Electrical Technology May-June 2023 (Power Systems) Eng_hlayiso.com_

Copyright reserved Please turn over SENIOR CERTIFICATE EXAMINATIONS/ NATIONAL SENIOR CERTIFICATE EXAMINATIONS ELECTRICAL TECHNOLOGY: POWER SYSTEMS 2023 MARKS: 200 TIME: 3 hours This question paper consists of 20 pages and a 2-page formula sheet. Downloaded from hlayiso.com.

[Audio] There is no information to summarize.. Electrical Technology: Power Systems 2 DBE/2023 SC/NSC Copyright reserved Please turn over INSTRUCTIONS AND INFORMATION 1. This question paper consists of SEVEN questions. 2. Answer ALL the questions. 3. Sketches and diagrams must be large, neat and FULLY LABELLED. 4. Show ALL calculations and round off answers correctly to TWO decimal places. 5. Number the answers correctly according to the numbering system used in this question paper. 6. You may use a non-programmable calculator. 7. Calculations must include: 7.1 Formulae and manipulations where needed 7.2 Correct replacement of values 7.3 Correct answer and relevant units where applicable 8. A formula sheet is attached at the end of this question paper. 9. Write neatly and legibly. Downloaded from hlayiso.com.

[Audio] The critical incident causes damage to equipment, requiring external repair services. A person may sustain a sudden and severe physical injury, necessitating external medical services. Alternatively, a person might sustain a physical injury, but external medical services are not necessary. In some cases, a person could experience a mental disturbance, requiring a restful recovery. Impedance refers to the ratio of the applied voltage to the resulting circuit current in an RLC circuit, where the applied voltage is the resultant of VR, VC, and VL. When the frequency increases, the current through a capacitor in an RLC parallel circuit also increases..

[Audio] The correct method to connect a wattmeter involves linking the voltage coil in series with the load and the current coil in parallel with the load. This approach guarantees precise measurement of active and reactive powers. The wattmeter calculates the real power consumed by the load by multiplying the voltage and current values. Additionally, it measures the phase angle between the voltage and current to determine the reactive power. Designed to provide accurate and efficient power consumption measurements, the wattmeter relies heavily on proper connection. Connecting the voltage coil in series with the load ensures that the voltage drop across the load is accurately recorded. Similarly, connecting the current coil in parallel with the load enables the accurate measurement of the current flowing through the load. This permits the calculation of real and reactive powers with precision. As a vital tool for assessing the power consumption of electrical devices and systems, the wattmeter plays a crucial role in ensuring efficient operation and maintenance. Precise measurements are essential for optimal performance and upkeep of electrical systems..

[Audio] The start-up current in an automatic star-delta motor starter is reduced because the line voltage in star is lower than the line voltage in delta..

[Audio] In phase refers to the relationship between the applied voltage and current in an RLC circuit connected to an AC supply, where the voltage and current waves have the same frequency and amplitude but differ in phase by 90 degrees. This means that the voltage wave leads the current wave by 90 degrees..

[Audio] The supply voltage is calculated using the formula V = √(Vr^2 + Vc^2), where Vr is the voltage measured between the neutral point and one of the phases, and Vc is the voltage measured between two phases. Substituting the given values, we get V = √(30^2 + 20^2) = √(900 + 400) = √1300 ≈ 36.06 V. Therefore, the supply voltage is approximately 36.06 V..

[Audio] The value of the inductance for the circuit to resonate at 3 kHz can be calculated using the formula XC = ωL, where ω is the angular frequency given by ω = 2πf, and L is the inductance. Substituting the values given, we get XC = 113,12 Ω = ωL, so L = XC / ω. Since ω = 2π(3 kHz), we have L = 113,12 Ω / (2π(3 kHz)) = approximately 59.45 μH..

[Audio] The current through the inductor is calculated by using the formula I_L = I_T - I_R. This formula indicates that the current through the inductor is equal to the total current minus the resistor current. By substituting the given values, we obtain I_L = 1.22 A - 0.2 A = 1.02 A..

[Audio] The national power grid has various stages, including generation, transmission, distribution, and consumption. As shown in the figure, each stage corresponds to specific voltages. Transformer (B) is a step-up transformer, elevating the voltage from the generation stage to the transmission stage. This reduction in current flowing through the transmission lines results in decreased copper losses due to the increased voltage. Transformer (D) is a step-down transformer, lowering the voltage from the transmission stage to the distribution stage. Both household and industrial loads can be powered by transformer (D), as it supplies a suitable voltage level for these types of loads. Transformer (A) produces a waveform, which represents the output of the generator. The waveform is a sinusoidal function of time, depicting the alternating current generated by the generator..

