Dynamic Grid Support in Low Voltage Grids Fault Ride-Through and Reactive Power/Voltage Support during Grid Disturbances

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1 Dynamic Grid Support in Low Voltage Grids Fault Ride-Through and Reactive Power/Voltage Support during Grid Disturbances Gustav Lammert University of Kassel Department of Energy Management and Power System Operation Kassel, Germany Tobias Heß TU Dresden Institute of Electrical Power Systems and High Voltage Engineering Dresden, Germany Maximilian Schmidt TU Dresden Institute of Electrical Power Systems and High Voltage Engineering Dresden, Germany Peter Schegner TU Dresden Institute of Electrical Power Systems and High Voltage Engineering Dresden, Germany Martin Braun University of Kassel Department of Energy Management and Power System Operation and Fraunhofer IWES Kassel Department of Distribution System Operation Kassel, Germany Abstract Faults in medium and high voltage grids lead to voltage sags in the low voltage level. Due to the voltage dip, distributed generators disconnect from the low voltage grid. Depending on the amount of disconnected generation, system stability could be compromised. With a dynamic grid support, distributed generators remain connected to the grid during faults, also called fault ride-through. Moreover, the feed in of reactive power supports the voltage during the fault. According to the present state of the art, no requirements exist for a dynamic grid support in low voltage grids. However, with a high penetration of distributed generation these requirements might change in the future. This paper investigates the dynamic grid support in low voltage grids. The impact of a dynamic grid support on a distribution grid is studied with IEEE/CIGRE benchmark models for low and medium voltage grids. Models of inverters, directly-coupled synchronous and induction generators are implemented. Furthermore, a present and a future protection system with fault ridethrough capability are added to the grid models. The inverters are equipped with a reactive current controller to give a voltage support during faults. To determine the effect of a dynamic grid support, faults in high and medium voltage grids with and without dynamic grid support are simulated and evaluated. The results have shown that the effect of the voltage boost through dynamic grid support in order to recover the voltage is marginal. However, with a high penetration level of distributed generators the voltage boost increases and the impact of faults can be further limited. This investigation has demonstrated the potential of dynamic grid support in low voltage grids to avoid the loss of a large amount of power and to improve the voltage recovery. Keywords Dynamic Grid Support, Fault Ride-Through, Low Voltage Ride-Through, Reactive Power/Voltage Support, Low Voltage Grid I. INTRODUCTION THE electrical power system is undergoing fundamental changes. Nowadays more and more generators are connected to the distribution grid. The increasing penetration level of distributed generators (DGs) in high, medium and low voltage grids has led to a policy change of the system operators. In order to ensure the security of supply, several grid operators have adapted their guidelines. Hence, DGs have to provide ancillary services just as conventional power plants. One of these ancillary services is dynamic grid support. Faults in high and medium voltage grids can lead to voltage sags at the point of common coupling (PCC) of the DGs in the low voltage level. According to the grid codes (GCs) for the medium and high voltage level, such as [] [] for Germany, different requirements for a dynamic grid support already exist. Considering the present state of the art, no requirements exist for the low voltage level. With an increasing penetration of DGs in low voltage grids, these requirements might change in the future to ensure the stability in the power system during grid faults. Based on this, a number of research questions arise:.) What is the amount of disconnected generation during grid faults in the high and medium voltage level?.) Is it possible to reduce the voltage dip in the grid caused by grid faults, by using a dynamic grid support in low voltage grids?.) Is a dynamic grid support required in low voltage grids of a future power system? Answering these questions, the paper describes a general investigation and provides new contributions in this research area. Further detailed investigations concerning this topic can be found e.g. in Bömer et al. [5] or Weise []. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,

