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Electrical quiz 2 > Quiz 2 > Flashcards

Quiz 2 Flashcards

1
Q

Why do we build lines?

A

Transfer power from one point to another

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2
Q

What are the prerequisites when building new lines?

A

Power, Voltage, Distance

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3
Q

How are new lines built?

A

One line at a time

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4
Q

where do we get opportunities to build ultimate grids?

A

Areas without distribution grid.
In very old grids.
Collection grids.
otherwise: make best with what we got.

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5
Q

What are examples of collection grids?

A

Wind Power
Wave Power
tidal Power

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6
Q

What is power flow?

A

Most important tool in power system operation and planning.

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7
Q

What analysis can be made on-line from monotoring the power flow?

A

State estimation
Security analysis
Economic analysis

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8
Q

What does on-line economic analysis of power flow allow us to determain?

A

Optimal operation
Loss coefficients
Optimal pricing

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9
Q

What analysis can be made off-line from monotoring the power flow?

A

Operation analysis
planning analysis

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10
Q

What does off-line planning analysis of power flow allow us to determain?

A

Network expansion planning
Power exchange planning
Security and adequacy analyses (Faults, Stability)

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11
Q

What are some problems with analysing power flow?

A

It’s only a snapshot of the system.
modeled using simple methods to a complex mathematical problem
solved using iterative techniques
accuracy vs. computing time

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12
Q

what needs to be known to calculate demand and/or generation of power in each bus?

A

bus voltages
load flow in lines and transformers.

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13
Q

What 4 quantities are associated with each bus?

A

– The real power
– The reactive power
– The voltage magnitude
– The phase angle between voltages

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14
Q

Slack bus provides the additional real and reactive power to meet
the losses. What quantities are specified?

A

Voltage magnitude and angle are specified

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15
Q

Voltage controlled buses. What quantities are specified?

A

Voltage magnitude and real power are specified

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16
Q

Load buses. What quantities are specified?

A

Real and reactive power are specified

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17
Q

How do we solve power flow equations?

A

iteratively

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18
Q

what steps are taken when solving power flow equations?

A

– Check that sufficient variables are known (2n)
– Give initial values to those voltages and angles, which are
unknown.
– Calculate the active and reactive power injections.
– Compare with known values of active and reactive power.
– Repeat the calculation until the accuracy between calculated and
known powers is sufficient.

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19
Q

What is optimal power flow?

A
  • The goal of an OPF is to determine the “best” way to instantaneously
    operate a power system.
  • Usually “best” = minimizing operating cost.
  • OPF considers the impact of the transmission system
  • OPF is used as basis for real-time pricing in many electricity markets.
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20
Q

What are inequality constraints in optimal power flow?

A

– transmission line/transformer/interface flow limits
– generator MW limits
– generator reactive power MVAr limits or capability curves
– bus voltage magnitudes

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21
Q

What available controls exist when optimizing power flow?

A

– Generator MW outputs
– OLTC transformer taps, phase-shift taps
– Reactive power compensation devices (switched capacitor
settings, SVCs, etc.)
– Load shedding
– etc.

22
Q

What are some common causes of faults in EPS?

A
  • Faults in apparatus
  • Branches and trees on OH-lines
  • Damage cables
  • Ice loaded OH-lines
  • Vandalism
  • Lightning
  • etc.
23
Q

what is a fault?

A

”An unwanted connection between phases or
between phases and ground”

24
Q

What fault causes the maximum current?

A

3-phases
3-phases and ground

25
Q

What fault causes the lowest current?

A

1-phase to ground (Earth faults)

26
Q

What is the most common fault?

A

1-phase to ground (Earth faults)

27
Q

what are some different types of grounding systems?

A

 Direct
 Isolated
 Resistans grounded
 Reactance grounded
 Resistans and reactance grounded

28
Q

What represents a small contribution to short circuits?

A
  • Individual turbines
  • Loads with rotating mass (motors without
    frequency converters)
29
Q

What represents a very small contribution to short circuits?

A

Loads with rotating mass (motors with frequency
converters)

30
Q

Describe short circuit current

A
  • Current due to a short circuit
  • Not only in the fault location
  • ”Keep” the voltage up
  • Significantly higher then the load current
31
Q

What does the mathematical model of the fault current consist of?

A

-sinus component, stationary solution, fault current
-DC component, transient solution
-all together: subtransient current

32
Q

how large will the subtransient current be?

A
  • > 600V: at generator; Is = 3 * Isc, otherwise; Is = 2,5*Isc
  • <600V: Is = 2*Isc
32
Q

For how long will the very large current caused by faults last?

A

less than 10 ms

33
Q

What are some mechanical impacts of fault current? (busbars, disconnectors, circuit breakers)

A

Busbars: bending
Disconnector: “welded” shut
Circuit breakers: May not be able to be operated

34
Q

What is short circuit power?

A

-made up
-generally used
-a measure on the strength of the grid

35
Q

What is short circuit capacity?

A

-made up
-generally used
-a measure on how much the system/component will stop short circuit current

36
Q

Describe: Circuit breaker

A

– Break up to fault current
– No visible breaking
– Remote operating possible

37
Q

Describe: disconnector

A

– Can not break current
– Visible breaking/lockable
– Remote operating often not possible

38
Q

Describe: switch

A

– Can break up to load current
– Often visible breaking

39
Q

Describe: Fuse

A

– Can break fault current
– Cheap
– Can ́t be “controlled”
– Up to 72 kV

40
Q

What is HVDC used for?

A

-Long distance power transmission
-transfering power between different frequencies

41
Q

Why do we use HVDC?

A
  • Transmission of large amount of power over
    long overhead lines
  • For crossing long submarine distances
  • HVDC enables transmission of more power
    with less Right of Way (ROW)
  • Control over the power exchanged between
    two areas
  • Flexibility of HVDC enables improvement of
    performance of the overall AC/DC system
  • Investment cost after a certain distance
42
Q

What are some Technical system aspects for DC applications (HVDC)?

A
  • Power Flow Control
  • Reactive Power Compensation
  • Transient stability
  • Inter-area oscillations
  • Voltage stability
  • Interconnection
  • Island Power Dispatch
  • Grid restoration, black start
  • Sub-synchronous Torsional Interaction (SSTI)
  • Environmental aspects
  • Grid loss reduction
  • DC grids
43
Q

What are the basic power flow control functions of HVDC?

A
  • Active-power control
  • DC-voltage control
  • AC-voltage or reactive-power control
44
Q

What are some additional power flow control functions of HVDC?

A
  • Frequency control
  • Flicker control
  • Harmonics mitigation
45
Q

What are some HVDC configurations?

A
  • Back to Back
  • Multi-terminal Systems
  • Point to Point Transmissions
    (Mono + Bipolar)
46
Q

Describe: Line-commutated Converters (LCC) (HVDC)

A
  • Active power control
  • Terminals demand reactive power
  • Reactive power balance by shunt bank switching
  • Minimum system short circuit capacity of twice rated
    power
47
Q

describe: Capacitor-commutated Converters (CCC) (HVDC)

A
  • Active power control
  • Weak systems, long cables
  • Reactive power from series capacitor
  • Minimum system short circuit capacity of rated
    power
48
Q

Describe: VSC-HVDC (HVDC light, HVDC+, HVDC MaxSine)

A
  • Active and reactive power control
  • Dynamic voltage regulation
  • Modular and expandable
  • Black start capability
49
Q

What methods are used for installing subsea cables?

A
  • Laying
  • Ploughing
  • Water jetting
  • Pre-excavation
  • Cover after laying

Electrical quiz 2 (2 decks)

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