Imagine you are building a bridge. No matter how solid it looks, it must undergo a series of strength tests before anyone can cross it. The same applies to transformers—before they are integrated into the grid, they must prove their resilience to voltage, load, temperature, and extreme operating conditions.

In this article, we will show you how we test our MarkoEco2 oil transformer.

You will learn:

✅ How we verify that the transformer "communicates" correctly with the grid and delivers the right voltage,
✅ How we test the quality of windings to prevent overheating and power losses,
✅ How we assess insulation strength to protect against overvoltages,
✅ How we check the transformer's energy efficiency and pressure resistance,
✅ How we ensure that mounting dimensions allow for seamless installation.

This is an insight into real quality control—from strict standards to practical tests that determine the reliability of the device.

Reading time: 3 minutes—just enough to gain knowledge that helps prevent transformer operation issues.

1. Voltage Ratio Measurement and Connection Group Test

This is the first and absolutely fundamental test that verifies whether the transformer "understands the language" of the power system it will be integrated into. Imagine a band – if the guitarist plays in a different key than the rest, the entire piece loses its harmony. The same applies to transformers – if the windings are misconfigured, voltages will become asymmetrical, causing inefficiencies, overheating, and even serious failures.

Why do we conduct this test?

Each transformer has its unique winding configuration and voltage ratio – the foundation of its proper operation. If the ratio deviates from the design specifications, the system may fail to deliver the correct voltage to consumers, leading to efficiency losses and overload risks.

This test allows us to verify two key aspects:

✅ Are the HV and LV voltages in line with the designed ratio? (e.g., 200 kV at input and 5 kV at output)
✅ Is the connection group correct? (e.g., Dyn5 – crucial for grid compatibility)

How do we conduct the test?

The transformer is connected to a test voltage source, and precise voltage measurements are taken at the winding terminals.

• Voltage ratio measurement – We check if the voltage ratio between the high-voltage and low-voltage windings matches the design specifications. For the MarkoEco2 transformer, this should be 200 kV / 5 kV.

• Connection group test – We verify whether the windings are correctly configured according to the standard, e.g., Dyn5.

The Dyn5 connection group means that the high-voltage winding is connected in a delta configuration, and the low-voltage winding in a star configuration with a 150° phase shift (5 × 30°). In practice, this means that if phase L1 on the HV side reaches its peak voltage, the corresponding LV phase will have a 150° phase shift.

What are the acceptable deviations?

IEC 60076-1 standards define the maximum allowable voltage differences:

⚡ ≤2% for HV windings
⚡ ≤4% for LV windings

If these values are exceeded, this may indicate:

❌ Winding configuration errors
❌ Issues with electrical connections quality
❌ Transformer entering an asymmetric state, increasing overheating and power loss risks

What if something is incorrect?

Incorrect test results serve as a warning signal that requires detailed analysis. If voltages deviate from the standard or the connection group does not match the design, we take several key steps:

• Checking internal connections – First, we verify whether the windings are correctly connected to the terminals and if the connection group configuration matches the documentation.

• Inspection of measurement equipment – Errors may result from incorrect test setup or calibration issues.

• Winding analysis – In case of significant deviations, it may be necessary to inspect the transformer using impedance measurement methods or open the unit for a physical examination.

Why is this test so important?

Voltage ratio measurement and connection group testing are fundamental steps in quality control. At this stage, we can detect potential issues that could later cause overloads, failures, or even damage to the entire power system.

It's crucial to remember that errors in connection group configuration can:

❌ Affect grid compatibility, leading to improper operation of other devices
❌ Cause dangerous working conditions, such as overheating and overloads
❌ Lead to unexpected power losses, reducing overall system efficiency

By conducting rigorous tests, we ensure that our transformers seamlessly integrate with the grid, delivering stable and predictable voltage that meets both technical and economic requirements.

Because in power engineering, just like in music – precision and harmony are the keys to reliable and efficient performance.

