Energetický partner

číslo 5, rok 2014, ročník II

Marián_UHER[1], Stanislav_MIŠÁK[2]

Analýza výkonových a napěťových poměrů v distribuční síti s obnovitelnými zdroji

Summary

This paper describes an analysis of power flow in a devoted part of electricity system, using a significant proportion of electrical energy from made by renewable sources. It describes using of computational software for the design and analysis of the resources working in the real power system, which gradually takes the philosophy of Smart Grids.

Keywords

Power flow analysis, Feedback effects, Smart Grid, Renewable resources, Energy storage unit

1 Introduction

With regard to recent trend involving in integrating spread non-traditional sources of electricity increasing demands for monitoring the behaviour of power systems [1]-[5]. The reason is the instability and stochastic supply of electricity such resources, whose operation in power system causes a change in the power and voltage conditions and power quality in general [6]-[8]. Existing electricity networks are greatly strained by this character of power delivery, because with such concepts do not count in its original philosophy of construction. They are designed for one-way power flow with an equal balance of electric power. Another type of electrical networks allowing integration renewable energy sources already in its concept is called the Smart Grid [9]-[12]. Transformation of the existing network on the Smart concept requires knowledge of the behaviour of existing networks.

Analysis of the imaginary electricity system equipped by elements of the smart technology, which was developed in the framework of the existing electricity system transformation to the first functional Smart Grid in the Czech Republic, will be described below, including climatic conditions, and the analysis of the characters of renewable resources [13].

2 Analyzed power system description

Analyzed power system is arranged imaginary grid, but equipped by real source corresponding Smart philosophy [9], [12], [13]. Also includes parts of energy consumption units that correspond to the actual values ​​and characters of the devoted distribution system actually transformed into the first functional Smart region in the Czech Republic [13].

As shown in the block diagram, the network can be divided into the part of the power resources and power consumption. It should be noted that the analyzed part of the distribution system is connected to the superior power system 35 kV.

Fig.1: Block diagram of analyzed power grid

Diagram also shows a series of control points (CP). These are the places where readings of following values were made:

· Effective value of the current

· Effective value of the voltage

· Active power

· Reactive power

· Power factor

2 Resources unit description

Units of power sources working in the analyzed networks have been selected to satisfy requirements of the philosophy of smart grids [9], [12], [13]. Stochastic power resources were chosen - wind turbine 2 MW (WPP) and photovoltaic power plant 1.1 MWp (PVP) and one source with a constant supply of electricity, i.e. cogeneration unit 1.5 MV·A.

The basis for the proposal mathematical models became database data from long-term measurements taken at specific existing sources installed directly in the emerging Smart Region [9]-[15].

These resources are in an environment of ATPDraw simulation program presented as current sources, working through their block transformers at 10kV voltage level. Daily power variability of individual sources has been simulated by the time dependence of the supplied current. The ratio of active and reactive current parts was ensured by changing the phase angle of the current source.

Fig.1: PVP power supply in case of typical a cloudy day

3 Power consumption part description

Part of electricity consumption is composed by four consumption points on the voltage level of 0.4 kV and a MV consumption point at voltage level of 35 kV, as shown in the block diagram. Daily load diagram was determined based on the requirement to simulate power consumption of residential areas of the city. The starting point became the definitions of typical daily load diagrams (hereafter DLD) decree of ERÚ č. 373 / 2001 Sb., which determines the assignment of daily diagrams class according to the character of daily power load. Thus, for character of LV power load was assigned DLD class 4 - consumption without heat utilization of electrical energy. Electricity Market Operator (OTE), then in its database offers relative values ​​of hourly power loads for each of the above class of DLD profiles for each day of the year, see Fig 3. Then daily load diagram was grossed up using known maximum values of power load of each supply points.

Fig.3: Power consumption – ratio values

4 Power flow analysis requirements

Power flow analysis in referred network was calculated for 4 operation variants that differed by simulated climatic conditions and the nature of operation of renewable sources:

· Variant 1 - system operation with a maximum contribution of energy produced from renewable sources. Cogeneration unit assumes the active component to total power consumption of all loads. PVP operated in a typically sunny day.

· Variant 2 - system operation with a maximum contribution of electricity generation from renewable sources. Cogeneration unit assumes the active component to total power consumption of all loads. PVP operated in a typical cloudy day

· Variant 3 – equal balance of power active component at the transfer point (CP1). Cogeneration unit "top up" the balance of active component in the transfer point to zero PVP operated in a typically sunny day

· Variant 4 - equal balance of power active component at the transfer point (CP1). Cogeneration unit "top up" the balance of active component in the transfer point to zero PVP operated in a typically cloudy day

In all control points variables described in paragraph 2 were monitored, for all types of network operations. From the values ​​obtained in this way were made an extensive database, enabling precise description of system behaviour in selected situations.

