SINGLE-WIRE ELECTRIC POWER SYSTEM FOR RENEWABLE-BASED ELECTRIC GRID

Transcription

1 SINGLE-WIRE ELECTRIC POWER SYSTEM FOR RENEWABLE-BASED ELECTRIC GRID Dmitry S. Strebkov, Stanisav V. Avramenko, Aeksei I. Nekrasov The A-Russian Research Institute for Eectrification of Agricuture 2, 1-Veshnjakovsky proezd, Moscow, , Russia te. (+7 095) , fax (+7 095) , e-mai Abstract Low cost and ow osses singe-wire eectric power system (SWEPS) for renewabe- based eectric grid was deveoped. The new technoogy of eectric power transmission uses ide operation regime of the transmission ine and reactive capacitive current for transmission of active eectric power. Three different SWEPS were constructed and tested: 230V, 10kV and 100kV each is of one kiowatt capacity. Resonance mode of osciation with frequency from 3 to 30 khz was used to provide the most efficient power transmission. Frequency converter and modified Tesa transformer are appied at the generator site to generate high frequency reactive capacitive current. Reversa Tesa transformer and standard rectifier and invertor were used at the user s end to convert the reactive high frequency eectric power to standard 50-60kHz eectricity. It was experimentay proved that SWEPS has quasi-superconductivity property for reactive capacitive current fow aong the ine even at high operation temperature of the eectric conductor. SWEPS has no resistance osses for foowing tested conductor materias of the ine: copper, auminum, stee, tungsten, carbon, water, damp soi. The principe of SWEPS operation is based on the fact, that in no oad operation mode the active current and magnetic fied of a singe-wire ine is equa to zero, whie the eectric fied of the conductor has maximum vaue and this eectric fied is exited by reactive current which is charging the capacitance of the singe-wire ine and the eectric oad. The resut of theoretica cacuation and experimenta study shows that SWEPS can be appied both for the energy transmission from renewabe powerfu generation site to a arge energy system and for transmission ines for connecting different parts of renewabe energy system. 1. INTRODUCTION Renewabe-based eectric grids are increasingy being viewed as an attractive aternative for providing power to rura communities. Technoogy options incude sma hydropower, biomass-powered generators, sma geotherma, PV, soar-therma, wind turbines and hybrid systems with back-up diese generator, which may be connected to the oca utiity. Impementation of renewabe-based technoogies for rura eectrification woud contribute to the socia and economic growth of the rura communities and woud serve sustainabe progress of the remote regions. The eectric grid faces specific probems of non-efficient operations, incuding transmission osses and the high cost of grid extension in remote sparsey popuated areas [Strebkov, 1994]. For exampe off-shore wind turbine, micro-hydro or geotherma generator are often ocated far from consumer and requires costy instaation of ong distance transmission ine which usuay has from 6% to 10% eectric oses. Hybrid system, with jointy operating sma power generators of equa capacity, faces the probem of joint eectromagnetic operation stabiity during renewabe energy potentia or eectric oad variation. We made computer simuation of soar power system, consisting of three or more soar power pant of equa capacity connected by superconducting eectric transmission ine. One soar power pant is ocated in Spain, another soar pant is instaed in Far East region of Russia, the third one is situated in Astrakhan region near Caspian Sea. The computer simuation shows that this distributed Europe-Asia soar power system is generating eectricity 24 hours per day 6 months a year and it does not require eectric accumuator or back up generator during the night. In winter season soar eectricity shoud be transmitted from Africa, India and Austraia and for this United Soar Eectric Power System new ow cost and ow osses eectric power transmission technoogy shoud be appied. The objective of this paper is to introduce ow cost and ow osses singe-wire eectric power system (SWEPS) for renewabe-based eectric grid instead of three phase network. 2. THREE PHASE NETWORK FOR ELECTRIC POWER TRANSMISSION It is known that the tota transmitted power over eectric transmission ine 2 2 = P Q (1) S +

