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Electrical Engineering > Electrical Systems > Flashcards

Electrical Systems Flashcards

1
Q

Why would we use a Main-Tie-Tie-Main (MTTM) over a Main-Tie-Main (MTM) configuration?

A
  • The extra tie breaker has extra cost but provides more reliability
  • In a MTM set-up if the tie breaker fails it will trip all breakers on both buses, for a MTTM a tie breaker failure only clears one bus
  • Tie breaker often gets operated and racked-out more than other breakers. In order to isolate, maintain or work on one bus, you need to rack and lock out the Bus B tie, which if you only had a MTM set-up, it means it performs double duty to isolate buses
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2
Q

Why do we have a neutral bar and a ground bar?

A

Neutral bar collects “grounded” conductors and ground bar collects “grounding” conductors. Grounded conductors carry the unbalanced load of phased conductors and grounding conductors carry fault current. At the main service panel the neutral bar would be bonded to the ground bar via the main bonding jumper. The neutral bar should not be bonded to the ground bar anywhere other than the main service panel.

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3
Q

Why are IR scans useful?

A

Can use thermal imaging to detect high levels of heat or cold spots which could indicate a problem. Early fault detection can help prevent permanent damage and unplanned downtime.

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4
Q

When would rotary UPS be suited for use?

A

Rotary UPS would be suited for environments with multiple short inrushes of power, such as where motors are constantly turned on and off.

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5
Q

When do you need an isolating switch?

A
  • When service disconnect is an oil switch or [air, oil, sf6, vacuum circuit breaker]
  • Located on supply side of service disconnect
  • Not required if service disconnect is rack-out breaker (metal clad switchgear, etc)
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6
Q

What would drive the selection between EMT, RMC, and IMC?

A

EMT is lightweight “thin-wall” metal conduit and commonly used.
IMC is galvanized steel tubing that can be used wherever RMC is used, but is lighter and easier to work with than RMC
RMC is heavy-duty galvanized steel conduit, and probably used in harsher environments since it is heaver than IMC

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7
Q

What is ZSI?

A

ZSI is zone selective interlocking.
System in a coordinated distribution system to limit stress on system by reducing the time it takes to clear electrical faults while maintaining coordination. Detects where the fault is and determines whether breaker will open to clear a fault based on trip delay settings or to immediately bypass it.

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8
Q

What is voltage drop?

A

The longer the length of circuit or cable, the greater the voltage loss. Impact can include problems with motors running more slowly or lights being dimmed. Can be compensated with larger cross-sectional sized cables to offer less resistance/impedance to current flow.

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9
Q

What is VLF tan delta test?

A

Diagnostic test on cable using AC voltage at low frequency and at lower voltage magnitude than withstand Hi-pot test. Can be used to evaluate overall condition of cables rather than localized defects.

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10
Q

What is VLF AC Hi-Pot testing

A

Very Low Frequency, AC Hi-pot testing. Uses 0.1 Hz instead of true 60Hz AC testing. Still a withstand test with many of the same problems as DC hi-pot. Designed to grow defects quickly

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11
Q

What is top of rack data center topology?

A

Two or more switches placed at top of rack at each server cabinet. Servers are cabled to both switches for redundancy. Utilizes cables efficiently, but increased costs and potential for overheating. Also difficult to manage in large deployments.

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12
Q

What is the difference between generator and alternator?

A

Alternator converts mechanical energy into AC electrical energy. Generator converts mechanical energy into either AC or DC energy. Alternator have rotating magnetic field and a stationary armature. Generators have a fixed magnetic field and rotating armature.

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13
Q

What is the BTU (heat) equivalent to 1 watt?

A

1 watt = 3.412 BTU/hr

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14
Q

What is short-time delay?

A

Short-time delay is used in conjunction with short-time pickup to control the time involved in postponing a short-time pickup trip. Mainly used for coordination with downstream breakers and fuses.

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15
Q

What is short time pickup?

A

Short-time pickup is used for selective tripping. It determines the amount of current the breaker will carry for a short period of time, allowing downstream protective devices to clear short-circuits without tripping the upstream device.

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16
Q

What is voltage sag?

A

A decrease in nominal voltage or current lasting 0.5 cycles to one minute.

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17
Q

What is overvoltage?

A

An increase in the nominal voltage for more than 3600 cycles or one minute.

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18
Q

What is long-time delay?

A

Long-time delay causes the breaker to wait a certain amount of time to allow temporary inrush currents, such as those encountered when starting a motor, to pass without tripping.

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19
Q

What is instantaneous pickup?

A

Used to trip the breaker with no intentional delay at any current between 2 to 40 times the breakers continuous ampere setting. (Ir)

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20
Q

What is inductance?

