GENERATORS AND LOADS MODELS TO INVESTIGATE UNCONTROLLED ISLANDING ON ACTIVE DISTRIBUTION NETWORKS

Transcription

1 GENERATORS AND LOADS MODELS TO INVESTIGATE UNCONTROLLED ISLANDING ON ACTIVE DISTRIBUTION NETWORKS Paolo MATTAVELLI, Riccardo SGARBOSSA Roberto TURRI University of Padova Italy ABSTRACT The number of distributed energy resources (DERs) connected to low voltage (LV) distribution networs has increased the concern on the unintentional/uncontrolled islanding operations. To evaluate the effect of the Distributed Generation (DG) on the uncontrolled islanding events in LV networ portions independent studies have been performed, considering different LV load dynamic characterization and inverter based models for a correct evaluation of the islanding issue. P/f and Q/V capabilities and regulation laws required by the DERs stated by the most relevant standards have also been considered. Field measurements, simulations in different simulating environments as Real Time Digital Simulator (RTDS), Simulin/Power System Blocset and DIgSILENT Power Factory are reported. INTRODUCTION To evaluate the effect of the most relevant capabilities and regulation laws required to distributed energy resources (DERs) [1-2] on uncontrolled islanding in distribution networs, two completely independent studies have been performed, based on: 1) field measurements to assess LV load dynamic behavior; 2) digital simulations performed in different simulation environments (RTDS, Simulin/DigSILENT). Suitable models for the inverter based generators and for the loads are essential for a correct evaluation of the phenomena. This need has recently led to the Joint Woring Group C4/C6.35/CIRED: Modelling and dynamic performance of inverter based generation in power system transmission and distribution studies. Considering that inverter based models in standard libraries do not include the several new DER capabilities and may be not adequate for dynamic simulations (EMTtime domain), additional models have been developed. More precisely: University of Padova realized models in the time domain (Power System Blocset), based on switching inverter operation. Subsequently, porting of the model in an equivalent RMS model to be adopted in DigSILENT was performed by simplification firstly to average models and then to RMS-phasor models, in Gianluca SAPIENZA, Giovanni VALVO, Cristiano PEZZATO, Alberto CERRETTI Enel Distribuzione Italy gianluca.sapienza@enel.com giovanni.valvo@enel.com cristiano.pezzato@enel.com alberto.cerretti@enel.com Ettore DE BERARDINIS CESI Italy deberardinis@cesi.it order to be used in dynamic simulation, together with other networ components (load, generator, etc). CESI and ENEL Distribuzione implemented in a Real Time Digital Simulator (RTDS) digital models of PV inverters, synchronous generators, loads and protection systems. The correct dynamic representation of the LV load depending on the voltage and the frequency [3-4] is important for the assessment of unintentional islanding operation. Considering the data in [3-4] outdated, ENEL decided to set up two field measurement campaigns in order to assess the current typical LV equivalent load behavior and to define updated sets of load model parameters. In any case, in order to cover different scenarios for uncontrolled islanding operation, analysis of islanded operation has taen into account both load representations, i.e. the old ones with the coefficient reported in 1993 [3-4] and the new ones as resulting from the two field measurement campaigns. STANDARDS AND CONNECTION RULES Recently, reference technical rules have been revised by standards, introducing for DERs protection interface systems and local control strategies, in order to integrate the growing number of DGs [1-2]. Permissive thresholds for voltage and frequency have also been introduced in compliance with the Fault Ride Through (FRT) philosophy [3-4], as shown in Fig. 1. a) b) Figure 1-a) Frequency permissive and restrictive thresholds. b) voltage FRT thresholds. Moreover, local controls set by standards are required: active and reactive power exchanged by DGs are function of the frequency and voltage levels, as shown in Fig. 2, introducing the droop characteristic Q=f (V) and P=f (f). CIRED2015 1/5

