Active Power Control of Hydro-Electric Power Unit ... - IEEE Xplore

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Active Power Control of Hydro-Electric Power Unit Auxiliary Synchronous Generator Connected to Distribution Systems M. O. Oliveira, M. C. Rodriguez Paz, A. S. Bretas, S. L. S. Severo, A. A. P. Lerm, Members, IEEE, and W. de F. Ciarelli

Abstract—This paper presents initial project and test results of an active power control scheme development for auxiliary synchronous generators of an AES-Distribution System connected Hydraulic Generation Unit. An open loop control system using TCSC switched by IGBT’s is proposed to compensate the inductive impedance of the synchronous generator and control the active power injection. Several switching angles were evaluated at different compensation levels and simulations are presented to analyze the effectiveness and robustness of the proposed controller project. Index Terms--. Synchronous generator, auxiliary service, distribution systems, TCSC, series compensation.

T

I. INTRODUCTION

HE safety and continuity of energy production from a Hydroelectric Power Plant (HPP) depends greatly on the auxiliary power system reliability. This reliability is dependent on the power supply methodology; both direct current (DC) and alternating current (AC) are possible configurations [1]. In AC supply systems the loads are primarily motors, illumination, heating elements and equipments for general use, where the voltage level is usually 220 or 380V. In DC systems, the common loads are control and protection circuits, transducers, startup of excitation systems, among others, where the usual voltages levels are 24 and 125V. The energy provided by auxiliary services systems is delivered to various consumption points within the plant through an internal distribution system and in the studied case, with ring configuration. This ring distribution system can operate in a radial configuration through operation switches located at strategic points, allowing the power flow through different paths, increasing system reliability [2]. The Synchronous Generators (SG) excitation system performs control and protection functions, which are essential This work was supported in part by the AES-Tietê Company and in part by CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico). M. O. Oliveira, M. C. Rodriguez Paz and A. S. Bretas are with the Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil, 90035-190 (phone: +55 51 33083291; e-mail: [email protected], [email protected], [email protected]). S. L. S. severo and A. A. P. Lerm is with Southern Federal Institute of Education and Technology, Pelotas, RS, Brazil, (e-mail: [email protected], [email protected]). W. de F. Ciarelli is with AES-Tietê Company, São Paulo, Brazil, (e-mail: [email protected]).

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for the correct performance of the Electric Power System (EPS). The excitation system can be classified into three categories according to the excitation power source, namely: direct current system (CC), alternating current systems (AC), and static systems [3]. The advent of statics excitation systems caused them to replace the original rotating systems, however, the old auxiliary synchronous generator were mechanically held in the turbine-generator set, objecting to not change the original dynamic characteristic. In this sense, the use of auxiliary generators in electrical power supply for auxiliary service, even if already installed, is not trivial, since they were originally projected as an excitation system of the main generator. Moreover, its mechanical connection with the primary machine makes the control system act only on the angular opening of the main generator, which does not allow active power regulation on the auxiliary machine. Developed originally for power transmission systems, FACTS devices (Flexible AC Transmission System) provide to the power system greater operational flexibility. In particular, a TCSC device (Thyristor Controlled Series Capacitor) can be used in optimizing stability margins, increasing the limits of power transfer and damping EPS inherent oscillations [4]-[8]. This paper presents the initial project and test results of an active power control scheme for an auxiliary generator mechanically connected to a Hydraulic Generation Unit Main Machine. The auxiliary generator is connected to a 400 V power system auxiliary bus. A TCSC is used on the active power control scheme, with its equivalent series reactance controlled without necessary change of the generator opening angle. II. BASIC CONCEPT OF TCSC TCSC is one of the best known series FACTS controllers, which has been used for many years to increase line power transfer capability as well as to enhance system stability [9][12]. Moreover, it can have various roles in the operation and control of power systems, such as scheduling power flow, providing voltage support, decreasing unsymmetrical components, reducing net loss, damping the power oscillation, providing voltage support, limiting short-circuit currents, mitigating subsynchronous resonance and enhancing transient stability [9]. TCSC module consists of three components: a