2. Winding Resistance Measurement – How Do We Test the Transformer’s Internal Strength?

Every athlete knows that strong muscles are essential to withstand prolonged exertion. In a transformer, the windings play this role—if they are weakened, the entire structure loses its efficiency. Measuring resistance allows us to ensure that the transformer will not only endure years of operation but also use every bit of energy effectively. This test checks whether the device is prepared for intense loads and will not overheat during long-term operation.

Why Do We Measure Winding Resistance?

Winding resistance is one of the fundamental parameters that define a transformer's technical condition. Even slight variations can cause excessive heating, power losses, and uneven phase loading.

Through this measurement, we can assess:

✅ Winding uniformity – whether the conductors are wound correctly and do not cause local overloads,
✅ Quality of electrical connections – whether there are loose contacts or soldering defects that could lead to overheating,
✅ Phase symmetry – whether each phase has similar resistance, ensuring stability and even load distribution,
✅ Potential mechanical damage or inter-turn short circuits – if one phase’s resistance deviates from the norm, it may indicate a short circuit or insulation degradation.

Measurement Methodology

Winding resistance is measured according to IEC 60076-1 standards, using the Kelvin four-wire method. This eliminates the influence of lead resistance, ensuring precise results.

Test Stages:

1️⃣ Connecting the measuring equipment – we use a DC power source with a current range of 1 to 10 A and precise voltage drop meters.
2️⃣ Stabilizing the values – we wait for the current to reach a stable level, eliminating thermal effects on the readings.
3️⃣ Recording the results – we measure resistance separately for each phase of the HV (high-voltage) and LV (low-voltage) windings.
4️⃣ Temperature correction – since resistance changes with temperature, results are converted to a reference value (typically 75°C).

What If the Results Deviate from the Norm?

If values exceed the acceptable range, additional analysis is required:

❌ Excessively high resistance – may indicate damaged soldered connections or improper winding,
❌ Excessively low resistance – could suggest an inter-turn short circuit, leading to localized overheating and potential failure,
❌ Phase asymmetry – may result from uneven winding, causing one phase to be overloaded and the transformer to operate unstably.

Why Is This Test Crucial?

Winding resistance measurement is one of the pillars of quality control. Even small deviations can result in increased energy losses, higher operating temperatures, and a reduced transformer lifespan.

Thanks to this test, we can guarantee that the transformer operates efficiently, avoids unnecessary power losses, and is ready for years of reliable service.

3. Insulation Strength and Resistance – The Protective Shield Test

In power engineering, insulation serves as a critical shield—standing between high voltage and the integrity of the transformer. If it is too weak, electrical breakdown can occur, leading to transformer damage or even system-wide failures.

Think of it like a knight’s armor—if its structure is compromised, the knight becomes vulnerable to attack. In transformers, however, the threat isn’t swords but extreme voltages and uncontrolled surges.

Insulation tests help determine whether a transformer can withstand electrical stress and maintain its properties over decades of operation.

What Is the Insulation Strength and Resistance Test, and Why Do We Perform It?

Insulation resistance and dielectric strength are fundamental parameters defining a transformer's safety. Measuring these values allows us to assess:

✅ The quality of insulating materials – whether the dielectric meets industry standards and withstands high voltages,
✅ The absence of mechanical damage – whether the insulation is free from microcracks caused during manufacturing, transportation, or installation,
✅ Environmental resistance – whether the insulation effectively protects against moisture, dust, and aging,
✅ The ability to withstand voltage surges and dynamic electrical loads, which may occur in normal power grid operations.

These tests verify whether the transformer will operate safely and reliably for decades.

How Do We Conduct Insulation Strength and Resistance Tests?

Transformer insulation tests consist of several key stages:

1. Insulation Resistance Measurement (R60) – A Long-Duration Test

Insulation resistance provides insight into the condition of the dielectric materials used in the transformer. This measurement is performed using a megohmmeter (e.g., 2.5 kV or 5 kV), and the reading is taken after 60 seconds, hence the designation R60.

