Substation Interview

Question 1

Power Transformers

Answer 1

-step up voltage for transmission or step down for local delivery

Question 2

Circuit Breakers

Answer 2

-protect the system from fault currents

Question 3

Isolators

Answer 3

-only operated when there is no load
-provide a visible physical gap in the circuit for maintenance safety

Question 4

Busbars

Answer 4

-thick conductive bars that connect incoming and outgoing lines

Question 5

Surge/Lightning Arresters

Answer 5

-protect the expensive equipment from voltage spikes caused by lightning or surging switches

Question 6

Current Transformers

Answer 6

-step down high current to a measurable level for relays

Question 7

Instrument Transformers

Answer 7

-step down high voltage for metering and protection

Question 8

Protective Relays

Answer 8

-detects faults
-sends trip command to breakers

Question 9

SCADA (Supervisory Control and Data Acquisition)

Answer 9

-allows operators to monitor and control the substation remotely

Question 10

DC Battery Bank

Answer 10

-If main power fails, the substation must be able to trip its breakers
-battery bank provides the emergency power for the control circuits

Question 11

IEC 61850

Answer 11

-modern international standard for substation automation and communication

Question 12

Earthing/Grounding

Answer 12

-protects personnel from “touch” and “step” potentials during a fault

Question 13

why are isolators needed/used if they are used under no load?

Answer 13

-visible break in the air gives technician 100% certainty the line is disconnected
-if opened under load, electricity will jump the gap creating a plasma arc that can melt equipment

Question 14

how do potential transformers protect?

Answer 14

-physically separates the dangerous 132kV lines from the control room
-if fault occurs, high voltage stays on the primary side, protecting humans and equipment on the secondary side
-for fault detection, the PT sends a voltage signal to a relay, if that voltage drops, the relay sees an undervoltage fault and tells the breaker to trip

Question 15

how do surge arresters protect against lightning/surges?

Answer 15

-(normal voltage) inside of an arrester is a column of varistors that have near-infinite resistance so no current flows through them to the ground
-(surge/lightning) when voltage spikes, the resistance of the ZnO blocks instantly drops to zero so the surge is shunted safely to the earth grid

Question 16

how do relays work?

Answer 16

-works as logic center, it doesn’t cut the power but it makes the decision to cut the power
-receives current from CTs and voltage from PTs, compares the data against pre set values, uses complex algorithms to distinguish between an inrush and a short/fault, then sends signal to breaker to trip, if necessary

Question 17

transmission substation

Answer 17

-very high voltage
-132kV - 765kV
-connects power plants to “bulk” grid

Question 18

distribution substation

Answer 18

-medium/low voltage
-4kV - 69kV
-smaller, often located within city neighborhoods

Question 19

why is undervoltage dangerous?

Answer 19

-when voltage drops, motors try and pull more current to maintain power
-can melt insulation and start fires
-Potential transformer monitors line voltage and if voltage drops, it trips breaker

Question 20

how do transformers work?

Answer 20

-two coils of wire, primary and secondary
-AC current flows into primary
-because its alternating it creates a changing magnetic field in the iron core
-changing magnetic field induces a voltage in wire
-this induces power into secondary coil
-if secondary has twice as many coils, voltage doubles, if it has half as many, voltage is cut in half