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series capacitor (C), bypass inductor (L) and bidirectional thyristors, as shown in Fig. 1. Also in parallel, same as in conventional series capacitor applications is a metal-oxide varistor (MOV) for overvoltage protection and a bypass circuit breaker (CB). A complete TCSC system may consist of several modules such as parallel controlled reactors and conventional series capacitors, all in series with the EPS to improve the overall power system performance. TCSC is a controllable power electronic element in the whole power circuit, and line impedance can be adjusted by phase control of its thyristor switches [13]-[14].

A. TCSC Configuration The equivalent impedance of TCSC consists of the partial cancellation of the capacitance of fixed capacitor for conducting part of the IGBT (Insulated Gate Bipolar Transistor), so for the fundamental frequency is [15]: X TCSC (α ) =

X C ⋅ X L (α ) X L (α ) − X C

where, XC and XL are the capacitor and inductor impedance respectively. Thus, the TCSC represents a parallel LC circuit for the generator current. By varying the angle α the TCSC impedance can change from its minimum capacitance, given by: X TCSC min = X C = 1 ωC

Fig. 1. Basic TCSC scheme.

(2)

This value can vary until the parallel resonance, when XC = XL (α) and XTCSC max theoretically becomes infinite. Decreasing α below the resonance angle, the TCSC impedance becomes inductive reaching the minimum value on α = 0:

III. PROPOSED ACTIVE POWER CONTROL SYSTEM Fig. 2 illustrates the proposed active power control system for active power regulation on the auxiliary synchronous generator. This system has the function of generating the control signal to trigger the power semiconductor to a series compensation of the TCSC type, implemented by switching inductors. The TCSC is responsible for the generation control of the auxiliary synchronous machine (Eg/Vt) and for the voltage and current synchronization supplied by generator to the AC bus. According to the diagram presented in Fig. 2, the data acquisition of the voltage and current generated by the synchronous machine is performed through potential transformers (PTs) and current transformers (CTs) [6]. The conditioning circuit has the task of adapting the voltage and current levels to compatible ones with values accepted by the analog-digital converters (A/D). It also is responsible for the the electrical isolation between the measurement system and the sampling circuit, and holds a pre-filtering function of the signals (anti-aliasing) [7].

(1)

X TCSC min =

X L⋅XC X L − XC

(3)

Thus, the TCSC has two operating ranges as a function of

α, α = 0° to α = αL lim and another from α = αC lim to α = 90°. IV. CASE STUDY Fig. 3 illustrates the power system studied in this work, where the auxiliary synchronous generator (AG) is mechanically connected on the axis of the main synchronous generator (SG), which is moved by a Hydroelectric Turbine (HT). In this configuration, the AG provides electric energy to auxiliary systems of the HPP through the a 400 V distribution system bus.

Fig. 3. Studied power system.

Fig. 2. Proposed control system.

Fig. 4 illustrates the simulation system built in Matlab environment [16], for this study. The TCSC is placed in series with the equivalent impedance of the auxiliary synchronous generator with propose of changing active power transfer from the rotating machine to the load (distribution system). In this analysis, the active power generated by the synchronous machine is injected directly on the load. The synchronous generator technical data are, rated power: 1MW, voltage:

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400V, stator resistance: 1.62Ω, stator inductance: 0.102Hy, and frequency: 60Hz.

In the open loop control scheme (without feedback signal of the main generator) the IGBT’s firing angle value was indicated manually, and this was maintained through the simulation (5 seconds). All simulations began with the bypass breakers closed (CB in Fig. 5), and after 0.5 seconds of simulation they are opened to incorporate the compensator. Since the bypass CB were opened the system switches the IGBTs with a constant firing angle of predetermined value. Fig. 6 illustrates the control system in open loop for the synchronous generator.

Fig. 4. Block diagram of the simulation circuit.

