Improved hvdc generation

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME

International Conference on Communication Systems (ICCS-2013) October 18-20, 2013 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 269

Improved HVDC Generation

Rajendra Bhamu

Electrical Engineering, BKBIET, Pilani, INDIA

[email protected]

ABSTRACT: This paper focuses on the design and implementation of a HVDC GENERATOR to utilize the renewable energy sources like wind & hydro at remote areas & to transmit power through hvdc lines to urban areas at low transmission losses. The system consist specific winding arrangement for forward & reverse conduction of individually connected coils emf through diode string in series on stator. The generator works on variable mechanical rotating power of renewable sources. By the direct hvdc generation the generating units can easily connected in series to build up voltage to hvdc level. The generator winding comprises of technology that have a smooth D.C. output with a very less ripple content & a very simple connection & low cost component. KEYWORDS: Distributed generation, HVDC generation, Renewable sources, HVDC transmission I. INTRODUCTION To overcome the increasing demand of electric power and to conserve the conventional sources for long time to upcoming generations, use of renewable sources is an alternative. The increasing power demand, advancement in technology, and environmental concerns are leading to increased interconnection of renewable energy source (RES) -based distributed generators to the utility grid [1]. Distributed generation (DG) is an alternative that is gathering momentum, and alter native energy sources such as photovoltaic, wind, fuel cells, etc., are likely to play an increasing role in ever-increasing power demands [2,3]. Therefore, to utilize the renewable sources of remote areas and feed power to the urban areas at long distance the hvdc transmission system is required. In the direct hvdc generation system it generates smooth dc power with very low ripple content at generator terminal. Therefore, the units can easily connected in series to build the voltage up to hvdc level to efficient transmission. There are many advantage of hvdc transmission over hvac transmission. A. Advantage of DC Transmission

More power can be transmitted per conductor per circuit and Use of Ground Return Possible

Smaller Tower Size and Higher Capacity available for cables No skin effect and Less corona and radio interference

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No Stability Problem and Asynchronous interconnection possible Lower short circuit fault levels and Tie line power is easily controlled

There are many advantages of hvdc transmission but some Inherent problems associated with hvdc generation & transmission given below.

Expensive convertors and Reactive power requirement Difficulty of voltage transformation and Difficulty of high power generation Generation of harmonics Absence of overload capacity

B. Advantage of hvdc generation The direct hvdc generation technique can solve some of inherent problems associated with hvdc transmission system. In direct hvdc generation, smooth dc power with very low ripple content is generated. Therefore, the units can easily connected in series to build the voltage up to hvdc level. It is useful for distributed generation.

Only one end converter required so reduced convertors cost: By direct generation of hvdc no requirement of Convertor Stations at generating end of a D.C. transmission link, only distribution end converter is required. Therefore, converter cost will reduce.

Less Reactive power requirement:- By direct generation of hvdc rectification required no reactive power but only in inversion reactive power is required.

Very less harmonics at generation end Difficulty of high power generation:- In direct hvdc generation no such problem of

commutation therefore, voltage speed and size are large. Thus, high D.C. power can be generated.

II. SYSTEM DESCRIPTION

The main parts of direct hvdc generating machine A. Stator The stator core is made of insulated steel laminations. The thickness of the laminations and the type of steel are chosen to minimize eddy current and hysteresis losses. The magnetic path, which comprises a set of slotted steel laminations called stator core pressed into the cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents, reducing losses and heating. CRGO laminations of 0.5 mm thickness are used to reduce the iron losses. [4] The stator frame is used to hold the armature windings. A set of insulated electrical windings are placed inside the slots of the laminated stator. The coils are connected in series with a diode string for the forward & reverse conduction. The output is collected at the terminals of the string at stator. No. of slots is twice of no. of coils. A.1 Diodes required for Forward conduction & reverse conduction of coils The diodes are required for forward and reverse conduction of coils. So diodes have a very less reverse recovery time. The rating of the diodes depends on peak voltage in coil. Calculation for peak inverse voltage of various diodes is obtained by the waveform of Individual coils shown in fig.3 the peak inverse voltage across forward diode string.



