A simulation of the actual installed controller will be carried out and an analysis of the adjustment of their respective parameters will be made, the manufacturer was based on the recommendations defined in the “IEEE Recommended Practice for Excitation System Models for Power System Stability Studies [4], [5], IEEE Std 421.5TM -2005”, specifically the ST6B model. The identification and validation process in hydroelectric plants usually concentrates on two subsystems: governor-turbine and exciter. Standard models GGOV1 and STB6 are preferred for the dynamical structures of governorturbine and exciter, respectively [6]. II. M ODELLING METHODS First we will proceed with a theoretical review of the most important parts and characteristics of the voltage regulator, then we will proceed with the simulation of the model to know the maximum variations of the parameters of the control loop of the Voltage Regulator, with which we will obtain a range of the generation unit operability. This paper follows the IEEE descriptive material intended to assist in the planning for design, development, and operation of small hydroelectric power plant control systems [7]. A. Baba Hydroelectric Power Plant description The Baba Hydroelectric Power Plant is located in the province of Los Rios and is part of the Baba Multipropurpose project, the Power Plant has two generation units with a total power of 42 MW. Figure 1 shows the inside and outside Baba plant where the two generators are located. Fig. 1. Baba Hydroelectric Power Plant The generators are of the synchronous type, vertical poles projecting out from the surface of the rotor, and the turbine is of the Kaplan type. The voltage regulation system consists of a Static Type Excitation (ST) system whose manufacturer is Voith. Table 1 describes the main characteristics of the generator. TABLE I M AIN PARAMETERS OF THE GENERATOR Turbine type Constructive shape Apparent Power Nominal Active Power Nominal Voltage Nominal current Power Factor Velocity Number of poles Frecuency Vertical Kaplan 2 X W1- IM8015 S = 23,40 MVA P = 21,06 MW V = 13,8 KV I = 979 A fp = 0,9 n = 225 rpm p = 32 f = 60 Hz B. Description of the functionality of the excitation system The excitation system provides field current to a synchronous generator, additionally it contains control and protection functions that guarantee a stable dynamic response in the Generation Unit. Among the main control functions are the voltage of the generator terminals, the reactive power and the ability to improve the stability of the power system [8]. The protection functions include the capacity limits of the synchronous machine, such as the regulation of low and high excitation, Volt Hertz limiter and minimum field current limiter. The voltage regulator has as its main parts: • Controller • Rectifier bridge • Collector Rings • Excitation Transformers The functional blocks of the excitation control in connection with the synchronous generator are shown in Figure 2. TABLE II - E XCITATION SYSTEM DATA Description Excitation current MAX Excitation current nominal Vacuum excitation current Excitation current in the air gap Excitation voltage maximum Excitation voltage nominal Excitation voltage in vacuum Excitation tension in the air gap Field resistance Ifm Ifn Ifo Ifag Ufm Ufn Vfo Ufag Rf Units 959 Amps dc 854 Amps dc 466 Amps dc 379 Amps dc 169.4 Volts dc 143 Volts dc —- Volts dc 58.2 Volts dc 119.6 mΩ D. Excitation system Model The excitation system at the Baba control panel is of the self-excited type with completely static excitation, none of the elements of the voltage regulator are rotating within the unit. The field voltage is obtained by means of the terminal voltage in the stator, which is conditioned by means of the excitation transformer, the rectification of this voltage is carried out through a fully controlled bridge rectifier, the angle The trigger variable of the rectifier thyristors is the actuator variable to obtain the appropriate field voltage and current at the unit operating point. The most traditional and practical structure of the synchronous machine that is suitable for the stability analysis of power systems was developed by the standard IEEE 421.5 [10], which has developed flexible models applied to most of the excitation systems found in synchronous generators. These models have a bandwidth of at least 3 Hz and a frequency deviation of 5% and is oriented to obtain the controller parameters suitable for field tests [11]. The model used by the manufacturer was the ST6B Static excitation system with field current limiter. The AVR shown in Figure 3 consists of a PI voltage regulator with feedback from the internal loop field voltage regulator. The field voltage regulator implements a proportional and integral control. If the field voltage regulator is not implemented, the corresponding parameters KFF and KG are set to 0. VR represents the limits of the power rectifier. The current upper limit IFD is included in this model. The power of the rectifier, VB, can be supplied from the terminals of the generator or from an independent source [12]. Fig. 2. Functional scheme of the excitation control system of a synchronous machine [9]. C. Excitation system data Next, Table 2 shows the excitation system data. Fig. 3. Model ST6B according to the IEEE421.5 standard - Static excitation system with field current limiter [11]. III. M ETHODOLOGY The application of generation schemes to hydroelectric power plants offers a series of advantages, based essentially on the greater flexibility of the controllers [13]. This section explain the methodology. A. General Description A general description of all the Matlag-Simulink is following in the next sections. 