Question 1 The diagram below is the schematic of a protection system for a 33/11kV double transformer substation in which the 33kV switchboard and the 11kV switchboard are at the same location. The...

Electrical Enginnering Protection 9
Answer:1
PROTECTION SCHEME OF DOUBLE TRANSFORMER SUBSTATION
The protection system for a 33/11 KV double transformer substation consists of following components:
· Transformers
· IDMT ( Inverse Definite Minimum Time ) relays
· ITS ( Inter Trip Send ) relays
· SBEF ( Stand By Earth Fault ) relay
· REF ( Restricted Earth Fault ) relays
· DOC ( Directional Over Cu
ent) relays
· Bucholz relay
· Measuring and indicating elements (Ammeter, voltmeter, Maximum Demand Indicator)
DETAILED DESCRIPTION OF THE SYSTEM COMPONENTS:
Transformer:
Substation uses three phase transformer with delta connected primary and star connected secondary. It step down the voltage from 33 to 11 KV. Further, different protection schemes are employed in the primary and secondary of the transformer.
Inverse Definite Minimum Time (IDMT) relays:
It is an Induction relay. The characteristics of the induction relay are Inverse Definite Time Relay. The time of operation cannot be less than the minimum time however great the CT cu
ent. The operating time of the relay is inversely proportional to the Plug setting multiplier (which depends on Fault cu
ent level), therefore such relays are known as Inverse Definite Minimum Time (IDMT) relays. So when the fault cu
ent value is more than the time taken to operate the relay will be minimized.
IDMT relays are installed to control a Circuit Breaker, which is very similar to the a
angements for AC Trip Coils.
Inter Trip Send ( ITS ) relay:
The Inter Trip Send relay is used to employ an intertripping protection scheme in a system to isolate the faulted circuit at both ends of the system. Inte
upting schemes use signals to convey a trip command to local and/or remote Circuit Breakers to isolate faulty sections of the network
Stand By Earth Fault (SBEF) Relay:
SBEF relay is introduced to prevent neutral earthing resistors (either liquid or air cooled) that may be damaged by long term earth fault cu
ent. It comprises a single CT in the neutral earth lead, often located in the (NER) Neutral Earthing resistor high voltage bushing, driving a single IDMT relay that is set to give a long operating time.
Nowadays this protection scheme is thought of more as a backup system for all other earth protection systems and as such it needs to discriminate with any other downstream earth fault protection on the same network. Sometimes a two stage SBEF scheme is implemented, with one stage set to operate slower and backing up the othe
Restricted Earth Fault (REF) relay:
The REF protection scheme is a unit or zone protection scheme because it covers only one winding of the transformer. This scheme operates only for faults within a restricted zone and not for fault cu
ent that pass through the zone.
Directional Over Cu
ent (DOC) relays:
Directional overcu
ent relaying refers to relay that can use the phase relationship of voltage and cu
ent to determine the direction to a fault. The directional relay operates only when fault cu
ent flows in a specified direction.
Bucholz relay:
Buchholz relay is mounted on the transformer equipped with an external overhead oil reservoir. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside...