IJEETC. InternationalJournalof. ElectricalandElectronicEngineering& Telecommunications.

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1 IJEETC InternationalJournalof ElectricalandElectronicEngineering& Telecommunications

2 Int. J. Elec&Electr.Eng&Telecoms Anoop Dhayani A P et al., 2015 Research Paper ISSN Special Issue, Vol. 1, No. 2, July 2015 National Conference on Emerging Trends in Electronics & Communication (ETEC-2015) 2015 IJEETC. All Rights Reserved DESIGN AND PERFORMANCE ANALYSIS OF HVDC TRANSMISSION SYSTEM UNDER DIFFERENT FAULT CONDITIONS Anoop Dhayani A P 1 *, Gurmeet Kaur A P 2 and S K Goel 3 *Corresponding Author: Anoop Dhayani A P, anoopdhayani@gmail.com The ability of the power system to return to its initial conditions subjected to a physical disturbance is termed as stability. HVDC power transmission is becoming more and more attractive due the significant progress in power electronic technology in last two decades. It is important to thoroughly understand the mechanism of the interactions between a HVDC system and AC network so that the HVDC control can be operated in a manner that enhances the stability and reliability of the entire power grid. This paper investigates the performance of HVDC system for steady state, DC and AC fault conditions. The HVDC transmission system has been proposed on the basis of simulation studies using MATLAB software package (Simulink Model). Keywords: High Voltage Direct Current (HVDC) transmission system, Voltage Source Converter (VSC), Rectifier, AC filter, Total Harmonic Distortion (THD), MATLAB simulink INTRODUCTION With the development of advanced power electronics based semiconductor devices; it becomes possible to fulfill the increasing demand of high efficiency and high quality of power transmission all over the world wide. Presently, the HVDC system has fully grown in long distance transmission for transmitting bulk amount of electric power by over head transmission system between asynchronously interconnecting AC systems [2]. A basic VSC- HVDC transmission system has demonstrated many benefits. The selection of HVDC is over HVAC is typically motivated and advantageous for bulk power transmission over long distances. Some advantages are as follows: 1. Low transmission losses over long distances. 2. Reduced harmonics. 1 Department of Electrical Engineering MVIET, Kaushami. 2 Department of Electrical Engineering, Quantum Global Campus, Roorkee, UK. 3 Director, WIT, Dehradun, UK. 60

3 3. Without reversing polarity and without interruption, it reverses the power within milliseconds. 4. Full control of power flow. 5. Control active and reactive power independently. 6. Improve stability of AC system. This paper describes a simple model of HVDC transmission system which is designed in order to analyze the performances under different fault conditions. COMPONENT STUDY The basic components that comprise a HVDC transmission system are the AC System, the converter stations (rectifier at sending and inverter at receiving end), converting transformer, smoothing reactor, AC and DC filters and control system. For assessment of transmission system, components that comprise the HVDC system are discussed below: Transformer Here the use of the transformer is not limited to step up (for long transmission) and to step down (for distribution of electrical energy) the voltages, transformers also plays an important role in the conversion of alternating current to direct current. The 1200 MVA converter transformer Yg, Y/Ä is modelled with three 1- Ö phase 3-winding transformer [1]. Poly-phase transformation of three-phase to six-phase or even higher is a step in the rectification process. In half wave and full wave rectifier such higher systems are particularly used, because of their relatively lower ripple components. Therefore, the requirement of large DC power, it becomes uncommon to convert 3-phase to 6-phase, 12-phase or even 24-phase, using transformer and suitable half wave or a full wave rectifier [3]. AC Filter One of the AC sides of a 12-pulse HVDC converter; current harmonics are higher in order. To limit this, AC filters are installed. DC Filter In operational mode HVDC converters creates harmonics due to which disturbances is created. In order to reduce these disturbances, specially designed DC filters are used. IGBT It is a semiconductor device which is controlled by the gate signal. IGBT consist of a series combination of resistor (R), inductor (L) and DC source voltage (V dc ). A switch (S) is connected in series, which is controlled by the logical signal (G > 0, G = 0) Figure 1: IGBT Circuit When V CE is positive and greater than V dc and the gate input signal is positive, i.e., G >0, IGBT turns on, whereas when V CE is positive and signal zero is applied as a gate input ie, G = 0, IGBT turns off. When V CE is negative, IGBT is in the off state. IGBT generally uses with an anti-parallel diode due to lack of 61

