Journal of Mechanics Engineering and Automation 5 (2015) 401-406 doi: 10.17265/2159-5275/2015.07.003 D DAVID PUBLISHING Transient Stability Assessment of Synchronous Generator in Power System with High-Penetration Photovoltaics (Part 2) Masaki Yagami 1, Seiichiro Ishikawa 1, Yoshihiro Ichinohe 1, Kenji Misawa 1 and Junji Tamura 2 1. Department of Electrical and Electronic Engineering, Hokkaido University of Science, Sapporo 006-8585, Japan 2. Department of Electrical and Electronic Engineering, Kitami Institute of Technology, Kitami 090-8507, Japan Abstract: In the previous paper [1], the transient stability of synchronous generator in power system with high-penetration PV (photovoltaic) was assessed by simulation analysis of a single-machine infinite-bus system model. Through the simulation analysis, we have obtained some conclusions in terms of the impact of high-penetration PV on the stability. However, for more accurate assessment of the transient stability, it is necessary to analyze various simulation models considering many other power system conditions. This paper presents the results of the analysis for the transient stability simulation performed for IEEE 9-bus system model, in which the effects of various conditions, such as variety of power sources (inverter or rotational machine), load characteristics, existence of LVRT (low-voltage ride-through) capability and fault locations, on the transient stability are investigated. Key words: High-penetration photovoltaics, LVRT (low-voltage ride-through), synchronous generator, transient stability. 1. Introduction After the Fukushima nuclear power plant accident in Japan, the investment in the field of PV (photovoltaic) power generation has been increasing. During 2013, a total of 6.9 GW of PV was installed in Japan, 400% increase beyond the installation in the previous year (1.7 GW). Cumulative installed capacity of PV systems in 2013 reached 13.6 GW [2]. With the increasing of PV capacity in the power system, the capacity of conventional synchronous generators needs to be reduced relatively. This leads to the lower system inertia, the higher generator reactance and the fewer frequency control generators, and hence the synchronous generator transient stability may negatively be affected. In the previous paper [1], the results of the simulation analysis of the synchronous generator stability for a single-machine infinite-bus system with Corresponding author: Masaki Yagami, Ph.D., associate professor, research fields: power system stability and FACTS (flexible AC transmission system). E-mail: yagami@hus.ac.jp. PV installed were presented. Through the simulation analysis, we have obtained the fundamental characteristics of the transient stability in the power system with PV installed, and it was concluded that the transient stability becomes better in the cases of the higher penetration PV without LVRT (low-voltage ride-through) capability. However, this conclusion is for a case study using the single-machine infinite-bus system model. For more accurate assessment of the transient stability and the dynamic characteristics of the power system with PV installed, it is necessary to analyze various simulation models considering many other power system conditions. In this work, the transient stability simulation is performed for IEEE 9-bus system model [3], in which the effects of various conditions, such as variety of power sources (inverter or rotational machine), load characteristics (constant impedance or power), fault locations (6 fault points) and existence of LVRT capability, on the stability are investigated. The
402 Transient Stability Assessment of Synchronous Generator in Power simulation is performed by using PSCAD/EMTDC software [4]. The paper is organized as follows: Section 2 describes the simulation model; Section 3 presents the simulation results and discussions; Section 4 gives conclusions. 2. Simulation Model 2.1 Power System Model Fig. 1 shows 9-bus power system model used in the simulation analysis. A synchronous generator of 500 MVA (SG1) and a large-scale PV plant of 500 MW (PV) or another synchronous generator of 500 MVA (SG2) are connected to an infinite bus through transformers and double circuit transmission lines. The capacity of PV or SG2 is equivalent to 50% for the power system capacity of 1,000 MVA. In the case with PV connected, the existence of LVRT capability is considered for PV inverter. LVRT characteristic considered in this work is simple. The PV inverter with LVRT capability maintains the connection to the grid even if its terminal voltage is dropped. On the other hand, the PV inverter without LVRT capability is disconnected from the grid when its terminal voltage drops below 70% of the nominal voltage [5], and the disconnection is maintained for remaining period of the simulation. The parameters used for each generator are shown in Table 1. AVR (automatic voltage regulator) and governor control systems which are the same models as those in Ref. [1] have also been included in each generator model. In the power system model, the symmetrical 3LG (three-line-to-ground) fault is assumed as network disturbance. The fault occurs at each fault point (F1-) at 0.1 s, the CBs (circuit breakers) on the faulted lines are opened at 0.17 s, and at 1.17 s, they are reclosed. 2.2 PV System Model The PV system model is shown in Fig. 2. It consists of PV module, inverter and LC (low-pass) filter. The PV module is represented with a voltage source, and hence the PV output is constant under the steady state. This means irradiation and PV cell temperature are constant during the simulation period. In general, transient stability is analyzed in a time window of a P/V = 0.5/1.0 0.017 j0.144 (j0.0373) j0.025 F2 SG1 500 MVA CB 20/500 kv 500/20 kv F1 F3 0.064 j0.322 (j0.0765) 3LG fault CB P/Q = 0.5/0.3 0.0238 j0.2016 (j0.0523) F4 0.078 j0.34 (j0.0895) P/Q = 0.5/0.0 j0.0152 500 MVA SG2 or PV 500 MW P/Q = 0.4/0.3 F5 P/Q = 0.2/0.2 0.020 j0.17 (j0.044) 0.034 j0.184 (j0.0395) V = 1.0 j0.0576 50 Hz, 1,000 MVA base Infinite bus Fig. 1 Power system model.
