Electrical
Describe our electrical redundancy and what happens if we lose one line up.
DISTRIBUTIVE REDUNDANCY!
Another term used is “LOAD SHEDDING”
Define and describe a Circuit Breaker, to include:
A circuit breaker not only controls the flow of electricity in a circuit, but also provides overload protection by tripping
Define and describe the LSIG settings found on the Trip Unit of a circuit breaker.
LSIG signify the type of protection and available adjustments on certain trip units
L - Long time setting
Overload Protection
S - Short time setting
Low level fault short circuit protection
I – Instantaneous setting
High level fault short current protection
G - Ground fault setting
Equipment ground fault protection
a. UPS: Primary and secondary function.
Primary – uninterruptible (U), continuous power to the load
Secondary - protected power (P), filters and conditions power, prevents damage to output
Explain the difference between Maintenance Bypass and Static Bypass. Describe in which situations Maintenance Bypass would be required.
Maintenance Bypass – used when performing maintenance on the internal components, diverts power away from the UPS altogether (externally)
Static Bypass – used when there is an internal fault or UPS failure, diverts power away from the rectifier-battery-inverter (internal)
Describe the type of batteries we use in our UPS systems, including:
Lithium-ion batteries
17 batteries in each of the 5 strings
b. How long we are able to keep load on battery (if loaded to design)
7 minutes
Float voltage
Float voltage of 570v (the voltage at which a fully charged battery is maintained to prevent overcharging)
Define and describe a Circuit Breaker, to include:
b. Major external components and function of each
charging handle – when in open status, this charges the spring so the breaker may close
manual push buttons – used to manually close/open the circuit
Position – racked-in, test, disconnected, racked-out (withdrawn)
Status – open, closed, tripped, charged, not charged, OK
LOTO – safety measure to ensure equipment is safely shutdown and isolated
trip unit (not seen) – part that opens the contacts of a breaker in the event of a fault
Assuming proper permissions, are any of these trips safe to reset without a thorough breaker inspection?
For Long Term settings, only if engineering team understands what caused the trip
Any other trips requires a technician
UPS: Model, Type, and Capacity
Schneider Galaxy VX
Double-Conversion
1MW
Describe a transformer, to include:
a. component function
b. types found on site
a.
to step-up or step-down voltages in circuits (AC)
note that DC cannot change voltages
b.
Delta – three loads connected in a triangle
Wye – three loads connected in a Y
*Wye transformers have a neutral wire which allow for single-phase (Delta does not)
Provide an in-depth explanation of the difference between Single-Phase AC power and Three- Phase AC power. Which one do we use for our critical loads and why.
Single-Phase AC power – one sine wave created by one hot wire and one neutral
Three-Phase AC power – three sine waves, 120˚ apart each with one hot wire and one neutral
Critical loads use 3-phase because it’s the most power efficient source
BREAKER STATUS
D.What does it mean when a breaker is tripped?
Open – no power thru circuit
Closed – power thru circuit
Tripped – fault in circuit
Charged – ready to close breaker
Not charged – not ready to close breaker
D.
Define the term Arc Flash. Why are they dangerous? In what maintenance scenario is an arc flash most likely to occur with the electrical system?
electrical explosion
Most likely occurs during EEM (Energized Electrical Maintenance), when closing a live circuit breaker
UPS: Major internal components and basic function of each
Rectifier – converts AC to DC power, then filters thru capacitors; trickle charges the batteries
Inverter – converts DC to AC power via Pulse Width Modulation (PWM), and again filters thru capacitors
Batteries – provides DC power to the inverter should the utility fail
Static Switch – internal to the UPS, diverts power from the rectifier-inverter-batteries to the load when there is a fault/failure within the UPS system
Back Feed Breaker – prevents the flow of electricity going backward
Difference between step up or step down transformer
Step-up transformers – voltage is higher on the secondary side, current lower
Step-down transformers – voltage is lower on the secondary side, current higher
Define AC and DC power and provide a brief description of each. Where are each utilized in the building?
DC power – direct current; used in batteries
•
AC power – alternating current; used everywhere else
Explain Open vs Closed Transition with respect to the electrical distribution system. Which do we use when going in and out of maintenance bypass? How about when load is transferred between Utility and generator?
Open transition – “break before you make” open contact with primary source, then closing the secondary (both contacts are open for a moment)
Closed transition – “make before your break” closed contact with primary source, then open the secondary (both contacts are closed for a moment)
During maintenance bypass, a closed transition is used
When load is transferred between utility and generator, an open transition is used
How are the Trip settings and the Arc Flash Labels related?
Arc flash ratings are determined by the trip settings
If the settings have changed, then the arc flash labels must be updated as it is inaccurate.
UPS: Modes of Operation
Normal
Static Bypass
Maintenance Bypass
On Battery
Effects of a transformer's turn ratio on output
Effects of a transformer’s Turn Ratio on output
turn ratio refers to the number of times a coil is wrapped (more turns, more voltage)
Define the following electrical parameters, to include their base measuring units and symbol.
Voltage, Current, Resistance, Power, Frequency, Phase
Voltage – pressure that pushes electrons thru circuit; Volts [V, KV]
Current – flow of electrons thru circuit; Amps [A]
Resistance – opposes current (flow); Ohms [Ω]
Power – work done in a circuit (P=IE), Watts [W, KW]
Frequency – the # of times AC completes full cycle in a second; Hertz [Hz]
Phase – distribution of AC power, a single-phase being one hot and one neutral wire
Explain the process of selective tripping. Why is it designed within the electrical system? How is it accomplished?
Selective Tripping means that when a fault occurs anywhere in the distribution system, one and only one breaker will trip
The tripped breaker will be upstream from the fault so a chain reaction would be prevented (cascading failure)
opening times are adjusted so the breakers closest to the faults open first
Define and describe UPS, to include:
UPS – Uninterruptible Power Supply
Sequence of Operation:
Under normal conditions, power goes thru rectifier, trickle charges the batteries, thru the inverter and to the load.
Alarming set points:
Over temperature
Inverter/Rectifier failure
Fan failure
Communication failure
Power Module failure
Define PDU and describe is function. Include in your description
PDU – Power Distribution Unit
Converts power (via transformer) and distributes to multiple devices
a. output power capacity
480v TO 415/240V and 208/120V
480V enters PDU and converts to dual tap 415/240V and 208/120V
Internal components:
Input breaker, voltage transformer, CT, sub-feed output breakers
Sequence of Operation:
Distributed to output thru the sub-feed output breakers