www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 Developing Model of Control Stratagem with Variable Speed Drive by Synchronous Speed in Micro‐hydro Plant Amam Hossain Bagdadee1 and Nazib Sobhan2 Energy, Asian Institute of Technology, Thailand amambagdadee@gmail.com Abstract This research focuses on the development of intrigue control for variable speed drives as an alternative to solve the problem of small hydro power plants. Detailed mathematical model for the system, including small hydropower Kaplan turbine shaft, mechanical and electrical machines is presented and validated simulation. Control strategy for the implementation of self‐
doubly fed induction machine drive has been developed for a wide range of speeds. The drive can operate at unity power factor. Possible applications of the analysis will be presented. As the positive aspects of the system, it is found that there is a direct interaction between electronic and other electrical converter utility that can be avoided by taking advantage of the proposed structure, which may lead to a better quality of energy production in terms of music. It may also lead to the elimination or reduction of harmonic filters that are used in the assembly, installation doubly fed induction generator. While the shortcomings of the system, a comparison of the conversion and the birth rate problem analysis and problem solving assembly was performed. While the plot remains the same, with one appliance and the doubly fed generator will increase slightly. Losses are also slightly larger due to the presence of a second. Key words Micro‐hydro; Variable Speed; Synchronous; Simulation Introduction
Hydroelectricity or hydro energy is one of the highly accepted forms of energy source amongst the other renewable energy sources. The produced energy unit is cheaper compared to the other sources of energy such as coal or nuclear power plants. It is also environment friendly as it does not emit carbon dioxide. It is the most widely used renewable source of energy as it gives 16% of the world electricity consumption. Depending upon the amount of power produced the hydro power plants can be classified as large, small and micro‐hydro power plants. There is no strict criterion for differentiating the size of the power plants and it also varies from country to country. Usually for a large hydro power plant, a massive civil work is required for the construction of water reservoir that demands a large initial investment. Constructing a dam often causes the environmental and ecosystem changes like shifting of inhabitants. The European Union took the decision to increase the green electricity production from the renewable energy sources (wind, hydro, photovoltaic etc.) by 2020 in order to reduce the production of carbon dioxide. The European strategy named as 20‐20‐20 which requires that by 2020 each country must produce at least 20% of its energy through renewable energy sources. Due to this strategy most of the countries (like Austria, Poland and Romania) are now utilizing their unused small or micro hydro sites. The major contribution from the hydro power to date is through large power stations which were built in last century. However nowadays, most of the large hydro sites in European and North American countries have been utilized and the ones which are available cannot be utilized because of the environmental concerns. On the other hand, many micro‐hydro sites are still available that need to be exploited and can be utilized in an efficient way for energy production. In Europe the installed micro‐hydro capacity ranges about 11500 MW sharing 1.7% in electricity production and 10% of total hydroelectricity. European Small Hydropower Association (ESHA) on March 1, 2012 stated that there are many small hydro power sites available in all over the Europe and only less than half of these have been utilized. About 68% of the existing micro‐hydro plants in Europe are based on ancient structures involving classical electromechanical sets. The operation of the existing micro‐hydro plants is based on fixed speed drives. Depending 88 International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres upon the load 2 and a governing system, a portion of the flow is allowed to pass through the turbine for regulating the frequency, thus only a portion of the total available power is being utilized. Purpose or Objective