2 where P and Q are active and reactive powers. The important parameter of transmission ine affecting energy transmission capabiity is surpus reactive capacity, which is depending on the regime of energy transmission. When the ine operates in the ide regime its surpus reactive capacity is equa to the capacity of ine s eectrica fied [Aexandrov, Smoovic, 1999]: Ρ = 0 S = Q = P (2) n λ where P n is natura power which is equa to the surge impedance Z oading of the ine. Z L 0 = = νl (3) 0 C 0 L 0 and C 0 are the specific inductance and capacitance of the ine. λ - waveength of the ine, which is equa to the variation of wave phase during the wave propagation aong ine of ength ω ν λ = = β (4) where ù=2π f, f - frequency of generator. β - the phase variation coefficient of the eectromagnetic wave. ν - the veocity of eectromagnetic wave propagation. No-oad operation mode is dangerous for eectric power transmission because of votage rise due to eectromagnetic wave osciation. When the generator frequency is equa to the resonance frequency of the ine the overvotage has maximum vaue. The votage coefficient of the ine where Ë is a generator votage and Q is quaity factor of the ine. In natura (nomina) regime of active eectrica power transmission the magnetic fied of the ine competey compensates the eectric fied of the ine and surpus reactive ine capacity is equa to zero. In this idea case reactive current and reactive power are equa to zero. (5) The ange θ between vectors of votage at the beginning of the ine () is equa to wave ength of the ine (0) and at the end of the ine θ = β the votage is stabe aong the ine (0) = () The active current and the active power transmission are controed by variation of ange θ and votage aong the ine. When a transmitted power is decreased and varies the compensation of eectric fied is not compete, the votage wi become higher and for its imitation shunt reactors are used to compensate the surpus reactive capacity of the ine. The equivaent circuit of this ine is simiar to series connection of active resistance and inductive impedance and such ine has no wave and resonance characteristics. Fexibe aternative current transmission systems with fast acting shunt reactors and series capacitate compensation contro system aows providing the stabe energy transmission over the ine ength km. The transmitted power is imited by resistance osses and by eectrica isoation of the air. The instaation cost of the kv aeria transmission ine is US$/km. Three phase a.c. 1,2MV, 10 GW, 1000km ong extrahigh-votage power transmission ine costs 1.31x10 6 US$/km, and the whoe power transmission system incuding transformer s substations and other eectrica equipment has the instaation cost 5.1x10 6 US$/km [Mogiis, 1991]. Very costy direct current transmission ines for onger distance (7000 km) and higher transmission capabiity (up to 70GW) are proposed. The instaation cost of 10GW, 1000 km, ±600kv bipoar d.c. transmission ine is 5.8x10 6 US$/km, and designed wasted power is 443MW (4.43%). In future direct current transmission wi provide even higher capabiity using advanced superconducting technoogy. The genera concusion regarding widey appied power transmission systems is that reactive power shoud be competey imited and compensated in order to provide stabiity of power transmission, to avoid the dangerous overvotage and to secure the osciation damping. 3. NEW TECHNOLOGY FOR ELECTRIC POWER TRANSMISSION In this paper we consider new technoogy for eectrica power transmission using ide operation regime and wattess capacitive circuating power for transmission of active power to the user s end of the ine. In the openended ine the active current and the magnetic fied of the ine are equa to zero, whie the eectric fied has maximum vaue and it is created by the reactive dispacement current which is charging the capacitance of the ine. The ange θ between votage vectors at the beginning and at the end of the ine is equa to zero. Practicay because we use the open-circuit ine we offer one-poe singe-wire eectric power system (SWEPS) instead of three-phase network [Avramenko, 1997, 1998]. The most important probems, which are to be soved:

3 1. How to provide the high density reactive capacitive current at the beginning end of the ine; 2. How to convert the reactive capacitance current and reactive power to active power and heat at the user s end of the ine. Schematic circuit of SWEPS is shown in Fig.1. Because the traditiona three phase Hz generators and other a.c. eectrica equipment are widey used we offer eectrica devices (back boxes), which can be instaed at the beginning, and at the end of transmission ine can provide eectromagnetic compatibiity of new technoogy with standard a.c. eectricity. Frequency converter and modified Tesa transformer with ferrite corn were appied at the generator site to generate high votage and high frequency reactive eectromagnetic power. Reversa Tesa transformer and standard rectifier and inverter were used at the consumer s end of high votage SWEPS to decrease the votage and to convert the reactive high frequency eectric power to standard three phase 50 Hz eectric power. For proper operation of SWEPS it is necessary to connect the neutra primary votage termina of reversa Tesa transformer to artificia natura capacitance ike an insuated metaic sphere or the frame of equipment. A reactive capacitive current fows trough Tesa transformer and provide resonance overvotage on its inductance impedance. Another technique of conversion of capacitive reactive power to active power is appication of diode-capacitor device which is usuay used in d. c. votage doubing circuit in ow votage SWEPS (Fig 1b). 4. CALCULATION OF SWEPS PARAMETERS In order to increase the transmitted power the operating frequency was significanty increased due to we-known equation for reactive power. [d] Q = 2πfcV² The quaity factor Q at frequency 10kHz is increased by 200 times comparing with 50Hz power system. The upper vaue of frequency kHz is imited by irradiation of eectromagnetic power. Effective radiated power Pir of the unoaded ine can be cacuated using known formua for transmitter s antenna For I = 100A, λ= m, f = 10kHz, Pir = 8x10³ W So at this frequency the radiated power is ow. Let us consider a singe-wire capacitive-inductive series resonant circuit without a corona osses connected to the Tesa transformer without the magnetic shunt (Fig.2.3). As the ine is open-ended the conduction current is equa to zero (Fig. 3). The Tesa transformer generates capacitive current, which is charging the capacitance of (6) the ine. For standard 50Hz 500kV ine the capacitive current is 1.13A/km, and reactive capacitive power is 0.98MVAR/km. A singe-wire overhead ine capacitance is defined under the known formua: C î = (7) 4 h 2 n d Where the ength of the ine d diameter of a conductor h distance between the earth and conductor For =1km, d=0.1cm, h=6m, C0 = 5505 mµf. The open ine is grounded through eakage current and dispacement current, which are distributed on a space, encosing a conductor. The dispacement current and votage depends on time and on coordinate. The equation of continuity for current Where gdxu eakage current g conductance of air u Cdx t dispacement current The equation for votages: Where votage drop across an inductive resistance. We obtain a set of equations for cacuation of parameters of singe-wire ine. u x i = x i = L t gu u + C t (8) (9) (10) These equations differ from known [Rasevich, 1976] by that the ohmic votage drop across resistance is equa to zero and the specific parameters g, L and C are considered for one conductor in reation to the ground but not for two-wire or three-wire ine. The soutions of the equations for operating compex votages and currents:

4 (11) ù (o) and ù () votage at the beginning and at the end of the ine As the ine is open-ended the current i() = 0 γ - coefficient of eectromagnetic wave propagation γ = á + iβ á damping factor r α = (12) L 2 C At high frequencies ωc > g, ωl > r The maximum votage is equa (13) (14) Where E is a votage of Tesa transformer. The capacitive current Ic = 2 ðfcv Assuming V(o) = E = 10kV, Q=10, f = 5kHz, C = 0,1µF, V ( = 15km) = 127,25kV The capacitive current Ic = 39,75 A The reactive power Q = 2 ðfcv² = 5,08 MVAR Energy storaged by the capacitor 0.1 µf. Takeoff active power from capacitor transmitted through eectronic key with switching frequency fo = 1kHz P = Ec x fo = MW 5. EXPERIMENTAL RESULTS Three different types of SWEPS were designed and tested: 230V, 10kV and 100kV, each is of one-kiowatt capacity. Tesa transformer has C-type uncosed magnetic circuit with ferrite core of mm diameter. Secondary high votage bobbin coi is wound upon ferrite core and it has 4-6thousand of winding turns. One termina of the secondary coi is in the center of secondary coi and from this termina the current is taken to singe-wire