2 In order to study the questions mentioned before, a short outline of the present practice of dynamic grid support requirements is made in the first part of the paper. The second part describes the model of the distribution grid, including the layout of the test grid, the models of DGs and loads and the protection system. In the third and last part of this paper the simulation results are presented and the effect of a dynamic grid support in low voltage grids is discussed. II. DEFINITION OF DYNAMIC GRID SUPPORT At all points the GCs for medium and high voltage grids define the dynamic grid support with the following requirements. The first requirement for DGs is to remain connected to the grid during voltage dips, also called fault ride-through (FRT). The second requirement for DGs is to inject reactive power to support the voltage during fault conditions. A. Fault Ride-Through DGs have to stay connected to the grid during grid disturbances depending on the duration and depth of the voltage sag. This requirement is called FRT, also known as low voltage ride-through or under voltage ride-through. The FRT characteristic corresponding to the German GCs [] and [] is depicted in Fig.. The two borderlines are representative for three phase faults and type DGs connected to the PCC. Type DGs are all DGs excluding directly-coupled synchronous generators. According to the GC for the medium voltage level [], type DGs have to remain connected to the grid for 5 ms in case of a voltage dip to pu. The draft for the high voltage level [] requires a connection of DGs during voltage dips to pu for a duration of ms. In general DGs have to remain connected to the power system if the voltage at the PCC drops on a value above the borderlines. If the voltage at the PCC drops on a value below the borderlines, tripping is allowed. The requirements in the TransmissionCode [] and the SDLWindV [] are equal and in comparison to the GC for the medium voltage level [] marginal different. Line-To-Line Voltage, U in pu B. Reactive Power/Voltage Support The reactive power/voltage support is required to minimize the voltage drop in the grid during the fault and to ensure a fast voltage recovery after the fault. This improves the stability of the power system. Depending on the voltage at the PCC the DG has to feed in a certain reactive current. The consumer reference arrow system is utilized in this investigation. The following control strategies for the DGs can be derived from this. In case of overvoltage the DG has to consume reactive power in order to decrease the voltage at the PCC. This is similar to an underexcited operation of a synchronous generator. In case of undervoltage the DG has to produce reactive power in order to increase the voltage at the PCC. This is similar to an overexcited operation of a synchronous generator. In Fig. the reactive power/voltage support characteristic is depicted. The various curves indicate different requirements depending on the GC. Required Reactive Current Deviation, ΔIQ in pu Fig.. Reactive Power/Voltage Support Characteristic. One parameter that varies is the droop k. Different ranges from k are possible. The droop determines the quantity of the required reactive current deviation depending on the voltage variation at the PCC. The second parameter that varies is the amount of reactive current deviation. The draft for the high voltage level [] requires a reactive current deviation of. of the nominal current of the DG. The third parameter that varies from ±.5 % to ± % of the nominal voltage U n is the range of the dead band, represented by the grey area. Within this range, the reactive current controller is deactivated. If the voltage variation ΔU exceeds the dead band, the reactive current controller is activated. For voltages in the normal operation area or steady state operation within the dead band other requirements exist. Fig.. Fault Ride-Through Characteristic. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,

3 III. MODEL OF THE DISTRIBUTION GRID The model of the distribution grid consists of different benchmark models for the medium and low voltage grid, models of DGs and loads as well as a protection system. A. Layout of the Test Grid The test grid exists of one medium voltage grid and several low voltage grids. The high voltage grid is represented by an aggregated external grid that works as a slack node. Medium Voltage Grid The medium voltage grid is based on a CIGRE benchmark model developed by Rudion et al. [7] with two sub-networks, illustrated in Fig.. Sub-network is a meshed ring system and characterizes a small town. Sub-network is a radial system and represents a rural area. Using the circuit breaker it is possible to connect both sub-networks. In normal operation the circuit breakers are opened. The connections within each subnetwork are realized with cables. Both sub-networks are connected via overhead lines to the high voltage grid. Each load in Fig. consists of a detailed low voltage grid. The loads at the busbars (BBs) and are associated with other medium voltage grids. These other medium voltage grids are aggregated to one constant PQ-load per BB. The peak load of the entire medium voltage grid, including the aggregated medium voltage grids, is 7 MVA. The transformers, which connect the medium voltage level with the high voltage level, have a rated apparent power of 5 MVA each. Low Voltage Grids The low voltage grids are created mainly based on Dickert et al. [8] and Scheffler [9]. In this context three different types of low voltage grids are developed. Each low voltage grid is modelled in detail according to the topology in Table I. All loads are equally distributed within the grid. TABLE I. Parameter Transformer Rated Apparent Power Peak Load of the Grid Number of Delivery Points Number of Feeders Topology PROPERTIES OF THE LOW VOLTAGE GRIDS Type of the Low Voltage Grid Rural Suburban Urban kva kva kva kva kva 5 kva 8 8 B. Models of the Distributed Generators To model a realistic distribution grid, three different types of DGs are integrated in the low voltage grids. The DG models are inverters, e.g. photovoltaic (PV) plants, directly-coupled synchronous generators and directly-coupled induction generators, e.g. micro combined heat and power (µchp) plants. Inverters The inverter model, including the reactive current controller, is based on Marinopoulos et al. [] and modified with a characteristic, which is displayed in Fig., to give a voltage support during faults ΔI Q = k ΔU - ΔI Q = k ± % U n Voltage Variation, ΔU in pu Fig.. CIGRE Benchmark Model of the Medium Voltage Grid [7]. Fig.. Implemented Reactive Power/Voltage Support Characteristic in the Inverter Model. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,

