What are the short circuit withstand levels for 400kV, 220kV, and 132kV transformers?
Ans: 400 kV transformers must withstand 63 kA for 1 second.
220 kV transformers must withstand 50 kA for 1 second and 40 kA for 3 seconds.
132 kV transformers must withstand 40 kA for 1 second and 31.5 kA for 3 seconds.
66 kV transformers must withstand 31.5 kA for 3 seconds.
33 kV transformers must withstand 25 kA for 3 seconds.
What is the purpose of tertiary winding in a 220/132/33 kV transformer?
Ans: A 220/132/33 kV transformer has a delta-connected (Δ) tertiary winding of 33 kV voltage level. The tertiary winding provides a closed path for circulating unbalanced currents. Delta-connected tertiary windings help in reducing third harmonics. It provides a closed path for third harmonic currents and prevents them from affecting the primary and secondary systems. It may supply local 33 kV loads. In a 220/132/33 kV transformer, the 33 kV tertiary winding is often used to supply the load of substations. This avoids the need for a separate step-down transformer, improving efficiency and reducing costs.
What is the MVA rating of the tertiary winding in a 315 MVA, 400/220/33 kV transformer?
Ans: The MVA rating of the tertiary winding in a 315 MVA, 400/220/33 kV transformer as per the WBSETCL specification is 5 MVA.
How does the tertiary winding help in harmonic suppression and stability?
Ans: In three-phase transformers, third harmonic currents (3rd, 9th, 15th harmonics, etc.) naturally occur due to the non-linearity of magnetizing currents. If a delta-connected tertiary winding is present, it provides a closed-loop path for these harmonic currents. The harmonics circulate within the delta winding instead of flowing into the power system, preventing voltage distortion in primary and secondary windings. So, it reduces harmonic content in the main windings (HV and LV). In Y-Y connected transformers, third harmonics can cause neutral displacement (floating neutral effect). Hence, a tertiary winding prevents the neutral shifting.
What are the short circuit withstand conditions for a transformer?
Ans: The transformer must withstand the thermal effects of a short circuit without insulation failure. The magnitude of current and the time duration for short-circuit withstand are the main factors of withstand capability. The maximum permissible winding temperature after short circuit must not exceed 250°C. Mechanical ability to withstand short circuit must also be considered. The transformer must be designed to withstand mechanical forces during a short circuit fault without deformation.
What is the purpose of the Dynamic Short Circuit Test, and how is it performed?
Ans: The Dynamic Short Circuit Test evaluates a power transformer’s ability to withstand mechanical and thermal stresses from high fault currents. During a short circuit, strong electromagnetic forces act on the windings, risking mechanical deformation, insulation damage, or structural failure. This test ensures the transformer remains operational under real-world fault conditions.
As per IS 2026 (Part-5) / IEC 60076-5, the test is conducted in a high-power laboratory, where the LV winding is short-circuited, and HV winding is energized with a controlled fault current (1.3 to 2 times full-load current). The fault is applied for 0.1 to 0.5 seconds per cycle, repeated 3 to 6 times with cooling intervals. Winding displacement, mechanical oscillations, and insulation integrity are closely monitored.
Post-test, SFRA, winding resistance, and leakage reactance are analyzed. A transformer passes if no mechanical deformation occurs, insulation remains intact, and leakage reactance deviation is ≤2%. This test is mandatory for 400 kV, 220 kV, and 132 kV transformers to ensure reliability in power networks.
How is the Lightning Impulse Test performed on a 220/132 kV transformer?
Ans: The Lightning Impulse Test checks the insulation strength of a transformer against high-voltage surges from lightning. It ensures the transformer can handle sudden voltage spikes without insulation failure. The test follows IS 2026 (Part-3) / IEC 60076-3 standards.
The transformer is placed in a high-voltage test bay. The HV and LV windings are connected, but only the winding under test remains open. The other terminals are grounded. A high-voltage impulse generator applies controlled surges. A voltage divider and oscilloscope record the response.
