Supply Voltage Effects on the Operation of Residential Air Conditioning Appliances: Experimental Analysis

Transcription

1 upply Vltage Effects n the Operatin f Residential Air Cnditining Appliances: Experimental Analysis J.M. Maza Ortega 1, M. Burgs Payán 1, J.M. Rmer Grdón 2 and M. Pinilla Rdríguez 2 1 Department f Electrical Engineering, University f eville Camin de ls Descubrimients, s/n evilla, pain phne: fax: jmmaza@us.es, mburgs@us.es 2 Department f Pwer Quality - Endesa-evillana Avda. de la Brblla, evilla, pain Abstract. The deregulatin f the electric energy market and the difficulties related t distributin netwrk such as the cngestin, the increasing grwth f the installed pwer density in urban znes, the grwth f the delivery-prvided energy and the spectacular increase that the peak pwer experiences are new prblems t be faced by the utility engineers., the knwledge f the lad behaviur is becming mre and mre imprtant in the planning and peratin f the electric energy distributin netwrk. In this paper, the influence f dmestic air cnditining equipment, ne f the mst energy cnsuming dmestic lads in pain, in the electrical energy distributin netwrk is addressed. The research is fcused n the analysis f the vltage supply effects n the peratin f hme air cnditining appliances frm an experimental pint f view. Key wrds Vltage regulatin, Air cnditining equipment, Lad mdelling, Distributin netwrk. 1. Intrductin The massive installatin f residential air cnditining units in pain and especially Andalusia has mtivated the interest f lcal utilities t characterize the behaviur f this type f electrical lad. The use f this electrical appliance has been grwing within the residential sectr during the last years due t its cst reductin. This change in the cmpsitin f classical residential lads can prduce a change f the cnsumptin pattern, bviusly affecting bth the planning and peratin f the pwer system. In this sense, ne f the imprtant tasks t be slved is t characterize the behaviur f the residential air cnditining units versus vltage variatins. Tw types f air cnditining technlgies are usually installed in the residential sectr. The first ne is based n a hysteresis cntrl f the temperature, perfrmed by switching n/ff the mtr driving the cmpressr f the air cnditining unit. On the ther hand, the inverter based applicatins are als used; in this type f appliance, the mtr driving the cmpressr is fed thrugh an inverter that adjusts the frequency in rder t precisely match the requirements f the thermdynamic prcess. In spite f the energy savings that can be achieved with the latter technlgy, the frmer type f air cnditining units is mre widespread installed due t its reduced initial cst. This wrk is the cntinuatin f the paper [1] which analyzes frm a theretical pint f view the tw type f air cnditining technlgies usually installed in the residential sectr. The paper cntent is as fllws. First, a descriptin f the labratry tests is included. Then, the electrical equipment used in the experimental validatin f the theretical studies is described. Finally, the results fr the tw types f air cnditining technlgies, the main cnclusins and sme suggestins fr future wrk are presented. 2. Descriptin f Labratry Tests The bjective f the labratry tests is t determine the influence f steady-state vltage variatins in the peratin f residential air cnditining appliances. The tests are fcused n the vltage dependence f fur main magnitudes: Demanded lad current. Pwer factr. Active pwer demand. Fig. 1 shws the diagram f the labratry benchmark system. The air cnditining appliance is supplied thrugh an auttransfrmer with a variable relatin f turns (variac), s that it is pssible t cntrl the secndary vltage. Althugh ther means t btain a variable supply vltage fr the air cnditining appliance, such as electrnic based pwer supplies, it has been cnsidered that a transfrmer is mre suitable. The auttransfrmer nt nly reprduces better the real cnditins f the appliance supply (almst sinusidal vltage wavefrm and linear netwrk impedance) but RE&PQJ, Vl. 1, N.5, March 2007