4 U N, P, Q in p. u. - - P Q -,,, -,, t in s Fig. 5. Performance of the Inverter Model during Grid Faults. In Fig. 5 the performance of the inverter model with the implemented current controller was verified. The curves are depicted in the consumer reference arrow system. In order to test the reactive power/voltage support characteristic, a three phase fault with a voltage dip of.5 pu and a duration of ms at the PCC was simulated. According to Fig., the inverter reduces the active current to pu and increases the reactive current to pu. Hence, the active power is reduced to pu and the reactive power is increased to.5 pu. After clearing the fault at the time of. s, the inverter recurs to the steady state operation with a feed in of pu of active power. Directly-Coupled Synchronous Generators For the synchronous generator the standard model from DIgSILENT [] with the parameters from Mecc Alte [] was taken. Additionally, a simple excitation system with a voltage controller for the generator was developed, to ensure the correct behavior during the fault. P, Q in p. u. - - P Q -,,,,, t in s Fig.. Performance of the Directly-Coupled Synchronous Generator Model during Grid Faults. In Fig. the performance of the directly-coupled synchronous generator during grid disturbances is shown in the consumer reference arrow system. A three phase fault with the duration of ms and a voltage dip of.5 pu at the PCC is simulated. The implemented excitation system guarantees the fast readjusting of the exciting voltage during the fault. During the disturbance, the synchronous generator contributes to the shortcircuit current, which is represented by the large peak in reactive power production at the time of ms. After the fault is cleared, at the time of. s, the synchronous generator starts to consume reactive power for a short period of time, which causes a delayed voltage recovery in the grid. Directly-Coupled Induction Generators The induction generator is based on the standard model from DIgSILENT [] with the parameters from VEM []. P, Q in p. u. 5 - P Q -,,,,, t in s Fig. 7. Performance of the Directly-Coupled Induction Generator Model during Grid Faults. In Fig. 7 the performance of the directly-coupled induction generator model during grid faults is depicted in the consumer reference arrow system. The behavior of the generator is tested during a three phase fault with a duration of ms and a voltage dip of.5 pu. In the steady state operation, before the grid fault occurs, the induction generator consumes reactive power, which is required for the magnetic field. At the time of ms the grid fault occurs and the generator contributes to the shortcircuit with a large peak of reactive power production, quite similar to the directly-coupled synchronous generator. After the fault is cleared at the time of ms, the generator starts to consume reactive power, which delays the voltage recovery in the grid. The generator turns back to the steady state operation when the transient oscillation subsides. TABLE II. SIZE AND ARRANGEMENT OF THE DISTRIBUTED GENERATORS IN THE LOW VOLTAGE GRID Distributed Generator PV Plant (Inverter) µchp Plant (Synchronous Generator) µchp Plant (Induction Generator) Type of the Low Voltage Grid Rural Suburban Urban kw kw 5 kw - - kva kva kva - In Table II the size and the arrangement of the DGs is shown. All DGs are installed in the low voltage level. Some of the low voltage grids have exclusively directly coupled synchronous or induction generators depending on the requirement of thermal power. The arrangement of the DGs within each low voltage grid is random. C. Models of the Loads All loads in the low voltage level are assumed as households. The loads are balanced and modeled with a constant PQ characteristic. The PQ characteristic of the loads causes a lower voltage boost in the grid because of the lower short-circuit current contribution of the DGs. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,

5 D. Protection System of the Distributed Generators To investigate the dynamic grid support in low voltage grids, the present protection system of the DGs must be modified in order to enable the FRT capability. The present and the assumed future protection settings are shown in Table III. The present protection settings are based on the GC for the low voltage level in Germany [] and represent the state of the art without dynamic grid support, especially without FRT requirement. If the voltage at the PCC drops for more than. s under a value of.8 U n, the DG disconnects from the grid. Parameter Voltage Threshold Time Delay (Tripping Time) TABLE III. Without Dynamic Grid Support PROTECTION SETTINGS Undervoltage Function With Dynamic Grid Support () () ().8 U n.5 U n.8 U n. s. s s The settings for a possible future protection system enable a