Generation of electric energy by renewable energy sources is a challenge that has to be carefully envisaged since presents both a potentially profitable enterprise and a source of problems to the complex operation of large electric power systems.
The article presents some aspects of grid-connected PV systems, especially the influence of PV system operation on voltage.
We well know that after introducing generation into the LVDN, the conditions in the network radically change. LVDN has been conceived, designed, constructed, and developed as radial networks with clearly defined active power flows – from higher voltage networks to lower voltage networks”. The injection of active power with the PV system into LVDN changes the habitual power flow direction so that the active power flows from the low voltage level towards the higher voltage level, i.e. towards secondary distribution substations (MV/LV).
Alteration of power flows in LVDN the usual voltage profile change, and, in most cases, unloading of LV circuits occurs, since PV systems maximum generation normally coincides with periods of LVDN high loading.
The LVDN is composed of a distribution transformer substation 10(20)/0,4 kV with one distribution transformer of rated power Sn=630 kVA and 8 low voltage circuits. Low voltage circuits are of underground cable type. The main circuits are realized by 1kV XP00-A 4x185 type cables, while the branches are realized with 1kV XP00-A 4x95. Figures 10 - 12 demonstrate voltage profiles for different operating conditions on the LVDN – maximum loading, 50% loading, and no-load conditions for different levels of PV generation. Fifteen different scenarios have been simulated (PL- LVDN loading, PPV-PV generation level):
S-1 – PL=100%, PPV =0% , S-2 – PL =50%, PPV =0%
S-3 – PL =0%, PPV =0% , S-4 – PL =100%, PPV =100%
S-5 – PL =50%, PPV =100% , S-6 – PL =0%, PPV =100%
S-7 – PL =100%, PPV =75%, S-8 – PL =50%, PPV =75%
S-9 – PL =0%, PPV =75% , S-10 – PL =100%, PPV =50%
S-11 – PL =50%, PPV =50% , S-12 – PL =0%, PPV =50%
S-13 – PL =100%, PPV =25%, S-14 – PL =50%, PPV =25%
By inspection of voltage profiles given in Figures 10 – 12 it can be noted that the profiles lack resemblance to typical voltage profiles inherent to radial distribution networks. This is so due to LVDN operation in closed loop form.
A voltage increase at nodes electrically close to the PV generator’s connection point is evident (SRO3, SRO4, SRO5, and SRO6).
The influence of PV generator connection point has also been analyzed and the results are presented in Figures 13 - 14. Namely, by performing power flow calculations for the LVDN operated both in closed loop and open loop form, voltage profiles for all network nodes were obtained and compared to the values obtained for LVDN operation without PV generation. Figure 13 demonstrates the overall relative voltage increase for all network nodes due to PV system current injection into LVDN operated in closed loop form, while Figure 14 demonstrates the same values quantities for LVDN operated in open loop form.
By inspection of Figures 13 and 14, a maximum voltage profile increase can be perceived at node denoted by SRO4, which suggests that by connecting the PV generator at node SRO4 a maximum voltage increase at all nodes of the network can be expected. Higher voltage increases in the LVDN, operated in open loop form, can also be noted.
Based on the information above we clearly understand the importance of voltage regulation in connections between transmission lines and solar panels.
That's why we offer technology that combines seemingly contradictory conditions - adapting line voltage for integration with solar panels, optimizing electricity consumption, and reducing losses in transmission lines.
REFERENCES
[1] Majdandžić Lj.: Solarni sustavi, Graphis, Zagreb, 2010
[2] Uredba o naknadama za poticanje proizvodnje električne energije iz obnovljivih izvora energije i kogeneracije, NN 33/07, 133/07, 155/08, 155/09
[3] The solar design company (PV*SOL) [online]
Available: http://www.solardesign.co.uk/pv.php [4] Photovoltaic Geographical Information System
Journal of Materials Processing Technology Vol. 201 (2008) No:1-3, p. 786-790
[8] Pregelj, A.; Begović, M.; Rohatgi, A.; Impact of inverter configuration on PV system reliability and energy production, Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, USA, 2002, p. 1388-1391
[9] Naiem, A. F., Hegazy Y., Abdelaziz A. Y., Elsharkawy M. A.: A classification technique for protection coordination assessment of distribution systems generation, Proceedings with of distributed 14th the international Middle East Power Systems Conference, Cairo, Egypt, 2010, p. 404-409
[10] Mandić, N.; Utjecaj VE Vrataruša na sigurnost i vođenje elektroenergetskog sustava, Časopis EGE, (2011), No. 1, p. 102-106
[11] Nimpitiwan, N., Heydt, G. T., Ayyanar, R., Suryanarayanan, S.: Fault current contribution from synchronous machine and inverter-based distributed generators, IEEE Transactions on power delivery, Vol. 22 (2007), No. 1, pp. 634- 641
[12] Nimpitiwan, N., Heydt, G. T., Blevins, J., Cummings, A. B.: Potential economic impact of fault currents contributed by distributed generation, Power Engineering Society General Meeting, Vol. 1 (2005), No. 1, pp. 678-683
[13] Phuttapatimok, S., Sangswang, A., Seapan, M., Chenvidhya, D., Kirtikara, K., Evaluation of fault contribution in the presence of PV grid-connected systems, Proceedings of the 33rd IEEE Photovoltaic Specialists Conference, San Diego, CA, USA, 2008, p. 1-5
[14] Power factory – Users guide, pp.: Digsilent
http://re.jrc.ec.europa.eu/pvgis/
[5] Čavlović M.: Tehnički uvjeti priključka jedinica povlašteni distribuirane proizvodnje - proizvođači, 1. Savjetovanje Hrvatskog ogranka međunarodne elektrodistribucijske konferencije, Šibenik, 2008, pp. SO4-02:1-10 [6] Labudović B.: Osnove primjene fotonaponskih
Dubravko Franković Marijana Živić Đurović Saša Saldić
Faculty of Engineering, University of Rijeka
