Experiment I Voltage Variation and Control

Transcription

1 ELE303 Electicity Netwoks Expeiment I oltage aiation and ontol Objective To demonstate that the voltage diffeence between the sending end of a tansmission line and the load o eceiving end depends mainly on the flow of eactive powe, Q and not active powe P, poviding that the X/R atio of the of the system is elatively lage. The decease of voltage at the eceiving end due to the flow of eactive powe limits the powe that can be deliveed to the load. Intoduction Powe and Reactive Powe Flow in Powe Systems By convention, the complex powe S is defined as: In eithe case. by convention * S I fo a load * S E I fo a geneato S P jq if the cuent lags the voltage S P jq if the cuent leads the voltage whee P is the eal powe and Q is the eactive powe. If we conside shot lines (tansmission lines whee all capacitances ae neglected) as shown in Figue 1 then the active powe tansmitted ove the line is P s cos cos Z Z Figue 1: Equivalent cicuit of a shot Tansmission line Because we make the assumption that the X/R atio is lage, Z X, R 0 and the pevious equation can be e witten as: s P X Similaly it may be shown fo the eactive powe that sin The Univesity of Sydney School of EIE 1 Semeste, 011

2 ELE303 Electicity Netwoks s Q cos X X Thus, fo a geneato feeding a lage system o fo a geneato feeding a lage system though a tansmission system, o fo a simple tansmission system, the fom of the powe flow equation and of the phaso diagam is the same. It must be noted that in all cases thee is a maximum value fo the powe that can be deliveed by a powe system and that, if the esistance and capacitance of the line ae neglected, occus when 90 o. oltage Regulation A voltage phaso diagam can be dawn fo the equivalent cicuit shown in Figue 1, by consideing the cuent, I, to be equal to the sum of two cuent, Ip and I q, that ae at ight angles to each othe. Ip is in phase with and Iq lags by 90. The esulting phaso diagam fo lagging p.f. load is shown in Figue. Fom this diagam we have s p q If we assume that angle is adequately small then fo the case of a lagging powe facto load we have RI XI s p p q Similaly it may be shown fo the eactive powe that P R Q X In the case of a capacitive load we have XI RI q p q P X Q R p P R Q X Figue : Phaso diagam fo a lagging powe facto Load The Univesity of Sydney School of EIE Semeste, 011

3 ELE303 Electicity Netwoks Similaly it may be shown fo the eactive powe that Also, if X/R > 1 then q P X Q R The flow of eactive powe Q detemines the voltage dop The flow of active powe P detemines the tansmission angle and these two statements ae substantially independent of each othe. A tansmission line absobs an inceasing amount of eactive powe as the load cuent inceases. This eactive powe is given by I X. Similaly a line will also geneate eactive powe equal to / X. If we conside constant voltage, then thee is a load fo which the tansmission line neithe absobs o geneates eactive powe. In this case In this case I X X I X X when a line is also teminated by an equal load of L / the voltage and the cuent ae in phase acoss the tansmission line and thee is no voltage dop s. The impedance L / is called suge impedance of the line and the powe tansmitted unde these conditions is equal to L P N L / which is called natual load o suge impedance load (SIL) of the line. Fo loads geate than P N the line absobs eactive powe and fo loads less then PN the line geneates eactive powe. It must be noted theefoe that in a powe system with an extensive tansmission and distibution system, consideable eactive powe can be absobed o geneated by the tansmission system itself with a consequent dop o ise of system voltage. oltage Regulation at onstant Load Powe Facto At the eceiving end of a tansmission system thee may be connected loads of vaying powe facto. If the sending end voltage of a tansmission line s is consideed constant it is of inteest to detemine the vaiation of fo vaying load at fixed powe factos. These calculations can be caied out fom the line equations. P cuves at vaious powe factos ae shown in Figue 3. Figue 3 shows the esult of such calculations. As might be expected fom ealie discussion, leading powe facto loads cause an incease in voltage whilst lagging powe facto loads can cause a sevee eduction not only of voltage but also of the maximum powe that can be deliveed. lealy, fo maximum powe tansfe and maintenance of voltage nea 1.0 pu it is necessay fo high powe factos to be maintained. The Univesity of Sydney School of EIE 3 Semeste, 011

4 ELE303 Electicity Netwoks Figue 3: oltage egulation at constant load powe facto Pe lab Wok 1. A lossless tansmission line has a total inductance of L and capacitance of. What is the limit powe it can tansmit? What is the load condition when it tansmits maximum powe? Why?. The sending end of a lossless tansmission line is connected with a geneato and eceiving end is connected with a load. Use a phaso diagam to demonstate the influence of the load popety (leading o lagging) on the voltage dop between the sending and eceiving end? Expeiment Setup Fo the expeiment setup the system consideed is shown in Figue 4. The equied connections fo the setup of the powe system simulato is shown in Figue 5. Figue 4: Expeimental setup The Univesity of Sydney School of EIE 4 Semeste, 011

5 ELE303 Electicity Netwoks Figue 5: onnection Diagam Tasks oltage egulation 1. Fo no load condition set the excitation of geneato G1 to poduce 0 at the Distibution Bus (Mete ML). This voltage is designated. Note the voltage at the Geneato Bus (Mete MD). This voltage is designated s.. onnect a 5% switched esistance load to the utilization bus. Incease the geneato excitation to poduce a voltage of 0 line at (Distibution Bus) and note the voltage s at the Geneato Bus, the line cuent I p, P, Q, S and powe facto (pf) at the sending end (Mete D), the line cuent I p,, P, Q, S and pf at the eceiving end (Mete L) and at the mete afte the distibution tansfome (Mete M). How is the diffeence in the measued powe values explained? Do not incease the geneato excitation cuent above 1.3 A. 3. onstuct a voltage phaso diagam fo this load and compae measued and calculated values of Geneato Bus voltage ( s ). 4. Now connect a 5% switched inductive load to the utilization bus, in paallel with the esistive load. Adjust the geneato excitation to maintain 0 at the Distibution Bus,. Recod, the line cuent I p, P, Q, S and pf at the Geneato Bus and also Mete L and Mete M. 5. Repeat this pocedue fo a 50% switched inductive load. 6. onstuct the phaso diagams fo the pevious loads. 7. Summaize the esults of the measuements, phaso diagams and analysis in a lab epot. oltage egulation at constant powe facto load The expeiment setup emains the same as above but the powe facto at each step below must be kept as constant. 1. Fo no load condition set the excitation of Geneato G1 to poduce 0 at the distibution Bus (). Note the voltage at the Geneato Bus (s). The Univesity of Sydney School of EIE 5 Semeste, 011

6 ELE303 Electicity Netwoks. onnect a load of 5% esistance and 5% inductive eactance to the system. Keep the geneated voltage s constant fo this incease in load and note the value of the Distibution Bus voltage. Measue and ecod the line cuent I p, P, Q, S and powe facto (pf) at the sending and eceiving end of the tansmission line and load connection point, which is at the Mete D, L and M. 3. Repeat this pocedue fo equal esistive and inductive loads of 50%, 75% and 100%. 4. Plot the vaiation of distibution voltage against powe (kw) deliveed and compae the cuve obtained with the cuves in Figue Repeat this pocedue fo esistive load only 5% to 100% as a efeence compaison. 6. Summaize the esults of the measuements and analysis in a lab epot. The Univesity of Sydney School of EIE 6 Semeste, 011