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87T - Transformer Differential Relay

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87T - Transformer Differential Relay

Description

This model implements a transformer differential relay (87T) with harmonic restraint and harmonic blocking functions. A differential relay operates when the difference between
two or more input electrical quantities exceeds a predefined value. It comprises up to three CT inputs (assuming there can be 3 transformer windings). A 2-slope restraint characteristic is available in this model.

The figure below shows the protection logic of 87T relay:

Table of Contents

Details on the Transformer Differential Model Functionality

The differential relay trips whenever the difference between two (or three) input currents exceeds a predefined value. It is sensitive to faults within the differential zone, and it should be set to make it insensitive to external faults.
This model implements two types of differential functions – unrestraint (87U) and restraint (87R) differential elements. The differential current is calculated as a phasor summation of the current inputs from the CTs. First, the current magnitudes are adapted
to the primary side based on the winding voltage levels of the transformer. Then, the magnitudes and angles of the currents are corrected from Y side to delta side according to the transformer winding connections in order to ensure the currents used by the differential element are in phase. It has to be noted that the input CT current polarity should be either “looking towards the transformer” for all inputs or “looking away the transformer” for all inputs and all the CTs are considered to be Y connected. Based on the rated current specified in the relay control panel, the per unit values of the currents are calculated. The rated current can be considered as the current seen at the secondary side of the CT at winding 1 during normal operating condition. The same current base is used for differential current settings.

  • The 87U element compares the differential current directly with the predefined 87U pickup current. The output of the 87U element operates when the differential current of one or more phases exceeds the pickup current.
  • The 87R element adds a 2-slope characteristic to the fixed operating current, as shown in Figure 3 - 16. The horizontal axis of this figure is the restraint current in pu whereas the vertical axis is the differential current in pu.
  • Two types of restraints are available:



Where Ii are the per unit input current phasors from each CT.
If and only if the differential current is larger than the pickup current and falls in the operating region (calculated from the two slopes), the 87R element will operate.

2-slope characteristics of differential relay


Harmonic restraint function is also provided in this model. This model implements a 2nd and 4th order harmonic restraint for better operation security. If the harmonic restraint function is enabled, the restraint current is calculated as follows:

Where   and  are the magnitudes of the second and fourth-order harmonics in the differential current.  and  are the harmonic blocking levels in percentage, which respect to the fundamental signal.


Harmonic blocking function is supported either for the 2nd, 4th, 5th harmonics individually or a combination of 2nd +4th or 2nd +5th harmonics. If the maximum level of the selected harmonics is reached, the 87R element operation is blocked as long as this
condition is satisfied. Internally, the comparison is made between the magnitude of the selected harmonics and the fundamental signal times the maximum harmonic levels in percentage, for example, IH2 is compared with IFund*L2 . If the former is larger, differential protection is blocked. Otherwise, harmonic blocking is not active.

Mask and Parameters

General parameters

NameUnitDescriptionDefault value
FrequencyHz

The nominal frequency of the measured current
signals. Both 50 and 60 Hz are supported within the model.

60

Primary/Secondary
connection

-

Transformer connection between winding 1 and
winding 2.

YY0

Primary/Tertiary
connection

-

Transformer connection between winding 1 and
winding 3.

N/A

Winding 1 voltage
(L-L)

VThe L-L voltage of winding 1.315000
Winding 1 CT ratio-The ratio of the CT connected at winding 1.240

Winding 2 voltage
(L-L)

VThe L-L voltage of winding 2.120000
Winding 2 CT ratio-The ratio of the CT connected at winding 2.600

Winding 3 voltage
(L-L)

VThe L-L voltage of winding 3.50000
Winding 3 CT ratio-The ratio of the CT connected at winding 3.20
Rated currentA

The base current (RMS) of the input signals and
differential current settings. User can use the CT
secondary current at winding 1 during normal
operating condition.

5
87U pickuppuThe pickup current for 87U element.8
Restraint type-

Select how the restraint current is calculated:

Sum: