Digital Relay for protection of high voltage transformers. Link to the research paper: https://www.researchgate.net/publication/262567749_Modeling_and_Testing_of_a_Digital_Differential_Relay_Using_MatLabSimuLink The digital protective relay is a protective relay that uses a microprocessor to analyze power system voltages, currents or other process quantities for the purpose of detection of faults in an electric power system, industrial process system or High voltage applications.

1. SWITCHGEAR AND PROTECTION
INNOVATIVE PROJECT ON
DIGITAL RELAY FOR
PROTECTION OF HIGH
VOLTAGE TRANSFORMERS

2. INTRODUCTIO
N
• Current differential relays are
widely applied to the protection
of electrical equipment due to
their inherent simplicity,
excellent sensitivity on internal
faults and high stability on
external faults. However, when
applied to power transformers
problem arises due to the
Current Transformers (CTs)
mismatch, CTs errors, and the
phase difference between the
primary and secondary currents
of the power transformer, which
makes accurate currents
comparison difficult to achieve.

3. THIS TYPE OF MATCHING CTs ADDS
ADDITIONAL BURDEN ON THE MAIN
CTs AND ADDITIONAL COST ON THE
PROTECTION SYSTEM. RECENT DIGITAL
DIFFERENTIAL PROTECTIVE RELAYS
OVERCAME THESE PROBLEMS BY
INTEGRATING NUMERICAL VECTOR
GROUP ADAPTATION AND NUMERICAL
ZERO SEQUENCE COMPONENTS
ELIMINATION INSIDE IT
DIFFERENTIAL TRANSFORMER PROTECTIVE RELAYS CAN BE
CLASSIFIED ACCORDING TO THE DESIGN INTO
ELECTROMECHANICAL, STATIC AND MORE RECENTLY DIGITAL
RELAYS. ELECTROMECHANICAL AND STATIC RELAYS USE MATCHING
CTs FOR ZERO SEQUENCE COMPONENTS ELIMINATION AND VECTOR
GROUP ADAPTATION.
Classification of
Relays

4. THE DIGITAL PROTECTIVE RELAY IS A PROTECTIVE RELAY THAT USES
A MICROPROCESSOR TO ANALYZE POWER SYSTEM VOLTAGES,
CURRENTS OR OTHER PROCESS QUANTITIES FOR THE PURPOSE OF
DETECTION OF FAULTS IN AN ELECTRIC POWER SYSTEM, INDUSTRIAL
PROCESS SYSTEM OR HIGH VOLTAGE APPLICATIONS.
Numerical Relay Digital Voltage Protection Differential Digital Protective
Relays
Digital Protective Relay

5. IMPORTANCE OF
MODELLING
• Protective relay modeling has
become a vital task to the
protection engineer in order to
choose its suitable setting and to
check its performance before
installing it in the field. Protective
relay simulation allows the
developed protection system to be
checked beforehand for any
potential problem, such as mal
operation and/or unsuitability.
• The relay model performance will
be tested by simulating various
internal and external faults on a
500/315 kV, 250 MW power

6. IF A FAULT OCCURS WITHIN THE PROTECTION ZONE OF A POWER
TRANSFORMER, THE FAULT CURRENT WILL BE If = I'1+I'2,
CONSEQUENTLY THE DIFFERENTIAL RELAY WILL OPERATE.
Current Differential Protection Algorithm

7. UNDER NORMAL CONDITIONS (EXTERNAL FAULT OR LOAD) THE
CURRENT ENTERING THE PROTECTION ZONE WILL IDEALLY EQUAL TO
THAT LEAVING IT, If = I'1-I'2 = 0. CONSEQUENTLY THE
DIFFERENTIAL RELAY WILL NOT OPERATE (RESTRAINT).

8. However, due to errors in the CTs and CTs mismatch, perfect
differential current measurement is not possible and the operate
quantity (I'1-I'2) is finite during an external fault or load
condition, and the differential relay may mal operate.
In order to provide adequate stability, a restraint quantity of
(I'1+I'2)/2 is required.
The operate quantity will now be compared against the restraint
quantity using the biased operating characteristic.

9. THE BIASED OPERATING CHARACTERISTIC OF
THE DIFFERENTIAL RELAY
The criteria uses a one slope percentage
bias characteristic given by:
Iop > iop0 when Ires ≤ ires0
Iop > k*(Ires-ires0) + iop0 when Ires  ires0
where:
Iop - The operating current.
Ires - The restraint current.
iop0 - The basic differential current setting.
ires0 - The bias current threshold setting.
k - The percentage bias setting, used when Ires 
ires0.

10. IN TRADITIONAL ZERO SEQUENCE ELIMINATION TECHNIQUE, THE
RELAY SENSITIVITY IS ONLY 2/3IF AND THERE IS NO SELECTIVE
FAULT INDICATION. ABOVE FIGURE ILLUSTRATES HOW THE
VECTOR GROUP ADAPTATION AND ZERO SEQUENCE
COMPONENTS ELIMINATION ARE MADE NUMERICALLY IN THIS
PROJECT.
Numerical Vector Group Adaptation and Zero Sequence Elimination

11. Simulink Model
Digital Relay
Subsystem

12. S-Function Block Codes

13. Primary Side Output

14. Secondary Side Output

15. • THE PROJECT INTRODUCED A DIGITAL DIFFERENTIAL PROTECTIVE RELAY
MODEL, DEDICATED FOR THREE PHASE POWER TRANSFORMER
PROTECTION OF ANY VECTOR GROUP.
• THE RELAY MODEL, WHICH IS BUILT USING MATLAB/SIMULINK, INCLUDES
THREE BLOCKS; THE FIRST ONE IS DEDICATED FOR ZERO SEQUENCE
COMPONENTS ELIMINATION AT A GROUNDED Y WINDING, THE SECOND
ONE IS DEDICATED FOR VECTOR GROUP ADAPTATION OF A STAR/DELTA
CONNECTED WINDINGS OF THE POWER TRANSFORMER, THE THIRD ONE
IS DEDICATED FOR MAKING A DIGITAL RELAY DECISION.
• TO EVALUATE THE RELAY MODEL PERFORMANCE IT IS SIMULATED IN
SIMULINK AND LINKED TO THE MATLAB INTERFACE THROUGH THE S-
FUNCTION.
• THE SCOPE OUTPUTS VERIFIED THE SATISFACTORY RESULT OF THE
DIGITAL RELAY.
Conclusions

16. SWITCHGEAR AND PROTECTION
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