NOTE: The winding may have any number of coil but for explaining the working principle only 12 coil 2 poles(1 pole pair) 3000 rpm or frequency of 50 HZ, the angle 훼=180(electrical degrees)/N=15 electrical degrees=0.8333(milli second) and Vm=200V is considered. Where (∗Vm=peak voltage in coil, ∗ 훼 = phase split of coils, ∗N=no of coils per pole pair)

D1 =Vm sin( 6훼) = Vm D2 =Vm sin(5 훼)+Vm sin(6 훼)= 1.9659 Vm D3,D4,D5,D6,D7,D8,D9,D10,D11,D12,D13 =Vm sin(5 훼)+Vm sin(6 훼)=1.9659 Vm

Because all the coils C2,C3,C4,C5,C6,C7,C8,C9,C10,C11,C12 attain the peak negative voltage as in fig3. And PIV across reverse diode string for diode

D14 = Vm sin(6 훼)= Vm D15= Vm sin(5 훼)+Vm sin(6 훼)=1.9659 Vm D16,D17,D18,D19, D20, D21, D22,D23,D24.D25,D26 =Vm sin(5 훼)+Vm sin(6 훼)=1.9659 Vm

Because all the coils C1, C2,C3,C4,C5,C6,C7, C8 C9, C10, C11,C12 attain the peak positive voltage as in fig.3. So PIV across forward diode string and reverse diode string is given in table.

FORWARD STRING REVERSE STRING DIODE PIV DIODE PIV

D1 Vm D14 Vm D2 1.9659 Vm D15 1.9659 Vm D3 1.9659 Vm D16 1.9659 Vm

D4 to D13 1.9659 Vm D17 to D26 1.9659 Vm Table 1: Peak inverse voltage of various diodes

A.2 Stator Winding (Armature Winding) The coil side of coils are placed 180 electric degrees apart as shown in fig.2. The connection of coils is in series with the diodes for forward and reverse conduction shown in fig.1. The coils are connected in series to build up the voltage. The coils are connected sequencely according to phase angle between coils. The positive and negative output terminals are taken out at two ends of coil connection. The connection is shown in fig. 1.

Fig. 1: Stator winding (connection of stator (armature) coils)



Forward diode string: D1,D2,D3,D4,D5,D6,D7,D8,D9,D10,D11,D12, D13

Reverse diode string: D14,D15,D16,D17,D18,D19,D20,D21,D22,D23,D24, D25, D26 The coil C1 terminals 1 1’,coil C2 terminals 2 2’,coil C3 terminals 3 3’,and so on all coil sides placed 180 electric degree apart and connected through forward and reverse diode string for conduction of positive induced and negative induced respectively. A.3 Winding for multipole machine The winding of a multipole machine be divided in sub section equal to number of pole pairs. All the subsections have winding in above explained manner fig. 1. And all the subsection be connected in series and two terminals are taken out for output. B. Rotor The rotor may salient pole rotor or cylindrical rotor. Poles are built with thin silicon steel laminations of 0.5mm to 0.8 mm thickness to reduce eddy current laminations. In case of high speed the rotors are manufactured form solid steel forging. The rotor is slotted to accommodate the field winding. Normally two third of the rotor periphery is slotted to accommodate the winding and the remaining one third unslotted portion acts as the pole. Generally rectangular aluminum or copper strips are employed for filed windings. The rotor is excited by direct current supply with slip ring to create magnetic field flux [4]. B.1 Rotor Windings In hvdc generator the rotor winding producing the magnetic field is made of a number of coils circuit equal no of pole pairs is energized with D.C. power fed through slip ring riding on the shaft. The field excitation is provided by a suitable fraction of output power using filter and chopper circuit because of D.C. output nature.

Fig. 2: Physical construction of direct D.C. generating machine



III. WORKING The working of the machine is mainly based on the nature of induced emf in coil will be sinusoidal according to Faraday`s law of emf induction & will be phase displaced by an angle 훼.The conduction of coil for the negative induced emf will take place through the reverse diode String and as the coil induced emf changes to positive conduction take place through forward diode string .So the induced emf equation in the coils is given below.

