Analysis of Transient Stability Enhancement Using Multi-Band Stabilizer on South Sulawesi Power Grid

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Analysis of Transient Stability Enhancement Using Multi-Band Stabilizer on South Sulawesi Power Grid

Jeremias Leda Department of Electrical Engineering The University of Atma Jaya Makassar

Makassar, Indonesia

Abstract Interconnected system has been operated widely for increasing electrical power reliability. Stability factor in such system is very important in which stability analysis is used to define whether the rotor of interconnected generators may be still in their synchronize speed after disturbance was happen or may loss their synchronization speed. Disturbance on power system could create electro-mechanics oscillation. One may damp such oscillation using power system stabilizer. The Multi-Band Power System Stabilizer (MB-PSS) is a concept to reduce oscillation effectively due to disturbances to avoid loss of synchronization.

This paper investigate and analyze the use of MB-PSS as a damping system in South Sulawesi power grid through a digital simulation study. The simulation result shows that MB-PSS could works nicely and more effective compare to generic PSS and may use to enhance system stability of South Sulawesi power grid.

Keywords multi-band; stabilizer; oscillation; power system

  1. INTRODUCTION

    Interconnection of power system is needed because enable one to take advantages of diversity of loads, availability of sources and fuel consumption in order to supply electricity to the consumers at minimum cost with a required reliability. However, there are a number of stability issues that limit the

    This paper introduce the advantages of a MB-PSS to reduce electro-mechanic oscillation of South Sulawesi power grid in Indonesia. It is well known that poorly damped of oscillations typically occur in power systems with longitudinal structure or weak ties. The supplementary excitation controllers are used as additional feedback signals to enhance system damping and to improve the dynamic stability of power systems. These controllers are known as power system stabilizers. They have been widely used for years with many types.

    The MB-PSS proposed in this paper has advantages of its simplicity, systematic design and combine functions of AVR and PSS. The performance of this controller is investigated for South Sulawesi power grid which is starting with collecting data, modelling, simulation, analysis, interpreting and then results discussion. Simulation work is done at laboratory of electrical engineering department at the University of Atma Jaya Makassar using multi-machine detail model under MATLAB Simulink environment.

  2. BASIC CONCEPT OF STABILIZER

    transmission capability, such as transient stability, dynamic stability and steady-state stability. This paper is going to focus on transient stability problems in which the South Sulawesi power grid in Indonesia is taking into the field to study.

    Various factors affect transient stability of a system such as the strength of transmission network within the system and of the tie lines to adjacent system, the characteristic of the generating units, including inertia of the rotating parts and electrical properties such as transient reactances and magnetic saturation characteristic of the stator and rotor iron. Another factor is the speed with which faulted lines can be disconnected and with the automatic reclosing transmission line, how rapidly lines can be restored to service. The speed with which the generator excitation systems give their respond is important in maintaining transient stability. According to the definition,

    TM _

    D+DA+DP

    K1+K1A+K1

    1 / Ms

    _

    K1P+DP

    0

    / s

    Torque Resultant K1 + D

    K1A+DA

    transient stability is the ability of generating systems to remain in sysnchronism during the period following a disturbances and prior to the time at which governors can act. Ordinarily the first swing of machines rotors will take place within about one second following the disturbance but the excat time depend on the machine characteristic and transmission line.

    Fig. 1. AVR + PSS Torque Characteristic Diagram

    PSS is designed to work together with generator excitation system in order to produce positive damping torque to ensure system stability in which can be explain by torque vector diagram as Fig.1 above. K1 is synchronizing torque, K1A is

    synchronizing torque by AVR and K1P is synchronizing torque by PSS. Where D is damping torque DA is damping torque by

    AVR and DP is damping torque by PSS. PT

    In principle generic PSS has three components as seen on Fig.2. They are washout, gain and phase compensation. The

    washout block serves as high pass filter with time constant high enough to allow signal associated with electro-mechanics oscillation to pass unchanged. The gain block determines the amount of damping introduced by PSS. Phase compensation block provides an approriate phase lead characteristics to compensate phase lagging between exciter input and generator electrical torque.

    Vref

    EXC

    f type

    AVR

    GEN

    Fig. 5. Connection Diagram of f type PSS

    PT

    Fig. 2. Generic PSS Block Diagram

    The input is rotor speed which came from the relation of change among mechanical power, electrical power and

    accelerating power as illustrated on diagram below. CT

    Vref

    P type

    AVR

    EXC

    Fig. 3. Power and Rotor Speed Relation Diagram

    GEN

    Fig. 6. Connection Diagram of P type PSS

    PSS has three types of application. The simplest type is single input type such as P, or f which may be used for individual generator oscillates against the system with frequency of 1 Hz approximately and named local mode power oscillation. For this case P type is more effective. The second type is used for the whole system oscillates with long distance and large power transfer system connection that known as inter-area mode power oscillation or long-cycle. In this case

    or f is more effective. The third type PSS is used for complex power oscillation mode such as local mode and inter- area mode. In this case multiple input is more effective. For instance P + or P + f type of PSS input. The connection of the three type of PSS is shown in next figures.

  3. MULTI-BAND POWER SYSTEM STABILIZER The multi-band power system stabilizer here is according to

    IEEE type PSS4b (Innocent Kamwa, Robert Grondin, IREQ Hydro-Quebec). Basically that type of stabilizer consist of three pass band that the low-pass, intermediate pass and high pass. Each of the three pass-band also have gain, lead-lag compensation and limiter respectively.

