MOM of Vestas, Chitradurga

Transformer Efficiency

Method 1:

Efficiency = Output / Input
= (Input - Losses) / Input
= 1- (Losses / Input) ; input = VI cos θ ; (say, losses/ VI = x)

Efficiency = 1 - (x/ (cos θ+x))
= 1 - ((x/cos θ)/(1+ x/cos θ)
Method 2:

Efficiency = (VA * cos θ) / ((VA * cos θ) + Losses) * 100

Example:

Transformer KVA = 1000 KVA; 11 KV/ 433 V, Full loadLosses measured = 12.5 KW
calculate efficiency in full load at .8 PF, .9 PF and UPF.

method 1:
Eff at .8 PF = 800/ (800+12.5) * 100 = 98.46%
Eff at .9 PF = 900/912.5 * 100 = 98.63%
Eff at UPF = 100/ 1012.5 * 100 = 98.76%

Method 2:
x= 12.5 / 1000= 0.0125 Eff= 1- (x / x+ cos θ)
Eff at .8 PF = 1- (0.0125/(.8+.0125))= 1- (0.0125/.8125) = 98.46%
Eff at .9 PF = 1- (0.0125/(.9+.0125)) = 1- (0.0125/.9125) = 98.63%
Eff at UPF = 1- (0.0125/1+ .0125) = 1- (0.0125/1.0125) = 98.76%

TERTIARY WINDING (Third Winding)

Tertiary / Stabilizing Winding:

In transformers there are two windings primary and secondary, but in some case you may see in addition to them one more winding named as tertiary winding.

This winding details were discribed in BS: 171-1959 as a delta winding used at a Star - Star Connected transformer or Auto transformer for any of the following usages;

1. Used to reduce the interference with communication circuit due to third harmanic currents in the lines and earth.
2. Used for facilitate the flow of Zero Phase Sequence Current
3. To reduce the third harmonic voltages in system and transformer
4. Injecting the reactive power for maintain the certain voltage level.( Tertiary connected with static capacitors or with synchronous condenser)
5. Auxiliary supply requirement at system which lower than primary and secondary winding voltages.

Its rating may be less apprx. 33.33% of main winding. This delta connected winding may be loaded or unloaded as per requirement. If it unloaded it called as stabilising winding. The design of tertiary based on the fault conditions.

IMPORTANCE OF PERCENTAGE IMPEDANCE

Importance of Percentage Impedance (%Z):

1. It decides suitability of transformer to the system
2. It limits Short Circuit Current (Inversely)
3. It causes voltage drop (Directly)

For transformers up to 2 MVA it is around 5%, Above 2 MVA it varies up to 25% also.

Note: %Z is depends on Resistance value (%Z = V R^2 + X^2). Resistance value may affect with temperature, so, all resistance and Impedance values must convert in to standard temperature of 75%.

PHASE - PHASE & PHASE - EARTH CLEARANCES IN AIR

Engineers who are all working in Project as electrical cable, line works, installations etc.. must know the clearances between phase - ph and also ph- earth to maintain the minimum values.

All over the world VDE (Germany) standards are famous to follow, In India we are following the CBIP standards, For our reference the standard values as follows

VDE - VERBAND DER ELEKTROTEKNIK ELECTRONIK INFORMATIONS TECHNIK
CBIP - Central Board of Irrigation & Power

Nominal (KV) Impulse (KV) CBIP (Ph-Ph) VDE (Ph-Ph) CBIP (Ph-E) VDE (Ph-E)

11 KV------------ 75 KV----------280mm --------110mm------- 140mm ----115mm
22 KV---------- 125 KV ---------330mm --------210mm------- 230mm---- 215mm
33 KV---------- 170 KV ---------350mm --------280mm------- 320mm---- 325mm
47 KV---------- 250 KV--------- 530mm------- 400mm------- -480mm---- 450mm
66 KV---------- 325 KV--------- 700mm------- 500mm-------- 630mm---- 610mm
110/132 KV-- 550 KV------- 1220mm------- 900mm------- 1050mm ---1000mm
132 KV --------650 KV------- 1430mm ----- 1100mm------ 1270mm --- 1150mm
220 KV-------- 900 KV------- 2000mm------ 1600mm----- 1800mm ----1600mm
220 KV -------1050 KV------ 2350mm ------2100mm----- 2080mm ----2100mm
400 KV------- 1425 KV------4000mm -----......-.........----- 3500mm ----..... -......

I hope this may helpful to you all Engineers. We welcomes all your suggestions, ideas, views about posts/blog and feedback for any poor performance in this regard.

PERCENTAGE IMPEDANCE

In our college studies we may know about Impedance. Impedance is nothing but the sum of Resistance and Reactance in circuit.

Z = √R↑2 + X↑2 where, R= Resistance; X= Reactance,

Than in General form, In A.C component circuit Z = V / I,

In Our transformer Impedance generally used to measure the Reactance as

X = √Z↑2 - R↑2

Once we entered in to professional these studied formulas, methods are not handy for get easy results, we must apply them at appropriate place,

Impedance Voltage: This is nothing but (HV) Primary voltage which required to create a full load current at short circuited secondary (LV).

