Give the reason for the need to have a free wheeling diode in chopper supplying

Question

Give the reason for the need to have a free wheeling diode in chopper supplying load. Give the primary advantage of SPWM inverter compared to a square inverter. List the main feature of an on-line UPS compared to an off-line UPS. A mains socket outlet gives a voltage waveform which has 5V of 5th harmonic, 2V of 7th harmonic and a fundamental voltage of 230 V. What is the total harmonic distortion of the waveform? List two power electronic switches that can be turned on and off using a control signal. Give 3 reasons for the interest in power quality by customers, utilities and equipment manufacturers. Give a brief description for (i) reliability and (ii) power quality List two (2) problems caused by poor power factor in industrial installations. Explain the meaning of the term "step potential". List two (2) aims of power system protection. What are the essential features of an off-line UPS? Describe the essential features of construction of a single phase transformer. If 660 MW is to be transmitted over 75 km, what would you estimate the required line-to-line voltage to be? Two (2) devices are connected in parallel for operational purposes. If one device is unavailable for 5 mins/year and the second for 8 mins/year, for what time of the year will the parallel system be unavailable? A computer system consumes 1.2 kW. A UPS uses a 240 V battery system with 80% efficiency in charging and discharging. How many joules of storage does the battery require in order to survive a mains ac supply interruption of 10 minutes?

Explanation / Answer

a. From the instant that electrical power is supplied to the circuit (by the closing of a switch say), the inductive load will accumulate stored energy. If an attempt is made to open the switch this energy will arc across the contacts of the switch, and could cause damage to the circuit components. Freewheeling diodes are placed across inductive loads to provide a path for the release of energy stored in the load when the load voltage drops to zero.

b.  SMPStransfers electric power from a source ( AC mains) to the load by converting the characteristics of current and voltage. SMPS always provide a well regulated power to the load irrespective of the input variations. SMPS incorporates a Pass transistor that switches very fast typically at 50Hz and 1 MHz between the on and off states to minimize the energy waste. SMPS regulates the output power by varying the on to off time using minimum voltage so that efficiency is very high compared to the linear power supply.

Inverter

This stage converts the rectified DC into AC using a power oscillator. The power oscillator has a small output transformer with a few windings at the frequency 20-100 kHz. Switching is controlled by a MOSFET amplifier. The output AC voltage is usually isolated optically from the input AC by using an Optocoupler IC for safety reasons.

c.

d. THD(%) = SQRT[(V2^2+ V3^2 + V4^2 + ... + Vn^2)] / Vt

THD= SORT(25+4)/230 =2.34%

e. A power electronic switch integrates a combination of power electronic components or power
semiconductors and a driver for the actively switchable power semiconductors. The internal
functional correlations and interactions of this integrated system determine several
characteristics of the switch.

GTO ,

g.

Power quality is measured by fluctuations in electricity, such as momentary interruptions, voltage sags or swells, flickering lights, transients, harmonic distortion and electrical noise (i.e. fewer such incidents indicate greater power quality).

Reliability is the continuity of electric delivery measured by the number and duration of power outages (i.e. fewer, shorter outages mean greater reliability)

Topology Reliability Cost Input Output Offline +
Fewer parts
lower operating temperature +
Lower initial cost (fewer parts)
Lower operating cost (less electricity) -
Extreme voltage distortion can require frequent battery usage +/-
Output frequency varies within a configurable range Online -
Many parts
higher operating temperature -
Higher initial cost (more parts)
Higher operating cost (more electricity) +
Accepts extreme voltage distortion without going to battery +
Output fixed to a configurable frequency

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