The applied impulse voltage depends on the transformer rating. For a 220/132 kV transformer, the 220 kV winding is tested at 1050 kVp and the 132 kV winding at 650 kVp. A standard lightning impulse wave (1.2/50 µs) is applied three times. This represents a real lightning strike. Next, two chopped wave impulses (1.2/3-5 µs) simulate system switching disturbances.
The oscilloscope records waveforms for analysis. Voltage distortion and neutral displacement are checked. The insulation must remain intact under the applied impulse. The transformer passes the test if there is no flashover, insulation breakdown, or waveform distortion. The Lightning Impulse Test is crucial for ensuring safe grid operation before commissioning.
What is the purpose of the Sweep Frequency Response Analysis (SFRA) Test?
Ans: The SFRA test checks the mechanical and electrical condition of a transformer. It helps find winding displacement, core movement, and insulation failures. Regular electrical tests cannot detect these faults. The SFRA test is useful after short circuits, transportation, or heavy faults to ensure the transformer is safe to use.
A low-voltage AC signal is applied to the winding. The voltage is ≤10V AC (RMS), and the frequency ranges from 20 Hz to 2 MHz. The transformer’s response is measured and compared with a reference signature. Any change in response shows possible internal damage. This can include winding deformation, core displacement, or insulation failure.
The test is done in two configurations. In the open circuit mode, one winding is excited with the SFRA signal, and the other windings are left open. This helps check the core and insulation condition.
In the short circuit mode, one end of the winding is shorted with all other terminals and grounded together, and the SFRA signal is applied to the opposite end. This helps detect winding deformation, shorted turns, and mechanical stress.
The SFRA test is a non-destructive method to find faults before they cause failure. It helps improve transformer reliability and safety in the power system.
What is the applied voltage range for SFRA testing?
Ans: The applied voltage range for Sweep Frequency Response Analysis (SFRA) testing is typically ≤10V AC (RMS).
- Minimum Voltage: 0.5V AC
- Maximum Voltage: 10V AC (RMS)
- Frequency Range: 20 Hz to 2 MHz
The voltage is kept low to prevent core saturation and avoid stressing the insulation. The goal of the test is to measure the frequency response of the transformer windings without affecting its normal parameters.
What are the different transformer oil tests, and why are they important?
Ans: Transformer Oil is tested as per IS 335. The following tests are specified for new insulating oil in transformers:
Breakdown Voltage (BDV) Test
Purpose: Measures the dielectric strength of the oil.
Requirement: Minimum 60 kV (RMS) after treatment. The same should be at least 30 kV before filtration.
Importance: Ensures the oil can withstand high electrical stress without breakdown.
Moisture Content Test
Purpose: Detects water contamination, which affects insulation strength.
Requirement: Below 72.5 kV: < 25 ppm
72.5 kV to 145 kV: < 20 ppm
Above 145 kV: < 15 ppm
For old oil it must be less than 50 ppm.
Importance: Prevents insulation failure due to moisture absorption.
Resistivity Test
Purpose: Measures the ability of oil to resist electrical conduction.
Requirement: 35 × 10¹² Ω-cm at 90°C and 1500 × 10¹² Ω-cm at 27°C.
Importance: Higher resistivity at both higher and normal temperature ensures better insulation properties for normal to maximum temperature limit.
Dielectric Dissipation Factor (Tan Delta) Test
Purpose: Evaluates energy loss in insulation due to impurities.
Requirement: Max 0.002 at 90°C.
Importance: Ensures minimal loss and higher efficiency.
Interfacial Tension (IFT) Test
Purpose: Measures surface tension between oil and water.
Requirement: ≥ 0.04 N/m at 27°C.
Importance: A lower IFT indicates oil degradation and contamination.
Flash Point Test
Purpose: Determines the minimum temperature at which oil vapors ignite.
Requirement: ≥ 140°C.
Importance: Ensures fire safety of the transformer.