A

It is defined as the property of a coil to resist any changes in electric current flowing through it. Mutual inductance happens when a secondary coil opposes current change in the primary coil.

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21
Q

What is IEEE 80?

A

Guide for safety in AC Substation grounding

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22
Q

What is harmonics?

A

Harmonics is the distortion of the ac waveform caused by nonlinear devices in a power system. Harmonics are multiples of the basic waveform, so a second order harmonics is at 120Hz and third order is at 180Hz, assuming a 60Hz power frequency.

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23
Q

What is ground fault pickup?

A

Controls the amount of ground fault current that will cause the breaker to interrupt the circuit. Can be set from 20% to 70% of max breaker rating, but no trip point setting exceeds 1200A to meet NEC 230.95 (A).

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24
Q

What is continuous amps (Ir)?

A

Continuous amps (Ir) varies the level of current the circuit breaker will carry without tripping. Ir is a percentage of the circuit breaker’s nominal rating (In), and can be adjusted from 20 to 100 percent of the breakers nominal rating.

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25
Q

What is considered medium voltage?

A

Medium voltage is a range between 1000V and 35kV

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26
Q

What is considered high voltage?

A

High voltage is between 35kV and 230kV.

Extra high voltage is anything above 230kV

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27
Q

What are capacitive loads?

A

Loads where current leads voltage.

Examples can be buried cables, capacitor banks, and capacitors used in various circuits such as motor starters

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28
Q

What is capacitance?

A

It it the amount of charge that is stored inside a capacitor at a given voltage.

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29
Q

What is an SF6 circuit breaker?

A

Breaker where sulfur hexafluoride gas is used as the arc quenching medium. Gas is electronegative gas with strong tendency to absorb free electrons. Very effective for high power and high voltage service.

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30
Q

What is an oil circuit breaker?

A

Breaker oil is used to vaporize arc in the oil when contacts separate.

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31
Q

What is an isolated redundant configuration?

A
  • Two single module UPS units are connected so that the output of the backup UPS unit supplies the bypass circuit of the primary UPS unit. Basically critical load never receives unconditioned power through the bypass circuit.
  • Disadvantage, back-up UPS is not normally used to drive the critical load, so it is costly and space-consuming system.
32
Q

How do you control harmonics?

A

IEEE 519 allows a max of 5% total allowable harmonic distortion on the voltage waveform. You might need to install oversized neutrals, so neutrals sized at 200% of load, or higher K-rated transformers to help deal with the harmonics. Or you can provide harmonic transformers to reduce or eliminate harmonics.

You can also add filters to remove the harmonics in the form of shunt filters, series filters, or active filters.

Shunt filters short-circuit the harmonic currents as close to the source of distortion as practical to keep the current out of the system.

Series filter blocks the harmonic currents, not often used since it is difficult to insulate and the load voltage is very distorted.

Active filters electronically supply the harmonic component of the current into a nonlinear load.

33
Q

How would you design an underground service?

A

1) Determine service load requirements
2) Determine pathway, determine burial depth that might be needed, and perform derating if burial depth greater than 30”
3) Perform neher-mcgrath calculations
4) termination temperature limits. (what the terminations are rated at)
5) spare pathway for future growth
6) manhole sizing and placement

34
Q

What is Voltage Interruption?

A

A total absence of voltage on one or more phase conductors for a period of time.

35
Q

What is voltage swell?

A

An increase in the nominal voltage or current lasting from 0.5 cycles to one minute.

36
Q

What is undervoltage?

A

A decrease in the nominal voltage for more than 3600 cycles (one minute)

37
Q

What are inductive loads?

A

Load where current lags voltage wave. Example can include transformers, motors and coils.

38
Q

What are the advantage of ring bus topology?

A
  • individual UPS modules can operate either independently or in parallel
  • load capacity is limited only by the size of the ring bus
  • Additional UPS modules or distribution equipment can be added to the bus without disturbing other sections oft the system by opening the appropriate sectionalizing switches
  • faults are isolated when the sectionalizing switches open automatically
39
Q

What are the advantages of DC Hi-Pot testing?

A
  • test equipment is relatively lightweight and portable
  • test procedure is simple to administer
  • cost of equipment is generally lower
  • power supply requirements are low power
  • test is widely available
  • test is useful in acceptance testing of new cables
40
Q

What are the advantages of GIS switchgear?

A
  • low maintenance
  • inherently arc resistant construction
  • much smaller footprint
41
Q

What are the advantages of PVC conduit?

A

Easy to bend, low cost, resistant to combustion.

42
Q

What are the advantages of SF6 breakers?

