Transformer technical Specifications; 990 KVA Transformer Technical Specifications

Transformers are backbone of electrical systems. Transformers are used for both stepping up and stepping down the voltage. While procurement of  Distribution Transformers there are following technical aspects which should be considered :-

For Tan-delta, Loss angle test visit link:-

http://electrialstandards.blogspot.com/2015/10/tan-delta-test-loss-angle-test.html

Below are the technical specifications of Oil type 990 KVA transformer :-

1.       Transformer voltage ratio should be 11 KV/ 433 V

2.       Vector group should be Dyn11

3.       Transformer Impedance at 75 degree Celsius should be 5%

4.       Transformer maximum no load losses should 1.4 KW

5.       Transformer Maximum full load losses should be 10.8 KW

6.       Transformer Oil Temperature rise without enclosure  should be 35 Deg C max over ambient 40 Deg C

7.       Transformer Winding Temperature rise without enclosure should be 40 Deg C max over ambient 40 Deg C

8.         Current density of LT and HT winding should be 3 A/mmsq

9.       Tapping on HT winding should be off load with +/- 5% in 2.5% step

10.    Design clearances for Phase to phase for 11 KV system should be 180 mm and Phase to earth for 11KV system should be 120 mm

11.   Design clearances for Phase to phase for 433 KV system should be 25 mm and Phase to earth for 433KV system should be 25 mm

12.    Transformer Type should be  Double Copper wound, three phase, oil immersed, with ONAN cooling

13.   Voltage variation on Supply side should be +/- 10%

14.   Frequency variation on Supply Side should be +/- 5%

15.   Insulation level for one minute power frequency withstand voltage should be 3KV for 433V system & 28KV for 11KV system

16.   Insulation level for Lightning impulse withstand voltage should be 75KV peak for 11KV system

17.   Short circuit withstand level should be 3 Sec with 5% impedance

18.   Noise level for Transformer should not exceed limits as per NEMA TR-1 with all accessories running measured as per IEC 551 / NEMA standard running measured as per IEC 551 / NEMA standard

For Transformer Oil Filteration process visit link:-
http://electrialstandards.blogspot.in/2015/10/transformer-oil-filtration-process.html

19.   Transformer tank design should be as below:-

	Tank	Type Tested Design
	Design	a) Completely sealed type with corrugated fins and with/without conservator tank as per site requirements  b) Completely oil filled or N2 cushion at top filled with positive pressure. N2 shall be technical grade in accordance with IS:1747  c) With bolted / welded cover
	Plate / Corrugated fin / tank features	a) Adequate for meeting mechanical & electrical withstand requirements, as per applicable standard. b) The tank and its sealing (gaskets, o-rings, etc.) shall be of adequate strength to withstand positive and negative pressures built-up inside the tank while the transformer is in operation. The maximum pressure generated inside the tank should not exceed 40kPa, positive or negative. c) Corrugated fins shall be built up of CRCA sheets of minimum 1.2mm thick. d) The corrugated tank wall shall ensure sufficient cooling of the transformer and compensate for the changes in the oil volume during operation.  e) The transformer shall be capable of giving continuous rated output, without exceeding the specified temperature rise. f) Internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank and HV & LV bushings mounted on Top cover.  g) All joints of tank and fittings shall be oil tight. The tank design shall be such that the core and windings can be lifted freely with cover. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. h) Tanks with corrugations & without conservator shall be tested for leakage at a pressure as per the applicable standard.
	Material of Construction	Mild steel plate with low carbon
	Plate Thickness	To meet the requirements of pressure and vacuum type tests as per CBIP manual
	Welding features	a) All seams and joints shall be double welded b) All welding shall be stress relieved for sheet thickness greater than 35 mm c) All pipes, stiffeners, welded to the tank shall be welded externally  d) All corrugated fins or expansion bellows provided shall be double welded.
	Tank features	a) Bottom with stiffeners & adequate space for collection of sediments b) No external pocket in which water can lodge c) Tank bottom with welded skid base d) Strength to prevent permanent deformation during lifting, jacking, transportation with oil filled. e) Minimum disconnection of pipe work and accessories for cover lifting  f) Tank to be designed for oil filling under vacuum g) Tank cover fitted with lifting lug

20.   Transformer oil should be Class 1 new mineral insulating oil, shall be certified not to contain PCBs. Naphthalene base with antioxidant inhibitor.

21.   Transformer winding should have following:-

	Winding
	Material	Electrolytic Copper
	Maximum Current Density allowed	Maximum 3 amp / sqmm
	Winding Insulating material	Class A, non catalytic, inert to transformer oil, free from compounds liable to ooze out, shrink or collapse.
	Winding Insulation	Uniform
	Design features	a) Stacks of winding to receive adequate shrinkage treatment. b) Connections braced to withstand shock during transport, switching, short circuit, or other transients.  c) Minimum out of balance force in the winding at all voltage ratios. d) Conductor width on edge exceeding six times its thickness. e) Transposed at sufficient intervals.  f) Coil assembly shall be suitably supported between adjacent sections by insulating spacers + barriers. g) Winding leads rigidly supported, using guide tubes if practicable.  h) Winding structure & insulation not to obstruct free flow of oil through ducts. i) Delta connection shall be done using Flexible cable.