Acidity (Neutralization Value) Test
Purpose: Measures acidic content, which can corrode transformer parts.
Requirement: Max 0.03 mg KOH/g
Importance: Prevents insulation deterioration due to acidic degradation.
How is the temperature rise test conducted, and what are the permissible limits for winding and oil?
Ans: How the Temperature Rise Test is conducted following the steps below.
Test Prerequisites: The transformer must be fully assembled, including its cooling system. That means, radiators, fans, , and oil pumps are fitted. The transformer is filled with insulating oil. Ambient temperature is measured and recorded.
Loading the Transformer: The transformer is subjected to a simulated load. Since applying the full rated load directly might be impractical, the test is often conducted using the short-circuit method. Here, the secondary winding is shorted, and a reduced voltage is applied to the primary winding to circulate the rated current through the transformer. This simulates full-load losses (copper losses) without requiring a full load on the secondary side.
Duration of the Test: The transformer is operated at rated current until thermal equilibrium is reached. Equilibrium means, when the temperature stops rising significantly. It is typically within 1°C over a specified period, such as 1 hour or 30 min. This may can take several hours to reach the equilibrium, depending on the transformer size and cooling system.
Oil Temperature Measurements: Oil temperature is measured using thermometers or thermocouples placed in the top oil. Sometimes the bottom oil temperature is also measured to calculate the temperature gradient.
Winding Temperature Measurements: Direct measurement of winding temperature is challenging during operation, so it is calculated indirectly. After reaching equilibrium, the test current is switched off, and the winding resistance is measured immediately and over time as it cools. The resistance change from its initial value before the starting of temperature rise test, is used to calculate the average winding temperature rise. For this we use a standard \[
\theta_w = \frac{(R_2 – R_1)}{R_1} \times (235 + t_1)
\]
Where \(\theta_w \)= Winding temperature rise
\(R_1\) = Winding resistance at initial temperature \(t_1 \)(before test). It is normally considered as ambient temperature as it is very close to ambient temperature.
\(R_2\) = Winding resistance after test
235 = Constant for copper windings (For aluminum windings, use 225).
What types of bushings are used in power transformers?
Ans: There are many types of bushings used in a high voltage power transformers. These are Resin Impregnated Paper (RIP) Bushings, Oil Impregnated Paper (OIP) Bushings, Porcelain Bushings, Polymer/Composite Insulator Bushings.
What are the insulation resistance requirements for a transformer core?
Ans: The insulation resistance of the core-clamp link must be more than 1 Giga-ohm.
What is the Nitrogen Injection Fire Prevention System (NIFPS), and how does it work?
Ans: The Nitrogen Injection Fire Protection System (NIFPS) is designed to prevent and extinguish fires in oil-filled transformers by employing a coordinated sequence of actions based on a “drain and stir” principle. Upon detecting internal faults or external fires through protective relays and heat detectors, the system activates and initiates the following steps:
Oil Draining: A quick-opening drain valve releases a predetermined volume of hot oil from the upper section of the transformer. This process reduces internal pressure and removes overheated oil that could fuel combustion.
Conservator Isolation: The Transformer Conservator Isolation Valve (TCIV) closes to prevent oil flow from the conservator tank to the main tank, thereby limiting the amount of oil available that could sustain a fire. The shutting of the TCIV accelerates the lowering of the oil level in the tank.
Nitrogen Injection: Nitrogen gas is injected under pressure into the bottom of the transformer tank. As the nitrogen rises, it stirs and cools the remaining oil, lowering its temperature below ignition levels. Additionally, the nitrogen occupies the space created by the drained oil, forming an insulating layer that separates the oil from oxygen, thus preventing combustion.
This integrated approach ensures rapid suppression of fires, typically within 30 seconds.
How Does a Tertiary Winding Suppress Unbalanced Load in a Three-Phase Transformer?