If the measured insulation resistance is too low, this may indicate moisture contamination, conductive impurities, or mechanical damage to the dielectric.

2. Dielectric Strength Test – Simulating Extreme Conditions

In this test, a test voltage is applied for 60 seconds to verify whether the insulation can withstand electrical stress without failure. This simulates the most extreme operating conditions a transformer might encounter.

• Test voltage ranges for the MarkoEco2 transformer:

  • HV → LV: 35 kV

  • LV → HV: 5 kV

  • Test frequency: 150 Hz

During the test, the transformer’s insulation must withstand these values without any signs of electrical breakdown, sparking, or material degradation.

3. Induced Voltage Test – Extreme Load Simulation

The induced voltage test evaluates the windings' ability to withstand long-term electrical stress. During this test, the voltage is increased to 200% of the nominal value and applied for 40 seconds, simulating real-world load conditions.

• Test parameters:

  • Test voltage: 200 kV

  • Frequency: 150 Hz

  • Duration: 40 seconds

If no breakdown or anomalies occur, this confirms that the windings and insulation comply with IEC 60076-1 standards and are ready for real-world operation.

What If the Results Deviate from the Standard?

If the tests indicate low insulation resistance or signs of electrical breakdown, we take the following diagnostic steps:

❌ Moisture analysis – checking whether humidity has weakened the insulation’s properties,
❌ Microscopic insulation analysis – assessing the dielectric structure for microcracks and mechanical damage,
❌ Checking the impregnation quality – poor insulation impregnation can lower dielectric strength.

If necessary, we carry out insulation drying and re-impregnation to restore full dielectric performance.

What Are the Consequences of Poor Test Results?

If the transformer does not meet insulation strength standards, it can result in:

❌ Increased risk of electrical breakdown, potentially leading to system-wide failures,
❌ Accelerated insulation aging, reducing reliability and shortening service life,
❌ Reduced surge resistance, increasing the risk of damage from sudden voltage spikes in the grid.

How Do We Ensure Maximum Insulation Strength?

At our company, we implement:

✅ High-quality insulation materials, resistant to extreme voltages and temperatures,
✅ Advanced impregnation techniques, improving resistance to moisture and aging,
✅ Additional protective layers, reducing the risk of mechanical damage to insulation,
✅ Strict quality control, including multi-stage testing at every phase of production.

The insulation strength and resistance test is a crucial transformer safety evaluation. Even the most precisely manufactured windings cannot ensure long-term, safe operation without durable and high-quality insulation.

Thanks to rigorous testing, we guarantee that our transformers are resistant to electrical stress, comply with IEC standards, and provide stability and safety for decades.

Because in power engineering, just like in bridge construction—a solid foundation ensures longevity.

4. No-Load Current Measurement – Is the Transformer an "Energy Guzzler"?

A transformer consumes energy even when it is not under load—the question is, does it do so efficiently?

The no-load current test evaluates efficiency—if losses are too high, it signals that something needs improvement. It is similar to assessing a car idling—an efficient engine should not consume unnecessary fuel.

What Is the No-Load Current Test and Why Do We Perform It?

The no-load current measurement determines a transformer's energy efficiency. It checks how much power the transformer consumes without load, meaning when voltage is applied, but no working current flows.

No-load losses mainly result from:

✅ Magnetic core losses – caused by eddy currents and hysteresis,
✅ Leakage current losses – minimal but can impact transformer efficiency over long periods.

This test is crucial for evaluating transformer efficiency—the lower the losses, the higher the efficiency, leading to lower operational costs.

How Do We Conduct the No-Load Current Test?

The measurement is conducted following IEC 60076-1 standards, applying the nominal voltage to the primary winding and measuring the current and losses in the transformer.

The acceptable no-load loss values for a standard power transformer should not exceed 2% of the rated power.

