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ENM125 Sustainable electric systems > Quiz 2 > Flashcards

Quiz 2 Flashcards

1
Q

Which is the most common fault?

A

1-phase to ground (earth faults)

80-90 % of all faults

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2
Q

Which faults have maximum vs lowest current?

A

Maximum: 3-phases, 3-phases and ground
Lowest: 1-phase to ground (earth fault)

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3
Q

Why do earth faults occur?

A

faults in components, branches on OH-lines, lightning, damage on cables etc.

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4
Q

Name the 5 types of grounding systems

A

Direct
Isolated
Resistance grounded
Reactance grounded
Resistance and reactance grounded

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5
Q

What is a grounding systems purpose?

A

It is used to fix the voltage towars ground and control fault current

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6
Q

What is fault impedance and why does it occur?

A

Impedance between conductors and/or ground. It reduces the fault current and thus can make it hard to detect.

This can arise due to several reasons, such as branch against OH-line

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7
Q

What is it called when there is no fault impedance?

A

Bolted short circuit. (This gives the maximum fault current)

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8
Q

What is a symmetrical fault?

A

A fault where all phases are affected so the system remains balanced, as a three-phased fault or 3-phases and ground fault

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9
Q

What can contribute to short circuits?

A

Main: the grid
Small: individual turbines, loads with rotating mass (motors without frequency converters)
Very small: loads with rotating mass (motors with frequency converters)

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10
Q

What is short circuit currance?

A

Current due to a short circuit, not only in the fault location. It “keeps” the voltage up. Current is significantly higher than the load current.

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11
Q

Fault current, notation

A

I_s

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12
Q

The equation for fault current, I_s

A

I_s = (Û/Z_sc) * ( - cos(ωt) + K * e^(-(R/L_sc)*t)

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13
Q

Which is the sinus component in the equation for fault current?

A

(Û/Z_sc) * ( - cos(ωt))

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14
Q

Which is the DC component in the equation for fault current?

A

K * e^(-(R/L_sc)*t

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15
Q

What is subtransient current?

A

In the beginning of faults (<10 ms). Might be a very high current but it will not last. (Important for mechanical dimensioning)

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16
Q

How to calculate the size of subtransient current?

A

If larger than 600 V:
At generator I_s = 3 * I_sc
Otherwise I_s = 2.5 * I_sc

If less than 600 V:
I_s = 2 * I_sc

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17
Q

How to calculate the force per length for two parallel lines?

A

F/l = (μ0 * I_1 * I_2)/(2 * π * d)

Where μ0 and π are constants
d is distance between lines
I_1/2 is current in the lines

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18
Q

What mechanical impact does fault current have on busbars, disconnectors and circuit breakers?

A

Busbars: Bending

Disconnectors: “Welded” shut

Circuit breakers: May not be able to operate

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19
Q

What current is used to calculate the thermal impact of fault current?

A

I_1 (for 1 s considered) I_s to short time to make an impact

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20
Q

How is I_1 calculated?

A

Standard for all types of cables, dependent on cross section and disconnecting/clearing time (t_sc)

If t_sc >= 0.5 s: I_1 = I_sc * sqrt(t_sc)

If t_sc < 0.5 s: I_1 = I_sc * sqrt(t_sc + 0.005)

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21
Q

How can short circuit power be described?

A

Made up, generally used and a measure of the strength of the grid

S_sc

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22
Q

How is short circuit power calculated?

A

S_sc = sqrt(3) * I_sc * U_N

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23
Q

How can short circuit capacity be described?

A

Made up, generally used and a measure on how much the component/system will stop the short circuit current

S_scc

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24
Q

How to calculate short circuit capacity?

A

S_scc = (U_N)^2/ x_c

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25
Q

Assumptions when looking at power system model for short circuit calculation?

A

Do not consider the load current

Transformers are just an impedance

26
Q

Method of reduction short circuit power

A

Reactance in series

Sectioning substations

27
Q

Mention 4 switching/operating/protecting devices

A

Circuit breaker
Disconnector
Switch
Fuse

28
Q

What does a circuit breaker do and how is it noted in circuits?

A

Breaks up to fault current
No visible breaking
Remote operating possible

Noted: —-x—-

29
Q

What does a disconnector do and how is it noted in circuits?

A

Can NOT break current
Visible braking/lockable
Remote operating often not possible

Noted: —-|——

framgooglat: used to ensure that an electrical circuit is completely de-energized for service or maintenance

30
Q

What does a switch do and how is it noted in circuits?

