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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
Assumptions when looking at power system model for short circuit calculation?
Do not consider the load current Transformers are just an impedance
26
Method of reduction short circuit power
Reactance in series Sectioning substations
27
Mention 4 switching/operating/protecting devices
Circuit breaker Disconnector Switch Fuse
28
What does a circuit breaker do and how is it noted in circuits?
Breaks up to fault current No visible breaking Remote operating possible Noted: ----x----
29
What does a disconnector do and how is it noted in circuits?
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
What does a switch do and how is it noted in circuits?
Can break up to load current Often visible breaking Noted: ______/ _________
31
What does a fuse do and how is it noted in circuits?
Can break fault current Cheap Can't be "controlled" Up to 72 kV Noted as a hollow resistance
32
What is the protection philosophy?
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
What is the most important tool in power operation and planning?
Power flow (see how the system works)
34
What can you see in on-line power flow analyses?
State estimation Security analysis Economic analysis (optimal operation, loss coefficients, optimal pricing)
35
What can you see in off-line power flow analyses?
Operation analyses Planning analyses (network expansion planning, power exchange planning, security and adequacy analyses (faults, stability))
36
Power flow problem description
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
What is associated with each bus in power flow problem?
The real power The reactive power The voltage magnitude The phase angle between voltage
38
3 types of buses represented in load flow calculations, which if voltage magnitude and angle are specified?
Slack bus (provides the additional real and reactive power to meet the losses)
39
3 types of buses represented in load flow calculations, which if voltage magnitude and real power specified?
Voltage controlled buses
40
3 types of buses represented in load flow calculations, which if real and reactive power are specified?
Load buses
41
What is admittance, how to calculate it and which unit?
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
Ohms law for an n-node network
I = y * U
43
What is OPF?
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
Ish how long can DC transmissionlines be? (HVDC)
> 50 km submarine > 1000 km overhead
45
What is some HVDC features?
- 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
What two things are power electronics direct controlling?
Voltage and frequency
47
For which application are back-to-back HVDC links used?
To interconnect two systems with different frequencies For power exchange between two close non-synchronised systems
48
What power flow control functions does HVDC have?
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
What three types of HVDC transmission technologies are there?
Line commutated converters (LCC) Capacitor commutated converters (CCC) VSC_HVDC
50
HVDC application range, from highest voltage and power to lowest (4 ranges)
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
5 installation methods for subsea cables
Laying Ploughing Water jetting Pre-excavation Cover after laying
52
Which type of point-to-point HVDC links exists?
Monopolar, bipolar
53
Give examples of two actions that would reduce the short circuit power
Divide the system into different sections Add a reactor in series with a line (Reactor = reactance?)
54
What is unique with a back-to-back HVDC facility?
The converter stations are placed next to each other
55
Why are earth faults so important in designing grids?
They are hardest to detect
56
What studies can NOT be made by a power flow calculation of a power system?
Optimal power production How the frequency changes due to load loss
57
What studies can be made by a power flow calculation of a power system?
The power flow through the lines If a transformer is overloaded The voltage magnitude at each bus. If a line is overloaded
58
I_b =
S_b/(sqrt(3) * V_b)
59
I_sc =
= U_N / (sqrt(3)* Z)
60
S_sc
= sqrt(3) * U_N * I_SC
61

ENM125 Sustainable electric systems (4 decks)

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