If losses exceed this threshold, possible causes include:

❌ Suboptimal core material – such as using steel laminations that are too thick,
❌ Poor core assembly – increasing eddy currents and thermal losses,
❌ Impregnation issues – which may cause additional vibrations and uncontrolled magnetic losses.

Load Losses and Short-Circuit Impedance

Load losses primarily depend on winding resistance and the quality of conductor connections. If they exceed design values, it may indicate:

• Incorrect mechanical tension in windings,

• Poor winding placement,

• Improper varnish impregnation, which affects insulation and heat dissipation.

According to IEC 60076-1, the short-circuit impedance should not exceed 5%.

• If impedance is too low, the transformer may be too sensitive to voltage fluctuations,

• If impedance is too high, the transformer may struggle to deliver power efficiently.

What Are the Consequences of Poor Test Results?

If no-load losses exceed acceptable limits, this may lead to:

❌ Increased operating temperature – shortening the transformer's lifespan,
❌ Reduced energy efficiency – leading to higher operational costs for users,
❌ Increased magnetic noise – caused by improper core assembly,
❌ Accelerated insulation aging – which, in extreme cases, can cause premature failure.

How Do We Minimize No-Load Losses?

At our company, we use:

✅ The highest-quality electrical steel with low magnetic losses,
✅ Precision-cut laminations to optimize magnetic flow,
✅ Advanced impregnation techniques to reduce uncontrolled vibrations in the core.

Thanks to these optimizations, our transformers achieve over 99% energy efficiency, resulting in tangible cost savings for users.

The no-load current measurement is a key test of transformer energy efficiency. It ensures that the transformer is not an "energy guzzler" and does not generate unnecessary losses.

By adhering to strict testing procedures, we ensure that our transformers are optimized for efficiency, meet IEC standards, and guarantee reliable long-term operation.

5. Pressure Strength and Durability Test – The Transformer Under Pressure

Imagine a transformer as a submarine. It must withstand immense pressure, both internal and external, to operate safely for decades. Pressure strength tests check whether the transformer's structure is sealed and whether there are any oil leaks that could lead to failure. It is similar to a structural test of a submarine hull before its first dive — if it leaks, the mission is not safe.

Why Do We Test the Transformer's Sealing?

Every oil-filled transformer is a hermetically sealed system in which insulating oil plays a crucial role—it cools and insulates the windings. Any leakage not only results in oil loss but also allows moisture and air to enter the transformer, leading to insulation degradation.

The pressure test allows us to check:

✅ Whether the transformer’s casing can withstand expected mechanical loads,
✅ Whether there are micro-leaks that could lead to oil loss,
✅ Whether the transformer’s design meets IEC mechanical resistance standards.

We Perform Pressure Tests in Two Main Stages:

1. Overpressure Test – Simulating Extreme Conditions

In this test, we introduce compressed air or nitrogen into the transformer at a specified pressure and observe how the structure responds to the stress.

If, after the test, the internal pressure remains within the expected range, it means that the transformer's casing is completely sealed. If the pressure drops below acceptable values, an inspection of welds, seals, and connection flanges must be conducted.

2. Pressure Variation Resistance Test – Simulating Real Operating Conditions

During operation, a transformer experiences pressure changes due to temperature fluctuations. The oil expands and contracts, and the casing must withstand this process. Therefore, we conduct a test that simulates cyclic pressure changes, reflecting real-world transformer operation.

Test parameters:

• Daily cycle simulation:

  • Oil temperature variation from -20°C to +90°C

  • Internal pressure fluctuates between 20-35 kPa

  • Transformer subjected to at least 100 cycles

Test objective: Ensure that structural components do not deform and that seals retain their properties.

Potential Issues and Consequences

If a transformer fails the pressure test, several issues may arise:

❌ Leakage in welded joints or flanges – even micro-leaks can lead to oil loss and moisture ingress,
❌ Casing deformation – if the transformer’s housing bends under pressure, it may indicate insufficient steel thickness or design flaws,
❌ Sealing problems – leaks can lower insulation quality and shorten the transformer’s lifespan.

