Transformer Testinf Part-3 (Ratio Test and vector group test)

(3) Turns Ratio / Voltage Ratio Test:

 Test Purpose:

• Turns Ratio Test / Voltage Ratio Test are done in Transformer to find out Open Circuited turns, Short Circuited turns in Transformer winding.
• The voltage ratio is equal to the turn’s ratio in a transformer (V1/V2=N1/N2). Using this principle, the turn’s ratio is measured with the help of a turn’s ratio meter. If it is correct , then the voltage ratio is assumed to be correct
• This test should be made for any new high-voltage power transformer at the time it is being installed.
• With use of Turns Ratio meter (TTR), turns Ratio between HV & LV windings at various taps to be measured & recorded.
• The turn’s ratio is measure of the RMS voltage applied to the primary terminals to the RMS Voltage measured at the secondary terminals.
• R= Np / Ns
• Where,
• R=Voltage ratio
• Np=Number of turns at primary winding.
• Ns= Number of turns at secondary Winding.
• The voltage ratio shall be measured on each tapping in the no-load condition.

 Test Instruments:

• Turns Ratio meter (TTR) to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
• Wheatstone Bridge Circuit

 Method No1 Turns Ratio Testing:

 Test Procedure:

• Transformer Turns Ratio Meter (TTR):
• Transformer ratio test can be done by Transformer Turns Ratio (TTR) Meter. It has in built power supply, with the voltages commonly used being very low, such as 8, 10 V and 50 Hz.
• The HV and LV windings of one phase of a transformer (i.e. R-Y & r-n) are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.
• Values are recorded at each tap in case of tapped windings and then compared to calculated ratio at the same tap.
• The ratio meter gives accuracy of 0.1 per cent over a ratio range up to 1110:1. The ratio meter is used in a ‘bridge’ circuit where the voltages of the windings of the transformer under test are balanced against the voltages developed across the fixed and variable resistors of the ratio meter.
• Adjustment of the calibrated variable resistor until zero deflection is obtained on the galvanometer then gives the ratio to unity of the transformer windings from the ratio of the resistors.
• Bridge Circuit:

• A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %
• This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the adjustable
  transformer as shown in the figure above and it should be changed until a balance occurs in the percentage error indicator. The reading on this indicator implies the deviation of measured turn ratio from expected turn ratio in percentage.
• Theoretical Turns Ratio = HV winding Voltage / LV Winding Voltage
• % Deviation = (Measured Turn Ratio – Expected Turns Ration) / Expected Turns Ration
• Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an associated high excitation current.
• Open turns in HV winding will indicate very low exciting current and no output voltage since open turns in HV winding causes no excitation current in the winding means no flux hence no induced voltage.
• But open turn in LV winding causes, low fluctuating LV voltage but normal excitation current in HV winding. Hence open turns in LV winding will be indicated by normal levels of exciting current, but very low levels of unstable output voltage.
• The turn ratio test of transformer also detects high resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge.

 Test Caution:

• Disconnect all transformer terminals from line or load.
• Neutrals directly grounded to the grid can remain connected

 Method No 2 Voltage Ratio Testing:

•  This test is done to check both the transformer voltage ratio and tap changer.
• When “Turns Ratio meter” is not available, Voltage Ratio Test is done at various tap position by applying 3 phases LT (415V) supply on HT side of Power transformer. In order to obtain the required accuracy it is usual to use a ratio meter rather than to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
• At Various taps applied voltage and Resultant voltages LV side between various Phases and phases& neutral measured with precision voltmeter & noted.

 Test Procedure:

• With 415 V applied on high voltage side, measure the voltage between all phases on the low voltage side for every tap position.
• First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.
• Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously.
• After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.
• Repeat the same for each of the tap position separately.
• At other taps values will be as per the percentage raise or lower at the respective tap positions.
• In case of Delta/Star transformers the ratio measure between RY-rn, YB-yn and BR-bn.
• Being Delta/Star transformers the voltage ratio between HV winding and LV winding in each phase limb at normal tap is 33 KV OR 33x√3 = 5.196 ,11 KV / √3 11
• At higher taps (i-e high voltage steps) less number of turns is in circuit than normal. Hence ratio values increase by a value equal to.5.196 + {5.196 x (no. of steps above normal) x (% rise per each tap)} 100
• Similarly for lower taps than normal the ratio is equal to 5.196 – {5.196 x (no. of steps above normal) x (% rise per each tap)}100

 Test Acceptance Criteria:

• Range of measured ratio shall be equal to the calculated ratio ±0.5%.
• Phase displacement is identical to approved arrangement and transformer’s nameplate.
• The IEEE standard (IEEE Standard 62) states that when rated voltage is applied to one winding of the transformer, all other rated voltages at no load shall be correct within one half of one percent of the nameplate readings. It also states that all tap voltages shall be correct to the nearest turn if the volts per turn exceed one half of one percent desired voltage .The ratio test verifies that these conditions are met.
• The IEC60076-1 standard defines the permissible deviation of the actual to declared ratio
• Principal tapping for a specified first winding pair: the lesser ±0.5% of the declared voltage ratio
• or 0.1 times the actual short circuit impedance. Other taps on the first winding pair and other winding pair must be agreed upon, and must be lower than the smaller of the two values stated above.
• Measurements are typically made by applying a known low voltage across the high voltage winding so that the induced voltage on the secondary is lower, thereby reducing hazards while performing the test .For three phase delta/wye or wye/delta transformer, a three phase equivalency test is performed, i.e. the test is performed across corresponding single winding.

 Test can detect:

• Shorted turns or open circuits in the windings.
• Incorrect winding connections ,and other internal faults or defects in tap changer

(4) Polarity / Vector group Test

 Purpose of Test:

• The vector group of transformer is an essential property for successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.

 Test Instruments:

• Ratio meter.
• Volt Meter. A Ratio meter may not always be available and this is usually the case on site so that the polarity may be checked by voltmeter.

 Test Circuit Diagram:

 

 Test Procedure:

• The primary and secondary windings are connected together at one point.
• Connect neutral point of star connected winding with earth.
• Low-voltage three-phase supply (415 V) is then applied to the HV terminals.
• Voltage measurements are then taken between various pairs of terminals as indicated in the diagram and the readings obtained should be the phasor sum of the separate voltages of each winding under consideration.

 Condition:(HV side R-Y-B-N and LV Side r-y-b-n)

• R and r should be shorted.
• Apply 415 Volt to R-Y-B
• Measure Voltage between Following Phase and Satisfy Following Condition
  

Vector Group	Satisfied Following Condition
Dyn1	Rb=Rn+Bn
	Bb=By
	Yy<Yb
Dyn11	Ry=Rn+Yn
	Yb=Yy
	Bb<By
Ynd1	RN=Ry+Yn
	By=Yy
	Yy<Yb
Ynyn0	Bb=Yy
	Bn=Yn
	RN=Rn+Nn