dynamic grid support, especially a FRT of the DG. If the voltage drops to pu at the PCC for a duration of less than. s, DGs remain connected to the grid and ride-through the fault. Furthermore, DGs have to stay connected during voltage dips between.5 U n and.8 U n for a duration of less than s. The assumed future protection settings, shown in the right part of Table III, enable the FRT capability of DGs and hence the first requirement for a dynamic grid support. The second requirement, reactive power/voltage support, is realized with the implementation of the reactive current controller in the inverter with the characteristic depicted in Fig.. IV. SIMULATION FRAMEWORK The simulation framework defines the different conditions under which all simulations are executed. The simulations are solved as RMS in DIgSILENT []. A. Voltage Dip Classification Voltage dips are characterized by the duration, depth and frequency of occurrence. The dip duration is determined by the fault clearing time of the protection system. The depth of the dip is determined by the feeder impedance, fault location and fault type. This investigation focuses on the analysis of three phase grid faults. Fault locations are in the high and medium voltage level. The fault impedance is X =. mω that causes a voltage dip of about pu at the fault location. The fault duration is ms. B. Evaluation Criteria The impact of the dynamic grid support is evaluated against two criteria. The first criterion is the amount of disconnected generating power caused by grid faults. This criterion describes the impact of grid disturbances without dynamic grid support. The second criterion is the rate of the voltage boost. This criterion describes the impact of the voltage dip limitation through dynamic grid support. Moreover, the second criterion is subdivided into two aspects. The first aspect is the rate of the voltage boost with FRT and without reactive power/voltage support. In this case the inverters inject only active current. The second aspect is the rate of the voltage boost with FRT and reactive power/voltage support. In this case the inverters inject also reactive current, respectively Fig.. The rate of the voltage boost is calculated by the difference of the arithmetic average of the voltages between and ms. C. Simulation Scenarios Two different scenarios are developed to investigate the dynamic grid support. In Table IV both scenarios with their different grid parameters are listed. The short-circuit power of the high voltage grid is based on Valov [5]. TABLE IV. Apparent Power of the Distributed Generators Apparent Power of the Loads OVERVIEW OF THE GRID PARAMETERS FOR EACH SCENARIO Grid Parameter A Scenario Sub-Network.5 MVA 5.9 MVA Sub-Network.8 MVA.857 MVA Total.7 MVA.7 MVA Sub-Network.8 MVA.5 MVA Sub-Network.5 MVA.75 MVA Short-Circuit Power of the High Voltage Grid Total.8 MVA.5 MVA B MVA 5 MVA Scenario A describes the base case and is used as reference. This scenario stands for a present power system with a low penetration level of DGs, a high consumption of households and a strong high voltage grid with a high short-circuit current contribution. Scenario B represents a future power system. This scenario describes a power system with a high penetration level of DGs, a low consumption of households and a weak high voltage grid with a low short-circuit current contribution. V. RESULTS The simulation results are shown in the Fig. 8 to and represent the evaluation criteria. The amount of disconnected generation, shown in Fig. 8 and, is without dynamic grid support in low voltage grids. The rate of the voltage boost, illustrated in Fig. 9 and, is with dynamic grid support in low voltage grids. The x-axis in Fig. 8 to shows the fault location BB number, respectively Fig.. The order of the fault location BB numbers correlates with the distance between BB and high voltage transformer of each sub-network. Using this order, it is easier to interpret the voltage dip in the grid depending on the impedance between fault location and DG. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,

6 As shown in Fig. 8, grid faults in the high voltage level at BB cause a disconnection of % of the generating power. Interesting to see is the impact of grid faults at BB and on the other sub-network. A fault at BB leads to a disconnection of about % of the generating power in sub-network. The grid impedance between fault location and DG determines mainly the amount of disconnected generating power. Comparing to Fig. 8, fault locations, and in Fig. are noticeable. Faults at these BBs cause a disconnected generating power of %, which is equal to.7 MVA, respectively Table IV. The amount of disconnected generation rises at these BBs. The main reason for that is the lower contribution of the short-circuit current from the high voltage grid. Hence, the impact of faults in the distribution grid is bigger. Teilnetz Teilnetz Teilnetz Teilnetz Teilnetz Teilnetz Teilnetz Teilnetz Disconnected Generating Power in % Disconnected Generating Power in % Fig. 8. Disconnected Generating Power in Scenario A. Fig.. Disconnected Generating Power in Scenario B. (a) 7 5 Fault Ride-Through without Reactive Power/Voltage Support Outlier Min./ Max. Value 5 %-/ 75 %-Quantile Arithmetic Average Median (a) 8 8 Fault Ride-Through without Reactive Power/Voltage Support (b) 7 5 Fault Ride-Through with Reactive Power/Voltage Support Fig. 9. Voltage Boost in Scenario A. (b) 8 8 Fault Ride-Through with Reactive Power/Voltage Support Fig.. Voltage Boost in Scenario B. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,