22.   Transformer Bushings and Terminations should have following technical specifications:-

	Bushings and Terminations
	Type of HV side bushing	Epoxy cast bushing, 630 Amp, interface type ‘C’ as per EN50180 and EN50181.
	Type of LV side bushing	Indoor, Epoxy resin cast, 1kv voltage class and creepage 31mm/KV
	Essential provision for LV side line bushing	It shall be complete with copper palm suitable for tinned copper busbar of size 100x12 mm
	Essential provision for LV side neutral bushing	In case of neutral bushing the stem and bus bar palm shall be integral without bolted, threaded, brazed joints. Bus bar size shall be 100x12 mm
	Arcing Horn	As per site requirements
	Termination on HV side bushing	Cable connection by screened separable connector kit.
	Termination of LV side bushing	Bus bar connections
	Minimum creepage distance of all bushings and support insulators.	31mm/kv
	Protected creepage distance	At least 50 % of total creepage distance
	Continuous Current rating	Minimum 20 % higher than the current corresponding to the minimum tap of the transformer
	Rated thermal short time current	26.3kA for 3 sec
	Bushing terminal lugs in oil and air	Tinned copper

23.   Current Transformers used for Transformers should have following Technical specifications:-

Current Transformers	All three phases and neutral on LV side
Maintenance requirements	Replacement should be possible by removing fixing nut of mounting plate without disturbing LT bushing
Accuracy Class	5P10
Burden	5VA
Type	Resin Cast Ring type suitable for outdoor use
CT ratio	990KVA -1500/5 Amp

24.    Transformer Name plate should consists of following Parameters:-

For Parallel Operation of Transformers Visit link:-
http://electrialstandards.blogspot.com/2015/10/parallel-operation-of-transformers.html

Following details shall be provided on rating and diagram plate as a minimum.

a)      Type/kind of transformer with winding material.

b)      IS/ IEC [R3] standard to which it is manufactured.

c)       Manufacturer's name.

d)       Transformer serial number.

e)       Month and year of manufacture.

f)        Rated frequency in HZ.

g)       Rated voltages in KV.

h)      Number of phases.

i)        Rated power in KVA.

j)        Type of cooling (ONAN).

k)      Rated currents in Amp.

l)        Vector group connection symbol.

m)    1.2/50µs wave impulse voltage withstands level in KV.

n)      Power frequency withstands voltage in KV.

o)      Impedance voltage at rated current and frequency in percentage at principal, minimum and maximum tap

p)      Load loss at rated current.

q)      No-load loss at rated voltage and frequency

r)        Continuous ambient temperature at which ratings apply in deg c

s)       Top oil and winding temperature rise at rated load in deg c;

t)        Winding connection diagram with taps and table of tapping voltage, current and power

u)      Transport weight of transformer

v)      Weight of core and windings

w)    Total weight

x)      Volume of oil

y)       Weight of oil

z)      Name of the purchaser

25.    Transformer to be designed for suppression of 3rd, 5th, 7th harmonic voltages and high frequency disturbances

26.    Transformer core should have following technical specifications:-

Core
Grade	High grade , non- ageing, low loss, high permeability, grain oriented, cold rolled silicon steel lamination
Core Design Features	a) Magnetic circuit designed to avoid short circuit paths within core or to the earthed clamping structures. b) Magnetic circuit shall not produce flux components at right angles to the plane of lamination to avoid local heating.  c) Least possible air gap and rigid clamping for minimum core loss and noise generation. d) Adequately braced to withstand bolted faults on secondary terminals without mechanical damage and damage/ displacement during transportation and positioning.  e) Percentage harmonic potential with the maximum flux density under any condition limited to avoid capacitor overloading in the system.  f) All steel sections used for supporting the core shall be thoroughly sand blasted after cutting, drilling, welding. g) Provision of lifting lugs for core coil assembly. h) Supporting framework designed not to obstruct complete drainage of oil from transformer

For Parallel operation of Transformers visit link:-
http://electrialstandards.blogspot.com/2015/10/parallel-operation-of-transformers.html

For Tan-delta, Loss angle test visit link:-

http://electrialstandards.blogspot.com/2015/10/tan-delta-test-loss-angle-test.html

Tan Delta Test (Loss Angle Test / Dissipation Factor Test)

The Tan Delta Test is also known as the loss angle test or dissipation factor test. It is one of the most reliable methods to evaluate the health of insulation in electrical equipment such as power cables, transformer windings, CTs, PTs, and bushings.

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Principle of Tan Delta Test

Any insulating material ideally behaves like a pure capacitor when connected across a power supply. In a pure capacitor, the charging current leads the applied voltage by exactly 90°.

However, in reality, no insulation is perfect:

• With ageing, dirt, and moisture ingress, a resistive (leakage) current component appears in addition to the capacitive current.

• Therefore, the total current leads the voltage by slightly less than 90°.

The loss angle (δ) is the angle between the total current and its ideal capacitive component.

