Industrial Electrical Engineering and Automation

Transcription

1 CODEN:LUTEDX/(TEIE-719)/1-7/(7) Industril Electricl Engineering nd Automtion Estimtion of the Zero Sequence oltge on the D- side of Dy Trnsformer y Using One oltge Trnsformer on the D-side Frncesco Sull Dept. of Industril Electricl Engineering nd Automtion Lund University

2 Tle of Contents 1 INTRODUCTION...3 THEORY ERIFICATION OF THE METHOD THROUGH SIMULATIONS IN MATLAB SIMPOWER LABORATORY TESTS AND RESULTS...6

3 1 Introduction This report dels with the estimtion of the zero sequence voltge on the D-side of Dy trnsformer y the use of only one voltge trnsformer (T) connected etween one phse nd ground on the D-side. Zero sequence voltge cn e used for phse to ground fult detection in non-directly erthed systems. Normlly, on the D-side of trnsformer the zero sequence voltge is mesured y the use of three Ts, ech of them connected etween ech phse nd ground on the primry side nd open delt connected on the secondry side. The method descried in this report llows D-side zero sequence voltge estimtion y the use of one T on the D-side nd three Ts on the y-side. If the y-side is t lower nominl voltge, the descried method could e ttrctive on n economic point of view. Though the descried method is generl nd works for every Dy trnsformer (nd with some djustments for ny trnsformer connection), the cses where it is economiclly dvntgeous re few. An exmple is the cse where distriuted genertor is connected to medium voltge non-directly erthed network through Dy step-up trnsformer nd zero sequence voltge on the D-side is necessry for protection purposes. 3 Theory The min ide ehind the method descried in this section for estimting the zero sequence voltge on the D-side is to use only one T on the D-side nd to use the informtion from three Ts connected on the y-side of the trnsformer. The estimtion of the zero sequence voltge is performed s descried elow. Two premises re necessry (Zorn Gcjc s PhD thesis): - Dy trnsformer trnsforms sequence quntities independently - positive nd negtive sequence re trnsformed ccordingly to: N j j = ϑ ϑ 1 y1e τy1e N = y (1) N = e = τ e jϑ y y N y jϑ where - ϑ is the ngle y which the primry-secondry quntities re shifted in the trnsformtion nd it is determined y the trnsformer group numer - τ is the trnsformer rtio - N nd N y re the numer of windings on the D- nd y-side respectively

4 4 Zero sequence quntities re not trnsformed in cse of Dyn trnsformer. Eqution (1) does not tke into ccount the voltge drop inside the trnsformer. This cn e esily ccounted for y modifying Eqution (1) s follows: τ e τz I e τe Z I = jϑ jϑ = jϑ () 1 y1 tr y1 y1 tr y1 = τ e τz I e = τe Z I jϑ jϑ jϑ y tr y y tr y where - Z tr is the trnsformer impednce s seen from the low voltge y-side If we express the phse voltges on the D-side in terms of their sequence components, we get: = c 1 (3) In this eqution the unknown is the zero sequence voltge. 1 nd re otined from Eqution () through the informtion collected on the y-side. By inserting Eqution () into Eqution (3) we get: ( y1 tr y1) jϑ τe Z I j 1 1 ϑ τ e y ZtrIy = = c 1 1 The previous is system of three equtions. Solving the first of these equtions for we find: jϑ τ = τe Z I e Z I (5) jϑ y1 tr y1 y tr y This is n estimtion of the zero sequence voltge on the D-side nd voltge is mesured. Any of the three equtions in (4) cn e used for the estimtion, ech requiring different phse voltge to ground to e mesured on the D-side. The zero sequence voltge on the D-side cn e ccurtely estimted if: - positive nd negtive sequence currents nd voltges on the low voltge y-side re known - step-up trnsformer impednce, rtio nd ngle group re known - one phse to ground voltge on high voltge D-side is known The dvntge of this method is the use of only one T on the high voltge D-side to estimte the zero sequence voltge rising fter ground fult in non-directly erthed system. The mjority of the distriuted genertors re lredy provided with current nd voltge trnsformers on the low voltge y-side, i.e. there re no dditionl costs to get the positive nd negtive sequence quntities used in (5). However, to dte, distriuted genertors re not commonly operted in islnd nd therefore, sensing the zero sequence voltge on the D-side of the step-up trnsform is not lwys needed. Insted, if islnd opertion is desired, sensing the zero sequence (4)