    PT Vref

    type

    AVR

    Transducer

    EXC

    Fig. 7. Representation of MB-PSS

    GEN

    Fig. 4. Connection Diagram of type PSS

    The need for effective damping of a wide range of electromechanical oscillations has motivated the concept of the MB-PSS. As its name reveals, the MB-PSS structure is based on multiple working bands. The low band is typically associated with the power system global mode, the intermediate band with the inter-area modes, and the high band with the local modes.

    The output of MB-PSS is feed into adder block before goes into the exciter block. On the other hand input of MB-PSS is design as electric power acceleration which consist of the rotor speed deviation and the change of the rotor speed d as shown in fig. 8 below.

    Fig. 8. Block Diagram of Exciter and MB-PSS

    Two different approaches ae available to configure the settings during MB-PSS tuning process:

    1. Simplified Settings: Only the first lead-lag block of each frequency band is used to tune the MB-PSS. The differential filters are assumed to be symmetrical band-pass filters respectively tuned at the center frequency FB, FI, and FH shown in fig. 7. The peak magnitude of the frequency responses can be adjusted independently through the three gains KE, KI, and KH.

    2. Detailed Settings: The designer is free to use all the flexibility to built into the MB-PSS structure to achieve nontrivial controller schemes and to tackle even the most constrained problem (for example, multi unit plant including an inter-machine mode, in addition to a local mode and multiple inter-area modes). In this case, all the time constants and gains have to be specified.

  4. SIMULATION, RESULT AND DISCUSSION

    Fig. 9. Single Line Diagram of Testing System

    The loads are modeled as constant impedances. A 135 MW electrical power transfer from GEN1 of area#1 to area#2 of GEN2 and area#3 of GEN3 and GEN4 is the main case study which is very stressed operating point.

    This system exhibits seven electromechanical modes of oscillations. Three inter-area mode which the generating units in one area oscillates against those in the other area. The frequency of this mode varies from 0.35-0.75 Hz depending on operating conditions. And four local modes which represent oscillations generating units internally within each area against system.

    The MB-PSS is used for each generating units. The test contains three inter-area. A comparison between the results of a generic PSS and without PSS due to different disturbances is presented below. Comparisons between resultant performances is needed. So we present the results values for the relation among the system with PSS dan without PSS graphically.

    Models are simplifications thus there are relatively small numerical error and will not be valid exactly. However when used appropriately the simulation models provide an important results and useful information. Results of simulation are as follow:

    During the simulation, there is assumption that input power is constant, no phase shifting between the rotor angle on each generating machine and the voltages respectively, bus no.7 is swing and finally fault location is on bus no.1.

    1. Single Phase to Ground Fault

      According to PLN, National Electric Power Company of Indonesia for the region of South Sulawesi, the total power installed in 2010 is 623 MW for supply the total load of 541 MW through the length of 1,926 km of transmission lines. Here, the grid-ring is simplified and created especially for this study of oscillation problem under Simulink configuration with 4 machine as shown in figure 9 which is used to examine oscillation problem. As shown in the single line diagram there are four generators, GEN1, GEN2, GEN3 and GEN4, and four

        1. /150 kV step-up transformers. There are 8 loads in the system except at bus no.7. The transformer and line

          degree

          No PSS

          Generic PSS

          MB-PSS

          time

          impedances for the system are given in Appendix and also data of generators. Some of the set up parameters based on their preset value by MATLAB.7-Simulink is acceptable due to the lack of information.

          Fig. 10. GEN 1 – GEN 2 Rotor Angle Deviation

          It can be seen that MB-PSS works better than Generic PSS and without PSS the deviation of rotor angle tends to produce instability problems.

          Degree No PSS

          Generic PSS

          MB-PSS

          time

          On the other hand if the system is designed without PSS thus the two fault will produce oscillation and potential to create electromechanical oscillations on the grid system.

  5. CONCLUSION

The work on this research has been done under MATLAB environment and shows that for both fault, single phase and three phase fault to ground:

      • MB-PSS is better than generic PSS and able to stabilize the grid system in which may damp the disturbances.

      • The MB-PSS signal can modulate the set point of the generator voltage regulator so as to improve damping of the system.

Fig. 11. GEN 1 – GEN 3 Rotor Angle Deviation

Again, it also can be seen that MB-PSS works better than Generic PSS and without PSS the deviation of rotor angle tends to produce instability problems.

  1. Three Phase Fault to Ground

  • The MB-PSS can work on both local area and inter-area of electromechanical oscillations.

APPENDIX SYSTEM DATA

TABLE II

GENERATORS, LOAD AND TRANSFORMERS DATA

degree

No PSS

Generic PSS

MB-PSS

time

Fig. 12. GEN 1 GEN 2 Rotor Angle Deviation

gree

MB-PSS

Generic PSS

No PSS

time

de

Fig. 13. GEN 1 GEN 3 Rotor Angle Deviation

In three phase fault to ground cases, the curve results also shows that MB-PSS works more effective than generic PSS and if without PSS the system oscillation tends to unstable.

Simulation results of the single phase fault to ground and the three phase fault to ground look similar in which MB-PSS works better than generic PSS.

Data Source: PT. PLN AP2B Sistem Sulawesi Indonesia

TABLE III GENERATORS IMPEDANCE

Data Source: PT. PLN AP2B Sistem Sulawesi Indonesia

TABLE IV

LINE TRANSMISSION IMPEDANCE

Data Source: PT. PLN AP2B Sistem Sulawesi Indonesia

ACKNOWLEDGEMENTS

This research was supported by Direktorat Penelitian & Pengabdian Masyarakat DITLITABMAS Ditjen DIKTI, National Ministry of Higher Education of Indonesia through Penelitian Hibah Bersaing No.005/PNL/PK-KOP.IX/2014 date 16 May 2014.

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