EEEN60371 Department of Electrical and Electronic Engineering
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EEEN60371
Faculty of Science and Engineering
School of Engineering
Department of Electrical and Electronic Engineering
Question 1
(a) In the context of overhead line design:
(i) Define the creepage distance/length of an insulator and state the reason this is an important insulator design parameter.
(ii) Explain the use of conductor bundles in high-voltage overhead lines.
(iii) State the shield wire’sfunctionality. [3, 3, 2 marks]
(b) A three-phase trident monopole overhead line (OHL) system rated at 132 kV has a 15.4 m total height at which the conductors are mounted. This OHL system has a 140 m ruling span length and it is designed to operate at 75 °C, on flat terrain. An ACSR Lynx conductor, with a 17.8 × 10-6 °C- 1 coefficient of linear expansion, is installed at 7 ºC with 16 kN tension. It has 840 kg/km mass and a 0.2091 Ω/km AC resistance at its designed thermal rating, which has to maintain a ground clearance of 6.7 m. The OHL is installed in an environment with maximum solar heat gain of 3.26 W/m, and at maximum operating temperature, it has a solar radiation loss of 8.20 W/m and convection loss of 41.26 W/m.
(i) Calculate the OHL’s maximum power transfer capacity.
SOHL=107.468 MVA
(ii) Calculate the ground clearance of the overhead line and thus state whether or not it meets the ground clearance requirements.
STC = 3.2406m
The OHL does not provide enough clearance and infringes the statutory clearance by 8.1 cm.
(iii) Calculate the wood pole hight modifications needed to make the ideal OHL design when corssing a motorway and 8.8 m of clearance is needed.
the wood poles should be 1.281m longer/taller. [2, 6, 2 marks]
(c) What is the main reason of formulation of Transient Recovery Voltage (TRV) during the AC circuit braker operation. Briefly explain a method to minimise the TRV the main principles by which they extinguish an arc. [2 marks]
(d) Next to the overhead line, there is a specially designed 132 kV AC to 1540 V DC substation to supply load to a large EV rapid-charging station. Its DC breaker is capable of interrupting a 12 kA short-circuit fault in the changing network that has negligible inductance. The breaker’s arc voltage is given by:
where, a = 1.2 × 106 VA/cm and VE = 34 V. X is the arc length in cmand Ia is the arc current. Calculate the arc current when the arc length has reached 3 cm.
Ia=8.448 A.
[5 marks]
Total [25 marks]
Question 2
(a) State the three major functions of conventional mineral oils and biodegradable ester liquids used in power transformers.
(b) The auto-transformer is the typical design used in transmission transformers owned by the National Grid UK. An auto-transformer usually contains a series winding, a common winding, and a tertiary winding. It is also fitted with a tapchanger.
(i) Draw a diagram to explain the concept of auto-transformer design and label each winding clearly.
(ii) List two benefits of having the tapchanger in a transformer.
(c) In the context of the electrical design of a transformer, the calculation of surge distribution along a winding is useful to understand the impact of transient overvoltages on a transformer. In a simplified case, a transformer winding is uniformly structured and divided into 10 sections. The winding has a total capacitance to ground Cg = 2500 pF and a total series capacitance Cs = 200 pF.
(i) Draw the equivalent circuit of the winding for an initial surge distribution calculation. Label the components in the circuit and give the value of each component.
Cs1= Cs2=…=Cs10= 2000 pF,
Cg1=Cg2=…Cg10= 250 pF.
(ii) Calculate the space coefficient a of the winding.
a = 3.54
(iii) Assuming that the neutral of the winding is grounded, sketch the initial voltage distributions along the winding.
(iv) A 150 kV impulse voltage with a zero rise-time and an infinite tail is applied to the winding. The withstand voltage of each sectional insulation system is 30 kV. State whether the winding can withstand the impulse voltage and briefly explain why.
The stress on the top sectional insulation is
vtop = 53. 1kv
(d) In the context of thermal rating of a transformer, it is important to understand the hot-spot temperature.