7 The second evaluation criterion is the rate of the voltage boost according to the fault location BB number, shown in Fig. 9 and as a boxplot. The rate of the voltage boost is divided into without, Fig. 9 (a) and (a), and with, Fig. 9 (b) and (b), reactive power/voltage support. The voltage boost is calculated by using the voltage values at the BBs in the low voltage grids. The main characteristics of a boxplot are: the median, shown as a red bar; the 5 %- and 75 %-quantile, shown as a blue box; the min. and max. value, shown as whiskers; the arithmetic average, shown as a blue cross and the outlier, shown as a red cross. Generally can be concluded that the activated reactive power/voltage support of the inverters leads to higher voltage boosts at the BBs in the low voltage grids. Moreover, the simulation results have shown, the higher the impedance between fault location and DG, the higher is the voltage boost at the low voltage BB. The impedances of the transformers and the lines have a major impact on the rate of the voltage boost. In Fig. 9 the voltage boost in the scenario A is depicted. A fault at BB in the medium voltage level, without reactive power/voltage support of the inverters, leads to an arithmetic average of all voltage boosts at the low voltage BBs of %. The median is.5 %. The max. value at one low voltage BB is about %. With reactive power/voltage support a fault at BB leads to an arithmetic average value of all voltage boosts at the low voltage BBs of about.5 %, which is nearly the same as the median. Faults at the BBs and in sub-network in the medium voltage level, have nearly no impact on the DGs in sub-network, which leads to outliers for the voltage boosts at the low voltage BBs in sub-network. The voltage boosts in scenario B, depicted in Fig., have increased in comparison to scenario A. The main reason for the increase is the lower contribution of the short-circuit current of the high voltage grid and the higher penetration level of DGs. A fault at BB in the medium voltage level, without reactive power/voltage support, leads to an arithmetic average of all voltage boosts at the low voltage BBs of %. The median is %. The max. value at any low voltage BB is %. With reactive power/voltage support the arithmetic average at BB rises to %. A max. voltage boost of about 8 % can be reached during a fault at BB. VI. CONCLUSIONS In this paper an investigation of a possible future ancillary service in low voltage grids, called dynamic grid support, is carried out. An overview of the different requirements for a dynamic grid support in medium and high voltage grids, according to the present GCs in Germany, is discussed. In order to evaluate the impact of a dynamic grid support in low voltage grids, a detailed model of a distribution grid is presented. Furthermore, a parameterization for the undervoltage protection system of the DGs and for the reactive current controller of the inverter, to enable a full dynamic grid support in low voltage grids, is suggested. Using this simulation model, several scenarios with different fault locations are developed to investigate the impact on a power system. Along with the conclusions the questions from the introduction are answered:.) The amount of disconnected generating power caused by grid faults, especially in the high voltage level, is not negligible. Moreover, a fault in the medium voltage level in sub-network leads to a disconnection of DGs in sub-network. The amount of disconnected power depends chiefly on the fault location or rather the impedance between fault location and DG..) Through a dynamic grid support in the low voltage level the voltage dip in the grid will be reduced. The FRT requirement together with the reactive power/voltage support is more efficient than without reactive power/voltage support. However, the effect of the voltage boost, in order to recover the voltage, through a dynamic grid support in the low voltage level is marginal in comparison to other investigations in the medium voltage level, such as Marinopoulos et al. []..) A dynamic grid support in low voltage grids can be required. The FRT requirement hinders the disconnection of a large amount of power, while the reactive power/voltage support reduces the voltage dip in the grid. REFERENCES [] Bundesverband der Energie- und Wasserwirtschaft e. V. (BDEW), Technische Richtlinie Erzeugungsanlagen am Mittelspannungsnetz Richtlinie für den Anschluss und Parallelbetrieb von Erzeugungsanlagen am Mittelspannungsnetz, Berlin, 8. [] Verband der Netzbetreiber e. V. (VDN), TransmissionCode 7 Netz- und Systemregeln der deutschen Übertragungsnetzbetreiber, Version., Berlin, 7. 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(VDE), Netztechnik/Netzbetrieb im VDE (FNN), Erzeugungsanlagen am Niederspannungsnetz Technische Mindestanforderungen für Anschluss und Parallelbetrieb von Erzeugungsanlagen am Niederspannungsnetz, Berlin,. [5] B. Valov, Auslegungskonzept des Netzanschlusses von PV-Kraftwerken zwecks Spannungsstabilisierung und voller Nutzung der Netzkapazität, Symposium Photovoltaische Solarenergie, Bad Staffelstein, 9. 8 th Power Systems Computation Conference Wroclaw, Poland August 8-,