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Effects of Distributed Generators from Renewable Energy on the Protection System in Distribution Networks Ahmad AbdelMajeed Robert Viereck Fred Oechsle Martin Braun Stefan Tenbohlen University of Stuttgart [email protected] DIgSILENT GmbH EnBW Regional AG [email protected] om University of Stuttgart University of Stuttgart [email protected] [email protected] [email protected] Abstract-- The increasing number of distributed generators especially from renewable energy requires investigating the impact of their integration on the protection system. This paper evaluates the effects of distributed generators on protection relays functionalities (main and backup protection) in the medium voltage grids. In order to obtain profound results, simulations were done on several realistic scenarios for existing medium voltage grid. As all short circuits were simulated in the time domain by using the software Power Factory from DIgSILENT GmbH, dynamic models for distributed power generation had to be implemented. The results were analyzed graphically and tested for negative influences on the protection functions. In addition, simulation was also done to study the effects of the realistic requirements for the increasing number of distributed power generation in the future. The results from these simulations are presented and discussed. Index Terms— Power distribution protection, distributed generators, simulation. I. INTRODUCTION In Germany, the number of small scale distributed generation (DG) units especially from renewable energy has increased sharply in the recent years and will grow up significantly in the near future especially the share of wind and solar energy in the total amount of energy generation through a targeted support from the federal government. Due to the low production capacities from DG units from renewable energy, they are normally connected to the medium or low voltage network, this results in problems of power quality and compliance with the voltage range. Currently, most of these DG units are not contributing to the short circuit current in case of fault, but they are disconnected from the network in case of high voltage drops, but the new directive from the Federal Association of German Energy (BDEW) states that the future installed systems must also contribute to the fault current in case of faults. So the target of this paper is to investigate this case and to study the side effect on the protection relays functionalities through simulations on a real existing medium voltage grid. Different worst case scenarios were defined. Dynamic models for the small scale DG units were used. The effects of integrating the DG units to the network were analysed mathematically in theory [1, 2, 3, 4] for a simple assumed small networks, these DG units could change the level and the direction of the short circuit current. This causes an increment for the short circuit currents, the direction of the short circuit currents varies depending on the location and penetration of the DG units. However this paper discusses in details the effects of integrating the DG units in a real existing medium voltage grid taking into account the current and the expected future expansion of integration of DG units from renewable energy. In this paper a real existing medium voltage network called “Freiamt“ is introduced with it’s protection relays, then the simulation setup is explained, afterwards simulation results are introduced based on three different scenarios; the first scenario shows the simulation results without integrating the current expansion of DG units, this scenario forms the basic for a later comparison, the second scenario shows the simulation results with integrating the current expansion of DG units, the third scenario shows the simulation results for double expansion of DG units. Later on a comparison figures show the effect of the DG units on the network protection. II. MEDIUM VOLTAGE GRID MODEL AND PROTECTION SYSTEM The simulations shown in this paper are performed in the time domain by using the software Power Factory from DIgSILENT GmbH [5] on a real existing medium voltage network called “Freiamt“. This network is one of the experimental networks of EnBW regional AG, it covers an area around 52 km² which has 4000 inhabitants, the expansion of renewable power generation and DG units exceeds the load in this network approximately by 1,5 GWh every year, this extra power is usually fed back into the 110 kV network level. More details about power production and consumption are shown in Table 1. Table. 1. Power production and consumption under the current expansion of renewable energy and users. Type Users Photovoltaic Wind Turbines Power 4,7 MW 2,5 MW 1,8 MW This network is monitored and protected by 5 relays; one DMT (Definite Minimum Time) directional over current relay monitors the two “Sexau” main feeders as shown in Fig. 1. The backup protection for these two feeders forms a distance protection for “Denzlingen”, another distance protection relay monitors the “Sexau” substation on the other direction. The reserve power supply of the substation “Sexau” is also monitored with a distance protection relay but this relay was not considered in this paper. The protection coordination settings for the relays are shown in Table. 