This set of Transformers MCQs focuses on “Transformers for HVDC Conversion”.
1. HVDC transformers are used widely _____________
a) for transmission purpose
b) for generation purpose
c) for supplying DC machines
d) for every use of transformer
Explanation: An HVDC converter (transformer) converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies.
2. HVDC converter transformers are generally __________________
Explanation: Almost all HVDC converters are bi-directional; they can convert either from AC to DC (rectification) or from DC to AC (inversion). A complete HVDC system always includes one converter operating as a rectifier (converting AC to DC) and another one operating as an inverter (converting DC to AC).
3. Insulation of HVDC transformers should be ___________
a) dry always
b) wet always
c) can be dry or wet
d) no need of insulation
Explanation: Because of the effect of moisture on the resistivity of insulation material, it is necessary to obtain and maintain a high level of dryness in the insulation of HVDC transformers. This is equally important in service as it is in the factory at the time of testing.
4. Tap winding is constructed so as to ____________________
a) maximize voltage regulation
b) minimize the voltage regulation
c) maximize the impedance regulation
d) minimize the impedance regulation
Explanation: The extent of the tap winding and its location such as to minimise impedance variation results in a high voltage being developed across it under impulse conditions, placing demands on the winding insulation design as well as the impulse withstand capability of the tapchanger itself.
5. Electronic converters for HVDC can be divided into _______
a) two types
b) three types
c) many types
d) only one type is possible
Explanation: Electronic converters for HVDC are classified into two main categories. Line-commutated converters- LCC (HVDC classic) are made with electronic switches that can only be turned on. Voltage-sourced converters- VSC are made with switching devices that can be turned both on and off.
6. Efficient LCC HVDC converters generally use __________
b) mercury valves
d) mechanical switches
Explanation: Line-commutated converters-LCC use switching devices that are either uncontrolled (such as diodes) or that can only be turned on (not off) by control action, such as thyristors. Though HVDC converters can be constructed from diodes, such converters can only be used in rectification mode and the lack of controllability of the DC voltage is a disadvantage.
7. HVDC LCCs have ______________
a) 1 degree of freedom
b) 2 degrees of freedom
c) 5 degrees of freedom
d) Many degrees of freedom
Explanation: As thyristors can only be turned on (not off) by control action, and rely on the external AC system to affect this turn-off process, the control system only has one degree of freedom – when to turn on the thyristor. This limits the usefulness of HVDC in some circumstances, some applications.
8. Which of the following is the problem with cast resin transformers?
b) Less efficiency
c) Low capacity
Expressions: The possibility of voids and of resin cracking is one problem which has been identified. One measure which can help to resist cracking is the incorporation into the resin of some reinforcement, such as, for example, glass fibre.
Sanfoundry Global Education & Learning Series – Transformers.
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