V1=Vmsin휔푡 (1) V2=Vm sin (휔푡- 훼) (2) V3= Vm sin (휔푡- 2훼) (3) V4= Vm sin (휔푡- 3훼) (4) V5= Vm sin (휔푡- 4훼) (5) V6= Vm sin (휔푡-5 훼) (6) V7 =Vm sin (휔푡- 6훼) (7) V8= Vm sin (휔푡-7훼) (8) V9= Vm sin (휔푡-8 훼) (9) V10= Vm sin (휔푡- 9훼) (10) V11= Vm sin (휔푡- 10훼) (11) V12= Vm sin (휔푡- 11훼) (12)

The waveforms are shown in fig.3 for the induced emf in coils. The nature of induced emf in the Coils for 360 (20 ms) electrical degrees is repeated again & again for rest of time. The change of polarity of induced emf form negative to positive and positive to negative will take place twice for each coil in a cycle of 360 electrical degrees in a sequence of phase angle after angle 훼.

Fig. 3: Waveforms of induced emf in coil for one cycle

The emf induced in various coils will add up in output for different time Intervals. The voltage in all coils will not add up in output for a very less time because as the emf in coils changes



from negative to positive the diode connected to that coil in reverse string will in reverse bias and not conduct for little time. Then again the diode connected in forward string will conduct and coil voltage adds up in output. The voltage of coil 1 will not add in output for 0>ωt>α/2 and add in output for α/2 >ωt>π and again in negative cycle will not add in output for π>ωt>π+α/2 and add in output for π+α/2> ωt>2π and voltage of Coil 2 will add in output for 0>ωt>α/2 and will not add in output for time α from α/2>ωt>3α/2 and then again add for time3α/2>ωt>π and again in negative cycle will add in output for π>ωt>π+α/2 and will not add in output for time α from π+α/2> ωt>π+3α /2 and then again add for time π+3α /2> ωt>2π. So the voltage in coils will not add up in output in Sequence and each coil voltage will not add for a time period of 2 α. The last coil voltage will not add for angle α. So the voltage equation for the different coils voltages add in output is given below.

V1 = 0 0 > 휔푡 > 훼/2, 휋 > 휔푡 > 휋 + 훼/2푉푚푠푖푛휔푡 훼/2 > 휔푡 > 휋, 휋 + 훼/2 > 휔푡 > 2휋 (13)

V2 = 푉푚푠푖푛(휔푡 − 훼 ) 0 > 휔푡 > 훼 /2, 휋 > 휔푡 > 휋 + 훼 /2

0 훼 /2 > 휔푡 > 3훼 /2, 휋 + 훼/2 > 휔푡 > 휋 + 3훼 /2푉푚푠푖푛(휔푡 − 훼 ) 3훼/2 > 휔푡 > 휋, 휋 + 3훼 /2 > 휔푡 > 2휋

(14)

V3 = 푉푚푠푖푛(휔푡 − 2훼) 0 > 휔푡 > 3훼 /2, 휋 > 휔푡 > 휋 + 3훼 /2

0 3훼 /2 > 휔푡 > 5훼 /2, 휋 + 3훼/2 > 휔푡 > 휋 + 5훼 /2푉푚푠푖푛(휔푡 − 2훼) 5훼/2 > 휔푡 > 휋, 휋 + 5훼 /2 > 휔푡 > 2휋

(15)



(16)



(17)



(18)

V7= 푉푚푠푖푛(휔푡 − 6훼) 0 > 휔푡 > 11훼 /2, 휋+> 휔푡 > 휋 + 11훼 /2


(19)



(20)



(21)





(22)

V11 =푉푚푠푖푛(휔푡 − 10훼) 0 > 휔푡 > 19훼 /2, 휋 > 휔푡 > 휋 + 19훼 /2


(23)


0 21훼 /2 > 휔푡 > 22훼/2, 휋 + 21훼/2 > 휔푡 > 휋 + 22훼/2푉푚푠푖푛(휔푡 − 11훼) 22훼/2 > 휔푡 > 휋, 휋 + 22훼 /2 > 휔푡 > 2휋

(24)

The conduction of the coils is repeats after each half cycle. The conduction of different coil for half cycle is shown in fig 4. This will repeats in next half cycle and so on. The fig.4 Represent a=α/2, b= 3α/2, c= 5α/2, d= 7α/2, e= 9α/2, f= 11α/2, g= 13α/2, h= 15α/2, i= 17α/2,j=19α/2,k= 21α/2, l= 22α/2 from point o to a voltage V1 will not add in output and from point a to b voltage V2 will not add in output and from point b to c voltage V3 will not add in output and this sequence be continue for further upcoming coils.

