This document summarizes the protection system and relay settings for the transformers and feeders in a power distribution system. It describes the overcurrent, earth fault, and differential protection provided. The philosophy is to select settings that allow for transformer inrush and motor starting currents while maintaining coordination between protective devices. Settings like instantaneous overcurrent pickups are set above full load and starting currents. Time multipliers and curves are chosen to provide acceptable grading. Earth fault settings are set at percentages of fault levels and transformer currents. Discrimination during emergency supply is also considered. Time-current coordination curves are included to demonstrate the assessment.

1. Introduction
This section of the report details the protection system, the protection grading
philosophy and the recommended relay settings.
Protection System Description
In general, the 33 kV and 6.6 kV Transformer feeders are protected by IDMT,
instantaneous over current and ground fault protection. In addition to this 25 MVA
& 12 MVA transformers are protected by Differential protection as well as
Restricted Earth Fault protection. Similarly all 6.6 KV motor feeders are protected
by thermal Overload, load jam protection, startup time supervision, startup time
counters etc. All 33 KV feeders have Siemens make 7SJ85 relays whereas in 6.6KV
level all feeders are protected by 7SJ66 relays of Siemens make. Power from MSDS-
II 33 KV Tie feeder is limited to 25 MVA. Relay pickup settings are calculated based
on this current values.
Overcurrent and earth fault protection on the 415V PCCs are provided by inbuilt
release MTX 3.5i.
Protection Grading Philosophy
The following philosophy was adopted in determining the recommended protection
settings detailed in
1.1 Transformer Feeder Protection
• HV instantaneous protection settings will be greater than 1.05 x full load current
(FLC) to allow for
Transformer inrush
• Transformer through fault current at its own base MVA is calculated to check
whether this value falls below PSM =10. If it happens to be greater than PSM=10
then PSM=10 is considered for TMS calculation of 51/51N.
• Overcurrent pick-up settings is selected such that downstream motor starting
current (if any) or highest short time current is bypassed with a minimum time
grading of 0.3sec.
• Overcurrent Time Multiplier and Curve type selected to give acceptable grading.
Mainly for Power transformer & dry type distribution transformers IEC normal
inverse & IEC very inverse curves are selected.
• Earth Fault Plug Setting set at 30% of minimum transformer FLC.
• Earth Fault time multiplier set as low as possible to coordinate with downstream
CDG-11 relays for standby E/F protection.
1.1 HT Motor Feeder Protection
• HV instantaneous protection settings will be greater than 1.1 x Motor starting
current at 100% Voltage to allow for Motor starting current
• Thermal O/L curve for the associated relay is selected based on the formula
provided in relay manual

2. • For motor startup time supervision time calculation is done as per following
formula as described in the relay manual
The delay time indicated for this protection will inherently overlook the startup
period of the motors.
• Overcurrent Time Multiplier and Curve type selected to give acceptable grading.
Mainly for Power transformer & dry type distribution transformers IEC normal
inverse & IEC very inverse curves are selected.
• Earth Fault Plug Setting set at 30% of minimum transformer FLC.
• Earth Fault time multiplier set as low as possible to coordinate with downstream
CDG-11 relays for standby E/F protection.
1.2 Feeder Protection
• Overcurrent Plug Setting set at approximately 110% of maximum load below
cable ampacity &
thermal protection to cables
• Overcurrent Time Multiplier and curve selected to give acceptable grading
• Earth Fault Plug Setting set at 20% of minimum earth fault level
1.3 Transformer Incomer LV Protection
Earth Fault Time Setting selected to operate withina maximum duration of 1
second at 50% of the
minimum prospective fault level.
1.4 Switchboard Protection
All devices are set to clear switchboard busbar faults withinswitchboard limits
which are 40 kA for 3
seconds at 33 kV and 31.5 kA for 3 seconds at 6.6 kV. Additionally, for LV
switchboards, devices are
set to clear switchboard busbar faults withina maximum duration of 1s.
1.5 Protection Co-ordination
The assessment of the overcurrent and earth fault protection co-ordination is
discussed below. The
time-current characteristic diagrams are included at the end of this section. The
protection coordination
assessment is based on the fault levels reported in Reference 33 and included in
Appendix
A of this report. Maximum fault levels are based on all four main generators plus
both grid infeeds
supplying peak load. Minimum fault levels are based on one gas turbine generator
supplying
minimum load.

3. Discrimination on the emergency system when fed by the emergency diesel
generator is also
considered.
The discussion of the protection co-ordination refers to the Time Current Co-
ordination (TCC) Curves
included at the end of the Report text. The current (Ampere) labels displayed on
the TCC Curves
represent the pickup current for relays and the full load current for transformers.
The TCC curves also
show the IEC thermal and dynamic damage points along with the transformer
inrush current, which
are all marked as ‘×’ on the curves.
FM IGBT transformer feeders (F-1- F-7):
As per HTC input frequent overloading of 175 % for 60 sec will occur.
Recommended curve is IEC long inverse curve.
Transformer damage curve and inrush currents are considered as per attached
Annexure-A (Transformer protection curve)
To limit the instantaneous phase o/c pickup during transformer inrush 2nd harmonic
& 5th harmonic blocking will be enabled in the relay.
IDMT Phase o/c pickup is set at 120% of Transformer F.L.C.