In the simulations carried out the nominal compensation of the generator was changed between 75% and 50% aiming to determine the TCSC compensation limit for this application. It is noteworthy that the TCSC can work in capacitive, inductive or manual mode. The inductive mode corresponds to firing angles from 0° – 50°, and the capacitive mode, firing angles from 70° – 90°, for which they get smaller impedances and therefore an increase active power transfer. A. TCSC Control Circuit The TCSC is conformed by a fixed capacitor in parallel with a reactor, which is controlled by IGBTs placed in each phases, as illustrated in Fig. 5.

Fig. 5. Command diagram of series capacitor controlled by IGBT’s.

B. Control System The TCSC compensator uses as feedback data the voltages and currents values to calculate the compensated equivalent impedance. Thus, the mentioned impedance indirectly determines the active power level to be transferred. A Proportional-Integral (PI) controller was latter constructed; however a manual open-loop control scheme was used in this initial study for active power control. In the proposed control system the IBGT’s firing use three PLL units (Phase-LockLoop) for synchronizing the firing angle with the generator line current, since the line voltage can vary greatly during the operation and hinder the active power control [17].

Fig. 6. Control system in open loop for synchronous generator.

V. SIMULATION RESULTS The test circuit used in the simulations is illustrated in Fig. 4. The simulations starts after defined the control block parameters of the TCSC. During the first 0.5 seconds, the TCSC was “bypassed” through the CB with the purpose to see the behavior of the active power supplied by the SG without compensation. After, the TCSC goes into operation and begins to regulate the impedance according to the percentage requested by predefined firing angles, thus the active power transfer to the load begins to change. It is noteworthy that the maximum time of the simulation was 5 seconds. The simulations were performed for different compensation levels, being considered values among 75% and 50%. For values outside these limits, problems of high fault currents where found, as control problems for load balancing between parallel lines in compensation near the upper limit, and subsynchronous problems in compensation for near to lower limit values [4]. Moreover, it was considered that the relationship between the inductance and capacitance reactance (XL/XC) of the TCSC should be fixed within the range 0.1 to 0.3. This considers that the resonance frequency does not coincide with, or is close to, two and three times the fundamental frequency [15]. Tables I and II present the parameters used in simulations, where simulated compensation of 75% and 50% for ratios of XL/XC of 0.1, 0.2 and 0.3. A. Compensation 75% Table I presents the parameters values used for simulation of a 75% compensation of the inductive reactance of the SG. This table details the capacitor and reactor values to circuit compensator, XC-TCSC and XL-TCSC respectively, as well as of equivalent capacitive value and inductance reactive, XL-TCSC and LTCSC.

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instability (magnitude) for α = 30° and instability reduction in for α = 75°. The active power for extreme values of 0° and 90° remained the same.

TABLE I SIMULATION PARAMETERS FOR COMPENSATION 75%

XL/XC

XC-TCSC [Ω]

CTCSC [F]

XL-TCSC [Ω]

LTCSC [H]

0.1

34.014

7.7984e-5

3.4014

0.0090225

0.2

34.014

7.7984e-5

6.8028

0.018045

0.3

34.014

7.7984e-5

10.20

0.02706

Fig. 7 illustrates the equivalent impedance variation (TCSC impedance plus SG impedance) during the simulation time. In this figure, different IGBT firing angles, between the following values: 0°, 30°, 75° and 90°, are shown with the purpose of studying the behavior of the compensator between its extreme values. Still, it is verified the equivalent impedance variation across the firing angle value of the IGBT. Thus, for α = 0° the equivalent impedance does not change and therefore it is expected that the active power provide by SG suffers no alteration. Now, for α = 90° the impedance remains constant at approximately 35Ω, and for switching angles of intermediate values (30° and 75°) there are instabilities in the impedance value.

Fig. 7. Equivalent impedance variation, XL/XC = 0.1.