1) Simulation system modelling: According to document No. VOITH 299411 provided by the manufacturer of the static excitation system, the model to be simulated is that shown in Figure 4. This comprises the stator voltage control loop in which the PSS is included and a field current feedback. Fig. 5. Stator voltage control loop [12]. TABLE III - D ESCRIPTION OF THE PARAMETERS OF THE EXCITATION VOLTAGE CONTROL LOOP Parameters Ug sp max(VHz) Ug sp min Tu Tf Kp Ti Kd Td PSS after UEL Kbr Description Maximum AVR reference allowed Minimum AVR reference allowed Transducer Time Constant Field Current Time Constant Proportional gain Integral time Derivative gain Derivative time constant PSS after UEL Rectifier Bridge Gain Tbr Rectifier bridge time constant Interval 0.9 - 1.2 0.7 - 1 20 ms 20ms 0.1 - 40 0.1 - 10s -40 - 0 0.1 - 10s 0o1 6.25 1.4ms o 1.7ms the PSS and the AVR droop compensator was included, but their influence on the analyses was not carried out Fig. 4. Full AVR model implemented by Voith 2) Simulation considerations: For the analysis and simulation of our model, the current limiter protections of the SCL stator, overexcited OEL, underexcited UEL, volt hertz VHZ and limiter of minimum field current MFCL, shall not be taken into account. We will explain them didactic but will not influence the simulation. In the control loop shown, the reference signal is the Set point generator voltage and the output variable is the unit field voltage, to be able to feedback the process the transfer function of the synchronous generator in vacuum was included. Load response was no included because the object of analysis is the response of the voltage regulator to the variation of the parameters of the controller. The values used are per unit. For simulation purposes, a step input of magnitude 1 p.u. The simulation and the entry of the system parameters is carried out in Matlab, the simulation of the model will be carried out in the Simulink application and a sweep of the influential parameters in the voltage regulator will be carried out to know the operating limits of the Generation Unit. 3) Stator voltage control loop: Figure 5 shows the control loop without the transfer function of the open circuit generator. Table 3 shows the description of the parameters of the excitation voltage control loop. B. MatLab - Simulink simulation The model shown in Figure 6 was used. The vacuum generator constants data were obtained with the nominal current and field voltage and in vacuum. Illustratively, the location of [1] W. Ali, H. Farooq, A. ur Rehman, and M. E. Farrag, “Modeling and performance analysis of micro-hydro generation controls considering power system stability,” in 2017 First International Conference on Latest trends in Electrical Engineering and Computing Technologies (INTELLECT), Nov 2017, pp. 1–7. [2] Y. Dai, T. Zhao, Y. Tian, and L. Gao, “Research on the influence of primary frequency control distribution on power system security and stability,” in 2007 2nd IEEE Conference on Industrial Electronics and Applications, May 2007, pp. 222–226. [3] M. Wu and P. Rastgoufard, “The application of reactive power control equipment in power system stability and security performance enhancement,” in IEEE Power Engineering Society General Meeting, 2004., June 2004, pp. 2114–2119 Vol.2. [4] K. Mslo, A. Kasembe, and M. Kolcun, “Simplification and unification of ieee standard models for excitation systems,” Electric Power Systems Research, vol. 140, pp. 132 – 138, 2016. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0378779616302383 [5] T. Xu, A. B. Birchfield, and T. J. Overbye, “Modeling, tuning, and validating system dynamics in synthetic electric grids,” IEEE Transactions on Power Systems, vol. 33, no. 6, pp. 6501–6509, Nov 2018. [6] T. Hosseinalizadeh, S. M. Salamati, S. A. Salamati, and G. B. Gharehpetian, “Improvement of identification procedure using hybrid cuckoo search algorithm for turbine-governor and excitation system,” IEEE Transactions on Energy Conversion, vol. 34, no. 2, pp. 585–593, June 2019. [7] IEEE, “Ieee guide for control of small (100 kva to 5 mva) hydroelectric power plants,” IEEE Std 1020-2011 (Revision of IEEE Std 1020-1988), pp. 1–56, April 2012. [8] B. C. A. Ricardo, “Metodologia para la identificacion del sistema de excitacion de un generador electrico de potencia para propositos de control,” Tech. Rep., 2010. [9] “Ieee recommended practice for excitation system models for power system stability studies,” IEEE Std 421.5-2005 (Revision of IEEE Std 421.5-1992), pp. 1–93, April 2006. [10] P. Kundur, Estabilidad y Control de Estabilidad de Potencia, 1994. [11] A. Greenwood, Electrical Transients in Power Systems - Second Editiod, Wiley and Sons Inc, 1970. [12] ARCONEL, “Pliego tarifario para las empresas electricas.” Agencia de Regulacion y Control de Electricidad (ARCONEL), Tech. Rep., 2017. [Online]. Available: http://www.regulacionelectrica.gob.ec/wp-content/ uploads/downloads/2017/01/Pliego-y-Cargos-Tarifarios-SPEE-2017.pdf [13] J. Fraile-Ardanuy, J. R. Wilhelmi, J. J. Fraile-Mora, and J. I. Perez, “Variable-speed hydro generation: operational aspects and control,” IEEE Transactions on Energy Conversion, vol. 21, no. 2, pp. 569–574, June 2006.