Fig. 4: Waveform for conduction of various coils points

IV. OUTPUT The output response of the system is analyzed by simulation with LTSPICE or MATLAB software taking voltage sources according to emf induced in coils of stator winding for 360 electrical degrees because of repetitive nature of same waveform after 360(1 cycle (20 ms)) electrical degrees and then Fourier series is used to find out d.c. component by conduction equations. The simulated output is shown in fig.7 Parameters of the Fourier series like the dc component is given by



V dc= 2Vm (1+ (N-1) cos (α/2)/π (25) Where Vdc = D.C. component of output

Fig. 5: Circuit for 12 coil machine simulated by LTSPICE software

If the peak voltage in a coil is 200V (Vm=200V). The output fig. 7 consists 1.507kV+13V (voltage drop across diodes) =1.520kV. The peak voltage is 1.507kV with a ripple fig. 6 of (1.496kV to 1.508kV) =12V. So a very smooth output response is obtained. Fourier series analysis of D.C. component is finding in output response. The D.C. component by above formula for N=12, Vm=200V and α=15 degree is Vdc= 2×200(1+ (12-1) cos7.5)/π =1.515kV- 13 V =1.502kV The dc component of the system is near to peak value.

Fig. 6: Output ripple wave ripple content of 12V



Fig. 7: Output wave form for 180 (20 ms) electric degrees

A. Hvdc Output So for a machine have a peak voltage in coil is Vm= 300V, 6 pole (3 pole pair) and no of coils is N=72 then no. of coil per pole pair equal to 24 so as the no. of coil increase α 0 then cos α/2→ 1 and the output is like as commutated output of D.C. generator and the value of output D.C. voltage is given by Vdc = (2×N×Vm)/π (output of coils per pole pair in fig. 8) So the output voltage Vo =3×output of coils per pole pair VO =3×4.555kV =13.665kV So hvdc voltage is obtained by a machine having large no. of coil and pole and connecting 15 to 20 units in series. And hvdc power transmitted to urban areas at low transmission loss.

Fig. 8: Output of one pole pair of 6 pole machine simulated by LTSPICE



V. CONCLUSION In hvdc generation technique it is very useful for utilization renewable sources like wind and hydro which are in abundance at remote areas. The solar cell can also be connected with this. The system also deals with some inherent problems of hvdc converter station. So some advantages of hvdc generation system

Series connection of units easy because of very smooth D.C. output. No synchronization problem in different units. High output power than rectification. Very useful for renewable sources Better for distributed generation.

The system can be implemented for generation of electrical power by conventational energy sources like coal, gas diesel and nuclear but it is very useful renewable energy sources like wind and hydro turbine. There is some complicacy in winding connection and maintenance is quite high due to large no of connection. The hvdc motor can be constructed by replacing diodes with power mosfet,s and controlling the gate pulse through microcontroller and providing dc input at terminals.

VI. ACKNOWLEDGEMENTS I would like to convey my sincere thanks to Mr. R K ALARIA (Assistant Professor, Electrical engineering Department), for your kind support and guideline for the project. I would also like to offer my sincere thanks to Mr. SATISH RAI (Assistant Professor, Electronics engineering Department) & Mr. SANDEEP SONI (Assistant Professor, Electrical engineering Department) for their support and help at various levels. REFERENCE [1] Power Quality Improvement Through Grid-interfacing RES Mehta G and Singh SP IETE JOURNAL OF RESEARCH | VOL 59 | ISSUE 3 | MAY-JUN 2013 [2] F Blaabjerg, Z Chen, and S B Kjaer,“Power electronics as efficient interface in dispersed power generation systems,” IEEE Transactions on Power Electronics, Vol. 19, no. 5, pp.1184 94, 2004. [3] W Kramer, S Chakraborty, B Kroposki, and H Thomas, “Advanced Power Electronic Interfaces for Distributed Energy Systems, Part 1, Systems and Topologies,” Technical Report NREL/TP-581-42672, 2008. [4] www.sjbit.edu.in_app_course-material_eee_vi_electrical machine design%5b10ee63%5d_eee-vi-electricalmachinedesign %5b10ee63%5d-notes BIOGRAPHY

Rajendra Bhamu was born in Sujangarh, Rajasthan, India in 1993. He is pursuing his B.Tech degree in Electrical Engineering from BKBIET Pilani (Rajasthan), India. His research interests focus on reliable & efficient power system through electronics & automation and utilization of renewable energy sources.