2 als des nt intrduce undesired effects such as high frequency vltage harmnics. AIR CONDITIONING UNIT DATA ADQUIITION TRANFORMER Fig. 1. Diagram f the labratry test bench. Tw different types f air cnditining appliances have been studied in the electrical pwer labratry: Cnventinal appliances. In this case, the inner temperature is cntrlled using a hysteresis band arund the reference temperature adjusted by the final user. As a cnsequence, the cmpressr f the heat pump is switched n/ff depending n the values f the inner and reference temperatures. The main disadvantage f this applicatin is the cntinuus switching peratins f the cmpressr when the reference temperature has been set up. This peratin mde leads t an increment f electrical pwer lsses, mechanical stress f the cmpressr, etc. On the ther hand, the ecnmic cst f this type f hme air cnditining unit is quite lw cmpared t the inverter based types. Inverter based appliances. The temperature is cntrlled in this case by adapting cntinuusly the cmpressr peratin t the heat pump requirements. Fr this purpse, the electrical mtr driving the cmpressr is fed up thrugh an inverter that changes the frequency in the prper manner. The main advantage f this type appliance is that is mre efficient that the cnventinal units. Hwever, this type f units is nt as ppular as cnventinal nes because f their higher initial cst. It is interesting t nte that the magnitudes t be measured nt nly are dependant n the vltage variatins but als t bth the inner and uter temperatures. Hwever, neither measurement nr cntrl f these parameters during the perfrmed tests exist. Only the reference temperature, set by the final user, is a cntrl variable during the tests. In this sense, further develpments shuld include measurement r cntrl f the actual inner and uter temperatures. In rder t establish adequate cmparisns between the btained data frm the tests in absence f inner and uter temperature measurements, the tests have been perfrmed starting frm similar electrical cnditins., the rated vltage has been always used t starting the peratin f the equipment, and nce the rated current f peratin has been established, the vltage is mdified t the desired value cntrlling the auttransfrmer. PC UPPLY 3. Descriptin f test equipment Tw cmmercial air cnditining appliances based n heat pump thermdynamic cycle, with a capacity f 3.5 kw (refrigeratin) - 4 kw (heating) and 1 kw f electrical rated pwer have been tested in the electric pwer labratry. Table 1 summarizes the main characteristics f bth cnventinal and inverter based appliances. Table 1. Technical characteristics f the cnventinal and inverter based air cnditining appliances. Functin Cl Heat upply ~/N, 230 V, 50 Hz Capacity (kw) Input (kw) Weight Inside (kg) 10 Outside (kg) 40 IP Cde Inside IP 20 Outside IP 24 It is interesting t nte that the units have the same rated characteristics. Hwever, the technlgy applied in each unit is quite different. The unit based n a hysteresis cntrl f the temperature is cmpsed by a single-phase inductin mtr with a starting capacitr. The technical dcumentatin f this air cnditining unit reveals that this capacitr is nt nly devted t the starting peratin but als t the reactive pwer cmpensatin task during nrmal peratin. The mtr frnt end is cnnected t the pwer supply thrugh a cntactr dedicated t switch it n/ff depending n the hysteresis cntrl f the temperature as shwn in Fig. 2. UPPLY Figure 2. Hysteresis cntrl f an air cnditining cmpressr. On the ther hand, the unit based n inverter technlgy is cmpsed by a three-phase inductin mtr. This mtr is fed thrugh a vltage surce inverter (VI) cnnected t the pwer supply thrugh a single-phase full-wave rectifier with a capacitive DC-link as shwn in Fig. 3. UPPLY M ON/OFF CONTROL f INVERTER COMPREOR T reference T ambient Figure 3. Inverter cntrl f an air cnditining cmpressr. M T reference T inner COMPREOR RE&PQJ, Vl. 1, N.5, March 2007