How Do We Ensure Maximum Transformer Strength?

✅ We use high-quality structural steel with enhanced pressure resistance,
✅ We precisely manufacture casing welds and conduct radiographic (RT) inspections,
✅ We apply advanced sealing techniques, eliminating the risk of leaks,
✅ We conduct tests on every transformer to guarantee the highest quality.

The pressure strength and durability test is one of the key assessments ensuring the transformer’s long-term and failure-free operation. Mechanical resistance, the absence of leaks, and the ability to withstand pressure changes are the foundation of every unit’s reliability.

Through professional testing, we ensure that our transformers can operate even under extreme conditions — without leaks, deformations, or the risk of failure.

6. Insulation Distance Measurement – Precision That Prevents Failures

In the world of high voltage, millimeters matter. If the insulation distance between conductors or between windings and the housing is too small, the consequences can be catastrophic—ranging from overheating to complete insulation breakdown. Insulation distance testing is one of those measurements that may seem trivial at first glance, but in reality, it is a key factor in ensuring transformer safety.

It’s like maintaining proper spacing between high-voltage lines—if the conductors are too close together, arcing can occur, leading to failures.

Why Do We Measure Insulation Distances?

Insulation distance refers to the space between conductive components in a transformer, preventing sparking, breakdowns, and excessive heat losses.

This measurement allows us to verify:

✅ Whether the transformer meets IEC 60076 standards for insulation distances,
✅ Whether there are any design flaws that could lead to short circuits,
✅ Whether there is a risk of dielectric breakdown during operation, especially during voltage surges,
✅ Whether the transformer can safely operate in high humidity and contaminated environments, which reduce insulation efficiency.

How Do We Measure Insulation Distances?

We conduct insulation distance measurements in several key areas of the transformer:

• Phase-to-ground distance – measuring the clearance between windings and the housing,

• Phase-to-phase distance – checking the spacing between phase conductors,

• Distances between conductive elements in both low-voltage and high-voltage areas,

• Clearance at connection points and terminals, where the risk of voltage arcing is the highest.

If the measured values deviate from the standards, this indicates:

❌ The winding design does not meet project specifications,
❌ Local electrical overloads could occur, leading to insulation breakdown,
❌ The transformer will not pass subsequent high-voltage tests.

Problems Caused by Improper Insulation Distances

If insulation distances are insufficient, the transformer may face several serious issues:

❌ Electrical breakdown – insulation breakdown voltage decreases if conductors are too close,
❌ Excessive heating – small clearances can cause local temperature rise, accelerating insulation aging,
❌ Operational issues – in high humidity or contaminated environments, breakdown voltage can drop significantly, leading to partial discharges and gradual insulation degradation.

How Do We Ensure Compliance with Insulation Standards?

✅ Precise design control – every transformer is engineered according to IEC standards, ensuring proper insulation spacing,
✅ Advanced insulation technologies – we use high-quality dielectric materials, maintaining safe distances even during long-term operation,
✅ Thorough testing at every production stage – from design verification to final high-voltage testing.

Insulation distance measurement is a critical stage in transformer testing. Proper clearances directly impact safety, reliability, and the ability to withstand high voltages.

Thanks to our rigorous testing, we ensure that our transformers are resistant to voltage surges, fully comply with IEC standards, and provide failure-free operation for decades—without the risk of insulation breakdowns or failures.

7. Installation Dimensions – Adapting to Reality

A transformer is not just a machine—it is a part of a larger system that must perfectly fit into the energy infrastructure. Even the best transformer in the world cannot fulfill its role if it does not fit into the designated installation space or if its connections do not meet customer requirements.

The installation dimension test is nothing more than a precise geometric verification of the device—ensuring that every screw, hole, and terminal is exactly where it should be. It’s like checking whether a key fits a lock—even the smallest discrepancy can mean that the transformer cannot be installed correctly.