ine. Externa neutra termina of the secondary coi has a zero potentia in reation to the ground. This neutra termina is isoated. Primary coi is wound around in proximity to the secondary coi. Primary coi has winding turns. The terminas of primary coi are connected to frequency converter. SWEPS has two Tesa transformers, connected by singe-wire ine. Reversa step-down Tesa transformer at the user s end has the same structure of cois, as a stepup Tesa transformer [Tesa, 1900]. As a materia of conductor copper, auminum, stee, tungsten were used. The diameter of wire is microns. The transmitted power is 1 kw at votage from 230 V to 100 kv. Diode-capacitor bock comprises 0.25 µf., 16kV capacitor. As a conductor we use aso nonmetaic conductive media, ike a carbon wire of 100mkm diameter with resistivity 100 Ohm m, pastic water tube of 10 mm diameter, pastic saucer with 10 mm ayer of damp soi, ITO conductive fim on the gass substrate. Conductive fim has a resistivity 30 Ohm/ and a thickness of 0.3 microns. The current, votage and power of SWEPS were measured by standard 50Hz devices at the beginning of the ine. As an eectric oad appiances a.c. motors and fiament amp were used. The parameters of the oad are measured by standard a.c. eectric meters. Singe-wire circuit was tuned by variation of the frequency. At the resonance mode the capacity of the oad is maximum. The experiments have shown that in the resonance mode the current transiting to the oad through set-down Tesa transformer in ten times exceeds a current transiting through the secondary coi and charging the natura capacitance. Transmitted power does not vary at any diameter and materia of a singe-wire circuit. The wire room temperature does not increase after severa hours of power transmission. The powerfu eectric osciation produces stationary waves in unoaded singe-wire ine. The waveength is defined by frequency of generator or frequency converter. But when the eectric oad is switched on, we have traveing waves. The reactive power transmission is carried out by eectromagnetic fied propagation aong the ine which one executes a roe of guiding system. Transmitted power of singe-ine-to-ground short is equa to zero because of a detuned circuit. Resonant frequency is depends on distributed capacitance and inductance of the Tesa transformer, the ine and the oad. At remova of ferrite core the resonant frequency was augmented in 2-3 times. The Tesa coi generates aso eectromagnetic waves of 4-5 cm ength, which is equa to diameter of secondary coi of a setup Tesa transformer. These waves were observed by connecting to the inner termina of secondary coi severa series connected fuorescent

5 amps. In oaded ine the transversa dark and ight areas dispaced. The size of each area was 2-3 cm. So the secondary coi of set-up Tesa transformer pays a roe of spira antenna, irradiating the eectromagnetic waves. The waveength is defined by a diameter of the resonator and waveguide, the functions which one are executed by secondary Tesa coi. SWEPS incudes mono-poar ow oss singe-wire ine. In a spark-gap of oaded singe-wire ine there was a pasma discharge of reactive power. We caed this reactive pasma as cod pasma. There is a great difference between this cod pasma discharge of reactive power and arcing short discharge of two-wire ine transmitting an active power. If a water ayer is incuded as part of the oaded singewire ine and spark-gap is created between conductor and the surface of the water, the cod pasma discharge between conductor and water is initiated. This cod pasma discharge does not change the temperature of the water and does not evaporate the water during 30 minutes of operation of ine. We use spring water as we as sea water and the water seems to be an idea superconducting materia for capacitive reactive power transmission. When arcing short was created by an active power in a spark-gap between water and standard 50Hz two-wire ine we can observe the spashes and evaporation of water. The industria eectrode boier is a good iustration of effective eectric power conversion to heat. The pasma discharge in spark-gap of unoaded singewire ine decreases and depends on natura capacitance of a body. Using this property of one poe singe-wire ine we deveop cod pasma coaguator for appication in medicine and chemistry [Avramenko and Stupin, 1997]. 