Transformer Differential Protection Calculations SEL 787

Dear All,

Protection Element Setting:

Functional Scope:

The SEL-787 has three differential elements (87R-1, 87R-2, and 87R-3).These elements employ

Operate (IOP) and Restraint (IRT) quantities that therelay calculates from the winding input currents.

Figure shows the relay characteristic. You can set the characteristic as either a single-slope, percentage differential characteristic or as a dual-slope, variable-percentage differential characteristic. Tripping occurs if the Operate quantity is greater than the curve value for the particular restraint quantity.

A minimum pick up level for the Operate quantity must also be satisfied.

The four settings that define the characteristic are:

O87P = minimum IOP level required for operation

SLP1 = initial slope, beginning at the origin and intersecting O87P at

IRT = O87P • 100/SLP1

IRS1 = limit of IRT for SLP1 operation; intersection where SLP2 begins

SLP2 = second slope must be greater than or equal to SLP1

The relay (SEL-787) use the transformer MVA rating as a common reference point, TAP scaling converts all secondary currents entering the relay from the two windings to per unit values, thus changing the ampere values

into dimensionless multiples of TAP. Throughout the text, the term “TAP” refers to the per-unit value common to

both windings. This method ensures that, for full-load through-current conditions, all incoming current multiples of

tap sum to 1.0 and all outgoing current multiples of tap sum to –1.0, with a reference direction into the transformer windings.

In this course, we will perform the setting calculation of 26MVA transformer with following data:

Type of cooling : ONAN / ONAF

Vector Group : Dyn11

Rated Voltage HV V : 132000

Rated Voltage LV V : 12000

Full Load current HV A : 87.48 / 113.72

Full Load current LV A : 1004.12 / 1305.35

Rated Power MVA : 20 / 26

% impedance at normal tap (12 no.) : 15.35% / 19.95 %

Type of tap changer : On load

Voltage at maximum tap V : 151800

Voltage at minimum tap V : 112200

Further we will calculate:

Protection Settings Calculations for Power Transformers

• Full load amperes

• Short circuit MVA

• Short circuit current at HV & LV sides

• relationship between short circuit MVA and percentage impedance of transformer

• Magnitude and phase angle error and its compensation

• CT error and its compensation

• Operating and restraining currents

• Pickup current calculations

• TAPN calculations

• Phase angle compensation settings

• Slope 1 & 2 calculations

• Harmonics blocking calculations.

  
  

Best Regards