Transient Stability Assessment of Synchronous Generator in Power 403 Table 1 Synchronous generator parameters. Generator parameters SG1 SG2 SG1 SG2 R a (pu) 0.003 0.004 X q (pu) 0.171 0.134 X l (pu) 0.102 0.078 T d (sec) 5.9 8.97 X d (pu) 1.651 1.22 T d (sec) 0.535 1.5 X q (pu) 1.59 1.16 T q (sec) 0.033 0.033 X d (pu) 0.232 0.174 H (s) 3.0 3.0 X d (pu) 0.171 0.134 LC filter Inverter PV PWM pulse I abc V abc abc/dq carrier Comparator PQ calculation P PV I d I d ref V d ref dq/abc V abc ref Q PV P PV ref Q PV ref I q ref I q V q ref Fig. 2 Control block diagram of PV inverter. few seconds to several tens of seconds. The assumption, therefore, may be valid for the transient stability analyses. The genetic PWM (pulse-width modulation) voltage source converter is used as PV inverter. The inverter controls the active and reactive power injected from the PV module to the system. To maintain the active and reactive power at the reference set points, the currents of the inverter are controlled by using vector control technique. The current limitation for over current is not considered in this model. Currently, most of the PV inverters are designed to operate at unity power factor [5], and therefore, the reference value of the reactive power is set to zero in each case. 2.3 Load Model A static load model can be expressed as algebraic functions of the bus voltage magnitude. The voltage dependency characteristic of load is represented by the exponential model [6]: a V P P0 V (1) 0 b V Q Q0 V (2) 0 where, P and Q are active and reactive components of the load when the bus voltage magnitude is V. The subscript 0 means the values of the respective variables at the initial operating condition. The parameters of this model are the exponents a and b. If these exponents equal to 0, 1, or 2, the model represents constant power, constant current or constant impedance characteristics respectively. In this simulation, two load models, the constant power and the constant impedance expressed by exponents of 0 and 2, are considered.
404 Transient Stability Assessment of Synchronous Generator in Power 3. Simulation Results 3.1 Impact of LVRT Capability The phase angle responses of SG1 in the case of fault point F1 are shown in Fig. 3. To compare the effects of the conventional rotating machine and PV system on transient stability, we have considered three simulation conditions in terms of the power sources as follows: (1) connecting SG2 instead of PV, (2) connecting PV with LVRT capability, and (3) connecting PV without LVRT capability. The load models of the constant impedance are used for all of the simulation conditions. As can be seen, the first peak of the phase angle swing is decreased in the case of PV without LVRT. In other words, risk of out-of-step of the synchronous generator becomes low in the power system with PV without LVRT installed. Fig. 4 shows the first peaks of the phase angle swings of SG1 in the case of each fault point. In all of the fault point cases, the first peaks of the phase angle swings in the case of PV without LVRT are restrained more than those in other cases. This is because the kinetic energy stored in the rotor of SG1 can be released quickly compared with other cases. Fig. 5 shows the kinetic energy responses of SG1 in the case of fault point F1. As can be seen, in the case of PV without Phase angle of SG1 (deg) 130 110 90 70 50 0 2 4 6 8 10 12 14 Fig. 3 Phase angle of SG1 for each power source (fault First peak of phase angle swing (deg) 140 F1 F2 F3 F4 F5 Fig. 4 First peak of phase angle swing of SG1 for each power source. Kinetic energy of SG1 (pu) 1.02 1.01 1.00 0.99 0.98 0.97 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Fig. 5 Kinetic energy of SG1 for each power source (fault LVRT, the kinetic energy stored in the rotor of SG1 can be released quickly compared with other cases. Fig. 6 shows the stability index, W c [1, 7-9], in each fault point case. W c is an integrated variation of the energy exchanged between the generator rotor and the power system; thus, the lower the value of the parameter, the shorter the convergence time of the swing. In this work, the simulation time of t = 30 (s) is used to calculate W c. As can be seen, in most of the fault point cases, W c is decreased in the case of PV without LVRT. Namely, the convergence of the kinetic energy swing becomes fast in this case.
