondary winding resistance
• Excessive secondary burden
• Abnormal ratio error
• Excessive phase displacement
• Connection problems

However, engineers should evaluate test results together with transformer specifications, test conditions, and applicable requirements. A single test result should not always determine the final condition of a transformer.

CT and PT Analyzer FAQs

What is a CT and PT analyzer used for?

A CT and PT analyzer checks the electrical characteristics of current transformers and potential or voltage transformers. Typical tests include ratio, polarity, excitation, resistance, burden, knee point, ratio error, and phase displacement.

What is the difference between a CT analyzer and a PT analyzer?

A CT analyzer mainly tests current transformers, while a PT analyzer focuses on potential or voltage transformers. A combined CT and PT analyzer can test both.

Why is CT knee point testing important?

Knee point testing helps engineers evaluate CT core excitation behavior and identify the point where the core enters strong saturation.

What is the normal CT ratio?

There is no single normal CT ratio for every application. The required ratio depends on the power system, primary current, secondary circuit, and transformer design. Engineers should compare the measured result with the specified CT ratio.

Can the RDHG-E test CT polarity?

Yes. The RDHG-E supports CT polarity testing as part of its multifunctional CT testing functions.

Can the RDHG-E test PT ratio?

Yes. The RDHG-E supports PT ratio testing with a ratio range of 1–10000 and an accuracy of ±0.2%.

What is the maximum knee point voltage of the RDHG-E?

The RDHG-E can perform CT knee point testing up to 30 kV.

What resistance range does the RDHG-E support?

The RDHG-E supports a resistance measurement range of 0–300 Ω.

How much CT burden can the RDHG-E measure?

The RDHG-E supports an AC burden measurement range of 0–300 VA.

How much test data can the RDHG-E store?

The RDHG-E can store up to 1000 groups of test data.

Is an MVCT tester the same as a CT analyzer?

An MVCT tester and CT analyzer can refer to similar multifunctional CT testing equipment. However, the actual functions vary by manufacturer and model. Users should compare specifications rather than rely only on the product name.

How Can Wrindu Support CT and PT Testing?

Wrindu provides electrical testing equipment for power utilities, substations, manufacturers, contractors, and maintenance teams.

The RDHG-E Variable Frequency Transformer Comprehensive Tester combines multiple CT and PT testing functions in one instrument. Its automatic measurement, data storage, USB transfer, reporting, and portable design can support both field and laboratory testing.

For engineers who need one instrument for common CT and PT tests, the RDHG-E provides a practical solution.