A
  • Superior arc quenching property of the gas. Very short arcing time.
  • Dielectric strength is 2-3 times compared to air. Thus can interrupt much larger currents.
  • Noiseless operation
  • Reduces space requirements and building installation costs
  • Can handle all known switching phenomena
  • Can adapt to environmental requirements, have an enclosed gas system that eliminates any exhaust during switching operations
43
Q

What are the advantages of VLF testing?

A
  • Test equipment is lightweight and portable
  • Test procedure is simple to administer
  • Test exposes prevalent defects, including cuts and voids in the insulation
  • Test faults cable defects without jeopardizing the rest of cable
  • Test does not induce space charges, so no concern about aggravating water trees
  • When a cable passes the test, it can be returned to service without concern about an impending cable failure
  • If a cable fails the VLF test, it must be replaced
44
Q

What are the advantages of a cable bus?

A
  • easy to install
  • good reliability
  • cheaper than bus duct
  • Easy to maintain
45
Q

What are the advantages of a centralized data center topology?

A
  • Easier to manage and add components.
46
Q

What are the advantages of a lithium ion battery?

A
  • Longer life
  • smaller in size
  • less weight - easier to install
  • Faster recharge time
  • able to withstand higher operating temperatures
47
Q

What are the advantages of a vacuum circuit breaker?

A
  • Reliable, compact, and long life
  • can interrupt any fault current
  • no fire hazards
  • no noise is produced
  • has higher dielectric strength
  • fast in operation, so ideal for fault clearing and suitable for repeated operation
  • almost maintenance free
48
Q

What are the advantages of an air-blast circuit breaker?

A
  • It is used where frequent operation is required because of lesser arc energy
  • risk free from fire
  • small in size
  • requires less maintenance
  • arc quenching is much faster
  • speed of the circuit breaker is much higher
  • time duration of arc is the same for all values of the current
49
Q

What are the advantages of an oil circuit breaker?

A
  • good cooling property because of decomposition
  • oil has high dielectric strength
  • it as an insulator between the earth and the live parts
  • the oil used will absorb arc energy while decomposing
50
Q

What are the advantages of bus duct

A
  • reduced costs from having less construction labor
  • faster installation speed
  • flexibility, since busbar trunking systems can be adjusted any time, and components re-used
  • high short-circuit strength and low fire load
51
Q

What are the advantages of cable trays for power distribution?

A
  • optimal for design flexibility
  • great for renovations, removal, and day 1 spare space
  • not limited by # of bends, (does not require junction box)
52
Q

What are the advantages of liquid type transformers?

A
  • smaller footprint
  • lower noise
  • higher overloadability, can run hotter, and therefore withstand higher overloads for longer times
  • Full diagnostics, (can just take an oil sample to see the condition of transformer similar to blood draw in humans)
  • Longer life
  • higher efficiency
  • Lower temperature rise (so runs cooler, generates less heat)
53
Q

What are the advantages of tan delta tests?

A
  • It is a nondestructive, diagnostic test using voltage levels up to 2x rated phase to ground voltage
  • cable can be graded as good, aged or highly degraded
  • cable can be monitored over time
  • priorities for cable replacement and/or cable rejuvenation can be planned and budgeted for based on accumulated test results
  • test sets are portable and power requirements are low
  • test can be performed while conducting a VLF hi-pot test
54
Q

What are the advantages of using a dry type transformer?

A
  • It’s environmentally safe.
  • Non flammable
  • Minimal maintenance
  • low initial cost, no fire suppression system needed, no containment system needed.
  • shorter lead times
  • reliable
55
Q

What are the advantages of zoned data center topology?

A
  • Usually most cost effective design and minimizes cabling costs.
56
Q

What are the advantages of a series rated panel system?

A
  • Cheaper since no need to fully rate every breaker since you’re relying on rating of main breaker.
  • sometimes desired to achieve better coordination between breakers (breakers that are rated for higher available fault might not coordinate well)
57
Q

What are the benefits of a fused disconnect switch?

A
  • is overall the safest option, if a issue is detected it immediately opens the circuit, no delays and no mistakes.
  • less expensive
  • do not require regular maintenance
58
Q

What are the benefits of closed transition?

A
  • Can prevent the disruption of outages caused by either manual or automated MV open transition switching operations.
59
Q

What are the different UPS topologies?

A
  • Parallel redundant
  • Isolated redundant
  • Distributed redundant
  • Communications link
  • Ring bus
60
Q

What are the different classes of redundancy?

A
  • Class 1 (Single Path, no redundancy)
  • Class 2 (single path, with redundant components)
  • Class 3 (concurrently maintainable and operable. Has redundant capacity components and multiple distribution paths. Only one path is typically active at any given time)
  • Class 4 (Fault Tolerant. Redundant capacity components and multiple electrical paths that are both active. )
61
Q

What are the disadvantages of DC Hi-Pot test?