5 Voltage and power analysis results

Results of the analysis of power relations at the transfer point at control point CP1, are presented in curves of active and reactive power at this point (note: sign convention with respect of appliances - plus sign characterizes the power consumption in a given control point, minus sign indicates power supply). From the curve of active power is evident obvious effect of operation of photovoltaic power plants at ideal climatic conditions. Maximum power at this point corresponds to the maximum power output PV, i.e. always around 12 hour. Of all the curves is also possible to observe the influence of peak power of WPP at 3 AM.

Fig.4: Active power curve at CP1

Fig.5:Reactive power curve at CP1

The same effects can be observed on the voltage waveforms at the transfer point see Fig 6. Maximum and minimum voltage values ​​are then presented in the table, see Fig 8. It should be noted that voltage value at any point in time does not exceed the limit specified by standard ± 5% [16].

Fig.6: Voltage condition at CP1

Tab.1: Voltage limit values in transfer point

| Voltage limit values in transfer point U (kV)
Hour | Variant 1 | Variant 2 | Variant 3 | Variant 4
4 | - | 34.968 | - | 34.970
12 | - | 34.945 | - | 34.948
14 | 34.980 | - | 34.982 | -
20 | 34.945 | - | 34.948 | -

Power curve of photovoltaic power and wind turbine are defined in the preceding paragraph. The requirements of analysis also show the power setting of cogeneration units as shown in Fig 7.

Fig.7: Active power of cogeneration unit

Cogeneration unit was tasked to balance active power ratios at the transfer point to zero by its output power within a variant 3 and 4. As is evident from Fig 7, cogeneration unit did not deliver any power to the network during the midday hours, but PVP and WPP caused substantial overflow of electricity to the external power grid, and even if the fact of the greatest industrial power load just in the same time. See Fig 4.

7 Proposed measures suppressing feedback effects of renewable energy resources to the operated electricity grid

It is possible to suggest some measures against the negative effects from the knowledge of renewable power and voltage conditions in the network. In this case, we are talking about the power storage system. World standards in the development of storage devices practically know two types of storage systems:

· Power storage - this reduces the power rapid changes in non-traditional sources during rapid climate changes battery - short charge and discharge times, big nominal power, smaller capacity

· Energy storage - used to supply power during peak of daily load diagram, battery - longer charge and discharge times, less power, big capacity

In this case, the effort is going to design the power storage system. This is based on the knowledge of the difference between the electricity PVP supplies at different climatic conditions (extreme conditions). Specifically 5034 kWh.

Battery arrays are presented in user interface of ATPDraw by capacitors, which are working to the grid of 690V. They are connected to the 10 kV voltage level grid over pulse modular inverter and block transformer of 10/0.69 kV. Switching frequency was determined at 4 kHz.

Discharging algorithm will depend on the chosen operation philosophy of storage station. The first step is to adjust the PVP power output level to 50%, or about 500 kW, (the remaining 50% will be accumulated) with respect to the monitoring of overflows electricity to external system. This will provide the opportunity to supply the stored energy in the late afternoon, when there is a lack of energy production from renewable sources.

Fig. 8: Connecting of energy storage unit to the 10kV power grid in time of 0.2 s – current condition

Fig.9: Connecting of energy storage unit to the 10kV power grid in time of 0.2 s – voltage condition

There is presented a connection of energy storage unit to the 10kV power grid level in time of 0.2 s, see Fig 8, Fig 9. It is possible to follow a switching frequency of 4 kHz in case of voltage condition as well as interaction between battery unit and the grid in case of current curves. This is a current drop in the maximum of its curve.

8 Conclusion

The network analysis clearly shows that the operation of this network is impossible without any regulation of output power supply. The analysis also clearly shows the influence of operation of renewable resources to operate system. Thus made analysis allows quantification of feedback effects of renewable resources in variable weather conditions. It is therefore possible to develop measures for their suppression. There was also described one of the measures, namely energy storage station.

Next step in the development of analysis will be directed to complete the development of a mathematical model of energy storage system. There will be established a philosophy of charging and discharging cycles and will analyze the impact of storage station to voltage and power ratios in the operation power system.

Acknowledgment

This work was supported by project SP2013/68 - Development of Smart management system of energy self-sufficient house.