5 5 voltge on the D-side cn e the most prcticl wy to detect ground fults on tht side. The word estimtion hs een used through ll the report insted of clcultion. The zero sequence voltge cn only e estimted ecuse of the uncertinties in trnsformer prmeters nd mesurement errors from CTs nd Ts involved in Eqution (5). Another source of error in the estimtion derives from neglecting the trnsformer mgnetizing current in Eqution (5). However, s it is shown in the following, the estimtion cn e very ccurte if the uncertinties re low. Also, the mgnetizing current of trnsformer is usully very smll. The physicl mening of the method is simple. On the y-side, ny informtion is lost on the neutrl point s (which is not physicl point) shift on the D-side. By mesuring one phse-to-ground voltge on the D-side we regin this informtion. All informtion out positive nd negtive sequence voltges is identicl on oth sides of the step-up trnsformer. Therefore the zero sequence voltge on the D-side cn e estimted. 3 erifiction of the method through simultions in MATLAB SimPower The method hs een implemented in MATLAB SimPower nd it proved to e ccurte. The estimted zero sequence voltge hs een compred with the mesured one on the D-side of the trnsformer, during phse to ground fult on tht side t k voltge level. Results from simultions re shown elow. x [] error [] Figure Time [s] Simultion of phse to ground fult on the D-side. ) mesured (dshed) nd estimted (dotted) zero sequence voltge. ) error etween the estimted nd the mesured zero sequence voltge

6 6 The mesured nd estimted zero sequence voltges re prcticlly undistinguishle. The mximum error is round 5, i.e. round,35% of system nominl phse voltge nd it cn e totlly neglected. The error depends on neglecting the mgnetizing current in the trnsformer. In the simultions it hs een ssumed mgnetizing current equl to,1 pu, i.e. mgnetizing impednce of 1 p.u. Angle errors, not reported, re lso very smll. 4 Lortory tests nd results Some tests hve een performed in the l to verify the method descried ove for estimting the zero-sequence voltge on the d-side of Ynd trnsformer. The fcts tht the d-side is the low voltge side nd tht the Y-side is grounded do not ffect the conclusions. The lortory experiment is descried in the figure elow. Figure 4.1 Set-up of the lortory experiment An Ynd trnsformer is fed through tp-chnger trnsformer from the net. The voltge on the Yn side is regulted so to get 11 phse-to-phse voltge on the d- side. On the d-side resistive lod is fed nd cpcitnces re connected so to simulte long line. On the Yn side ll phse currents nd voltges re mesured. On the d-side phse to ground voltge is mesured. Also the zero-sequence voltge is mesured on the d-side through three open delt connected voltge trnsformers, not shown in the figure for ske of clrity. All mesurements re fed in DSpce nd phsors clcultions re performed in DSpce through one-cycle Fourier lgorithm. Finlly, the new lgorithm for zero sequence estimtion is lso implemented in DSpce. A phse to ground fult is simulted on the d-side y connecting one phse directly to the cpcitnces neutrl point. Results elow show the mesured zero-sequence voltge, the zero sequence voltge estimted through the new lgorithm nd the fulted phse voltge. It is noted tht efore fult occurrence, the ngles of the estimted nd mesured zero sequence voltge hve ctully no mening, eing the voltge idelly equl to zero.

7 7 It is seen tht the zero sequence voltge estimted through the lgorithm reproduces firly the mesured zero sequence voltge, lredy fter less thn cycle. After the trnsient, the two voltges re prcticlly equl. The smll differences re minly due to uncertinties in the converting fctors for DSpce input signls. 8 6 [] 4 [degrees] time (s) Figure 4. Phse to ground fult on the d-side. ) Phse voltge (solid), mesured (dshed) nd estimted (dotted) zero sequence voltge. ) mesured (solid) nd estimted (dshed) zero sequence voltge ngle 15 1 [] 5-5 [degrees] time (s) Figure 4.3. Phse to ground fult on the d-side. ) Phse voltge (solid), mesured (dshed) nd estimted (dotted) zero sequence voltge. ) mesured (solid) nd estimted (dshed) zero sequence voltge ngle