2. for the medium voltage level. Thus, the conventional power plants will have enough time to start. This so-called "faultride-through" ability is the base of all models in this paper. A generic model of a Doubly Fed Induction Generator (DFIG) is used to model wind turbines. The model was designed by DIgSILENT GmbH and used widely. A single induction generator is controlled by a model which simulates the mechanical and electrical properties, as well as the controllers and protection equipment. The PV inverter was modeled as self-guided, pulse-width modulated inverter. This allows active and reactive power to be controlled and thus makes it possible to run through voltage dips without disconnection from the grid. III. SIMULATION SETUP Fig. 1. “Freiamt” medium voltage network diagram Table. 2. Relays Coordination Settings Relay Position Denzlingen in Direction Sexau (1) Sexau in Direction Helgenreuthe (2) Sexau in Direction Rathaus (3) Sexau in Direction Denzlingen (4) Activation Current 300 A 1st level 0,1 s 2nd level 0,6 s 3rd level 1,0 s End time 1,7 s 440 A 0,3 s - - - 440 A 0,3 s - - - 300 A 0,1 s - - 0,5 s Originally, DG Units were not meant to contribute to grid faults. For this reason their protection equipment will disconnect them in case of voltage drop in short circuit situations (i.e. grid faults). Due to the enormous growth in renewable energy, especially wind energy, a fault in the transmission grid during periods of weak conventional generation would lead to a high unbalance of load demand and generation, which in turn could lead to wide spread outage. To prevent the network from such cascaded results and failures, the German Association of Energy Economics (BDEW) has released a grid code [6,7] allowing distributed generators connected to the high voltage grid (> 110 kV) to be separated from the network in the earliest 150 ms after fault inception . A similar grid code is now coming into effect To view the contribution of DG units to short circuit currents in the medium voltage gird in case of fault, simulations in the time domain were done. The fault applied to the network was a 3-phase fault in all cases. The main goal of the simulations is to see the effect of distributed generation on the protection system. Hence the fault location had to be varied in order to see different fault scenarios. This meant a large number of simulations which would take a lot of time to conduct and to analyze the results. To automate this tedious task, the built-in scripting language of Power Factory DPL (DIgSILENT Programming Language) was used to run a series of simulations automatically and provide the combined results for analysis. The basic structure of the script can be seen in the flow chart in Fig. 2. In this context, a path is a set of lines and nodes between two points in the grid. In this case the grid was separated into three paths to reduce the calculation time. The fault location was calculated based on predefined step size. The resulting set of files was merged (by superposition method) and plotted to obtain graphical information for the analysis. Fig.2. Flow chart illustrating the basic structure of the script IV. SIMULATION RESULTS Simulation results were introduced based on three different scenarios; the first scenario shows the simulation results without integrating the current expansion of DG units, this scenario forms the basis for a later comparison, the second scenario shows the simulation results with integrating the current expansion of DG units, the third scenario shows the simulation results for double expansion of DG units. A. Scenario “1” To obtain results for the impact of DG units production on the protection coordination, several simulations were carried out without feeding from DG units. These figures form the basis for later comparison. The following Figures show the behavior of the phase current IL1 (phase A) representing the symmetrical shortcircuit current I”K in case of three-phase short circuit. The overcurrent relay located in “Sexau” was deactivated during these simulations in order to observe the levels of backup protection. Fig. 3. shows the short circuit current flow as would be measured from the backup protection relay in “Denzlingen” for the complete feeder in the direction of “Helgenreuthe”. Fig.4. and Fig.5. show the current flow, as it would be seen from the DMT relay in “Sexau” in the direction of “Helgenreuthe” and “Rathaus” feeders respectively. As expected, the short circuit current decreases with increasing distance from the short circuit position but never goes to a value below 1 kA. Fig. 3. clearly shows the different stages and levels of the distance protection which acts as backup protection. The simulation were also compared with IEC 60909 standards, the simulations and the calculations have a very low deviations from each other. Fig. 3. I”K in the path between “Denzlingen” and “Sexau” as a function of time and distance (without DG)- Relay :Denzlingen Fig. 4. I”K in the path between “Sexau” and “Helgenreuthe” as a function of time and distance (without DG)- Relay :Helgenreuthe Fig. 5. I”K in the path between “Sexau” and “Rathaus” as a function of time and distance (Without DG)- Relay: Rathaus B. Scenario “2” As a first comparison to the case without integration of DG units, all DG models which are used in the simulations are assumed to operate in their nominal operational power, this would be a cold, sunny winter day in accordance with a strong wind, this is only a hypothesis as a "worst-case scenario". Furthermore, all distributed generation units were treated as if they would meet the new guidelines for generating units in the medium voltage network. The protection relays have also been disabled for the following charts to visualize the influence of the network over the time. As seen from the following Figures (6-8), the lowest short circuit current is also within the range around 1 kA; and the activation condition for the over current relay is met. The short circuit current is no longer constant at a fixed value but it has small oscillations, this leads to a non-ideal sharp boundaries in the backup protection triggering, as the measured fault impedance is swinging in the same rate. Fig. 6. I”K in the path between “Denzlingen” and “Sexau” as a function of time and distance (with DG)- Relay :Denzlingen Fig. 7. I”K in the path between “Sexau” and “Helgenreuthe” as a function of time and distance (with DG)- Relay :Helgenreuthe Fig. 8. I”K in the path between “Sexau” and “Rathaus” as a function of time and distance (With DG)- Relay :Rathaus For the over current protection there is no negative influence because the short circuit current sometimes is slightly larger than the case without integrating of DG units. This is caused by the interaction of different circumstances; for short circuits near “Sexau” substation the voltage at “Sexau” substation is almost zero, the same as in the case without integration of DG units. Because of that the same short circuit current is delivered by the feeder. Added to this a small contribution from the healthy path (without faults) can be observed. This behavior is similar to the short circuit behavior of a transmission line with tap. Additionally the power consumption of loads in the healthy path is mainly provided by the distributed generation in this path. Since the load is reduced by the short circuit, due to the voltage drop, part of the current generated in the healthy path will also flow towards the short circuit. The effect is only observed at short circuits near the central substation “Sexau” and if the generation in the healthy feeder outweighs the generation in the affected feeder. By the current integration of DG units from renewable energy, the difference can be only considered in case of a fault in the “Rathaus” feeder because the short circuit current contribution in case of fault in “Helgenreuthe” feeder is small and can be neglected. Different behavior of the back-up protection for short circuits between the two paths can be explained by the imbalance of the distributed generation units. The path “Helgereuthe” contains about 70% of the installed capacity of distributed generators and can therefore feed in much more current in case of short circuit in the “Rathaus” path than the other way around. This implies that distributed generators play only a minor role in reducing the short circuit current in the faulty path. This is particularly the case of three-phase short circuits as they occur here. The short circuit separates the corresponding feeder into two separate zones and all the DG units behind the short circuit (as seen from the relay in “Sexau”) cannot participate in the short-circuit current. Even if all currently installed distributed generation units would meet the new standards for generating facilities, only a slightly different behavior for the protection system would be observed. However, this would not lead to a failure of protection. Another point for this would be supporting the producer's own protection systems; in case of a sharp voltage drop when three-phase faults occur, the producers would be allowed to separate their units from the grid before the backup protection could occur as such in action. For all the faults which could be affected by the DG units are at least in the second zone of distance protection, with a release time of 0.6 s. This time is significantly higher than the allowed time for them to remain connected to the grid (0.3 s for V<0.45 p.u). This would be 0.6 s after all DG units are already disconnected [6, 7]. Fig. 9. and Fig. 10. show the comparison of the short circuit current for the backup protection with and without the integration of DG units. Comparing the short circuit current for both cases, then the aforementioned oscillations on the short circuit current can be seen clearly. The reason for the oscillations is the converter model of the PV–models. As a contribution to short-circuit current, these oscillations are negligible, as they swing within the range of ±50 A. Furthermore, for the backup protection the short circuit current is reduced but this reduction is so small in comparison to the