Transformer Oil Testing; IS:335 for Transformer oil; Transformer BDV values

---

Transformer Oil: Functions, Production, Deterioration & Testing Standards

Transformers are the backbone of modern electrical power systems, ensuring efficient transmission and distribution of electricity. While the transformer itself is vital, transformer oil is equally integral for the reliable operation of oil-immersed transformers. Although dry-type transformers are available, oil-filled transformers remain the most widely used across the globe due to their superior dielectric and cooling properties.

---

Functions of Transformer Oil

Transformer oil (also known as insulating oil) serves two major purposes:

1. Dielectric Strength – It provides high electrical insulation between energized components, reducing the risk of breakdown.
2. Cooling Medium – Heat generated in transformer windings and core is dissipated through transformer oil, ensuring safe temperature rise.

---

Production of Transformer Oil

Transformer oil is a hydrocarbon product derived from crude oil. Its main constituents include:

• Naphthenic hydrocarbons
• Paraffinic hydrocarbons
• Aromatics

The manufacturing process involves refining crude oil into Transformer Oil Base Stock (TOBS), followed by finishing steps to meet international specifications.

Production Steps:

1. Distillation of crude oil
2. Acid treatment for impurity removal
3. Neutralization of acidic residues
4. Water wash for purification
5. Hot air blowing for stability improvement
6. Clay treatment of TOBS → Finished Transformer Oil
7. Hot filtration → Ready-to-use transformer oil

---

Deterioration Aspects of Transformer Oil

Transformer oil undergoes chemical and physical degradation over time. The main causes include:

• Moisture contamination
• Chemical decomposition of hydrocarbons
• Oxidation leading to sludge and acids
• Gas contamination from arcing or partial discharge
• Electrical stresses under high voltages
• Thermal stresses from overload conditions
• Accumulation of oxidation products
• Physical contamination (dust, fibers, etc.)

These factors reduce insulation strength, increase acidity, and cause sludge formation, ultimately leading to transformer failure if left unchecked.

---

Precautions in Transformer Oil Sampling

Transformer oil testing is highly sensitive to contamination. During sampling:

• Avoid exposure to dust, moisture, or rain/fog.
• Ensure sampler’s hands do not touch the oil.
• Prevent condensation if oil temperature is lower than ambient air.
• Always use clean, dry glass bottles prepared with:
1. Tap water wash
2. Soap solution wash
3. Hot distilled water wash
4. Acetone rinse
5. Oven drying at 110 °C for 1 hour
6. Final vacuum drying

---

Transformer Oil Testing Standards

To ensure quality, transformer oils must comply with national and international standards:

• IS-335 (India)
• ASTM D3487 (USA)
• IEC 60296 (International)
• BS-148 (UK)
• JIS-2320 (Japan)

---

Key IS-335 Specification Parameters

Property	Test Method	Equipment	Design Value
Appearance	Visual	–	Clear, no sediments
Density	IS-1448 (P16)	Hydrometer	≤ 0.89 g/ml @ 29.5°C
Viscosity	IS-1448 (P25)	Constant temp. bath	≤ 27 cSt @ 27°C
Interfacial Tension	IS-6104	IFT meter	≥ 0.04 N/m
Flash Point	IS-1448 (P21)	Pensky-Martens	≥ 140°C
Pour Point	IS-1448 (P10)	Pour point apparatus	≤ -6°C
Neutralization Value	IS-1448 (P2)	Titration setup	≤ 0.03 mgKOH/g
BDV (Breakdown Voltage)	IS-6792	BDV tester	≥ 30 kV (unfiltered), ≥ 60 kV (filtered)
Dielectric Dissipation Factor (Tan δ)	IS-6262	Tan delta test set	≤ 0.002
Resistivity	IS-6013	Resistivity cell	≥ 1500×10¹² Ω-cm @ 27°C
Water Content	IS-13567	Karl Fischer titrator	≤ 50 ppm
Corrosive Sulphur	IS-335 Annex-B	Oven test	Non-corrosive
Oxidation Stability	IS-335 Annex-C	Oxidation setup	Neutralization ≤ 0.4 mgKOH/g, Sludge ≤ 0.1%
Ageing Characteristics	IS-12177	Ageing oven	Tan δ ≤ 0.2, Acidity ≤ 0.05 mgKOH/g, Sludge ≤ 0.05%

---

NABL Requirements in Transformer Oil Testing Labs

• Maintain equipment history cards
• Calculate measurement uncertainty
• Regular calibration from NABL-accredited labs
• Participate in inter-laboratory comparisons
• Perform replicate testing for accuracy

---

Case Study: BDV Test Uncertainty Analysis

A sample transformer oil was tested for Breakdown Voltage (BDV):

• Test results (kV): 68, 74, 72, 60, 68, 64 … Average = 69.82 kV
• Type A Uncertainty: 0.818 kV
• Type B Uncertainty (Resolution + Calibration): 1.1115 kV
• Combined Uncertainty: 1.38 kV
• Expanded Uncertainty (95% confidence): ±2.76 kV

📌 Final Result: 69.82 ± 2.76 kV @ 95% CL

This demonstrates the importance of uncertainty evaluation for reliable transformer oil diagnostics.