This research work is mainly focused on modeling and simulating a variable speed drive for a micro‐hydro generation system. It is an effort for developing a variable speed drive as an alternative solution to the micro‐hydro generation system offering the autonomous or independent operation with improved power quality. The main objectives of the entire work can be described as:  To develop the mathematical model for observing the dynamics and the steady state operation of an autonomous micro‐hydro generation system having variable speed drive. The objective is to model the hydro turbine, drive train, electrical machines and power electronic converters.  To develop the control strategy for an independent or autonomous variable speed drive (VSD) system producing power at unity power factor. Mathematical Modeling of the System
The mathematical model for each component of the topology is presented. A proper derivation of mathematical model for the system is necessary to have a better analysis of the system dynamics in transients and steady‐state operation. The coming sections cover the models for a Kaplan turbine, mechanical shaft, PMSM, DFIG and a DC‐
link. Turbine Model The turbine is basically used to convert the hydro energy into mechanical energy that can later be converted into electrical energy. Depending upon the principle of operation, hydro turbines can be classified into two groups: i). Impulse turbines. ii). Reaction turbines. 1) Impulse Turbines This type of turbine is suitable for the sites where the available water flow rate is slow and water head is high. All the available potential energy in water head is completely converted to kinetic energy in the nozzles and the pressure inside the runner is constant at atmospheric level. As the pressure is constant throughout the impulse turbine, therefore the velocity is the same at the inlet and outlet of the turbine. Pelton turbine is the practical example of impulse turbines that have been used in many high‐head power applications. FIG 1: IMPULSE TURBINES 2) Reaction Turbines Reaction turbines are different compared to the impulse turbines as potential energy is partly converted to kinetic energy at the start of guide vanes, while the remaining potential energy is gradually converted to kinetic energy in the runner. The pressure in the runner varies as flow rate varies in the runner tube. As the pressure drops through the vane of the reaction turbine, therefore the velocity at the outlet is higher compared to the inlet. Depending on the direction of flow, reaction turbines are further classified as radial‐flow and axial‐flow turbines. Radial‐flow turbines are suitable for medium‐head and medium flow rate, whereas axial‐flow turbines are suitable for low‐head and fast flow rate. Francis turbines are a practical example of radial‐flow turbines, while Kaplan turbines are an example of axial‐flow reaction turbine. The system under study is suggested for a 89 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 micro‐hydro power application where the flow rate is quite high with low water head, so the simplified model of a Kaplan turbine with some assumptions will be given. FIG2: REACTION TURBINE Kaplan Turbine The Kaplan turbine is being widely used in low‐head power applications. It is a highly efficient turbine and in some applications an efficiency of 90% or even higher is recorded. A schematic diagram of a Kaplan turbine is presented in Figure 3 showing the wicket gate (guide vanes) and runner blades. The wicket gate is a special feature of the Kaplan turbine making it different compared to a simple propeller. The wicket gate angle can be adjusted depending upon the desired flow. FIG3: KAPLAN TURBINE Proposed System and Modeling The proposed system is shown in Fig. 4 and represents a micro hydro power station. In this system it is considered as a power station model, including an IG, excitation capacitor bank, hydro turbine and SVC. FIG. 4 PROPOSED SYSTEM DIAGRAM. Hydraulic System Modeling A nonlinear model assuming inelastic water column is considered for the hydro turbine and waterways. This model was chosen because it is adequate for studies involving large power and frequency variations so that it is appropriate for large‐signal time‐domain simulations. 90 International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres Considering a simple hydraulic system with a penstock, unrestricted head and tail race, and with either a very large or no surge tank, and assuming yet a rigid conduit and incompressible fluid, the basic hydrodynamic equation are: U  Ku G H (1) P  K p HU (2) a
dU
  g ( H  H 0 ) (3) dt
L
Q=AU (4) Normalizing (1)‐(4) based on rated values, the following expressions that completely describe the water column and turbine features are obtained: Hn  (
U 2
) (5) Gnn
Un
1
(6) 
H n  H 0 n TW s
TW 
Tmn 
LU b
LQb