3 Fig. 4 shws the labratry arrangement used fr the tests f these electrical lads. In this pht, bth the internal and external units f the air cnditining appliances, the auttransfrmer used t feed up these lads, and the measurement equipment based n a pwer quality analyzer can be seen. The auttransfrmer data are shwn in Table 2. Intensidad (p.u.) 1,30 T28 T24 T26 T22 Figure 5. Cnventinal unit: Demanded current as a functin f the supplied vltage [ ] p.u. and the reference temperature. Ptencia Activa (p.u.) Figure 4. Labratry arrangement. Table 2. Auttransfrmer electrical data. Rated pwer (kva) 1.5 Frequency (Hz) 50 Input vltage (V) 250 Output vltage (V) IP Cde IP Experimental results This sectin presents the results f the prpsed experimental validatin f the theretical results. Tensin (p.u.) Figure 6. Cnventinal unit: Active pwer as a functin f the supplied vltage [ ] p.u. and the reference temperature. Factr de Ptencia x ,00 99,80 99,60 99,40 99,20 Tensin (p.u.) Figure 7. Cnventinal unit: Pwer factr as a functin f the supplied vltage [ ] p.u. and the reference temperature. A. Cnventinal Air Cnditin Appliances Figures 5 t 7 shw the dependence f the demanded current, active pwer and pwer factr with respect t the vltage supply. Nte that, as stated in a previus sectin, the experiments have been perfrmed with different reference temperatures, being n cntrl f the inner r uter temperature. The reference temperatures shuld be within the interval frm 21ºC t 24ºC. Hwever, a wider temperature interval ranging frm 22ºC t 28ºC has been studied in rder t better asses the equipment behaviur. The fllwing cnclusins can be derived analyzing the previus figures: The current demanded by the cnventinal unit in case f vltage reductin is higher than the rated current. Hwever, the active and reactive pwer cnsumptins are reduced cmpared t their respective rated values. The pwer factr f the unit is almst cnstant fr the vltage interval used in these experiments. In this sense, the capacitr design is quite accurate. It is interesting t nte that these experimental results are perfectly crrelated t the theretical nes presented in the cmpanin paper [1]. The previus results crrespnd t vltage variatins within ±10% f the rated vltage. In fact, this is the permitted limit f the vltage variatins in public distributin netwrks impsed by the Eurpean Nrmative EN50160 [2]. Hwever, t asses the unit behaviur in case f lwer vltages and their cntributin t the vltage cllapse phenmenn [3,4] shuld be interesting t further reduce the lwer vltage limit. In this sense, figures 8 t 10 present the results in case f a vltage range varying frm 0.75 p.u t 1.1 p.u. f the rated vltage RE&PQJ, Vl. 1, N.5, March 2007

4 Intensidad (p.u.) Ptencia Activa (p.u.) 1,60 1,55 1,50 1,45 1,40 1,35 1,30 T28 T24 T26 T22 0,75 0,78 0,80 0,83 0,85 0,88 Figure 8. Cnventinal unit: Demanded current as a functin f the supplied vltage [ ] p.u. and the reference temperature. 0,75 0,78 0,80 0,83 0,85 0,88 Tensin (p.u.) Figure 9. Cnventinal unit: Active pwer as a functin f the supplied vltage [ ] p.u. and the reference temperature. derived. This expressin is critical t btain an adequate lad mdel representatin that can be used in planning r perating tasks. The apparent pwer can be frmulated as [5]: 2 = Y U + I U + (1) Thus, the lad is represented by a cmbinatin f simple lads f cnstant admittance-impedance (Y -Z ), cnstant surce current (I ) and cnstant apparent pwer ( ). In the same way, it is pssible t btain a expressin f the demanded current as: I = = Y U + I + (2) U U Table 3 shws the value f these parameters that better fits the btained experimental data. Table 3.Cnventinal unit: btained parameters frm experimental data. Parameters Temperature 28 ºC 26ºC 24ºC 22ºC Y Current I Factr de Ptencia x ,00 99,80 99,60 99,40 99,20 99,00 0,75 0,78 0,80 0,83 0,85 0,88 Tensin (p.u.) Figure 10. Cnventinal unit: Pwer factr as a functin f the supplied vltage [ ] p.u. and the reference temperature. In case f analyzing this wider range f vltage variatins, the fllwing cmments can be pinted ut: The demanded current presents the same behaviur that in the previusly analyzed vltage variatins range. Hwever, Fig. 8 reveals that in case f lw vltages, the demanded current increases cnsiderably. The active pwer demanded by the cnventinal units in case f lw vltages, i.e. belw 0.9 p.u., des nt fllw the behaviur previusly described. Fig. 9 shws that the active pwer increases in these cases. Assuming that the pwer factr is almst cnstant, as seen in Fig. 10, the active pwer equals the apparent pwer. Thus, the increase f active pwer is prduced because the vltage decrease is cmpensated by an increase in the demanded current. Using the experimental data btained in the labratry, the cmputatin f the mathematical expressins crrelating current versus vltage can be B. Inverter Based Air Cnditin Appliances Figures 11 t 13 shw the dependence f the demanded current, active pwer and pwer factr with respect t the supply vltage. These curves have been represented versus the supplied vltage cnsidering the reference temperature as a parameter. Intensidad (p.u.) Ptencia Activa (p.u) T28 T24 T26 T22 1,05 1,00 0,95 0,90 0,85 0,80 Figure 11. Inverter based unit: Demanded current as a functin f the supplied vltage [ ] p.u. and the reference temperature. 1,05 1,00 0,95 0,90 0,85 0,80 0,75 Figure 12. Inverter based unit: Active pwer as a functin f the supplied vltage [ ] p.u. and the reference temperature RE&PQJ, Vl. 1, N.5, March 2007