6. DISCUSSION The eectrostatic anaogy is one of visua arguments of operation principes of SWEPS. The transversa eectromagnetic waves are propagated aong the ine and these waves can have any frequency, incuding zero. The structure of wave fied in a transversa pane is identica to eectrostatic fied and stationary magnetic fied. The step-up Tesa transformer generates during haf-cyce the charges of high density and high eectrostatic potentia. The free charges are moving aong the ine from generator site with high potentia to the user s end with sma potentia and this capacitive charging current is stipuated by Couomb forces. These charges are moving on the surface of wire and this current is not affected by Ohm s aw and Joue aw. So Tesa transformer is operating during one hafcyce as eectrostatic generator continuousy generating free charges and supporting high potentia at the generator site. In the foowing haf-cyce there is a change of the sign of charges, which are recharging the ine capacitance but the potentia difference between the generator and end of the ine is saved and charges of other sign are moving aong the ine to the oad. A dispacement current in the space surrounding the wire corresponds to change of an eectric fied strength. The dispacement current as we as capacitive charging current is not affected by Ohm s aw and Joue aw. Another component of dispacement current takes into account moving charges and poarization in dieectric surrounding a wire. Poarization osses can be used for direct conversion of ide power to heat. But this effect has quite different nature than Joue osses physics. We found out the very high temperature increase of fresh wood when we use it as a conductor materia for oaded singewire ine. This simpe method can be used for fast wood drying and one can find a ot of materias, which can be used to provide heat from reactive eectric power using poarization osses mechanism. One hundred years ago Nicoa Tesa has deveoped his apparatus for transmission of eectric energy using singewire technoogy [Tesa, 1956]. In 1900 there was no photovotaic industry, radio engineering, aser technoogy and superconductivity. Now we better understand the theory and appication possibiities of SWEPS. N. Tesa considered that one termina of secondary and primary coi of step-up and step-down transformers must be connected to the earth. That means that singe-wire ine can be appied ony to the power transmission aong the earth. Now we know that eectric power can be transmitted to any body, not connected to the earth, for exampe, to air baoon, pane or even to sateite. We even don t need to appy a step-down transformer for singe-wire power transmission (Fig.1a) and we can use for power transmission non-metaic conducting media, ike isoated water tubes, cabes made from carbon or conducting oxides on gass etc. We deveoped SWEPS using aser beam as a singe-wire ine [Strebkov, Avramenko and Nekrasov, 1999]. Laser beam creates ionized conducting channe in the air with ions concentration cm -3. Stepup frequency Tesa transformer generates high votage (more than 1000kV) potentia and traveing eectromagnetic waves which fow aong this conducting channe. At the votage eve of 1000 kv the transmitted power may reach the vaue of 1000 Mw, depending on the frequency and capacitance of the oad. Another fied of SWEPS appication is an eectric transport. We offer eectric transport system using hybrid eectric car and pubic transport: bus, tram, troeybus, metro, eectric train using singe troey ine, isoated from the earth [Strebkov, Avramenko and Nekrasov, 1999]. 5W 12V experimenta mode of singe-troey car was constructed and tested. New principes of eectric power transmission, using capacitive and dispacement current in singe-wire one

6 poe circuit in future can be appied for construction of United Goba Soar Eectric Power System for the word. 7. CONCLUSIONS 7.1 Singe-wire eectric power system for renewabebased eectric grid can be appied instead of three-phase network. SWEPS uses one poe singe-wire open-tuned circuit, capacitive and dispacement current for transmission of active power. Modified step-up Tesa transformer was appied at the generator site to generate high frequency reactive capacitive current. Reversa stepdown Tesa transformer or diode-capacitor bock was used at the user s end to convert high frequency reactive power to standard a.c. 50 Hz or d.c. eectricity. 