Transient Stability Assessment of Synchronous Generator in Power 405 W c (s) 1.2 1.0 0.8 0.6 0.4 0.2 Fig. 6 F1 F2 F3 F4 F5 Stability index W c for each fault point. than that in the case of the. As a result, the kinetic energy stored during the period of acceleration is released quickly. The kinetic energy responses of SG1 are shown in Fig. 9. The first peaks of the phase angle swings and the stability index W c of SG1 in the case of the use of each load model are shown in Fig. 10. As a whole, values in the case of the are slightly larger in most cases than those in the case of the. Namely, the generator stability becomes slightly lower in the case of the. 3 130 Phase angle of SG1 (deg) 110 90 70 Active power of SG1 (pu) 2 1 0-1 50 0 1 2 3 4 5 Fig. 7 Phase angle of SG1 for each load model (fault point F1). 3.2 Impact of Load Characteristics Fig. 7 shows the phase angle responses of SG1 in the cases where each load model and PV without LVRT are considered. The load model of the constant impedance or the constant power, represented by Eqs. (1) and (2), is used in the simulation. As can be seen, the first peak of the phase angle swing in the case of the is smaller than that in the case of the. Fig. 8 shows the active power of SG1. After the fault clearing, the active power of SG1 in the case of the becomes slightly larger 0.1 0.2 0.3 0.4 Fig. 8 Active power of SG1 for each load model (fault Kinetic energy of SG1 (pu) 1.02 1.01 1.00 0.99 0.98 0.97 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Fig. 9 Kinetic energy of SG1 for each load model (fault
406 Transient Stability Assessment of Synchronous Generator in Power First peak of phase angle swing (deg) 140 1.2 F1 F2 F3 F4 F5 (a) First peak of phase angle swing of SG1 characteristics, fault locations and existence of LVRT capability, on the stability are investigated. Through the simulation results, it is concluded that the risk of out-of-step of the synchronous generator becomes low in the case of the use of PV without LVRT capability because the disconnection of PV system after the network disturbance increases the power load of the synchronous generator. Acknowledgments This work was supported by research grant from Japan Power Academy and JSPS KAKENHI Grant Number 24656179, 15H03956. References W c (s) 1.0 0.8 0.6 0.4 0.2 F1 F2 F3 (b) Stability index W c Fig. 10 Comparison of and constant impedance load. 4. Conclusions With the increasing of PV capacity in the power system, the transient stability of the conventional synchronous generator may negatively be affected due to the lower system inertia and the higher synchronous reactance. In this work, the transient stability simulation is performed for IEEE 9-bus system model, in which the effects of various parameters, such as variety of power sources (inverter or rotational machine), load F4 F5 [1] Yagami, M., Hasegawa, T., and Tamura, J. 2012. Transient Stability Assessment of Synchronous Generator in Power System with High-Penetration Photovoltaics. Journal of Mechanics Engineering and Automation 2: 762-8. [2] Photovoltaic Power Systems Programme. 2014. Trends 2014 in Photovoltaic Applications. Report IEA-PVPS T1-25. [3] Anderson, P. M., Fouad, A. A. 2000. Power System Control and Stability. New York: John Wiley & Sons, Inc. [4] Website of PSCAD. https://hvdc.ca/pscad.html. [5] Kobayashi, H., and Suzuki, A. 2010. Stable Operation Technique for PCS of PV Power Generation at Grid Recovery after Voltage Sag. System Engineering Research Laboratory, Rep. No. R09015. (in Japanese) [6] Kundur, P. 1994. Power System Stability and Control. New York: McGraw-Hill, Inc. [7] Yagami, M., Murata, T., and Tamura, J. 2004. An Analysis of Optimal Reclosing for Enhancement of Transient Stability. Electrical Engineering in Japan 147: 32-9. [8] Yagami, M., and Tamura, J. 2007. Improvement of Power System Transient Stability Using Superconducting Fault Current Limiter. Trans. on Systems, Signals & Devices 2 (2): 197-211. [9] Yagami, M., and Tamura, J. 2012. Hybrid Operation of Fault Current Limiter and Thyristor Controlled Braking Resistor. Journal of Energy and Power Engineering 6: 1-8.