The Power System Stabilizer (PSS) Types And Its Models

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The Power System Stabilizer (PSS) Types And Its Models Saeed Shakerinia Department of Electrical Engineering, Borujerd Branch, Islamic Azad university, borujerd,iran Shakeriniasaeed@yahoo.

1 The Power System Stabilizer (PSS) Types And Its Models Saeed Shakerinia Department of Electrical Engineering, Borujerd Branch, Islamic Azad university, borujerd,iran Abstract The aim of this study was to investigate the power system stabilizer and its types and models. Power system stability is the ability to maintain a stationary state in an electrical system after a disturbance has occurred. This disturbance can for instance be loss of generation, change in power demand or faults on the line. The PSS introduces a signal that optimally results in a damping electrical torque at the rotor. This torque acts opposite of the rotor speed fluctuations Other solutions on the oscillation problem exist. Keywords: Power System Stabilizer, PSS, Electrical Power Engineering Introduction The main reason for implementing a power system stabilizer (PSS) in the voltage regulator is to improve the small signal stability properties of the system. Back in the 1940 and 50s the generators were produced with a large steady state synchronous reactance. This led to reduction in field flux and to a droop in synchronising torque. The result was a machine with poor transient stability, especially when it was connected to a weak grid. To solve this transient stability problem, a fast thyristor controlled static excitation system was later introduced. This installation eliminated the effect of the high armature reaction, but it also created another problem. When the generator was operated at a high load and connected to a weak external grid, the voltage regulator created a negative damping torque and gave rise to oscillations and instability. An external stabilizing signal was therefore introduced as an input to the voltage regulator. This signal improved the damping of the rotor oscillations and the device was called power system stabilizer (PSS). The PSS introduces a signal that optimally results in a damping electrical torque at the rotor. This torque acts opposite of the rotor speed fluctuations Other solutions on the oscillation problem exist. A single machine connected to an external grid is often used to explain the dynamics of an electrical network. In 1952 Heffron and Phillips developed a model for this setup, and this model contained an electromechanical model of a synchronous generator with an excitation system. Control Action and Controller Design The action of a PSS is to extend the angular stability limits of a power system by providing supplemental damping to the oscillation of synchronous machine rotors through the generator excitation. This damping is provided by a electric torque applied to the rotor that is in phase with the speed variation. Once the oscillations are damped, the thermal limit of the tie-lines in the system may then be approached. This supplementary control is very beneficial during line outages and large power transfers [1,2]. However, power system instabilities can arise in certain circumstances due to negative damping effects of the PSS on the rotor. The reason for this is that PSSs are tuned around a steady-state operating point; their damping effect is only valid for small excursions around this operating point. During severe disturbances, a PSS may actually cause the generator under its control to lose synchronism in an attempt to control its excitation field [3]. 154

2 Fig 1 : Lead-Lag Power System Stabilizier A lead-lag PSS structure is shown in Figure 1. The output signal of any PSS is a voltage signal, noted here as VPSS(s), and added as an input signal to the AVR/exciter. For the structure shown in Figure 1, this is given by This particular controller structure contains a washout block, stw/(1+stw), used to reduce the over-response of the damping during severe events. Since the PSS must produce a component of electrical torque in phase with the speed deviation, phase lead blocks circuits are used to compensate for the lag (hence, lead-lag ) between the PSS output and the control action, the electrical torque. The number of lead-lag blocks needed depends on the particular system and the tuning of the PSS. The PSS gain KS is an important factor as the damping provided by the PSS increases in proportion to an increase in the gain up to a certain critical gain value, after which the damping begins to decrease. All of the variables of the PSS must be determined for each type of generator separately because of the dependence on the machine parameters. The power system dynamics also influence the PSS values. The determination of these values is performed by many different types of tuning methodologies. Other controller designs do exist, such as the desensitized 4-loop integrated AVR/PSS controller used by Electricité de France [4] and a recently investigated proportional-integralderivative (PID) PSS design [5]. Differences in these two designs lie in their respective tuning approaches for the AVR/PSS ensemble; however, the performance of both structures is similar to those using the lead-lag structure. The input signal for the PSSs in the system is also a point of debate. The signals that have been identified as valuable include deviations in the rotor speed ( = mach - o), the frequency ( f), the electrical power ( Pe) and the accelerating power ( Pa). Since the main action of the PSS is to control the rotor oscillations, the input signal of rotor speed has been the most frequently advocated in the literature [6,7]. Controllers based on speed deviation would ideally use a differential-type of regulation and a high gain. Since this is impractical in reality, the previously mentioned lead-lag structure is commonly used. However, one of the limitations of the speedinput PSS is that it may excite torsional oscillatory modes [8,9]. A power/speed ( Pe-, or delta-p-omega) PSS design was proposed as a solution to the torsional interaction problem suffered by the speed-input PSS [10]. The power signal used is the generator electrical power, which has high torsional attenuation. Due to this, the gain of the PSS may be increased without the resultant loss of stability, which leads to greater oscillation damping [11]. A frequency-input controller has been investigated as well. However, it has been found that frequency is highly sensitive to the strength of the transmission system - that is, more sensitive when the system is weaker - which may offset the controller action on the electrical torque of the 23machine [12]. Other limitations include the presence of sudden phase shifts following rapid transients and large signal noise induced by industrial loads [11]. On the other hand, the frequency signal is more sensitive to inter-area oscillations than the speed signal, and may contribute to better oscillation attenuation [13-14]. The use of a power signal as input, either the electrical power ( Pe) or the accelerating power ( Pa = Pmech - Pelec), has also been considered due to its low level of torsional interaction. The 155