(i) Based on a rated temperature-rise test report, the top oil temperature is 70 °C, the average winding to average oil gradient is 10 °C. Assuming the hot-spot factor of 1.3, calculate the hot-spot temperature of this transformer at the rated condition.
HST = 83 ℃
(ii) Briefly explain why an oil immersed transformer has a short-duration overloading capability.
Total [25 marks]
Question 3
(a) In the context of cable thermal rating:
(i) State any two sources that can generate heat.
(ii) Provide two suggestions that can help increasing cable thermal rating.
(b) The radial electric field distribution Ex within the dielectric insulation of a single core high voltage cable can be calculated as follows:
where Vis the applied voltage,
r and R are the inner and outer insulation radii, respectively,
x is the distance from the conductor axis.
(i) During the manufacturing process, an air bubble was accidently trapped in the dielectric insulation. Discuss what failure mechanisms would most likely occur when the cable is put into the service.
(ii) Comment on the impact of the bubble position on triggering the potential failure process.
(iii) For HVDC application, space charge phenomena may occur in the cable insulation. Briefly explain the potential detrimental effect as a result of space charge accumulation.
(c) In the context of high voltage substation design:
(i) Briefly describe the concept of insulation coordination.
(ii) Briefly explain the definition of step voltage with support of a diagram.
(d) A semi-infinite overhead line with a surge impedance of 400 Ω feeds into a 150 m length of cable with a surge impedance of 40 Ω. The cable is then terminated by a 60 Ω load. A 1 p.u. positive impulse voltage with a zero rise-time and an infinite tail reaches the cable from the overhead line at t = 0 µs. The travelling wave velocity is 3.0 x 108 m/s in the overhead line and 1.5 x 108 m/s in the cable.
(i) Construct a Bewley Lattice diagram for the time period t = 0 µs to t = 6 µs.
(ii) Find the voltage at the 60 Ω load at t = 6 µs.
V=0.2599 p.u.
Total [25 marks]
Question 4
(a) In the asset management of transmission plant it is often desirable to use an asset health index (AHI) to determine which items need attention and the most appropriate action to be taken. To develop an asset health index it is necessary to understand the failure modes of the plant and its components and the indications that can be used to diagnose them. As examples, a transformer replacement AHI indicates the need for the replacement of a complete transformer and a transformer safety AHI indicates the possibility that a transformer or its components might fail in away that causes danger.
For power transformers installed in a large high voltage network:
(i) Give three typical failure modes that should be considered when assessing the health of a transformer and briefly describe each one.
(ii) Name three condition assessment techniques that provide indications that can be used to help determine the health of a transformer or its components and briefly explain how each one relates to the transformer failure modes.
(iii) Draw a diagram that illustrates clearly the relationship between indications, failure modes and asset health indexes in the development of AHIs for transformer replacement and transformer safety
(iv) Explain why AHIs for transformer replacement and transformer safety might be different for a particular transformer
(v) When designing an asset health index determination algorithm; with particular reference to failure modes, what are the two most important characteristics that the algorithm output should have?
(b) When designing an efficient testing and maintenance regime for network equipment it is necessary to understand the place and use of routine tests, special tests, analysis and inspection in the plan.
(i) Draw a diagram illustrating at a high level the work flow and decision process for a typical testing and maintenance regime. Include routine tests, special tests, results analysis and remedial actions.
(ii) Give three characteristics that should be true in a good routine test
(iii) Give two characteristics that are necessary in a useful special test
(iv) Give an example of a continuous on-line monitoring technique and briefly explain the advantages and disadvantages of the widespread adoption of the technique to a large population of assets.
(c) Three identical parallel circuits each contain three items of plant connected in series with probabilities of failure for the plant items of 0.1, 0.2 and 0.05 per year. Give you answers to three decimal places.
(i) In a one year period what is the probability of failure of each circuit?
0.316
(ii) What is the probability of all three circuits failing within a one year period assuming they are not repaired if they fail?
0.0316
Total [25 marks]
2024-01-15