Power And Distribution Transformers Sizing Calculations – Part Six


 

Subject Of Previous Articles
Article
Glossary of Sizing Power and Distribution Transformers,
 
 
Resources used to calculate basic ratings of power and distribution transformers
 
the selection factors for the Power and Distribution Transformers
 
Applicable calculations procedures for sizing of power and distribution transformers
 
1-Applicable procedures for calculating power and distribution transformer ratios,
2-Applicable procedures for calculating power transformer efficiency,
3-Applicable procedures for calculating power transformer voltage regulation,
4-Special Cases In Transformers Sizing Calculations: Secondary Unit Substations
 
 
 
Note: I’d like from all of you to review our course “EP-3: Electrical Procurement – Transformers Courseto be more familiar with the contents of our new articles about the Power and Distribution Transformers sizing calculations.
 
Today, we will explain other special cases for Power and Distribution Transformers sizing calculations; Paralleled Transformers.
 

 


 
Special Cases In Transformers Sizing Calculations

 

2- Paralleled Transformers
 

  

 
Here we will explain the limiting conditions of connecting transformers in parallel and loading/sizing considerations that must be followed. Firstly, let’s see what is meant by Paralleled Transformers?
 
 
 
If two or more transformers are connected to a same supply on the primary side and to a same load on the secondary side, then it is called as Paralleled transformers.
 
 
Fig-1: Paralleled Transformers
 
Figure-1 shows two transformers connected in parallel. The transformers are connected in parallel by connecting the similarly marked terminals together. For example, terminal X1 of T1 to terminal X1 of T2, terminal X2 of T1 to X2 of T2, etc.
 

 

 

Importance for using Paralleled Transformers

1- Increased Load:
When load is increased and it exceeds the capacity of existing transformer, another transformer may be connected in parallel with the existing transformer to supply the increased load.
2- Non-availability of large transformer:
If a large transformer is not available which can meet the total requirement of load, two or more small transformers can be connected in parallel to increase the capacity.
3- Increased reliability:
If multiple transformers are running in parallel, and a fault occurs in one transformer, then the other parallel transformers still continue to serve the load and the faulty transformer can be taken out for the maintenance.
4- Transportation is easier for small transformers:
If installation site is located far away, then transportation of smaller units is easier and may be economical.

  

 
Conditions for Parallel Operation of Transformer
 
1- Mandatory conditions for Parallel Operation:
  • Same Phase angle shift (vector group are same),
  • Same Frequency rating,
  • Same Polarity,
  • Same Phase sequence.
 
2- Mandatory Conditions for Optimal Parallel Operation:
  • Same primary and secondary voltage ratings,
  • Same Turns Ratio,
  • Same Percentage Impedance and X/R ratio,
  • Identical Position of Tap changer,
  • Same KVA ratings.
 

 

 
The factors that should be considered when paralleling transformers
 
  1. Increased Fault Levels,
  2. Circulating Currents,
  3. Limiting kVA.
 

  

 
1- Increased Fault Levels
 
The paralleling of the secondaries of transformers increases the short circuit current available, and, therefore necessitates higher interrupting capacity and more expensive secondary switchgear (bus, breakers, etc.).
 
Example#1:
 
 
 
Fig-2
 
Referring to Figure-2, what is the total transformer let-through current at the secondary bus?
 
Answer:
 
1. IT = ILT(T1) + ILT(T2) = 2ILT(T1) = 2 [1500/( 3 x .48 x 0.055) = 2 (32803) = 65.6 kA
 

  

 
2- Circulating Currents
 
If the Mandatory Conditions for Optimal Parallel Operation do not exist, circulating currents will flow between and through the secondary windings of both transformers.
 
Definitions:
The circulating current is the current flowing at no load in the high and low voltage windings, excluding exciting currents.
Full load current is the current flowing in the transformer with a load connected, absent of exciting and circulating currents.
Disadvantages of Circulating Currents:
  • Lower the transformer efficiency
  • Reduce the maximum amount of load that the combined transformers can carry.
 