3 Pa signal is one of the two involved in the 4-loop AVR/PSS controller from [15], even though the tuning method related to this design approach is valid for other input signals. Control and Tuning The conflicting requirements of local and inter-area mode damping and stability under both smallsignal and transient conditions have led to many different approaches for the control and tuning of PSSs. Methods investigated for the control and tuning include state-space/frequency domain techniques [4,5], residue compensation [8], phase compensation/root locus of a lead-lag controller [16], desensitization of a robust controller [17], pole-placement for a PID-type controller [5], sparsity techniques for a lead-lag controller [4] and a strict linearization technique for a linear quadratic controller [1]. The diversity of the approaches can be accounted for by the difficulty of satisfying the conflicting design goals, and each method having its own advantages and disadvantages. This is the crux of the problem of low frequency oscillation damping by the application of power system stabilizers. Overview of different PSS structures In order to provide a damping torque signal, the PSS could use the rotor speed deviation from the actual rotor speed from the synchronous speed (Δωr) as an input. Other parameters, which are easily available and measurable, could also be used to provide the damping torque. These signals could be electrical frequency, electrical power or the synthesized integral of electrical power signal. In the measurements of input signals, different types of signal noise could be present. The stabilizer has to filter out this noise in order to feed the AVR with a steady signal, which could damp the actual rotor oscillation [17]. An explanation of different PSS s is given in the following sub chapters. 1- Speed-based stabilizer The simplest method to provide a damping torque in the synchronous machine is to measure the rotor speed and use it directly as an input signal in the stabilizer structure. Ordinary variations in speed, frequency and power must not generally enter the PSS structure and thereby affect the field voltage [18]. The washout constant should be chosen according to these criteria [11]: 1.It should be long enough so that its phase shift does not interfere significantly with the signal conditioning at the desired frequencies of stabilization. 2. It should be short enough that the terminal voltage will not be affected by regular system speed variations, considering system-islanding conditions, where applicable. 2- Frequency-based stabilizer This type of stabilizer has the same structure as used in the speed-based stabilizer mentioned above. By using the system frequency as an input signal the low frequency inter-area oscillations are better captured. These oscillations are thereby better damped in a frequencybased stabilizer, compared to the speedbased stabilizer. Oscillations between machines close to each other are not well captured by the frequencybased stabilizer, and the damping of the local oscillations is then not highly improved. The frequency signal may also vary with the network loading and operation. An arc furnace nearby could for instance create large unwanted transients in the measurement signal, and the PSS might produce a unwanted behaviour of the generator [14]. 3- Power-based stabilizer Power and speed of the rotor are in a direct correlation, according to the swing equation described below: 156

4 Where the damping coefficient is set to zero. The electrical power is easy to measure and also use as an input signal. Mechanical power is a more problematic value to measure. In most power-based stabilizers this mechanical power is threated as a constant value and the rotor speed variations are then proportional with electrical power. Change in the mechanical loading will then be registered by the PSS and it will create an unwanted output signal. A strict PSS output limiter must in those cases be established to prevent the PSS to contribute under a change in generator loading. This will reduce the overall PSS performance and the power system oscillations will not get as damped as wanted. Electrical power as an input signal will only improve the damping of one oscillation mode. Several oscillating frequencies in the network require a compromise solution of the lead/lagfilter [14]. 4- Integral of accelerating power-based stabilizer As mentioned as a drawback of the speed-based stabilizer, a filter has to be implemented in the main stabilizing path to reduce the contribution of lateral and torsional movements. This filter must also be applied in the pure frequency- and power-based single input stabilizers. The Integral of accelerating power-based stabilizer was developed to solve the filtering problem and also take mechanical power variations into account [4, 14]. 5- Multi-band stabilizer The motivation for developing a new type of stabilizer was that the lead/lag compensating filters in the older structures could not give an accurate compensation over a wide range of oscillation frequencies. If the network suffers from low- and high frequency oscillations, the tuning procedure of the single-band stabilizers have to compromise and will not achieve optimal damping in any of the oscillations. The multi-band stabilizer has three separate signal bands, which can be tuned individually to handle different oscillation frequencies. Some Model of PSS The power system stabilizer (PSS) model provides an input (VPSS) to the excitation system model to improve damping of system oscillations. A variety of input signals may be used depending on the particular design. ENTSO-E, an association of the European electricity transmission system operators, selected the Common Information Model (CIM) standards of the International Electrotechnical Commission (IEC) as a basis for its own CIM standards. These standards aim at ensuring the reliability of grid models and market information exchanges. In 2013, ENTSO-E adopted a new standard for grid models exchange called the Common Grid Model Exchange Standard (CGMES). The CGMES is a superset of the IEC CIM/CGMES standards (belonging to IEC CIM16). It was developed to meet necessary requirements for the transmission system operators, which exchange data in the areas of system operations, network planning and integrated electricity markets. All the CIM/CGMES regulators models are included in NEPLAN Power System Analysis Tools. 157