(7) ag H b ag AH b
0
P
1
( b )
(U  U NLn ) H n Pbn (8)  Pn kVAb
n n
Gn  At g n (9) At 
1
(10) g FLn  g NLn
Where the subscript “b” denotes base value, subscript “n” denotes normalized, per unit (pu), value. Figure block diagram which represents the nonlinear model of the hydraulic turbine. FIG. 5 NONLINEAR MODEL OF HYDRAULIC TURBINE Usual governor systems are composed by a pilot valve and a gate servomotor. The pilot valve is controlled by the frequency controller which controls the direction and flow of hydraulic fluid; while the gate servomotor, in accordance with that flow, controls directly the position of turbine gate. A typical transfer function (TF) adopted to represent the governor system is presented below: GGS ( s ) 
gn
Ka

u TG s 2  s
(11) Electrical System Modeling For the modeling of the electrical system which consists of IG, excitation capacitors, inductive filter and SVC, as shown in Fig. 4, the following simplifying assumptions are considered: 
The IG is considered an ideal voltage source, balanced and undisturbed. 
The DC bus capacitor associated with the SVC is considered an ideal voltage source. 
The inductance of the SVC output filter are identical and of equal value. 91 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 Based on these assumptions, an electric circuit diagram of the system is shown in Fig.6. FIG. 6 ELECTRIC CIRCUIT DIAGRAM OF THE PROPOSED SYSTEM Now, apply the voltage and current Kirchhoff laws in the circuit of Fig. 6, it is possible to obtain the state space representation of that system by the following equations: X abc (t )  A abc X abc (t )  Babc u abc (t )  Fabc Wabc (t ), (12) where:  ia 
i 
b
l 
 u1 pwm 
iGa 
x   c ; u  
; w   ; u
v
iGb 
 2 pwm 
 a
 vb 
 
 vc 
A abc
 Rf

 Lf

 0



 0

 1

 C

 0


 0

0
Rf
Lf
0

0
2
3L f
1
3L f
0
1
3L f
2
3L f
1
3L f
1
3L f
Rf
Lf
0
0
1
CRC
0
1
C
0
0
1
CRC
0
1
C
0
0
1 
3L f 
1 

3L f 

2 
3L f  B  1
 ; abc
3L f

0 


0 

1 

CRC 
 2 1 1 
0 0
0 0
 1 2 1 



 1 1 2 
1 0 0

 ; Fabc  
C 1 0
0 0 0
0 1
0 0 0



 0 0
 0 0 0 
0
0 
0
. 0
0

1 
From the equation (12), using arrays of suitable transformations in synchronous coordinates, it is possible to decompose the original system in a normalized system, composed of only two coordinates, direct and quadrature, and eliminating the time‐varying terms. Therefore, it is obtained the state space representation in dq coordinates given by: x dq (t )  A dq x dq (t )  Bdq u dq (t )  Fdq w dq (t ), (13) where:
92 International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres x dq
A dq
 Rf

 Lf

 


 1
 CZ base

 0



Z base
Lf
Rf
0
Lf
0
1
CZ base
 id 
i 
u d 
iGd 
q
   ; u dq    ; w dq    ;
vd 
u
 q
 iGq 
 