Why Do We Test Installation Dimensions?

During the transformer’s design phase, every dimension is carefully calculated. However, production is a process where unexpected deviations may occur. Measuring installation dimensions allows us to verify:

✅ Whether the transformer fits the foundation or structure it will be attached to,
✅ Whether the spacing of mounting holes aligns with customer requirements,
✅ Whether the connection terminals are positioned correctly, ensuring safe and efficient cable attachment,
✅ Whether transportation and installation will proceed smoothly, avoiding the need for modifications on-site.

How Do We Conduct Dimension Control?

We measure the installation dimensions at several critical points on the transformer:

• Length, width, and height of the housing – to ensure it fits within the designated installation area,

• Spacing of mounting holes – to confirm they align with the foundation,

• Positioning of electrical terminals – they must comply with the design to allow fast and secure cable connections,

• Height and placement of radiators and other external components,

• Distances between structural elements – ensuring there is adequate service space for maintenance access.

Dimensional Standards and Tolerances

Each transformer design specifies maximum permissible dimensional deviations, which cannot affect proper installation.

Deviations beyond these values may indicate that the transformer will not fit within the infrastructure, requiring additional modifications and potentially delaying system commissioning.

Potential Issues and Consequences of Incorrect Dimensions

❌ Foundation mismatch – if mounting holes are misaligned by just a few millimeters, on-site drilling adjustments may be necessary, delaying installation,
❌ Cable connection problems – incorrectly positioned terminals can make proper cable attachment impossible,
❌ Difficult servicing and maintenance – if components are too close together, routine inspections and repairs may become more complicated and costly,
❌ Transport issues – a transformer exceeding its designed dimensions may not fit into designated transport systems or may require additional transportation permits.

How Do We Ensure Precision?

✅ Advanced measurement technologies – we use laser measurement systems and 3D scanners to verify transformer dimensions with the highest accuracy,
✅ Strict quality control at every production stage – dimensions are checked not only on the finished transformer but also during the assembly of individual components,
✅ Comprehensive documentation for the customer – each transformer is delivered with a detailed dimension report, ensuring there are no surprises during installation.

The installation dimension test is a key stage in quality control, preventing problems during transformer installation. Thanks to precise measurements, we can guarantee that our transformers fit seamlessly into the energy infrastructure and are ready for immediate installation.

Every transformer that leaves our facility undergoes rigorous testing with one ultimate goal—absolute reliability. This is not just a checklist but a multi-stage control process designed to predict and eliminate potential failures before the device is put into operation.

We understand that the transformer is the heart of the power system. If it fails, the consequences can be severe—ranging from power supply disruptions and costly downtime to infrastructure failures.

Why Choose Energeks?

✅ Safety Proven by Numbers
Our transformers comply with IEC 60076 standards and maintain measurement deviations below 2%, ensuring high precision and long service life.

✅ Exceptional Efficiency – Less Loss, More Savings
Thanks to optimized design and the use of high-quality materials, our transformers achieve over 99% efficiency, significantly reducing operational costs.

✅ Tailor-Made Solutions for Each Client
We customize each transformer project to fit the client’s infrastructure, considering dimensions, electrical parameters, and specific mechanical and environmental resistance requirements.

✅ Trust Built on Experience
Our solutions are used in hundreds of industrial plants, power stations, and energy networks across Europe. Every transformer we deliver is the result of collaboration between a team of engineers dedicated to raising quality standards in the power industry.

✅ Comprehensive Support – From Design to Service
Working with Energeks is more than just purchasing a transformer—it means access to expert consulting, precision testing, and ongoing technical support at every stage of operation.

If your company is looking for proven, reliable, and safe transformers that can withstand even the most demanding conditions, Energeks is the right choice. Our products guarantee energy stability, long service life, and trouble-free operation for decades.

📩 Contact us today to discuss technical specifications and find the best solution for your needs.

Sources: Technical Documents energeks.com