2 These control strategies are aimed at facilitating the electrical system stability but they can potentially increase the ris of uncontrolled islanded networ operation. a) b) Figure 2 - a) Inverters could be required to participate to the voltage regulation using Q injections from a minimum of to the maximum of with respect to the rated power; b) gradual limitation of the generated P according to an over-frequency statism s o =2.4% [1-2]. LV LOAD CHARACTERIZATION For the load static and dynamic characteristics, the approach proposed in [3-4] has been adopted, where the active P out and the reactive Q out load powers depend both on frequency and amplitude of the voltage waveform, i.e.: P out = P 0 {[ f ( pf ) 1] ( u p ) pu } 1sT 1 u o { Q out = Q 0 {[ f ( qf ) 1] ( u p ) (1) qu } 1sT 1 u o where u p = [ u ( 1 1sT 1 ) u o ] and P o and Q o are the active and reactive power respectively absorbed at nominal frequency f 0 and nominal amplitude u 0. Moreover, pf, pu, qf and qu are parameters that describe different type of loads (residential, industrial, agriculture) and the voltage and frequency deviation from their nominal parameters are denoted as u = u u o and f = (f f o ) /f o, respectively. The above-mentioned parameters describe the variation of the load as a function of voltage and frequency, therefore the campaign has been realized by using a suitable synchronous generator able to supply the LV networ independently from the MV/LV transformer, as shown in Fig. 3. MV Networ 20/0.4 V G ~ Synchronous generator Figure 3 Test field realization Tests have been carried out at fixed voltage to determine pf, qf, at fixed frequency to find pf, qf, according to the following equations: V Other lines Residential loads Other load pu = log P P0 log up u0 qu = log Q Q0 log up u0 pf = 1 f ( P P 0 1) = f 0 f f 0 ( P P 0 1) qf = 1 f ( Q 1) = f 0 ( Q 1) Q 0 f f 0 Q 0 where P 0, Q 0, U 0, f 0, are the initial active power reactive power, supply voltage, frequency, for each test. The initial and final values for the active/reactive power at the corresponding values for the voltage or frequency have been determined by interpolating several measurements, for the pu, qu characterization, as shown for the sae of example in Fig. 4. Figure 4 Example of P and Q variation as a function of u p Table 1 shows the mean values measured during these tests. The values are quite different from the IEEE campaign and they refer to a limited number of tests. This difference may be justified by the different type of electrical equipment currently used. Residential loads Other loads pf qf pu qu Table 1 Field tests results (average values). (2) CIRED2015 2/5

3 The T 1 parameter has been determined by applying step voltage variations using the tap changer of the Primary Substation HV/MV transformer. T 1 value has been confirmed to be about 0.1 s. INVERTER AND LV NETWORK DISTRIBUTION MODEL DIgSILENT inverter model RMS vs Simulin/Time domain model (Case A) In this study a detailed inverter model, with embedded voltage and current regulations typical for photovoltaic applications, has initially been developed. In order to study the uncontrolled islanding operations the average values and the phasors representations are superior to the detailed model in terms of computational requirements and numerical convergence issues. Although different conversion module configurations can been considered [5], the analysis has focused on the single-stage system and on the two level Voltage Source Inverter (VSI). A three-phase inverter with EMI filter in addition to the inductive output filter, as shown in Fig. 5, has been considered. Figure 6- Switching and average inverter model The representation has been developed in Maltab/Simulin environment and compared with DIgSILENT software using RMS simulations. The results show a good match between the two environments. i a L v f 1 AB L 1 i b v CA L v f 2 BC L 2 i c L f 3 L 3 C f 1 C f 2 C f 3 R Cf 1 R Cf 2 R Cf 3 Figure 5 Three phase inverter with L 1,2,3 inductive filter and EMI filter composed by L f1,2,3,c f1,2,3 and R Cf1,2,3 as dumping element of the filter (if present). The switching function approach has been initially used to fully represent the switching operation of the inverters, as depicted in Fig. 6. However, for most of the islanded analysis, the detailed switching operation has been neglected, in order to reduce simulation time and facilitate convergence, and an average model, where each state variable is averaged over the PWM switching period, is adopted, as generally shown in Fig. 6. The averaging model is more efficient and with less numerical convergence problem than the detailed switching model. Moreover the presented average model has been transferred in the d,q coordinates using a Par Transformation, and completed with the required current, voltage and Phase-Looed Loop (PLL) regulations and with the required protection