Quiz 2 Flashcards
Which is the most common fault?
1-phase to ground (earth faults)
80-90 % of all faults
Which faults have maximum vs lowest current?
Maximum: 3-phases, 3-phases and ground
Lowest: 1-phase to ground (earth fault)
Why do earth faults occur?
faults in components, branches on OH-lines, lightning, damage on cables etc.
Name the 5 types of grounding systems
Direct
Isolated
Resistance grounded
Reactance grounded
Resistance and reactance grounded
What is a grounding systems purpose?
It is used to fix the voltage towars ground and control fault current
What is fault impedance and why does it occur?
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
What is it called when there is no fault impedance?
Bolted short circuit. (This gives the maximum fault current)
What is a symmetrical fault?
A fault where all phases are affected so the system remains balanced, as a three-phased fault or 3-phases and ground fault
What can contribute to short circuits?
Main: the grid
Small: individual turbines, loads with rotating mass (motors without frequency converters)
Very small: loads with rotating mass (motors with frequency converters)
What is short circuit currance?
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.
Fault current, notation
I_s
The equation for fault current, I_s
I_s = (Û/Z_sc) * ( - cos(ωt) + K * e^(-(R/L_sc)*t)
Which is the sinus component in the equation for fault current?
(Û/Z_sc) * ( - cos(ωt))
Which is the DC component in the equation for fault current?
K * e^(-(R/L_sc)*t
What is subtransient current?
In the beginning of faults (<10 ms). Might be a very high current but it will not last. (Important for mechanical dimensioning)
How to calculate the size of subtransient current?
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
How to calculate the force per length for two parallel lines?
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
What mechanical impact does fault current have on busbars, disconnectors and circuit breakers?
Busbars: Bending
Disconnectors: “Welded” shut
Circuit breakers: May not be able to operate
What current is used to calculate the thermal impact of fault current?
I_1 (for 1 s considered) I_s to short time to make an impact
How is I_1 calculated?
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)
How can short circuit power be described?
Made up, generally used and a measure of the strength of the grid
S_sc
How is short circuit power calculated?
S_sc = sqrt(3) * I_sc * U_N
How can short circuit capacity be described?
Made up, generally used and a measure on how much the component/system will stop the short circuit current
S_scc
How to calculate short circuit capacity?
S_scc = (U_N)^2/ x_c
Assumptions when looking at power system model for short circuit calculation?
Do not consider the load current
Transformers are just an impedance
Method of reduction short circuit power
Reactance in series
Sectioning substations
Mention 4 switching/operating/protecting devices
Circuit breaker
Disconnector
Switch
Fuse
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—-
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
What does a switch do and how is it noted in circuits?
Can break up to load current
Often visible breaking
Noted: ______/ _________
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
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
What is the most important tool in power operation and planning?
Power flow (see how the system works)
What can you see in on-line power flow analyses?
State estimation
Security analysis
Economic analysis (optimal operation, loss coefficients, optimal pricing)
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))
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
What is associated with each bus in power flow problem?
The real power
The reactive power
The voltage magnitude
The phase angle between voltage
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)
3 types of buses represented in load flow calculations, which if voltage magnitude and real power specified?
Voltage controlled buses
3 types of buses represented in load flow calculations, which if real and reactive power are specified?
Load buses
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 ℧
Ohms law for an n-node network
I = y * U
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
Ish how long can DC transmissionlines be? (HVDC)
> 50 km submarine
1000 km overhead
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))
What two things are power electronics direct controlling?
Voltage and frequency
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
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
What three types of HVDC transmission technologies are there?
Line commutated converters (LCC)
Capacitor commutated converters (CCC)
VSC_HVDC
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)
5 installation methods for subsea cables
Laying
Ploughing
Water jetting
Pre-excavation
Cover after laying
Which type of point-to-point HVDC links exists?
Monopolar, bipolar
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?)
What is unique with a back-to-back HVDC facility?
The converter stations are placed next to each other
Why are earth faults so important in designing grids?
They are hardest to detect
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
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
I_b =
S_b/(sqrt(3) * V_b)
I_sc =
= U_N / (sqrt(3)* Z)
S_sc
= sqrt(3) * U_N * I_SC