4 reverse blocking capability. Here series R s -C s snubber circuit is also connected in parallel with the IGBT. Snubber capacitor C s prevents unwanted dv/dt triggering whereas snubber resistance R s is placed in order to limit the discharge current magnitude [5]. 12-Pulse converter/converter Unit In HVDC converter station, the conversion from AC to DC and vice-versa is done with the help of 3-phase bridge converter. In present scenario, 12-pulse bridge converter is used as basic converter units because of the reduced filtering requirements in HVDC transmission. 12-pulse bridge converter is obtained by connecting two identical 6-pulse bridges in series except that AC supply voltage to the two bridges are shifted by 30 degree in phase [7]. This is usually happened by supplying one 6-pulse bridge by a star-star connected 3-phase transformer on DC side and other 6-pulse bridge by a star-delta connected 3-phase transformer on the AC side [6]. Figure 2: 12-Pulse Converter with Synchronous 12-Pulse Generator The rectifier and the inverter (voltage source converter) are interconnected through a transmission line of 300 km and 0.5 H, 1 ohm smoothing reactor. Voltage source converter employed a fullycontrollable switch as IGBTs. Fully controllable switches are preferred for high voltage Figure 3: HVDC Model Without Fault 62

5 Figure 4: HVDC Model with DC Fault and AC Fault applications with relatively high switching frequencies. PWM techniques are employed to the switches to reproduce a sinusoidal waveform on the AC side. This sinusoidal waveform is filtered by the AC filters and the phase reactor. This results in low harmonic content of the reproduced waveform. To build a three- phase VSC, a two-level converter is the simplest topology that can be used [4]. SIMULATION RESULT AND ANALYSIS Without fault at rectifier and inverter side, the simulation of voltage and current represents in figure 5 and 6. In these figures it is clearly seen that voltage at rectifier side (sending end) is more fluctuating in nature at time t=0.395s. After elapse time voltage at inverter side (receiving side) become constant. The value of voltage and current at inverter side are almost identical to the rectifier side with a little Figure 5: Sending Voltage and Receiving Voltage Figure 6: AC Transmission Line to Line Voltage and Three Phase Voltages 63

6 delay. At steady state operation, in case of without DC fault time at maximum firing angle, the current Id remains constant and Vdc have also near about constant value. When firing angle going to decrease current Id start to increasing slowly. In comparison with rectifier side more steady current and voltage characteristics is obtained at inverter side. Figure 7 shows the output result of the total harmonic distortion. The result shows how much current is distorted, which come out from rectifier. In case of DC fault the value of voltage at sending end is fluctuating. When fault occur, the value of voltage is zero at transmission line as shown in Figure 8. After clearing the fault the value of voltage becomes constant at Figure 7: Total Harmonic Distortion Figure 9: AC Transmission receiving end and going for steady state condition. During the HVDC AC fault on DC transmission the value of voltage is fluctuating and become zero at sending end and receiving end respectively as shown in Figure 10. Figure 11 shows the AC fault on AC transmission which is a result in between Vabc and Figure 10: HVDC AC Fault DC Transmission Figure 8: DC Fault V Sending and V Receiving Figure 11: AC Fault AC Transmission 64

7 Figure 12: Total Harmonic Distortion Vab_VSC2. Figure 12 shows the output result of THD. CONCLUSION This paper shows a simple approach of HVDC model by considering power electronic devices for control the overall system in order to improve power transfer as well as to achieve reliability in the power transfer. Software based studies of transient disturbances have been carried out using the Simulink in MATLAB. The system has also been simulated for steady state condition and also for different fault conditions at both the rectifier and inverter sides. It has found that current and voltage strongly depend on the types of fault. The HVDC system has been used to transmit power from a 500 kv, 50 Hz network to 200 kv, 50 Hz network. The receiving end and sending end AC systems are separated by 300 km DC transmission line. The three phase AC load has the parameters: Active power 10 MW, reactive inductive power 2 KVAR and reactive capacitive power as 2.1 KVAR. The analytical results obtained in this proposed model can be a useful tool in system design and optimization. REFERENCES 1. Andersen B and Barker C (2000), A New Era in HVDC, IEE Review, Vol. 46, No. 2, pp Ashfaq Husain (2008), Electrical Power Systems, 5 th Edition, Ch. 23, pp , CBS Publishers and Distributors. 3. Bimbhra P S (2012a),Power Electronics, 4 th Edition, Ch. 4, pp , Khanna Publishers. 4. Bimbhra P S (2012b),Power Electronics, 5 th Edition, Ch. 6, pp , Khanna Publishers. 5. Epameinondas (Minos) Kontos (2013), Control and Protection of VSC-Based Multi-Terminal DC Networks, M.Sc. Thesis, Department of Electrical Sustainable Energy, Delft University of Technology, Delft, the Netherlands, September 25, Chap. 2, pp Irving L Kosow (2003),Electric Machinery and Transformers, 2 nd Edition, Chap. 14, pp , Prentice-Hall. 7. Zakir Hossain M, Md. Kamal Hossain, Md. Alamgir Hossain and Md. Maidul Islam (2014), Performance Analysis of a High Voltage DC (HVDC) Transmission System Under Steady State and Faulted Conditions, TELKOMNIKA Indonesian Journal of Electrical Engineering, Vol. 12, No. 8, pp

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