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DEE-24106 ELECTRIC POWER SYSTEMS

Enrigue Acha and Sami Repo
27th of February, 2017

Attempt ALL guestions

The numbers in sguare brackets in the right-hand margin indicate the marks allotted to the
part of the guestion against which the mark is shown. These marks are for guidance only.

An electronic calculator may be used provided that it does not have a facility for either
textual storage or display, or for graphical display.
If a calculator is used, intermediate steps in the calculation should be indicated.

O1 (a) State what is the main objective of a power flow study. [1]

(b) State the three types of buses used in conventional power flow solutions and
their specified and calculated variables. [2]

(c) For the circuit shown in Fig. 1 determine the nodal voltage solution using one
iteration of the power flow Newton-Raphson method. Select bus 1 to be the
slack bus, with a voltage magnitude of 1 p.u. and 0 phase angle. The voltage
magnitude at bus 3 is also kept at 1 p.u. To start the iterative solution, assume 1

 

 

 

 

 

 

 

 

 

 

 

pu voltage magnitude and 0 phase angles in bus 2 and also assume a 0 phase
angle at bus 3. [3]
2
Xn=0.1p.uv. P12=0.5pu.
Owr=0 p.u.
J
Pas=0.5p.v.
r j=
Puo0dpu. X13=0.2p.u.
Or=0.1p.u.
P15=0.8p.v.
Or:=0.2p.u.
Fig. 1. 01

1) 3
02 — The power circuit shown in Fig. 2 undergoes a three-phase-to-ground short-
circuit fault in bus 3.

(a) Calculate the short-circuit fault current assuming a flat voltage profile of 1 p.u.
in all buses just before the fault occurs and zero fault impedance, i.e. Z;= 0+j0.

[2]

(b) Determine the faulted nodal voltages in all buses, in seguence guantities and in
phase guantities. [2]

(e) Calculate the current flows in the three transmission lines, two transformers and
two generators in seguence guantities and in phase guantities. [R]

Xo=0.1 puu. X0=0.1 pu. Xu =0.1 p.u. XGy=0.1 p.u.

   

 
 

 

  

 

 

 

 

03 (a)Explain with your own words what is (n-1) criteria and how it is

(b)

(e)

(d)

04

used. [2]

Price areas of day-ahead electricity market is a method for transmission network
congestion management. Explain the basic idea of price areas and how it will resolve
congestion in the network. [1]

Extra high voltage transmission lines are typically over-compensated. Explain what
over-compensation means, what conseguences it has for power flow on transmission
line, and how it may be mitigated. [2]

Fxplain why rotor current limits of synchronous generators may endanger power system
voltage stability. [1]

Figure 3 gives the power-angle curves of a power system where synchronous generator
supplies power through two parallel lines into a stiff network. A fault occurs in the
middle point of another line. The fault is successfully disconnected after some time by
opening the circuit breakers at both ends of faulted line, at the same time. P = active
power, ö = power angle, sub-indexes 0 and c corresponds to situations before the fault
and at the time when the fault is disconnected, and sub-indexes e and m correspond to
electrical and mechanical variables.

X>=0.1 p.u.

2/5
a) Estimate if cases A and B are stable based on the egual area criterion and figures. Draw the
accelerating and decelerating areas and the maximum angle into both figures. Explain your
estimations. [3]

b) Justify why power curves are different for conditions before fault, during fault and after
fault. [1]

c) Determine a rough estimation of how the generator output power, rotating speed and power
angle functions during different points of case A. Draw the necessary figures to explain
your estimations. [2]

Case A Case B
Pe — before fault Pe — before fault

  

  

P

Pe — after fault Pe — after fault

  
 
   

 

Pe — during fault

 

Pe — during fault

 

Fig. 3. 04

O5 —Deseribe in chronological order what it is expected to happen in a well-functioning
power system, during and immediately after the sudden disconnection of a large
synchronous generator. [6]

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