The magnitude of the circulating current
 
The magnitude of the circulating current that flows depends on which of the above optimal conditions do not exist. Let’s consider each of these optimal conditions to determine what occurs if they do not exist as follows:
 
 
a- If the turns ratios are not equal
 
A circulating current flows even if there is no load on the transformers. This circulating current flows because the voltage is different on the secondary side of the transformer.  In figure-1, if transformer T1 has a higher turns ratio than transformer T2, then T1 will have a lower secondary voltage than T2.
 
b- If the transformers do not have equal impedances
 
Transformers will not equally share the load. Current will divide between the two transformers, but not necessarily equally. .  In figure-1, if transformer T1 has a higher impedance than transformer T2, then more current will flow through T2 than T1. This is the same principle as the principle of current dividing between two paralleled impedances. If unequal impedances exist, one transformer can easily overload.
 
c- Different Phase shift and polarity
 
Paralleling two transformers with different phase shifts (e.g., Δ-Δ to Δ-Y) or different polarities (e.g., subtractive to additive) will cause large secondary circulating currents to flow.
 

 
 
 
3- Limiting kVA
 
Two dissimilar transformers (where one of the Mandatory Conditions for Optimal Parallel Operation doesn’t exist) may be operated in parallel, but the following two conditions must be met:
 
  1. The circulating current should not exceed 10% percent of the full load rated current of either transformer.
  2. The total load (combination of the circulating currents and full load current) should be limited to a value below the full rated current in either transformer. This total load limit is called the limiting kVA or the maximum kVA load of two transformers that are being operated in parallel.
 
 
Example#2:
 
Referring to Figure-1, what is the circulating current magnitude that flows between the paralleled transformers, and what is the limiting kVA of the two transformers that are being operated in parallel?
 
Answer:
 
 
Icirc = (kV1 - kV2)/[(Z1kV1)/(100*I1) + (Z2kV2)/(100*I2)]
 
Where:
 
  • kV1 and kV2 are the kV secondary voltages for transformers T1 and T2 at no-load.
  • Z1 and Z2 are the percent impedances for transformers T1 and T2.
  • I1 and I2 are the full load current for transformers T1 and T2.
 
I1 = kVA1/( 3 x kV1) = 12000/( 3 x 14.0) = 494.9 A
I2 = kVA2/( 3 x kV2) = 10000/( 3 x 13.9) = 415.4 A
 
Icirc = (14.0 - 13.9)/[(9 x 14.0)/(100 x 494.9) + (8 x 13.9)/(100 x 415.4)] = 19.15 A
 
The circulating current must be less than 10% of the smaller of I1 or I2.
 
Icirc <(0.10 x I1)<(0.10 x 494.9) = 49.5 A
Icirc <(0.10 x I2)<(0.10 x 415.4) = 41.5 A
 
The circulating current is less than 41.5 A and therefore it is acceptable.
 
kVAlimit = Zmin x [(kVA1/Z1) + (kVA2/Z2)] x 0.9 = 8 x [(12000/9) + (1000/8)] x 0.9 = 18600 kVA = 18.6 MVA
 
So, Total power supplied to the secondary bus cannot exceed 18.6 MVA. If more than 18.6 MVA of load is connected, transformer T2 will overload resulting in possible damage.
 

  

 
Additional precautions must be considered before paralleling transformers
 
  • If paralleling two transformers with load tap changers, the automatic sensing relay which controls the tap position of the transformers must be set up in a master/slave configuration. Both load tap changers must “stay in step” (stay on the same taps). The master/slave configuration system should allow one transformer’s sensing relay to control the tap changer on both transformers.
  • If paralleling transformers that have a high secondary current, consider the impedance of the cables or bus that is used to parallel the secondary windings. Small changes in impedance may have a large effect on load sharing.
 

  

 
Summary for Parallel Transformers Connections
 
Transformer
Parallel
Connection Types
 
Equal
Loading
 
Unequal
Loading
 
Overloading
Concerns
 
Circulating
Currents
 
Recommended
paralleling Connection
Equal impedances—
Equal ratios—
Same kVA
Yes
No
No
No
Yes
Equal impedances—
Equal ratios—
Different kVA
No
Yes
No
No
Yes but Not Optimal
Unequal
impedances—
Equal ratios—
Same kVA
No
Yes
Yes
No
Not Optimal
Unequal
impedances—
Equal ratios—
Different kVA
No
Yes
Yes
No
Not Optimal
Unequal
impedances—
Unequal ratios—
Same kVA
Yes
No
Yes
Yes
Not Optimal
Unequal
impedances—
Unequal ratios—
Different kVA
No
Yes
Yes
Yes
Not Optimal
Different Phase shift
Different polarity
Not allowed
 
 

 
In the next article, we will continue discussing other special cases for Power and Distribution Transformers sizing calculations which are:

  • K-Factor Transformers,
  • Transformers with Large Motor Loads.

So, please keep following.

  


 

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