A

Can break up to load current
Often visible breaking

Noted: ______/ _________

31
Q

What does a fuse do and how is it noted in circuits?

A

Can break fault current
Cheap
Can’t be “controlled”
Up to 72 kV

Noted as a hollow resistance

32
Q

What is the protection philosophy?

A

The fault needs to be disconnected as fast as possible

Always more than one protection, back-up needed

Selectivity, as little part of the system shall be disconnected as possible

33
Q

What is the most important tool in power operation and planning?

A

Power flow (see how the system works)

34
Q

What can you see in on-line power flow analyses?

A

State estimation
Security analysis
Economic analysis (optimal operation, loss coefficients, optimal pricing)

35
Q

What can you see in off-line power flow analyses?

A

Operation analyses
Planning analyses (network expansion planning, power exchange planning, security and adequacy analyses (faults, stability))

36
Q

Power flow problem description

A

Its a snapshot of the system

Knowing demand and/or generation of power in each bus

Most used tool in steady state power system analysis

The solution technique is a balancing act bw accuracy vs computing time

37
Q

What is associated with each bus in power flow problem?

A

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

38
Q

3 types of buses represented in load flow calculations, which if voltage magnitude and angle are specified?

A

Slack bus (provides the additional real and reactive power to meet the losses)

39
Q

3 types of buses represented in load flow calculations, which if voltage magnitude and real power specified?

A

Voltage controlled buses

40
Q

3 types of buses represented in load flow calculations, which if real and reactive power are specified?

A

Load buses

41
Q

What is admittance, how to calculate it and which unit?

A

Y = 1/Z

an expression of the ease with which alternating current ( AC ) flows through a complex circuit or system

Unit: siemens (S) or the old one mho ℧

42
Q

Ohms law for an n-node network

A

I = y * U

43
Q

What is OPF?

A

Optimal power flow, is the “best” way to instantaneously operate a power system. Often by minimizing operational cost. Used as the basis for real-time pricing. Considers the impact of the transmission system

44
Q

Ish how long can DC transmissionlines be? (HVDC)

A

> 50 km submarine
1000 km overhead

45
Q

What is some HVDC features?

A
  • Transmission large amount over long OH lines
  • Can cross long submarine distances
  • Enables transmission of more power with less ROW (right of way - physical space)
  • control over the power exchanged between two areas
  • Flexibility of HVDC enables improvement of the overall AC/DC system
  • Investment cost (lower cost if long enough distance compared to AC (AC reactive power compensation))
46
Q

What two things are power electronics direct controlling?

A

Voltage and frequency

47
Q

For which application are back-to-back HVDC links used?

A

To interconnect two systems with different frequencies

For power exchange between two close non-synchronised systems

48
Q

What power flow control functions does HVDC have?

A

Basics (as a regular system):
Active-power control
DC-voltage control
AC-voltage or reactive power control

Additional:
Frequency control
Flicker control (flimmer)
Harmonics mitigation

49
Q

What three types of HVDC transmission technologies are there?

A

Line commutated converters (LCC)
Capacitor commutated converters (CCC)
VSC_HVDC

50
Q

HVDC application range, from highest voltage and power to lowest (4 ranges)

A

Highest: LCC or CCC with OH line (800 kV, 7000 MW)
Mid-high: VSC with OH line (650 kV, 2500 MW)
Mid-low: HVDC with oil impregnated cable (500 kV, 2000 MW)
Low: VSC HVDC with extruded cable (300 kV, 1000 MW)

51
Q

5 installation methods for subsea cables

A

Laying
Ploughing
Water jetting
Pre-excavation
Cover after laying

52
Q

Which type of point-to-point HVDC links exists?

A

Monopolar, bipolar

53
Q

Give examples of two actions that would reduce the short circuit power

A

Divide the system into different sections
Add a reactor in series with a line

(Reactor = reactance?)

54
Q

What is unique with a back-to-back HVDC facility?

A

The converter stations are placed next to each other

55
Q

Why are earth faults so important in designing grids?

A

They are hardest to detect

56
Q

What studies can NOT be made by a power flow calculation of a power system?

A

Optimal power production
How the frequency changes due to load loss

57
Q

What studies can be made by a power flow calculation of a power system?

A

The power flow through the lines
If a transformer is overloaded
The voltage magnitude at each bus.
If a line is overloaded

58
Q

I_b =

A

S_b/(sqrt(3) * V_b)

59
Q

I_sc =

A

= U_N / (sqrt(3)* Z)

60
Q

S_sc

A

= sqrt(3) * U_N * I_SC

61
Q
A

ENM125 Sustainable electric systems (4 decks)

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