:::: CT PT Metering Set ::: |
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CONSTRUCTION |
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Oil Cooled , Out Door Type
C.T.P.T. Metering Sets Are Made For Use On 11 Kv To 33 Kv Systems & Are Suitable For Mounting On Structures . BASICALLY A SET CONSISTS OF : 1. C.R.G.O. Cores 2. H.V. & L.V. WDG. 3. M.S. Tank with Secondary Terminal Box 4. Bushing & Transformer Oil SPECIAL FEATURES : 1) A Pressure Device Is Provided On The Top Cover Of The Tank Which Opreates In Case Of Abnormal Internal Pressure Development Due To Abnormal Operating Conditions. 2) The Set Is Much More Compact Compared To C.T..P.T. Sets Of Other Makes .  |
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| Applications | ||
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As The Accuracy Of The C.T.- P.T. Set Is High & As It Is Connected Beforethe Transformer , Measurement Of Power Is Much More Accurate Compared To The Metering Done On L.T. Side , The Transformer Losses Are Also Included In The Measurement . |
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| CT. PT. Metering Set | ||
| Connection Diagram | ||
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Connections are done as per terminal markings on the connection diagram shown below |
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| Dimensions | ||
| Overall maximum dimensions (approx.) of different C.T.- P.T sets are as below | ||
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| Residual Voltage Transformer | ||
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Residual Voltage Transformer
Is Used To Detect Unbalanced Voltage In Three Phase System &
To Supply Voltage To Directional Earth – Fault Relay. For Directional Earth – Fault Relay , It Is Necessary That The Voltage Applied To The Voltage Coil Of The Relay Corresponds In Phase To That Of The Current In Current Coil Such Voltage Will Be The Residual Voltage Of The System & Will Be The Phasor Sum Of The Three Line - To - Earth Voltages . Residual Voltage Can Be Achived By Connecting Secondaries Of Three Single Phase V.Ts. Connected In Three Different Phases , In Open Delta Fashion . It Is , However , Economical To Use Three Phase V.Ts. Instead Of Three Nos. Single Phase R.V.Ts. Suitable Upto 33 Kv System Voltage |
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In Normal Three Phase Three Limb V.T. When One Phase Experiences Earth Fault, The Resultant Flux Due To Two Healthy Lines Returns Through Transformer Limb Of Faulty Line, Including A Heavy Short Circuit Current In The Wdg. On The Limb . Also Voltage Induced In Secondary Open – Delta Wdg. Is Not True Residual Voltage . To Overcome This Difficulty Two Separate Limbs Are Provided In The Magnetic Circuit Of Three Phase Voltage Transformer To Form Five Limb Voltage Transformer As Shown In Fig. ( I ). |
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5 LIMB Construction Of Residual Voltage Transformer |
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The Use Of An Open – Delta
Connected Wdg. To Give True Residual Voltage Is Demonstrated
In Fig.( Ii ) With Three Cases : A) Under Healthy Condition All Three Phases Will Be Balanced & Hencce Residual Voltage ‘ Vr ’ Will Be Zero. B) In System With Unearthed Neutral, If There Is Earth Fault On One Line , Neutral Of V.T. Will Be Shifted, Such That Each Healthy Phase Wdg. Will Have Line – To – Line Voltage With 60 Phase Difference . The Resultant Open – Delta Voltage Shall Be Therefore , The Resultant Open – Delta Voltage Shall Be Therefore, Three Times Phase Voltage . ( Vr = 3 Vs ) C) In Solidely Earthed System , Neutral Of The V.T. Does Not Get Shifted Due To Earth – Fault On One Line. Therefore, The Resultant Open – Delta Residual Voltage Remains Phase Voltage Only. ( Vr = Vs ) |
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Primary Wdg. Of R.V.T. Is Connected In Star . The Secondary Wdg. Is Connected In Open – Delta. Some- Times Additional Secondary Wdg. Is Provided In R.V.T. For Measuring Purpose & Is Connected In Star . Ratio Of R.V.T. For Measuring Purpose Generally Specified As Line – To Line – Primary Voltage To Open – Delta Residual Voltage. IT , Therefore Becomes Necessary To Mention System Neutral Condition So As To Select Phase Voltage Of The Open – Delta Wdg. Correctly . When R.V.Ts. Are Used Along With Capaccitor Bank, Capaccitor Bank Gets Dischared Through Primary Wdg. |
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