---

Final Thoughts

Transformer oil is not just a filler medium—it is the lifeline of oil-immersed transformers. Regular monitoring, correct sampling, and strict adherence to IS, IEC, and ASTM standards ensure long service life and prevent catastrophic failures. With NABL-compliant testing, utilities can ensure transformer reliability, grid stability, and reduced maintenance costs.

 

Protection in Solar Panels;Bypass diode

For protection of Solar panels bypass diodes are used details for protection of solar panel is as described below:-

Bypass diode is also known as free-wheeling diode and is connected in parallel to every solar cell or for group of solar cells. It is not advisable to use bypass diode for every solar cell as it is too costly so it is used for group of solar cells. Bypass diode is connected in parallel with opposite polarity to a solar cell. As we know that solar panel is constructed using individual solar cells and solar cells are made from layers of silicon semiconductor materials. One layer of silicon is treated with a substance to create an excess of electrons. This becomes the negative or N-type layer. The other layer is treated to create a deficiency of electrons, and becomes the positive or P-type layer similar to transistors and diodes. During normal operation of solar panels all solar cells are forward biased and bypass diode is reverse biased as bypass diode acts as open circuit. The damaging effects of hot-spot heating may be avoided through the use of a bypass diode. Bypass Diodes prevent the current(s) flowing from good, well-exposed to sunlight solar cells overheating and burning out weaker or partially shaded solar cells by providing a current path around the bad cell.

Bypass diodes are basically used for protection in case of shades/Dark. Normally one bypass diode is used for every 15 solar cells. Therefore for 60 cell module bypass diode required are 4 nos.

The voltage across the unshaded solar cells depends on the degree of shading. For example, if one cell is completely shaded, then the unshaded solar cells will be forward biased by their short circuit current and the voltage will be about 0.6V (this is taken as an example). If the cell is only partially shaded, the some of the current from the good cells can flow through the circuit, and the remainder is used to forward bias each solar cell junction, causing a lower forward bias voltage across each cell. The maximum power dissipation in the shaded cell is approximately equal to the generating capability of all cells in the group. 

Above figure is self explanatory showing how bypass diode provides protection from Hot Spot heating. 

Process by Which Hot-Spot heating is prohibited by Reverse bias Diodes

If a solar cell is reverse biased due to a mismatch in short-circuit current between several series connected cells, then the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the external circuit rather than forward biasing each good cell. The maximum reverse bias across the poor cell is reduced to about a single diode drop, thus limiting the current and preventing hot-spot heating.

Mono-Crystalline Vs Poly-Crystalline Solar Panels

Comparison between Mono-crystalline and Multi-crystalline Solar Panels:-

Both types of solar panels use Silicon crystalline panels. Only difference is the purity of silicon used during manufacturing process. More purity of silicon we use better will be the efficiency of solar panels, but more purity we achieve is at the expense of more cost.

Mono-Crystalline Solar panels

Typical Mono-Crystalline solar panels are typically dark black in colour. Also corners are missing in these types of solar panels. They have uniform look which indicates that they have high purity of silicon. Mono-crystalline solar cells are usually made out of silicon blocks which are cylindrical in shape.

Poly-crystalline Solar Panels:-

Pole crystalline solar panels are usually light or dark blue colour, some patches are lighter than others. Poly-crystalline solar panels are usually perfect rectangular and these don’t have round edges. Raw silicon is melted and poured into a square mold, which is cooled and cut into perfectly square wafers.

Mono-Crystalline solar cells have slighter higher efficiency than poly-crystalline solar cells. But Poly-crystalline solar panels are slightly cheaper to manufacture than mono-crystalline panels. Also there is tighter spacing of cells in poly-crystalline solar panels than mono-crystalline panels which will defeat the higher efficiency of mono-crystalline solar panels. Also poly-crystalline solar panels have better performance at higher temperature than mono-crystalline solar panels have better performance at higher temperature than mono-crystalline solar panels.

There is effective utilization of space in Poly-crystalline solar panels than mono-crystalline solar panels as there is wastage of space in mono-crystalline solar panels due to shape of single crystals, So slighter lower efficiency will get offset by better utilization of space, Which will leads to higher efficiency per m2 in case of Poly-crystalline solar panels.

Mono-crystalline Vs Poly-Crystalline Solar Panels

1.     Mono-Crystalline solar panels have the highest efficiency rates as these are made out of the highest-grade silicon. Typical efficiency of these panels is 15-20% as compared to poly-crystalline solar panels which have efficiency range of 13-15%. This is due to lower silicon purity.