 vq 

1
CRC




Z base 

Z base
Lf 
 ; Bdq  L
f
 


1 

CRC 
0
 1 0 
 0 1

 ; F  1
dq
0 0
CZ base


0 0
0
0

1

0
0
0 
. 0

1
Control System Design To analyze the characteristics and system behavior under different operating conditions, and help controllers design, the proposed system was developed in a simulation environment of software Matlab®. In this environment, the whole system, what includes hydraulic, electric and control systems, is represented by block diagrams, whose blocks are defined based on the mathematical equations of the system model. To present the project of the control system, it is divided in two parts: one relating to frequency control and the other relating to the voltage control. Frequency Control Design The frequency of the generated voltages is directly related to the active power balance of the system. The principle of the control is in the feedback of the measured frequency, which can be obtained using the method of frequency tracking based on the measurement of the generated voltages. The hydraulic system presents a peculiar characteristic due to the water inertia where the initial power surge is opposite to that of the direction of change in gate position. Because of that, the hydraulic turbine is considered a “non‐minimum phase” system. Thus, for a stable performance of the controller, the frequency control TF shall have a time constant that slows its output signal (control signal), and consequently, the performance of the distributor, until the inverse response have been extinguished. It was then considered, the TF shown in for frequency control, given by: C freq ( s )  RT
sTR
(14) 1  sTR
The arrangement of the frequency control actuators, now located in the direct loop is shown in the block diagram of Fig. 7. Where ωref and ω are the angular velocity reference of the generator, in pu, and its actual angular velocity, also in pu, respectively. FIG. 7 BLOCK DIAGRAM OF THE FREQUENCY CONTROLLER IN CONJUNCTION WITH THE ACTUATORS OF THE TURBINE The TF of frequency controller in direct loop has the same characteristic of a proportional‐derivative (PD) controller with a derivative time constant, represented by the following equation: 93 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 CGS ( s )  kp 
kD s
kp  ( kpN  k D ) s
(15) 
1  Ns
1  Ns
Considering the integrative action present in the actuators TF, the use of a PD controller meets the requirements of regulation in the transitory time. For the design of the controller gains, it is necessary to know the hydraulic parameters of the plant, especially the parameters of the actuators. Table 1 presents the parameters considered for the hydraulic actuators. TABLE 1 ACTUATORS PARAMETERS CONSIDERED FOR FREQUENCY CONTROLLER DESIGN Parameter Value K a 8 TG 0.01 Figure 7 shows the simulation results comparing the responses of the gate opening and mechanical torque of the turbine, considering the parameters defined in Table 2, which are obtained considering the real parameters presented in Table 3. By this figure it can be seen a direct relationship between mechanical power and the gate opening, that is, both curves show similarity, differing only in amplitude and due to the non‐minimum phase characteristic of the turbine. TABLE 2 PARAMETERS OF THE HYDRAULIC SYSTEM MODEL Parameter Value At 1.041667 (pu) H0n 1.0 (pu) UNLn 0.020833 (pu) TW 0.655308 (pu) Pbn 1.1 (pu) gFLn 0.98 (pu) gNLn 0.02 (pu) TABLE 3 REAL PARAMETERS OF THE HYDRAULIC SYSTEM Parameter Value P 5 kW L 30 m A 0.066 m2 U 1.5 m/s H 7 m So it is possible to design the frequency controller only from the dynamic characteristic of the actuators and considering the rest of the plant as a gain. Once the parameters of the plant to be controlled are known, and using the Matlab® tools, it is possible to tune the controller gains in order to obtain the desired response. Thus, it was defined the controller gains as presented in Table 4. TABLE 4 FREQUENCY CONTROLLER GAINS Parameter Gain kP 1.5 kD 0.9 N 0.01 Using the first order holder (FOH) method, which was chosen because it does not introduce additional time delay in the model, the TF’s in the discrete domain are: G freq ( z ) 
1.33 106 z 2  5.307 106 z  1.323 106