Fig. 8 shows the active power variation in function of the impedance compensation change, which varies according to the IGBT’s firing angle. As observed in this figure, for α = 0° there was no change in the active power supply by the SG due to no change of the equivalent impedance for this angle (see Fig. 7). For α = 90°, there is an increase in active power of 0.25MW to 0.85MW, with an oscillation period of 1 second. For intermediate firing angles, there are instabilities in the active power, being more critical to α = 30°. For α = 75° the active power increase was higher than the case of α = 90°, however, with a slight instability in the power value which fluctuates between 1MW and 1.3MW. Fig. 9 shows the active power variation for a ratio XL/XC = 0.2. In this case, there is an increase in the active power

Fig. 8. Active power variation, XL/XC = 0.1.

Fig. 9. Active power variation, XL/XC = 0.2.

In Fig. 10, the ratio XL/XC is 0.3 where the active power presents the same behavior as in Fig. 8, i.e., the instability of the active power supplied for SG increases for low firing angles. For α = 0° the system does not present equivalent impedance variation and therefore, there is no variation of active power. Fig. 11 presents the active power behavior for different ratios XL/XC with the same compensation rate. It is observed that smaller the ratio, lower is the power instability for firing angles in the inductive operation mode (0° < α < 50°). For values of α > 50°, it is observed that the active power has a tendency to a constant value the higher is the ratio XL/XC, however, the time delay which the power is stable is greater than 5 seconds. The oscillatory time period could generate problems in the closed loop control system, especially in the

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case involving the SG synchronization with the HPP auxiliary service bus.

values: 0°, 30°, 75° and 90°. It can be seen that the equivalent impedance increases for both α = 75° and α = 90°, and remained constant at 25Ω and 23Ω respectively. In those cases, there is no instability in the impedance value. For which it is expected that the active power supplied to the load is stable. For switches low angles (α = 0° e α = 30°) no changes in the equivalent impedance value are obtained.

Fig. 10. Active power variation, XL/XC = 0.3.

Fig. 11. Equivalent impedance variation, XL/XC = 0.1.

In Fig. 12 it can be observed the behavior of the SG active power supply for a compensation of 50%. In this case, the active power remained constant and stable for all switches angles considered. For α = 0° and α = 30° there were no changes in the power value due to the no impedance compensation of the generator for these angles. For α = 90°, there is an increase in active power of 4.6MW, with oscillatory period of 1.2 seconds. On the other hand, for α = 75° the increase power, obtained for the same compensation of 50%, reached a value of 5.2MW. However, the transitory period was 2 seconds, higher than if α = 90°. Fig. 11. Active power variation as a function of XL/XC (α = 0°).

B. Compensation 50% Table II shows the parameters selected for simulation of 50% compensation of the inductive reactance of the synchronous generator. TABLE II SIMULATION PARAMETERS FOR COMPENSATION 50%

XL/XC

XC-TCSC [Ω]

CTCSC [F]

XL-TCSC [Ω]

LTCSC [H]

-5

0.1

22.6760

11.6977e

2.2676

0.006015

0.2

22.6760

11.6977e-5

4.5352

0.01203

22.6760

-5

6.8028

0.018045

0.3

11.6977e

Fig. 11 shows the equivalent impedance variation when the IGBT’s firing angle was changed between the following

Fig. 12. Active power variation, XL/XC = 0.1.

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Fig. 13 presents the active power variation for a ratio XL/XC = 0.2. For this relationship and for the firing angle of α = 90° and α = 75°, the active power increase was less than the previous case reaching the values of 0.5MW and 0.51MW respectively (4.1MW unless the case corresponding to XL/XC = 0.1). On the other hand, the increase in the ratio XL/XC had generated instability in the power values for α = 30°.

•

For firing angle α = 75° (capacitive operation mode) there was no instability of active power, independently of the XL/XC ratio. VII. ACKNOWLEDGMENT

The authors wish to thank the American Energy System Tietê (AES-Tietê) for the financial support and Southern Federal Institute of Education and Technology (IFSul) and Federal University of Rio Grande do Sul (UFRGS) for the facilities offered. VIII. REFERENCES [1] [2]

[3] [4] [5]

[6] Fig. 13. Active power variation, XL/XC = 0.2.

