PARALLEL OPERATION OF TRANSFORMERS

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The Parallel Operation of Electrical Equipments Mainly Generator, Transformer & UPS will leads finest operation and more efficiency when compare than working as Idle in operation.

Here, I am taking step to elaborate " PARALLEL OPERATION OF TRANSFORMERS ". Also this article covers, What are the Main Criteria needed to meet Success operation of Paralleling of Transformers.

Now let us Start with Generators. The following Factors are to be considered while conducting Parallel Operation of Two & More Generators having Equal Rating.

1. Voltage.

2. Phase Sequence.

3. Frequency.

The above 3 Factors are well known Factors for Everyone. But Another Important to be considered for the Paralleling.

4. Generator Internal Impedance : It have more interesting topic according to their Types, It has divided into Three Categories a) SubTransient b) Transient c) Permanent.

Now the Condition for Transformer also same, but Three more additional in the Mentioned list.

"FACTORS FOR PARALLELING OPERATION OF TRANSFORMERS"

1. Voltage Ratio.

2. Phase Sequence.

3. Frequency.

4. Impedance [% Z].

5. Vector Group.

6. Winding Class.

The First Three Factors are well known by us, Now we need more detailing of rest of the Factors.

IMPEDANCE [%Z]

When Impedance voltage is applied across one winding of a Transformer it produces normal full load current to flow through the secondary winding when its terminals are short circuited. It is expressed as as percentage of the applied voltage. For example, in a 11 kV / 440 V Transformer having an impedance voltage of 5 % full load current across the 11kV windings, and the 400V winding is short circuited at the terminals. The assumption that there is complete magnetic coupling between the Primary & Secondary windings is an ideal condition. In Practical Transformers there is always a certain amount of leakage flux which results in a slightly lower flux linking the secondary than the flux in the Primary. This results in a voltage drop inside the Transformer. This, in addition with resistance drop in the windings is known as the leakage Impedance of the Transformer.

The resistive component of short circuit impedance gives parameter for estimating load losses. These losses include Eddy current losses in the conductors and structure as a small portion. Their contribution is materially enhanced due to Harmonic currents in Load. Exact determination by the test is difficult and simplified test @ low current suffers from the disadvantage of a high multiplying Factor. But it is expected to give representative values.

Impedance is the best factor to consider FAULT LEVEL of Circuit Breaker at both Ends, i.e Primary & Secondary End Protection. Now we will see importance of Transformer Impedance. 

According to Transformer, The following Standards of National & International which are reserved for Transformers. The Product Manufacturer should follow Standards of both and should implement their best quality.

INTERNATIONAL STANDARDS:

 IEC 60076 - Part - V describes Minimum Fault level permit table according to the System Voltage. Here, Indian Standard IS 2026 gives more clarity of Transformer Percentage Impedance.

Now we will go with Example:

According to IS 2026 Transformers up to 630 kVA, Percentage Impedance level should be equal & less than of 4%

Transformer of 11 / 0.433 kV of 500kVA with %Z = 4%

At Primary End Fault Current will be 656A or 0.656 kA @ 11kV end.

At Secondary End Fault Current will be 16668A or 16.66 kA @ 433V end. 

Important Note:

Some of the Transformer Manufacturer should cross the limit for the Transformer Impedance voltage against Standards, if they design 11 / 0.433 kV of 500kVA with %Z = 5%, according to this transformer ,

At Primary End Fault Current will be 525A or 0.525 kA @ 11kV end.

At Secondary End Fault Current will be 13334A or 13.33 kA @ 433V end. 

If Equal Rating of Transformer having different Impedance level, then it is difficult to make Parallel Operation. Here you have also to Consider IEC Standard to make the system as Neat in Operation. 

Higher Value in Impedance result in lower Value of Short Circuit Current.

Voltage Rating of up to 24kV - Short Circuit Apparent Power will be 500 MVA.

So for the 11kV systems, the HT Breaker should have Short Circuit Current Carrying Capacity should be Equal to & Less than of 25kA.

While Ordering Transformer, The Technical Team should verifer Whether Primary Winding of Transformer will carry Short Circuit Current Rating of 25kA for 3 Sec. 

The above factor is important to consider the Cost of the Transformer. 

VECTOR GROUP:

Phase Rotation is Always Anti-Clockwise. [International Adopted Convention] Use the Hour Indicator as indicating the Phase Displacement Angle. 

There are 12 Hours on a Clock, each hour represents 30 Deg. Round the Clock will be 360 Deg. The Minute set on 12 O'Clock and replaces the line to Neutral Voltage of the HV Winding. This Position is always the reference point. Because Rotation is Anti- Clock wise 1 = 30 Deg. Lagging [LV Lags HV with 30 Deg.]

Group – I     :    Zero Phase Displacement   [ Yy0, Dd0, Dz0 ]

Group – II    :    180° Phase Displacement   [ Yy6, Dd6, Dz6 ]

Group – III   :    -30° Phase Displacement   [ Yd1, Dy1, Yz1 ]

Group – IV   :    +30° Phase Displacement   [ Yd11, Dy11, Yz11 ]

Step-up Transformers: It should be Yd1 or Yd11

Step-Down Transformers: Dy1 or Dy11

Grounding Purpose Transformers [Zig-Zag Transformers]: It should be Yz1 or Dz11

Generating Transformer: : Dyn11

Furnace Transformer: YNyn0        

Here I would like to Mention familiar Vector Groups used for Solar Industry. 

Question:

If One Transformer having 11/0.433kV 1000 kVA Rating with Vector Group of Dyn11 & Another Transformer having 11/0.433kV 1000 kVA Rating with Vector Group of Dyn5. Is it possible to make Parallel Operation?

Answer : No. It is difficult to make operation even though if Transformer Ratings are same. Both of the Transformer having different Vector Group. So it is not possible to make Synchronization. 

WINDING CLASS:

Here I Mentioned Parallel operation of Transformer with different Winding Class type. [Here I updated only limited winding Configuration].