(16) z 2  1.99 z  0.99
C freq ( z ) 
94 91.05 z  91.04
(17) z  0.99
International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres Considering that it is a system of mechanical action, there are real limitations imposed by the actuator to the controllers design. Negative actions are not physically possible to be realized in practice, creating limitations on the speed and amplitude of control actions. Synchronization and Frequency Tracking The method of synchronization and frequency tracking proposed is inspired in [16] and [17]. It has great importance in both the voltage and frequency controls, being responsible for synchronization of SVC compensation currents through the synchronism signals (sine and cosine) and instantaneous frequency measurement. The synchronization method is presented in Fig8 (a). The use of two low pass filter (LPF), being the first of 4th order and the second of 2nd order, with respective TF presented in (18) and (19), make it possible to obtain the synchronism signals from what is obtained the phase angle. LPF1 ( s ) 
1
(18) s 4  4Qn s 3  (2  4Q 2 )n2 s 2  4Qn3 s  n4
LPF2 ( s ) 
1
(19) S 2  2Qn s  n2
With the purpose of obtaining the instantaneous frequency of generated voltages, which compared to a reference value generates the error input of the frequency controller, it is applied a single‐phase Phase Locked Loop (PLL) algorithm. The diagram block of the PLL is presented in Fig. 8(b). (a)
(b) FIG. 8 (A) SYNCHRONIZATION METHOD DIAGRAM BLOCKS; (B) PLL DIAGRAM BLOCKS. Table 5 presents the parameters considered for LPF design and gains of PI controller applied to PLL. TABLE 5 LPF PARAMETERS AND PLL PI CONTROLLER GAINS Parameter Gain k
ωn , Q 60 Hz, 0.7 P _ PLL 0.005335 I _ PLL 1.137 k
Voltage Control A major disadvantage of IG operating in isolated areas is that the variation in load at the generator terminals considerably influences the amplitude of the voltage generated by the induction machine, even if the rotor speed is maintained constant. The voltage at the terminals of the SEIG is governed by the excitation capacitance, the rotor speed and the power and power factor associated to the applied load, since the terminal voltage decreases with the increase of the difference between reactive power (VAR) provided by excitation capacitors and the reactive power required by the generator and applied loads [9, 18]. Therefore, it is possible to use a SVC in order to regulate the voltage generated by SEIG during variations of loads, since it can operate absorbing or injecting reactive power into the system. Figure 9 shows a simplified block diagram of the control system of the SVC in synchronous dq axis with two control loops being employed to control the SVC. Both loops, internal and external, use proportional‐integral (PI) controllers. The dc‐link voltage is kept constant at a reference value by the control of d axis SVC current. Similarly, the control of ac voltages amplitudes generated by the IG is accomplished by controlling the reactive power flow, represented by q axis SVC current [18]. 95 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 FIG. 9 BLOCK DIAGRAM OF THE CONTROL SYSTEM OF THE SVC. The comparison of dc bus voltage (vdc) to a reference value (vdc*) is the input of a PI controller, which generates the d axis reference current (id*). The d axis inverter current (id) controls the active power flow through the SVC. Similarly, the error between the IG output voltage in the d axis (vd) and its reference value (vd*) is the input of another PI controller, generating the q axis reference current (iq*). The q axis inverter current (iq) controls the reactive power flow through the SVC. The errors between the reference currents obtained from the outer loop voltage controllers and the measured currents are the inputs of PI current controllers, which generate the control signals in the dq axis (ud and uq). The TF’s which define the electrical system behavior are obtained from (13), considering the parameters presented in Table 6. In the sequence, the TF’s can be discredited using the FOH method assuming the following form: Gvd ( z ) 
vd 4.564 107 z 2  1.79 106 z  4.564 107