VI. CONCLUSION This paper presents the initial results of an ongoing study of the active power control in auxiliary synchronous generators connected to a distribution system. In this sense, the TCSC switched through IGBTs was used. The simulation analysis led to the following conclusions: ¾ Compensation 75%: • It was observed that an increase in the ratio XL/XC, increases the instability level of the active power supply for the firing angle α = 30°. For α = 75° this instability was reduced. For α = 0° and α = 90° the active power supply for SG remains the same. • There was no change in the oscillatory time for α = 90° and no change in impedance values for α = 0°. • A low ratio XL/XC generates smaller power instability for firing angle from 0° < α < 50°. • For higher rates of XL/XC greater was the tendency of the active power stability in cases where α > 70° (capacitive operation mode). ¾ Compensation 50%: • There was a greater variation of active power reaching the value of 5.2MW to the following situations: XL/XC = 0.1 and α = 75°. • Smaller the ratio XL/XC, lower the instability in the power values at the expense of a reduction of power transferred for the SG with the TCSC operating in inductive mode.

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16] [17]

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IX. BIOGRAPHIES Mario Orlando Oliveira (M’09) was born in Capiovi. Misiones, Argentina, on May 13, 1979. He received the Eletromechanical Engineering degree from the National University of Misiones (UNaM), Argentina, in 2005 and M.Eng. degree from the Federal University of Rio Grande do Sul (UFRGS), Porto Alegre, Brazil, in 2009. Currently, he is researcher of the Energy Study Center to Development (CEED) and auxiliary professor of the UNaM. His research interests include electrical machines protection and modeling, faults detection and location. Martin Cruz Rodriguez Paz (M’10) was born in Posadas, Misiones, Argentina, on March 03, 1982. He received the Eletromechanical Engineering degree from the National University of Misiones (UNaM), Argentina, in 2007 and M.Eng. degree from the Federal University of Rio Grande do Sul (UFRGS), Porto Alegre, Brazil, in 2010, where he is currently pursuing the D.Eng degree in electric power systems. His research interests include electrical machines protection and control, faults detection and location. Arturo Suman Bretas (M’98) was born in Bauru, São Paulo, Brazil, on July 5, 1972. He received the E.E and M.Eng. degrees from the University of São Paulo, Brazil, in 1995 and 1998 respectively, and the Ph.D. degree in electrical engineering from Virginia Polytechnic Institute and State University, Blacksburg, VA, in 2001. Currently, he is an Associate professor of the Federal University of Rio Grande do Sul, Porto Alegre, Brazil. His research interests include power system protection, control and restoration. Sérgio Luiz Schubert Severo (M’99) was born in Santa Cruz do Sul, Rio Grande do Sul, Brazil, on October 2, 1964. He received the E.E and M.Eng. degrees from the Federal University of Rio Grande do Sul, Brazil, in 1992 and 2003 respectively. Currently, he is an Associate professor of the Southern Federal Institute of Education and Technology (IFSul), Pelotas, Brazil. His research interests include power system automation and power electronics. André Arthur Perleberg Lerm (M’99) was born in Pelotas, Rio Grande do Sul, Brazil, on April 16, 1964. He received the E.E degrees from the Catholic University of Pelotas, Brazil, in 1986, and the M.Eng. and the Ph.D. degrees from the Federal University of Santa Catarina, Brazil, in 1995 and 2000 respectively. Currently, he is an Associate professor of the Southern Federal Institute of Education and Technology (IFSul), Pelotas, Brazil. His research interests include power system modeling and control. Wagner de Freitas Ciarelli was born in São Paulo, São Paulo, Brazil, on June 7, 1987. He received the Electrical Engineer degree from the University of São Paulo, Brazil, in 2009. Since 2010 he works at AES Tietê, an energy generation company of AES in Brazil, currently he is coordinator of real time operations and has interests in generation units and power plants auxiliary services reinforcements.