7.2 Three different 1 kw capacity SWEPS were tested: 230 V, 10 kv and 100 kv. Resonance mode of osciation with frequency from 5 to 15 khz was used to provide the most efficient power transmission. The transmitted active power is proportionate to the frequency, capacitance of the oad and quadrate of the oad votage. SWEPS and three-phase transmission ine have the same parameter affecting energy transmission capabiity and this parameter is surpus reactive capacity of the ine, which is equa to the capacity of the ine s eectrica fied. So both ines have the same transmission capabiities in the range from 1 W to 10GW. Both the predicted and observed eectric osses of singe-wire ine are consideraby ess than the osses predicted for three-phase network. It is known from the theory of eectricity that capacitive and dispacement currents are not affected by Ohm s aw and Joue aw. It was experimentay proved that SWEPS has quasisuperconducting property for capacitive and dispacement current. SWEPS has insignificant resistance osses for foowing tested conducting materias: copper, auminum, stee, tungsten, carbon, conducting ITO oxides on gass, isoated water tubes, damp soi. Conducting channe in the air ionized by aser beam was offered as a singe-wire ine for SWEPS SWEPS is one of the most promising eectric power transmission technoogies for renewabe-based eectric grid. This technoogy may be recommended both for the power transmission from a powerfu generation site to eectric grid and for transmission ine for joining together different parts of energy system. The computer simuation of distributed soar power system, consisting of severa soar power pants instaed in Spain, in European part of Russia and Far East of Russia, connected by ow oss transmission ine, resuted that this power system is generating eectricity 24 hours a day 6 months a year and does not require eectric accumuator or back-up generator during the night. Another promising possibiities incude singe-troey eectric transport, isoated from earth and powered by soar power system, soar driven cod pasma generator and compact extra high votage equipment. REFERENCES Strebkov D.S. (1994). Deveopment of soar energy in Russia. Therma Engineering, Vo 41, #2, Aexandrov G.N., Smoovic S.V. (1999). Fexibe ines for eectric energy transmission over ong distances. Prospective directions in deveopment of Eectric Power Industry and Eectrica Engineering Equipment. V Symposium Eectrica Engineering 2010, October 1999, Moscow Region, Mogiis D. (1991). Hydro-Quebec. The principes and practice of transmission system panning. IV Internationa NICOLA TESLA Symposium, September 23-25, Proceedings. Serbian Academy of Sciences and Art, Knez-Mihaiova, 35, Begrade, Strebkov D.S., Avramenko S.V., Nekrasov A.I. (1999). The method and apparatus for eectric power transmission. Russian Patent # Priority caimed Pubished in Russian Patent Buetin # 36 of Strebkov D.S., Avramenko S.V., Nekrasov A.I. (1999). The method of power suppy of eectric transport and apparatus for its reaization. Russian Patent # Priority caimed Pubished in Russian Patent Buetin # 25 of Avramenko S.V. (1998). The method for eectric power suppy and device for its reaizations. Russian Patent # Priority caimed Pubished in Russian Patent Buetin # Avramenko S.V. (1997). Apparatus and method for singe-ine eectrica transmission. European Patent # Priority caimed (Russia). Pubished in European Patent Buetin of Tesa N., (1956) Lectures. Patents. Artices. Beograd Tesa N. (1900). Apparatus for transmission of eectrica energy. US Patent # dated May 15, 1900 Rasevich D.W. (1976). High votage technique. Energy Pubishing House, Moscow, Tamm I.E. (1976). Fundamenta theory of eectricity. Science Pubishing House, Moscow, 133, Avramenko S.V., Stupin I.V. (1997). The apparatus for tissue coaguation. Russian Patent # Priority caimed Pubished in Russian Patent Buetin # 36 of

7 1. High frequency converter 2. Step-up high frequency Tesa transformer 3. Singe-wire ine khz 4. Diode-capacitor bock 5. Thyristor eectronic key 6. Eectric oad 7. Step-down Tesa transformer 8. Eectric capacitance 9. Rectifier Fig. 1 a Low votage singe-wire ine V b High votage singe-wire ine kV

8 du 1 h u du 2 u u + dx x dx S i x x+dx i i + dx x gdxu + Cdx u t dx Fig. 2. Currents and Votage Drops in Singe-Wire Line x L o x c Fig. 3. Equivaent Circuit of SWEPS XL inductactive impedance of Tesa transformers and singe-wire ine Xc capacitance of the ine and the oad