Question

For the above SMES system shown in Figure 1,

Voltage Source Converter using IGBT DC-DC chopper using IGBT Three-phase AC (from generator terminal bus) Epc SMES coil Transformer 66/2.0[kV] DC link capacitoir

i) Why is the step down transformer used?

ii) In the voltage source converter component, what are the roles of the diodes and IGBTs?

iii) Why is the capacitor used between the voltage source converter and the DC-DC chopper?

iv) Is the current through the SMES coil flowing from top to bottom or from bottom to top or ​from both directions? Explain your answer.

Answer 1

i) Why is the step down transformer used?

Generator BUS Voltage is 66kV, which is too high to use on a converter. The power electronic switches(IGBT,MosFET, Power BJT) available for rectifying or converting voltage source is of upto 1.2kV. Thus 66kV is too high to handle. There it has to employ a step-down transformer which reduces the voltage to 2kV which can easily be handled by two IGBTs.

ii) In the voltage source converter component, what are the roles of the diodes and IGBTs?

The Voltage Source converter is applicable for both the operations i.e Rectification (AC-DC) & Inverter (DC-AC). SMES(Superconducting Magnetic Energy Storage) is a device that can store magnetic energy in a Reactor (specifically Inductor) for a long time. This energy is used accordingly for power quality improvement. In simple words the reactor can both absorb the excess reactive power from system (Rectification operation) & feed the reactive power back to line when there is deficiency of reactive power (Inverter Operation) in the line. Thus Voltage profile of the power lines are kept healthy.

iii) Why is the capacitor used between the voltage source converter and the DC-DC chopper?

The link Capacitor ius used to smoothen up the rectified DC voltage and act as a constant DC Voltage source. While inverter operation is carried out this capacitor acts as the DC Source.

iv) Is the current through the SMES coil flowing from top to bottom or from bottom to top or ​from both directions? Explain your answer.

Current through the Coil is unidirectional as it stores the magnetic energy. Magnetic energy can be stored in a inductor when the fluxlinking the coil is not changing or collapsing. It needs to be constant flux. To build up constant flux a constant DC cuurent has to flow through the coil. Thus it stores magnetic energy.