2. Mono-Crystalline solar panels have long life usually these solar panels manufacturer’s will give 25-year warranty.
3. Mono-Crystalline solar panels have better performance than poly-crystalline panels at low light conditions.
4. Process of Manufacturing Poly-crystalline solar panels is simpler and less costly than mono-crystalline. Also wastage of silicon during manufacturing is lower in poly-crystalline solar panels.
5. Poly-crystalline solar panels have better performance than mono-crystalline solar panels.
6. Mono-crystalline solar panels have thin-films which will tend to be more aesthetically pleasing since they have a more uniform look compared to the stippled blue color of polycrystalline silicon.

Power factor variation with load;Why Power factor low during Starting of Motor?

During starting of Induction motor , motor draws very high current as power factor at the start is very low. Power factor is very low because the magnetizing component of current  is very high at starting. At Start Magnetizing component current is very high as it has to overcome the reluctance offered by air gap between stator and rotor. This leads to higher current withdrawal during starting.

You can also say that angle between core-loss component and net current increases. As power factor is angle between core-loss component and net current, then there will be decrease in power factor.

During starting Power factor starts from zero and keeps on increasing and will be maximum at full load current. Rated Power factor of motor will be achieved at Rated current of motor. Usually it is power factor at start is considered as 0.2.

Motors usually have rated Power factor between 0.80 -0.90.

Power factor varies as per load connected to the motor.

Chart for the same is shown below:-

From above you will see that there is variation in Power factor depending upon the load. 

Circle diagram for the power factor is shown as below:-

Variable Refrigerant flow Air conditioning; VRV System

Variable Refrigerant Flow (VRF) Systems

Variable refrigerant flow type Air conditioners also known as Variable Flow Volume i.e. VRV, Where variable refrigerant means the ability of the system to control the amount of refrigerant flowing to multiple evaporators i.e. indoor units. Actually VRV consists of multiple Indoor units and all are connected to one single outdoor unit.

VRV also provide the individual settings of all Indoor units and it can handle both heating and cooling in different zones simultaneously. VRV were originally manufactured by Diakin, Japan. These are more widely used for Industrial applications where ducting isn’t possible.

 There is one problem that Design of VRF systems is more complicated and requires additional work compared to designing a conventional direct expansion (DX) system.

Let’s how VRV is different from other Air conditioning systems:-

Split Air-conditioning

Split type air conditioning you have often seen which consists of one indoor unit along with one oudoor unit i.e. every indoor unit and outdoor unit will constitute a single split AC.

Few advantages and Disadvantages of Spilt AC’s

Advantages:-

1.    These AC’s have low initial cost

2.      Ease of installation

3.      No ducting required

4.      Each system have own control.

Disadvantages

• Distance between indoor and outdoor unit should not be greater than 100- 150 ft otherwise the performance will suffer.

• Limited air throw.

Multi-Split Systems

This system operates similar to split type air- conditioning system however difference is that in this case there are ‘multiple’ evaporator units connected to one external condensing unit.  These type of systems were designed mainly for small to medium commercial applications. These are basically used where ducting isn’t possible.

Each indoor unit has its own set of refrigerant pipe work connecting it to the outdoor unit.

Advantages of Multi-splits

•      No need of duct work installation.

·         System efficiency improves then individual Split unit

•      Multi-splits are suitable for single thermal zone applications i.e. either for cooling mode or heat mode.

Drawbacks

• Main Drawback is that Individual system control not possible.

• In this systems whole system will turn OFF or ON completely in response to a single thermostat. These systems are therefore not suitable for areas/rooms with variable heat gain/loss characteristics.

Variable Refrigerant Flow Or VRV

These type of air conditioners are similar to the multi-split systems which connect one outdoor section to several evaporators main difference is that in multi-split systems there is only one controller which turns OFF or ON completely in response to one master controller. But in VRV systems can adjust the flow of refrigerant to each indoor evaporator.

The control of refrigerant is achieved by continually varying the flow of refrigerant through a pulse modulating valve. Opening of Pulse modulating valve is done by the microprocessor receiving information from the thermistor sensors in each indoor unit.

The indoor units are linked by a control wire to the outdoor unit which responds to the demand from the indoor units by varying its compressor speed to match the total cooling and/or heating requirements.

VRV systems are efficient than air conditioning options and usually save 10- 20% electricity. But they have some high initial cost. Today in modern technology  when there is inverter controlled technology has arrived which will leads to as many as 48 or more indoor units to operate from one outdoor unit .

With VRV refrigerant piping runs of more than 200 ft are possible, and outdoor units are available in sizes up to 240,000 Btuh.

VRF systems are engineered systems and use complex refrigerant and oil control circuitry. The refrigerant pipe-work uses a number of separation tubes and/or headers.A separation tube has 2 branches whereas a header has more than 2 branches. Either of the separation tube or header, or both, can be used for branches. However, the separation tube is NEVER provided after the header because of balancing issues