A
  • provides no diagnostic info about the cable since it’s pass/fail
  • does not replicate AC service conditions
  • May create space charges that polarize water trees and lead to failure long after the test
  • Weather is a factor that must be considered when conducting the test since windy or humid conditions can affect the leakage current
  • Cable must be taken out of service to test it
62
Q

What are the disadvantages of PVC?

A

Physically weak compared to metal conduits. PVC can break and splinter, and needs to be supported when installed in longer runs due to sagging.

63
Q

What are the disadvantages of SF6 circuit breakers?

A
  • relative high cost
  • Heavy duty switching can generate harmful by-products from decomposition
  • Imperfect joints can lead to leakage, so continuous monitoring is required.
  • Arced SF6 gas is poisonous
  • Can become a breathing hazard if collected
  • Special facilities are required for the transportation of gas, transfer of gas, and for maintaining the quality of the gas. The deterioration of quality of the gas can affect the performance and reliability of the SF6 breaker.
64
Q

What are the disadvantages of VLF test?

A
  • Withstand test, so cable might fail. Test is designed to expose weak spots and grow them to failure
  • Cables with several defect locations could be problematic, as the test does not identify all defects with one test. Several test-fail-repair cycles may be required.
  • VLF test equipment generally not commercially available for systems above 35kv
  • VLF test is not a diagnostic test, does not provide data points that can be used to assess the health of the cable
  • cable must be taken out of service to test
  • test duration is relatively long, 30-60 minutes
65
Q

What are the disadvantages of a fused switch

A
  • need to be replaced whenever it trips.
  • does not offer magnetic protection, only thermal
  • fuses age and degrade over time which can cause nuisance tripping
  • using individual fuses for three phase power can result in single phasing and equipment damage.
66
Q

What are the disadvantages of air-blast circuit breakers?

A
  • requires additional maintenance
  • air has relatively lower arc extinguishing properties
  • contains a high capacity air compressor
  • from air pipe junction, there may be a chance of air pressure leakage
  • there is a chance of high rate rise of re-striking current and voltage chopping
67
Q

What are the disadvantages of bus duct?

A
  • may encounter issues of condensation or water ingress which can lead to short circuit, (if used outdoors)
  • need to be evaluated for loose connections which can be difficult to do.
68
Q

What are the disadvantages of cable bus?

A
  • not permitted in hazardous locations unless otherwise noted,
  • more susceptible to physical damage
  • need to make sure the ratings of the cable bus can work with the terminations and ratings of the connecting equipment/switchboard/gear
69
Q

What are the disadvantages of closed transition?

A
  • Safety concerns from risk of parallelling 2-sources with the potential to be out-of-phase when the switch/breaker is closed, causing a catastrophic fault
  • Utilities may prohibit closed transition due to risk of back-feed
  • Paralleling sources also requires a lot of generator syncing and sync checks which is a complex process, and a lot of coordination, Complexity is what we want to reduce especially in MV switching operations
70
Q

What are the disadvantages of oil circuit breakers?

A
  • It will not permit high speed of interruption
  • it takes a long arcing time.
  • There is a possibility of explosion and fire
71
Q

What are the disadvantages of series rated panel?

A
  • the system is rated as a whole, so any change to its components means it would need to be recertified which can be impossible as sometimes manufacturers no longer produce those products.
  • can cause safety issues, ex a downstream panel is relying on the series rating of an upstream panel, but the upstream panel gets replaced and the designer doesn’t realize or know the downstream panel was relying on the replaced panels rating.
72
Q

What are the disadvantages of tan delta tests?

A
  • There isn’t a vast data base of VLF tan delta data to use for comparison with the test results
  • Only provides an overall condition assessment, but there could still be a localized defect in the cable that goes undetected
  • Com cable terminations and splices use high stress grading materials that may cause the tests to incorrectly show that a healthy cable is highly degraded
  • the cable must be taken out of service to conduct the test
73
Q

What are the disadvantages of vacuum circuit breakers?

A
  • cost of breaker is excessively high at higher voltages. uneconomical at voltages exceeding 28kv
74
Q

What are the disadvantages of zoned topology?

A

End of row switching might need to run cable back to end of row switch, and if every server is connected to redundant switches, this cabling can exceed what is required in top-of-rack architecture.

75
Q

What is the difference between real and apparent power?

A

Apparent power is the calculation of power based on measured voltage and measured current. Measured in VA
Real power is the power that is actually consumed. Measured in watts

Electrical Engineering (7 decks)

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