[Audio] This is a three-phase star-connected system delivering power to a balanced three-phase star-connected load. The given data includes the line voltage, phase impedance, and power factor. We will calculate the phase voltage across the load, which is equal to the line voltage divided by the square root of three, or approximately 227.04 volts. Next, we will calculate the phase current through the load, which is equal to the total power divided by the product of the line voltage and the cosine of the phase angle. Then, we will determine the true power, which is equal to the product of the line voltage and the phase current. Furthermore, we will find the phase angle, which is equal to the arctangent of the imaginary part of the complex power divided by its real part. Moreover, we will calculate the reactive power, which is equal to the product of the line voltage and the sine of the phase angle. Additionally, we will discuss why this is an inductive load, as it has a power factor less than one, indicating that the current lags behind the voltage. We will also explain how the power factor can be improved by adding capacitors in parallel with the load to compensate for the inductive reactance. Finally, we will describe the effects of an improved power factor on a system, including reduced energy losses and increased efficiency. Two advantages of an improved power factor to the consumer are reduced energy bills and increased reliability..

[Audio] Transformers use electromagnetic induction to transfer energy from the primary winding to the secondary winding..

[Audio] The given data reveals that we have a three-phase delta-star transformer with a primary voltage of 6 kV, a turns ratio of 25:1, and a power output of 50 kW with a power factor of 0.9. To determine this transformer's performance, we require calculating four essential parameters: the secondary phase voltage, the secondary line voltage, the apparent power, and the primary line current..

[Audio] The cage rotor consists of three main parts: the squirrel cage, the rotor bars, and the end rings. These parts work together to convert electrical energy into mechanical energy in an induction motor. The squirrel cage provides a path for the rotor currents to flow, while the rotor bars carry these currents and interact with the magnetic field generated by the stator windings. The end rings help to distribute the rotor currents evenly around the circumference of the rotor. By understanding the function of each part, we can better appreciate how the cage rotor operates in an induction motor..

[Audio] The combined true power of the two motors is calculated by adding the true powers of each motor. Therefore, the combined true power is 125 kW..

[Audio] The contacts that create the interlocking function in this circuit are MC1 and MC2..

[Audio] Relay control systems have two main disadvantages compared to PLC control systems. They are less flexible and require more manual intervention to make changes to the control logic. Additionally, they are more prone to errors due to the complexity of wiring diagrams and the potential for human error during installation..

[Audio] A relay and a contactor are both used to switch electrical circuits on and off, but they differ in their construction and operation. A relay is an electromechanical device that uses an electromagnetic coil to operate a set of contacts, which can be normally open (NO) or normally closed (NC). When the coil is energized, it moves the contacts to either close or open the circuit. On the other hand, a contactor is a type of relay that is specifically designed for high-power applications, such as industrial control systems. It typically consists of multiple sets of contacts and is designed to handle higher currents and voltages than a standard relay. In addition, a contactor often has a more robust construction and may include features such as thermal protection and overload protection..

[Audio] The manual sequence starter without a timer is designed to ensure safe starting of motors. Each overload is connected in series with the contactor coil to prevent the motor from being started if it has overheated previously. This prevents damage to the motor and ensures its longevity. The MC1 N/O2 contact is used to prevent motor 2 from being started while motor 1 is still running. This ensures that the two motors do not operate simultaneously, which could cause a fault or even a safety hazard..

[Audio] Voltage frequency control refers to the ability of a Variable Speed Drive (VSD) to adjust the speed of an induction motor by varying its frequency. This is achieved through the use of pulse-width modulation (PWM) techniques, which allow the VSD to generate a variable-frequency AC supply to the motor. By adjusting the frequency of the supply, the VSD can control the speed of the motor, allowing it to operate within a specific range. This control is particularly useful in applications where the load requires a constant torque, such as in pumps, fans, and conveyor systems..

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[Audio] The voltage in a delta connection is equal to the voltage in a star connection multiplied by √3. This is known as the turns ratio. The efficiency of a transformer is given by the equation η = P/Plosses, where P is the real power output and Plosses is the total power loss. Since the power loss is proportional to the square of the current, the efficiency of a transformer increases as the current decreases. The frequency of a transformer is given by the equation..