5 Fig 2. Power system PSS Diagram PSS Simple model : PSS Simple model Fig 3. IEEE Stablizing Model With Dual-input Signal : Conclusion: In this paper, the concept of power system stabilizer and types and models that have been investigated.power system stabilizer (PSS) controller design, methods of combining the PSS with the excitation controller (AVR), investigation of many different input signals and the vast field of tuning methodologies are all part of the PSS topic.the power system stabilizer (PSS) model provides an input (VPSS) to the excitation system model to improve damping of system oscillations. A variety of input signals may be used depending on the particular design. 158

6 References [1] IEEE, "IEEE Recommended Practice for Excitation System Models for Power System Stability Studies," in IEEE Std (Revision of IEEE Std ), ed: IEEE, 2006, pp [2] D. Mota, "Models for Power System Stability Studies, Thyricon(R) Excitation System," Trondheim Patent, [3] R. Grondin, I. Kamwa, G. Trudel, L. Gerin-Lajoie, and J. Taborda, "Modeling and closed-loop validation of a new PSS concept, the multi-band PSS," Power Engineering Society General Meeting, 2003, IEEE, vol. 3, p. 1809, July [4] B. Pal and B. Chaudhuri, Robust Control in Power Systems. Boston, MA: Springer Science+Business Media, Inc., [5] W. G. Heffron and R. A. Phillips, "Effect of a Modern Amplidyne Voltage Regulator on Underexcited Operation of Large Turbine Generators," Power Apparatus and Systems, Part III. Transactions of the American Institute of Electrical Engineers, vol. 71, pp , [6] J. Machowski, J. W. Bialek, and J. R. Bumby, Power system dynamics: stability and control. Chichester: Wiley, [7] E. C. C. C.-E. f. a. C. W. EU. (2008). EU action against climate change. Available: [8] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Power Explained - Theory, Designe and Application, 1. edition ed.: John Wiley & Sons, [9] I. Kamwa, R. Grondin, and G. Trudel, "IEEE PSS2B versus PSS4B: the limits of performance of modern power system stabilizers," Power Systems, IEEE Transactions on, vol. 20, pp , [10] Statnett. (2008). Funksjonskrav i kraftsystemet. Available: [11] P. Kundur, N. J. Balu, and M. G. Lauby, Power system stability and control. New York: McGraw-Hill, [12] K. Bjørvik and P. Hveem, Reguleringsteknikk. Trondheim: Høgskolen i Sør Trøndelag, Avd. for teknologi, Program for elektro- og datateknikk, [13] IEEE, "IEEE Guide for Identification, Testing, and Evaluation of the Dynamic Performance of Excitation Control Systems," in IEEE Std , ed, 1990, p. 45. [14] G. R. Bérubé and L. M. Hajagos, "Accelerating-Power Based Power System Stabilizers," p. 10, Year not known. [15] K. Kiyong and R. C. Schaefer, "Tuning a PID controller for a digital excitation control system," Industry Applications, IEEE Transactions on, vol. 41, pp , [16] A. Murdoch, S. Venkataraman, R. A. Lawson, and W. R. Pearson, "Integral of accelerating power type PSS. I. Theory, design, and tuning methodology," Energy Conversion, IEEE Transactions on, vol. 14, pp , [17] N. Martins and L. T. G. Lima, "Eigenvalue and Frequency Domain Analysis of Small Signal Electromechanical Stability Problems," [18] STRI, "SIMPOW, Power System Simulation Software, USER MANUAL (Beta release)," vol. 10.2, ed,

ANALYTICAL AND SIMULATION RESULTS

6 ANALYTICAL AND SIMULATION RESULTS 6.1 Small-Signal Response Without Supplementary Control As discussed in Section 5.6, the complete A-matrix equations containing all of the singlegenerator terms and