5 Factr de Ptencia x ,00 99,80 99,60 99,40 99,20 Figure 13. Inverter based unit: Pwer factr as a functin f the supplied vltage [ ] p.u. and the reference temperature. The fllwing cmments can be pinted ut frm the btained experimental data: The demanded current and active pwer des nt fllw a clear behaviur with respect t the vltage variatins. In this sense, a lt f pints ut f the trend curve determined by the majr part f the experimental data are advised. This fact can be mtivated by the actin f the cntrl system that cntinuusly regulates the frequency f the mtr driving the cmpressr unit. The pwer factr f this type f units is again almst ne. Hwever, in this case a small amunt f reactive pwer is injected t the pwer system. This fact is mtivated by the capacitr installed in the DC-link between the unit mains and the inverter. Table 4 shws the parameters used in the crrelatin between the demanded current and the vltage. Nte that the parameters are in this case quite scattered with respect t the temperature due t the behaviur f the curves shwn in Fig. 11. Table 4. Inverter based unit: btained parameters frm experimental data. Parameters Temperature 28 ºC 26ºC 24ºC 22ºC Y Current I Cnclusins The main cnclusins derived frm this experimental apprach fcused n the effects f the vltage variatins n the peratin f hme air cnditining appliances are summarized as fllws: Cnventinal appliance The air cnditining lad can be successfully mdelled as a cmbinatin f three simple cnstant lads (Z, I and ). A mderate vltage reductin ( U 10%) increases the current lad and reduces the Inverter appliance pwer demand. This behaviur has been theretically fully explained and validated in an experimental study. Higher vltage reductins increase the reactive pwer demand as the capacitr drastically reduces the reactive pwer cmpensatin. Frm an energy efficiency pint f view, this type f equipment is mre suitable than the cnventinal ne, because the cmpressr always wrk in the ptimal perating pint required by the thermdynamic prcess. Nevertheless, this type f devices is a nn-linear lad, generating harmnic currents leading t a number f well-knw prblems. A greater dispersin in the experimental results exists, justified by the actuatin f the cntrl system f the equipment that cntinuusly varies f the frequency f the mtr driving the cmpressr. An increase f the demanded current exists when vltage variatins ccurs as in the case f cnventinal units. This type f lads can be mdelled as cnstant pwer lads in case f vltage variatins lwer than 10% f the rated vltage. In spite f this fact has nt been clearly appreciated in the develped experimental study, the theretical analysis cnfirms this assumptin. In case f deeper vltage variatins, this equipment usually is discnnected autmatically by the undervltage relay assciated t the DClink. In this sense, the inverter based air cnditining units are nt as critical as the cnventinal nes in case f majr vltage variatins. It is imprtant t nte that the experiments have been perfrmed withut any measurement r cntrl f the inner and uter temperatures. In this sense, a line f future research will be t include in the mdelizatin f the air cnditining lads the influence f these tw new variables. Acknwledgement The authrs wish t acknwledge Endesa-evillana fr the financial supprt f this wrk. References [1] J.M. Maza, M. Burgs, J.M. Rmer and M. Pinilla, "upply Vltage Effects n the Operatin f Residential Air Cnditining Appliances: Theretical Analysis", RE&PQJ, Vl. 1, N.5, March 2007

6 Internatinal Cnference n Renewable Energy and Pwer Quality - ICREPQ-07, evilla, [2] EN Vltage characteristics f electricity supplied by public distributin systems. [3] A. Kurita, T. akura, "The Pwer ystem Failure n July 23, 1987 in Tky", Prceedings f the 27 th Cnference n Decisin and Cntrl, Austin, Texas, December 1988, pp [4] K. Tmiyama, J.P. Daniel,. Ihara, "Mdelling Air Cnditined Lad fr Pwer ystem tudies", IEEE Trans. n Pwer ystems, Vl. 13, N. 2, May 1998, pp [5] L.M. Hajags, B. Danai, "Labratry Measurements and Mdels f Mdern Lads and Their Effect n Vltage tability tudies", IEEE Trans. n Pwer ystems, Vl. 13, N. 2, May 1998, pp RE&PQJ, Vl. 1, N.5, March 2007