functions. At last we obtain a dynamic model expressed in amplitude and phase of voltages and currents instead of instantaneous sinusoidal values. Figure 7 - LV distribution networ used Case A With this modeling approach, the tested LV networ is shown in Fig. 7. It presents a DG unit, three loads, a MV/LV Transformer and cables lines, whose lengths are shown in Fig. 7, and characterized by R=0.164 Ohm/m, X= Ohm/m and B= µs/m. The switching of the breaer starts the islanding events. RTDS model (Case B) After CESI residential loads characterization, a large 4- wires unbalanced LV grid has been implemented by ENEL-CESI in the Real-Time Digital Simulator (RTDS) installed in the ENEL Smart Grid Test Center of Milano. The RTDS allows to simulate a large number of power CIRED2015 3/5

4 system components and control model in real-time, in the time domain, using the Dommel algorithm. The LV grid, represented in Fig. 8, is composed by three feeders (urban, rural, and urban-rural mixed feeder), single and three-phase loads (with the caracteristics and dynamics derived by the CESI load caracterization) and 18 PV inverter-based distributed generators (6 threephase 10 Wp, 3 three-phase 20 Wp, 4 single-phase 6 Wp, 5 single-phase 3 Wp). Each generator is regulated in P,Q mode, where the P setpoint is generated using the P-f characteristic represented in Figure 2 b). Indeed, the Q set-point is generated using the Q-V characteristic represented in Fig. 2a. Generators are synchronized to the grid voltage using a PLL and the coupling to the grid is performed using a transformer with short-circuit impedance equal to 6%. Finally, the interface protection and the capability curve has been modeled for each generator. The LV grid is fed, from the MV grid, by a Dy11 MV/LV transformer. The grid neutral wire is connected to the transformer secondary winding neutral point. Lines are represented using the PI section model, where the zero-sequence impedance taes into account the neutral wire effect. DIgSILENT Power Factory are reported here, but similar results were obtained with the switching and average Simulin models. The tests have been carried out modifying the loads and inverter rated powers, in order to analyze the islanded grid behavior under different initial conditions. Frequency [Hz] P g [W] P L [W] P g = P L P g < P L P g > P L Time [s] Voltage p.u. Time [s] Figure 9 - Frequency and voltage at the LV Busbar 0.4 V Figure 8 - RTDS model of the LV grid (Case B) EXAMPLES OF SIMULATION RESULTS In this wor parametric analyses considering various combinations of Q(V) and P(f) regulations have been performed, taing into account the inertia of rotating machines and the initial active and reactive power unbalance, in order to identify the role of the different parameters in determining islanding conditions. In the following, few samples of the results obtained are presented and discussed. An Example of Results for Case A The inverter model has been tested with dynamic load models, including the residential and industrial model parameters of [3]. Results show how uncontrolled islanding operations occur in a LV networ. Furthermore, the influence of the standards regulation requirements leads to islanding events starting from different power balance conditions between loads and generator. Due to space constraints, only results performed with Fig. 9 shows frequency and voltage of the islanded LV grid portion between the standards imposed thresholds. The uncontrolled islanded condition is facilitated by the regulation of active and reactive power, but also due to a regulation effect of the dynamic load model considered. Examples of Results for Case B RTDS simulation results are shown in Table 2. In this case the load model parameters are based on the updated measurements reported in Table 1. It may be observed that: 1. With only PV static generation, without any Q(V) and P(f) regulation, islanding is not possible even in case of small power unbalance. The same occurs, if part of PV generation is substituted by synchronous machines. 2. With only PV static generation, with Q(V) and P(f) regulation with hysteresis (according to CEI 0-21 Italian rules), islanding is not possible even in case of small power unbalance. Instead, if hysteresis is excluded for P(f) regulation (lie requested in RfG rules from ENTSOE), stable islanding operation is CIRED2015 4/5