short circuit current and can only show a minor delay in tripping, the delayed trips are shown as white areas. Fig. 9. I”K Difference in case of short circuit in “Helgenreuthe” pathRelay: Denzlingen Fig. 10. I”K Difference in case of short circuit in “Rathaus” pathRelay: Denzlingen A. Scenario “3” The simulations with the current expansion already show a slight influence on the protection as the DG units from renewable energy are increasing and expanding significantly. Simulations were also done to check the effect of double expansion of DG units on the protection system. The nominal power output of all PV- systems was doubled, as well as the output of the wind turbines, the resulting generation values can be found in Table 3. This configuration can be used to represent an estimated growth in a range of 5 – 10 years. A scaling factor of 1.1 was used for the loads to the take increasing load demand into account. Fig. 13. Scenario “2” Back up protection levels in the path “Denzlingen”“Sexau”-“Helgenruthe” Table. 3. Power production and consumption under expected expansion of renewable energy and users Type Users Photovoltaic Wind Turbines Power 5,1 MW 5,0 MW 3,6 MW Fig. 11. and Fig. 12. show the resulting back up protection levels in the paths of “Helgenreuthe” and “Rathaus” respectively. Fig. 14. Scenario “2” Back up protection levels in the path “Denzlingen”“Sexau”-“Rathaus” Fig. 11. I”K difference in case of short circuit in “Helgenreuthe” pathRelay: Denzlingen Fig. 15. Scenario “3” Back up protection levels in the path “Denzlingen”-“Sexau”-“Helgenruthe” Fig. 12. I”K difference in case of short circuit in “Rathaus” pathRelay: Denzlingen The simulation results show that the increased generation capacity increases the already described effects in the current expansion of renewables. Erratic behaviour of the protection is not observed. The impact on primary and backup protection will remain small so that they can be neglected. The following Figures (13-16) show similar behaviours of short circuit current, the changes of the current flow in the various paths, as well as the shift of the levels of backup protection as in the previous section. Fig. 16. Scenario “3” Back up protection levels in the path “Denzlingen”-“Sexau”-“Rathaus” V. CONCLUSION In this paper, simulations in the time domain on an existing MV network were performed (in the time domain) with dynamic models to investigate the side effect of integration DG units from renewable energy on the protection system. The simulation shows that the small scale distributed generation has no significant impact on the protection in the medium voltage network. While the expected reduction of short-circuit current due to the infeed by the small scale generation can be observed, the effects remain minimal. These simulations were assumed as “Worst case scenario” so that all the renewable energy sources were feeding with their nominal power at the same time. But the fluctuation of wind energy and the geographical position of Germany (relatively few sunny days) lead that the simulated scenarios can probably be found in reality. For symmetrical short circuit, there was no effect on the overcurrent protection, the operating ranges of the distance protection tend to under reach only slightly which can be ignored. Unbalanced faults, which are much more common in medium voltage grids, could not be simulated due to inappropriate models. The lower short-circuit current, and different fault detection by the protection relays could lead to malfunction in those cases. For a full assessment of the impact of small scale distributed generation on medium voltage protection, new models for the generation have to be developed. REFERENCES [1] F T Dai “Impacts of distributed generation on protection and autoreclosing of distribution networks” Developments in Power System Protection (DPSP 2010). 10th IET International Conference on Managing the Change. [2] Martinez, J.A. ; Martin-Arnedo, J., “ Impact of distributed generation on distribution protection and power quality” Power & Energy Society General Meeting, 2009. PES '09. IEEE . [3] Kwon, Seongchul ; Shin, Changhoon ; Jung, Wonwook , “Evaluation of protection coordination with distributed generation in distribution networks” Developments in Power System Protection (DPSP 2010), 10th IET International Conference on Managing the Change. [4] Kauhaniemi, K. ; Kumpulainen, L., “Impact of distributed generation on the protection of distribution networks” Eighth IEE International Conference on Developments in Power System Protection, 2004. [5] DIgSILENT Power Factory is a leading high-end power system analysis tool for applications in transmission, distribution, generation, industrial and railway systems, wind power and Smart Grids. [6] BDEW: Erzeugungsanlagen am Mittelspannungsnetz - Richtlinie für Anschluss und Parallelbetrieb von Erzeugungsanlagen am Mittelspannungsnetz ,June 2008. [7] BDEW:Regelungen und Übergangsfristen für bestimmte Anforderungen in Ergänzung zur technischen Richtlinie: Erzeugungsanlagen am Mittelspannungsnetz - Richtlinie für Anschluss und Parallelbetrieb von Erzeugungsanlagen am Mittelspannungsnetz, July, 2010.