The tan delta (tan δ) is defined as:

$$\tan δ = \frac{I_r}{I_c} = \frac{(V/R)}{V \cdot 2πfC} = \frac{1}{2πfCR}$$

Where:

• $I_r$ = resistive (leakage) current

• $I_c$ = capacitive current

• $R$ = insulation resistance

• $C$ = capacitance of insulation

• $f$ = applied frequency

👉 Thus, tan δ ∝ 1/f, meaning at lower frequencies, tan δ values are higher and easier to measure.

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Why Low Frequency is Used

Tan Delta Test is always carried out at very low frequency (0.1 Hz – 0.01 Hz) because:

• At high frequencies, capacitive reactance becomes very small, leading to large capacitive current.

• Resistive current is independent of frequency.

• Hence, at high frequencies, capacitive current dominates, making tan δ values extremely small and difficult to measure.

• Low frequency testing ensures measurable tan δ values without excessive power requirements.

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Tan Delta Testing Methodology

1. Isolation: Disconnect the equipment under test (cable, transformer bushing, winding, CT, PT, etc.) from the circuit.

2. Application of Test Voltage: Apply a very low frequency test voltage, starting from normal operating level.

3. Step-up Voltage: If initial results are satisfactory, gradually raise the voltage up to 1.5 – 2.0 times rated voltage.

4. Measurement:

   • A tan delta controller records tan δ values.

   • A loss angle analyzer compares tan δ values at different voltages.

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Interpretation of Test Results

• Stable Tan δ Across Voltages: Indicates good insulation condition.

• Rising Tan δ With Voltage: Suggests ageing, moisture ingress, or partial discharge activity in insulation.

• Trend Analysis: Comparing present results with past records provides insights into the rate of insulation deterioration.

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Applications

• Power Cables – to assess insulation ageing and moisture ingress.

• Transformers – windings and bushings are frequently tested.

• Rotating Machines – insulation of stator windings.

• CTs/PTs – insulation condition monitoring.

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✅ Conclusion:
The Tan Delta Test is a non-destructive diagnostic tool widely used in the field to predict the remaining service life of insulation systems. It is an essential test for condition monitoring, preventive maintenance, and ensuring reliability of electrical equipment.

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Parallel Operation of Transformers; Conditions for Parallel operation of Transformers

Parallel Operation of Transformers: Benefits and Conditions

Transformers are the backbone of electrical systems in industries and utilities. To ensure reliability, efficiency, and flexibility, it is often advisable to connect transformers in parallel. This article explains the advantages of using transformers in parallel and the essential conditions that must be satisfied for safe and effective operation.

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Benefits of Using Transformers in Parallel

1. Improved System Availability

When multiple transformers are connected in parallel, the system becomes more reliable. If one transformer fails or is taken out of service for maintenance, the load can be transferred to the remaining transformers. This ensures continuous power supply and higher availability of the electrical system.

2. Higher System Efficiency

Transformers operate at maximum efficiency near their full load. With parallel operation, the load can be distributed among fewer transformers during light load periods, keeping them closer to full capacity. As demand increases, additional transformers can be switched on. This leads to optimized efficiency across varying load conditions.

3. Greater Flexibility for Load Growth

Parallel operation allows easy adaptation to future load changes. Instead of replacing an existing transformer with a larger unit, additional transformers can be connected in parallel. This provides scalability and operational flexibility in the power system.

4. Improved System Economy

Adding transformers in parallel is often more economical than installing a single large transformer. It avoids the cost of replacing existing equipment while ensuring effective utilization of available transformers. This approach also reduces the risk of under-utilization of oversized transformers.

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Conditions for Parallel Operation of Transformers

While parallel operation offers many advantages, it is only effective when certain technical conditions are met. Failure to follow these requirements can lead to circulating currents, unequal load sharing, overheating, or even equipment failure.

1. Identical Voltage Ratio

Transformers connected in parallel must have the same voltage ratio.

• If ratios differ, circulating currents will flow between transformers even under no-load conditions.

• Since transformer impedance is very low, even a small voltage mismatch can cause large circulating currents and excessive I²R losses.

2. Equal Impedance and X/R Ratio

• Transformers should have the same per-unit (p.u.) impedance to ensure proportional load sharing according to their ratings.

• If the X/R ratio differs, one transformer may operate at a higher power factor than the other, leading to unequal load distribution.

• This condition is especially critical in three-phase banks of single-phase transformers.

3. Same Polarity

Transformers must have the same polarity before connection.

• Opposite polarity results in induced voltages of opposite directions, which can cause short circuits or dangerously high circulating currents at the output.

4. Same Phase Sequence

In three-phase systems, all transformers must follow the same phase sequence.

• A mismatch in sequence leads to severe short circuits.

• This condition is of utmost importance for safe parallel operation.

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Conclusion

Parallel operation of transformers enhances system reliability, efficiency, flexibility, and economy. However, strict adherence to technical conditions such as identical voltage ratio, impedance, polarity, and phase sequence is essential. By following these guidelines, industries can ensure safe and effective use of transformers in parallel operation.

👉 For detailed explanation of the Tan Delta (Loss Angle) Test, click here.
👉 For specifications of Oil Type 990 KVA Transformers, visit this link.

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