(20) iq
z 2  1.618 z  1
Gv dc ( z ) 
Gid iq ( z ) 
vdc 0.01065 z  0.01063

(21) id
z 1
iq 0.4498 z 2  0.0003595 z  0.449
id


(22) ud uq
z 2  1.996 z  0.9976
TABLE 6 ELECTRICAL SYSTEM AND GI PARAMETERS Parameter Value Capacitor Bank 40 μF Output Filter (LF / RF) 2.5 mH / 0.03Ω DC‐Link Capacitor 4700 μF/900 V Switching frequency 10 kHz IG rated voltage 220 V(∆ connection) Voltage frequency 60 Hz IG power 5 HP IG Speed 1730 rpm (4 poles) Stator resistance 0.66 Ω Rotor resistance 0.25 Ω Stator leakage reactance 0.929 Ω Rotor leakage reactance 0.929 Ω Rotor inertia 0.034 kg.m2 Power Factor 0.81 The parameters of the PI controllers can then be defined, as presented in the following: 96 Cvd ( z ) 
0.151z  0.149
(23) z 1
Cvdc ( z ) 
2.881z  2.479
(24) z  0.99
International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres Cid  iq ( z ) 
0.2888 z  0.2782
(25) z 1
Simulation and Experimental Results
The generation system shown in Fig. 10 has been simulated with Matlab® and implemented in laboratory. The generation system is composed by a set of primary machine and SEIG, SVC with an inductive output filter, and control system consisting of a DSP TMS320F28335 and sensing devices of voltages and currents. In both, simulation and experimental setup, it was considered the parameters shown in Tables 1, 2, 3, 4, 5 and 6. At the experimental setup, in order to represent the behavior of the hydraulic turbine and hydraulic actuator, it was employed a 7.5 HP induction motor, driven by frequency inverter as the primary machine. The model of the DSP was chosen due to high processing speed (150 MHz), its 16 analogue input channels and multiple digital output channels, fundamental for the reading of system variables and to the frequency and voltage control proposed and especially to its feature of floating point operation, whose benefits simplify the development of the programming algorithm. The response of inner control loops of the voltage controller can be simulated short‐circuiting the inductive filter output and fixing values for dq axis reference current (id* and iq*). The expected response is presented in Fig. 11, with the dq axis currents generated by SVC following the reference currents imposed. The external control loop response of AC voltage is presented in Fig.12, where the controller responds appropriately to changes in reference (vd *). Figure 13 shows the sequence of loading of the DC bus capacitor of the SVC. At time 6.5 seconds, the first contactor is activated and the terminals of the SVC are connected to the AC bus through resistors to limit the starting current. During this period, the IGBTs are disabled and the SVC operates as an uncontrolled rectifier preloading the DC‐
link capacitor, increasing the DC bus voltage to approximately 315 V. As soon the DC bus voltage achieves the steady‐state, the second contactor is activated and the starting resistors are removed from the circuit. At about 14.5 seconds, the IGBTs of the SVC are enabled, and control starts to work, so that the DC bus voltage is controlled at approximately 450 FIG10. THE INITIAL GENERATION SYSTEM SHOWN TIME PERIOD WITH MATLAB FIG.11 AC VOLTAGE PRESENTED TIME PERIOD 97 www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 FIG.12 DC VOLTAGE PRESENTED TIME PERIOD FIG.13 COUNTENEOUSLY TIME PERIOD GENERATION PRESENT BY MATLAB FIG.14 CONTINEOUSLY AC VOLTAGE TOME PERIOD FIG.15 SYNCHRONIZATION IN DIFFERENT TIME PERIOD IN DC VOLTAGE Conclusions
In this chapter it is proposed advances in the modeling and control of micro hydro power stations based on nonlinear modeling of hydraulic turbine, considering real application parameters and limitations. Nonlinear model, in comparison to the linear model, is more appropriate for large‐signal time‐domain simulations, which is desirable for studies involving large variations in power output and frequency. 98 International Journal of Power and Renewable Energy Systems (IJPRES) Volume 2 Issue 2, 2015 www.as‐se.org/ijpres It was proposed the hydraulic and electric system modeling, the last composed by IG, excitation capacitors and SVC, being the SEIG considered as a source of balanced sinusoidal three‐phase voltage and constant frequency. The system modeling has allowed the design of frequency and voltage controllers, from which were obtained simulation and experimental results. The model validation is obtained from the comparison of simulation and experimental results, which showed very similar responses. Small differences between the results are associated with the non‐modeled dynamic of the IG, once it was considered an ideal voltage source in the modeling, and the dynamic associated to damping in the real system that have not been considered, particularly with regard to the frequency regulation. Moreover, the whole control system is implemented digitally by means of a DSP, which makes faster the development of the algorithm and also of easy adaptation if needed to be applied in other plants using SEIG. Finally, this chapter contributed in the literature regarding the use of non‐linear modeling of hydraulic turbines and the use of PD and PI controllers applied to cases of micro hydro power plants, where it is possible to control the flow of water and avoid that during periods of water shortages this system stop operating. REFERENCES
[1]
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Chauhan, Y.K., Jain, S.K., and Singh, B.: ‘A Prospective on Voltage Regulation of Self‐Excited Induction Generators for Industry Applications’. IEEE Transactions on Industry Applications, 2010, 46, (2), pp. 720‐730 [7]
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