5 possible only if the generation is greater than the load. This is possible because the P(f) may reduce the generation power, according to the frequency growth, until a new balance condition will be restored. The same occurs, if part of PV generation is substituted by synchronous machines. 3. With only PV static generation, with or without any Q(V) and P(f) regulation, transient islanding is always possible even in case of small power unbalance. In this case the generation could be disconnected before fast reclosing (operated on MV side) only if narrow frequency thresholds are activated. On the other hand, if part of PV generation is substituted by synchronous machines, it is possible to disconnect the generation during the fast reclosing only if a large power unbalance is present. Again this phenomenon is due to the inertia increment of rotating machines. Of course, the above mentioned results have to be considered as qualitative and not quantitative due to the variability and uncertainty of all factors influencing the transient to reach the island operation and during the island operation itself: a) V and f measurement methods; b) actuation periods of Q(V) and P(f) regulations; c) loads behavior in function to V and f; d) power factor; e) generators and loads inertia. These factors are not completely determinable because, for some of them, a reference to standards does not exist. CONCLUSIONS Before new networ code on Requirements for Generators (RfG), narrow band frequency thresholds hindered stable island and guaranteed, with very high probability, disconnection before fast reclosing operated by the circuit breaer at the beginning of MV feeder. After RfG requirements (partly introduced in some Countries), the possibility of uncontrolled islanding operation has significantly increased. The conclusions of the present study are, anyway, qualitative, because studies are valid only for the used loads models and generators models; in particular, the different dynamic response of the loads intentionally adopted by the two studies, highly influences the behaviour of the phenomena. In particular, it should be better investigated the dynamic characterization of loads (P and Q variation function of V and f, inertia, cos, etc) in order to achieve sufficient level of details, not below that of IEEE 1993 report [3] which is old and focused on the creation of aggregations at HV level. With regards to generators, different generators capabilities strongly influence the system behaviour. Finally, further potential influencing factors/elements are not still fully investigated: fast voltage support, power system stabilization, synthetic inertia, power factor regulation [6], and energy storage systems. Such elements should be included as well in dynamic studies, in order to assess their effects on the system. REFERENCES [1] CEI Comitato Elettrotecnico Italiano, Standard-CEI 0-16, Reference technical rules for the connection of active and passive consumers to the HV and MV electrical networs of distribution Company, [2] CEI Comitato Elettrotecnico Italiano, Standard-CEI 0-21, Reference technical rules for the connection of active and passive users to the LV electrical Utilities, [3] IEEE Tas Force on Load Representation for Dynamic Performance, 1993 Load Representation for dynamic performance analysis, IEEE Transactions on Power Systems, Vol. 8, No.2. [4] K. Tomiyama et al., 2003, Modeling of Load During and after System faults based on Actual Field Data,Proceedings IEEE Power Engineering Society General Meeting [5] Y. Xue, L. Chang, S. Bæhøj Kjær, J. Bordonau, and T. Shimizu, 2004, " Topologies of Single-Phase Inverters for Small Distributed Power Generators:An Overview", IEEE Trans. on Power Electronics, Vol.19,No. 5, pp [6] L. Cocchi, A. Cerretti et al. Influence of average power factor management on distribution networ power losses" CIRED 2015 Lyon conference. Generation type No regulation Q(V), P(f) Regulation Q(V), P(f) with histeresys CEI 0-21 Regulation Q(V), P(f) without histeresys RfG (soglie strette non attive) P%>0 Stable island (soglie strette non attive) P%>0 Transient island, not permanent operation Narrow tresholds enabled:<0.5 s Narrow tresholds disabled: <0.7 s, scatto V Narrow tresholds enabled:<0.8 s Narrow tresholds disabled: <1 s, scatto V Narrow tresholds enabled: <0.6 s, Narrow tresholds disabled: <5 s, trip V Narrow tresholds enabled:<1.5 s Narrow tresholds disabled: <9 s, large strshold trip Narrow tresholds enabled: <0.6 s Narrow tresholds disabled:<5.2 s, large treshold trip or trip V Narrow tresholds enabled:<3 s Narrow tresholds disabled: <14 s, large strshold trip Disconnection before FR (0.6 s) if P%> 0% If P%> 45% if P%> 10% If P%>99% if P%> 10% if P%> 99% Table 2 RTDS simulation results. CIRED2015 5/5