** Percentage Impedance Voltage (3 phase Transformer)

= √3 X 100 X (HV Applied voltage to get full load current / HV Rated voltage)

*** Without knowing the Impedance it is unable to measure the Short Circuit withstand current.

** Trans withstand Short Circuit Current = 100 X Full Load Current (Rated Current) / Percentage Impedance

Measurement of Impedance:

1. Short the LV side

2. Start to apply 5 or 10% of rated voltage at primary side

3. Gradually increase the voltage level up to current flow in shorted LV side is full(Rated Current) value.

4. Then measure the applied voltage at primary as Va.

* Take rated primary voltage as Vr

Percentage Impedance (% Z) = (Va / Vr) X √3 X 100

Example:

In 25 MVA, 220 / 11 KV Transformer YNy0, HV rated current 65.61 A, after short flow of rated current in primary, the applied voltage is 20094 V.

% Z = (20094 / 220000) X √3 X 100 = 0.09133 X √3 X 100 = 15.81 % (At Measured Temperature)

We must express the %Z and other losses etc.. at 75 C only then it will be apprx. %Z = 15.99%.

NOTIFICATION OF NEXT POST

Whenever i am visiting site most of the ambitious persons asked me about
What is percentage Impedance in Transformer?
How to calculate Percentage Impedance in Transformer?
Use of Percentage Impedance in Transformer?
What is the relationship between % Z and Short Circuit current?


For all the above questions we will put end by today at this blog, but due to some other work i am unable, it is sure you will get those all at within a day or two.

BUSHING METAL PART MELTING

Objective: Electrical Connections are very important if they improper it may cost the following effects on man and materials, and ready to destroy the whole system also, The poor connection in Bushing metal part causes the following,

1. Melting in any side of connections, usually low melting point material side.
2. Over heating in contact places
3. Over heating increases chances of early oil leak in bushings
4. Servicing difficult once the part melted/ burred
5. The over heating will affect the next weakest point which is near.

Once it start melt it is unable to replace it may possible to cut and remove the entire metal part then only it possible to change

Further detailed photos you may visit www.sagar33@vox.com

MAGNETIC BALACE TEST

Magnetic Balance test used to find the distribution of Flux (Magnetic), Core Assembly Condition and defect in winding (If any).

It may check at HV Side or LV Side as per requirement/ site condition.

For Star Connection, apply the supply at U & N measure the values at V & N as well as W & N, then repeat the apply at V & N measure other two values, repeat it to apply at W & N.

For Delta connections, apply voltage must be in Phase - Phase like U & V, V & W and W & U.

For Ex:

U & N = (V & N) + (W & N); 240 = 198 + 42

Example : Results of 16 MVA, 110/ 33 KV Transformer, Vector Group of Dyn 11 (Delta Star 11)

Magnetic Balance at HV side

1U - 1V 1V - 1W 1W - 1U

417.0 (Appl) 401.0 22.0

232.0 419.0 (Appl) 189.0

20.0 402 420 (Appl)

Magnetic Balance at LV Side:

2u - 2n 2v - 2n 2w - 2n

238 (App) 208 31

122 241 (App) 119

38 207 245 (App)

RATIO TEST

Calculating the Specified Ratio:

If Star – Star Transformer: Ratio K = HV Specified voltage / LV Specified Voltage

If Delta – Star Transformer: Ratio K = 1.732 * (HV Specified voltage / LV Specified Voltage)

If Delta – Delta Transformer: Ratio K = HV Specified voltage / LV Specified Voltage

Procedure:

1. In Ratio Meter just rotate the dial and directly measure the value.

2. By apply separate voltage and measure using Meters.

i) Apply Single Phase/ phase to Phase/ All three phases as per availability. Measure the output.

ii) For easy reference apply 2 phase, that means 1st U & V,2nd V & W, finally W & U.

iii) The output must be V out = Applied voltage/ Specified Ratio (K)

Error:

Calculating the error is E = Specified Ratio ± Measured Ratio

( +) % Error = (100 * (Specified Value/Measured value)) – 100

( - ) % Error = (100 * (Measured value/ Specified Value)) – 100

Posted By: B.K, boopathymail@gmail.com, transtechindia33@gmail.com

TRANSFORMER TESTING AT SITE

Everyone knows before commissioning or charging any equipment it must be need to test before putin to service, like that the new transformer is also need to ensure everything is fine. So, some tests are required to confirm. those are all called Pre-commissioning tests.some important tests are mentioned here for easy ref,

i) Insulation Resistance or Meggar Value with PI (Polarization Index)
ii) Turns / voltage Ratio
iii) Magnetic Balance
iv) Magnetizing current
v) Oil tests (BDV, PPM and DGA (for future ref))
vi) Protections (Differential, IDMT, Bucholz Relay,OTI, WTI, PRV, OSR, MOG, Fans and pumps)
vii) Operation of OLTC or OCTC/OCKT
viii) Vector Group Test

Testing methods will be described in next post.

TRANSFORMER TESTING

As per IS 2026 the following Routine tests are carrying out in transformers at testing stage.
1. Measurement of Winding resistance
2. Voltage Ratio/ Ratio Test
3. Measurement of Impedance voltage/ Short Circuit impedance
4. Measurement of NO-Load loss and current
5. Dielectric tests (Induced Over Voltage and Separate Source Voltage)
6. Magnetic Balance and Current
7. Load Loss
8. Insulation Resistance
9. Vector Group
10. Oil BDV
11. Capacitance and Tan delta

Tests at site will be discussed in next post.