Ans: Normally, in a balanced system, the vector sum of the three-phase fluxes is zero. Suppose, the secondary winding of a three phase experiences an unbalanced load. That means one phase has higher current than others. The flux produced in the core by each phase winding becomes unequal. Thus under unbalanced conditions, the sum is no more zero. This leads to a residual or zero-sequence flux in the core. This zero sequence flux distorts the magnetic balance in the core. This magnetically unbalanced core creates unbalanced voltage in three phase system.
A delta-connected tertiary winding provides a closed-loop path. The residual flux caused by unbalanced load induces a circulating current in that closed delta winding. This circulating current generates a compensating magnetic flux that opposes and cancels out the residual or zero sequence flux in the core. As a result, the net residual flux in the core is minimized, stabilizing the transformer voltage. Therefore, the tertiary winding reduces voltage fluctuations in the main windings, keeping phase voltages closer to their ideal values.
We may consider the following points regarding a tertiary winding, too. The residual flux could drive part of the core into saturation. The third winding prevents this localized core saturation. The tertiary winding also helps in circulating third-harmonic components, preventing their propagation into the power system.
How does an On-Load Tap Changer (OLTC) function?
Ans: In electrical system load fluctuations lead to voltage variations. To maintain supply voltage within specified limits, a power transformer requires a tap changer. There can be two types of tap changers, off-load tap changers and on-load tap changers. Here, we will discuss the functional process of on-load tap changers or OLTCs. These tap changers shift between adjacent tap positions while the transformer remains loaded. Hence, these allow voltage adjustment without interrupting the power supply.
Switching Circuit of OLTC: The diagram represents a typical circuit arrangement for an OLTC in a single-phase winding. For three phase transformer the same arrangement is provided for each phase winding.
The setup consists of moving contacts (a and b) and contactors (C-1 and C-2). Also there is reactor in between C-1 and C-2, as shown. The reactor has a central tap connected to one-half of the transformer winding. Under normal conditions, both moving contacts a and b remain connected to fixed tap-changer contacts, and both contactors (C-1 and C-2) remain closed, allowing load current to flow through them as well as both halves of the reactor coil.
During a tap change, for example from tap position 2 to tap position 3, the process occurs in a step wise manner
Opening Contact C-2: Initially, C-2 is opened, and moving contact b is switched to the fixed contact of tap 3.
Closing Contact C-2: Once b is in position, C-2 is closed, which shorts the winding section between taps 2 and 3 across the reactor R. The reactor helps control the circulating current that arises due to the voltage difference between the two tap positions.
Opening Contact C-1: After C-2 is secured, C-1 is opened, and moving contact a is switched to fixed contact 3.
Closing Contact C-1: Finally, C-1 is reclosed, completing the transition from tap position 2 to 3.
The reactor between C-1 and C-2 is carefully selected to provide adequate reactance. This helps ensure that the circulating current remains at a minimal level.
What is the tap range normally followed in an on-load tap changer?
The range normally followed in an on-load tap changer is 1.25% voltage variation per step in 16 equal steps. That means tapping range is ±10% of the nominal voltage.
Why is an oil pot provided in a breather of a transformer?
The oil pot at the bottom of the breather prevents direct atmospheric moisture from entering the silica gel. The incoming air must pass through a small layer of oil, which traps moisture particles. The oil in the pot also prevents dust from entering the transformer. It creates a barrier between the silica gel and outside air.
What are the recommended temperature settings of the cooling controller of a power transformer?
Based on standard practices, the typical temperature settings for cooling system operation in power transformers are:
- Cooling Fans (ON/OFF settings):
- ON at: 70°C (Oil Temperature)
- OFF at: 65°C (Oil Temperature)
- Oil Pump for Forced Oil Cooling (ON/OFF settings):
- ON at: 75°C (Oil Temperature)
- OFF at: 70°C (Oil Temperature)
- Alarm Settings:
- Oil Temperature Alarm: 85°C
- Winding Temperature Alarm: 90°C
- Trip Settings:
- Winding Temperature Trip: 95°C