References

[1] Delfino, F.; Denegri, G.B.; Invernizzi, M.; Procopio, R., "An integrated active and reactive power control scheme for grid-connected photovoltaic production systems," Power Electronics Specialists Conference, 2008. PESC 2008. IEEE , vol., no., pp.1463,1468, 15-19 June 2008 doi: 10.1109/PESC.2008.4592142

[2] Nezamabadi, P.; Gharehpetian, G.B., "Electrical energy management of virtual power plants in distribution networks with renewable energy resources and energy storage systems," Electrical Power Distribution Networks (EPDC), 2011 16th Conference on , vol., no., pp.1,5, 19-20 April 2011

[3] Delgado, J.; Santos, B.; de Almeida, A.T.; Figueira, A., "Solutions to mitigate power quality disturbances resulting from integrating intermittent renewable energy in the grid of Porto Santo," Electrical Power Quality and Utilisation (EPQU), 2011 11th International Conference on , vol., no., pp.1,6, 17-19 Oct. 2011 doi: 10.1109/EPQU.2011.6128870

[4] Prokop, L., Mišák, S., Dvorský, J.: Prediction System for Energy Production from WPP. In Proceedings of the 11th International Scientific Conference Electric Power Engineering 2010, Brno:VUT Brno, 2010, 978-80-214-4094-4

[5] Mišák, S., Prokop, L.: Wind Power Plant Production Prediction Using Various Models. In sborník konference EPE 2009, Ostrava:VŠB-TU Ostrava, 2009, 65-72, ISBN 978-80-248-1947-1

[6] Procházka, O., Gurecký, J., Mišák, S.: Feedback effects definition of alternative electric energy sources connected to the distribution power network. In sborníku vědecké konference EPE2010. Ed. Jiří Drápela, Jan Macháček, Brno:University of Technology, Faculty of Electrical Engineering and Communication, Department of Electrikal Power Engineering, 2010, 861, ISBN 978-80-214-4094-4

[7] Palizban, O.; Rezaei, M. A.; Mekhilef, S., "Active and reactive power control for a hybrid system with photovoltaic panel, wind turbine, fuel cells, electrolyzer and super capacitor in off-grid mode," Control System, Computing and Engineering (ICCSCE), 2011 IEEE International Conference on , vol., no., pp.404,408, 25-27 Nov. 2011
doi: 10.1109/ICCSCE.2011.6190560

[8] Prokop, L., Mišák, S.: Analysis of Photovoltaic Power Plant running. In sborníku EPQU 2011

[9] Karnouskos, S., "Future smart grid prosumer services," Innovative Smart Grid Technologies (ISGT Europe), 2011 2nd IEEE PES International Conference and Exhibition on , vol., no., pp.1,2, 5-7 Dec. 2011
doi: 10.1109/ISGTEurope.2011.6162832

[10] Prokop, L., Mišák, S.: Off-Grid Power Systems. In 2010 9th International Conference on Environment and Electrical Engineering. Ed. P.Janik,D.Lehmann, Z.Leonowicz, H.Schwarz, Wroclaw:Wroclaw University of Technology, 2010, 978-1-4244-5374-0

[11] Prokop, L., Mišák, S., Kačor, P.: Results from Hybrid Off-Grid Power System Operation Analysis. In sborníku konference EEEIC 2011. Ed. Zbigniew Leonowicz, Wroclaw:, 2011, vol. 10., 1-3, ISBN 978-1-4244-8781-3

[12] Ipakchi, A.; Albuyeh, F., "Grid of the future," Power and Energy Magazine, IEEE , vol.7, no.2, pp.52,62, March-April 2009
doi: 10.1109/MPE.2008.931384

[13] Futuremotion, [on-line], URL:<http://www.futuremotion.cz/smartgrids/cs/index.html> [quot. 2011-05-12

[14] Mišák, S., Prokop, L.: Problem of Wind Power Plant Engineering in the Czech Republic. In Proceedings of the 2nd International Conference on Power Engineering, Energy and Electrical Drives, 2009, 100-115, Instituto Politécnico De Setúbal, Universidade de Lisboa

[15] Krejčí, P., Prokop, L., Mišák, S.: Analysis of Long time WPP PowerFlows Measurement. In sborník konference EPE 2009, 2009, 6, ISBN 978-80-248-1947-1

[16] ČSN EN 50160, Voltage characteristics of electricity supplied by public distribution networks.

[1] Ing. Marián Uher, Katedra elektroenergetiky, Fakulta elektrotechniky a informatiky, VŠB-Technická univerzita Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, tel.: (+420) 597 323 449, e-mail: marian.uher@vsb.cz.

[2] Doc. Ing. Stanislav Mišák, Ph.D, Katedra elektroenergetiky, Fakulta elektrotechniky a informatiky, VŠB-Technická univerzita Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, tel.: (+420) 597 329 308, e-mail: stanislav.misak@vsb.cz.