XIX International Conference on Electrical Machines - ICEM 2010, Rome
The Dynamic Interaction of Independent Power Producer Synchronous Machines Connected to a Distribution Network in ATP-EMTP F. A. M. Moura, J. R. Camacho (IEEE-SM), M. L. R. Chaves and G. C. Guimarães Abstract-The main task in this paper is to present a performance analysis of a distribution network in the presence of two synchronous generators in parallel, with its speed and voltage regulators modeled with TACS – Transient Analysis of Control Systems, for distributed generation studies. This type of configuration is common in systems where renewable fuels are used as sugar cane bagasse or straw. Must be highlighted that these generators are driven by steam turbines, and the whole system with regulators and the equivalent of the power authority system in the point of connection (CCP - Common Coupling Point) are modeled in the “ATP-EMTP - Alternative Transients Program”. Technical questions studied here refer to steady-state voltage profile, voltage stability, voltage dip due to a balanced three-phase fault, load rejection, distribution line outage and the behavior of machine regulators facing the aforementioned contingencies. Results show that, in some cases, the independent power producer (IPP) can be a risk to the physical integrity of the system and in other cases can be very beneficial to the distribution system in the point of connection.
Emin - minimum exciter output voltage (applied to generator field), Ve = f ( E f ) - saturation function, Vmax - maximum limit for the voltage regulator output voltage (pu), Vmin - minimum limit for the voltage regulator output voltage (pu), E f - field voltage (pu),
S n - rated aparent power, U n - rated voltage, L - length, RA - armature resistance (pu), xL - armature leakage reactance (pu), xd - direct axis reactance (pu), xq - quadrature axis reactance (pu),
Keywords—distributed generation, synchronous generator, voltage profile, voltage regulator, speed regulator.
I.
x`d - direct axis transient reactance (pu), x`q - quadrature axis transient reactance (pu),
NOMENCLATURE
x``d - direct axis sub transient reactance (pu), x``q - quadrature axis sub transient reactance (pu),
Vt - voltage at the independent generator bus bar (pu), Vref - reference voltage (pu),
x0 - zero sequence reactance (pu), T `d 0 - direct axis transient short-circuit time constant (s), T `q 0 - quadrature axis transient short-circuit time constant (s),
K a - voltage regulator gain, K e - exciter constant related to self-excited field, K f - time gain for the voltage regulator stabilizing circuit,
T ``d 0 - direct axis sub transient short-circuit time constant (s), T ``q 0 - quadrature axis sub transient short-circuit time constant (s), H - inertia time constant (s), P - pole number, f - frequency (Hz),
Ta - voltage regulator amplified time constant (s), T r - voltage regulator input filter time constant, Te - exciter time constant, T f - time constant for the voltage regulator stabilizer circuit (s),
Emax - maximum exciter output voltage (applied to generator field), ____________________________
ωs - synchronous speed (rad/s), G = flyball gain, Tfb = flyball time constant, T1 = first time constant for the control system, T2 = second time constant for the control system, T3 = third time constant for the control system, T4 = water departure time constant (hydraulic turbine), T5 = turbine time constant (hydraulic or thermal).
This work has the monetary assistance of the Brazilian Ministry of Education and CNPq - National Council for the Scientific and Technological Development. Institutional support of Universidade Federal de Uberlândia through the Rural Electricity and Alternative Energy Sources Laboratory. Fabrício A. M. Moura, Universidade Federal de Uberlândia (e-mail:
[email protected]). José R. Camacho, Universidade Federal de Uberlândia (e-mail:
[email protected]). Marcelo L. R.Chaves, Universidade Federal de Uberlândia (e-mail:
[email protected]). Geraldo C. Guimarães, Universidade Federal de Uberlândia (e-mail:
[email protected]).
1
978-1-4244-4175-4/10/$25.00 ©2010 IEEE
II. INTRODUCTION
is based in one of the models that are the basis for excitation regulators [2], [3] and [4]. According to the data input, this model can be reduced to four basic forms. The model used in this work for the voltage regulator can be seen in Figure 1, it is the type I model, one of the most complete designs recommended by the IEEE.
T
HE interest for distributed generation has increased considerably over the years due to the restructuring in the Brazilian energy sector. With the increasing demand for biofuels it has become common the ethanol production in sugar mill production plants, the electrical energy generation in such plants gained focus in the national energy scene. Such plants are increasing their production and are building larger installations all over the country. Consequently, an increase exists in the number of synchronous generators owned by sugar mill plants. Some of them are connected to the local power authorities’ medium level voltage. This fact added to the current need to benefit from different forms of primary energy, technological advances and the awareness on environment conservation, is the way to induce and contribute to the dissemination of independent electrical power production. Therefore, it is an emerging force the need to understand the influence of such aspects in the operation and design of electrical energy distribution networks. Among the analysis to be made, the monitoring of voltage levels in the Common Coupling Point (CCP), before and after the presence of the Independent Power Producer (IP), as well as the analysis of load rejection, the outage of distribution lines and balanced three-phase short-circuit are made necessary. Moreover, the response of the synchronous machine controls, such as the speed regulator and voltage regulator are the subject of the studies in this paper.
Fig. 1. Voltage regulator model.
B. Speed Regulator The speed regulator was implemented based in one of the simplest IEEE models, and often used in transient stability studies programs. Figure 2 presents the block diagram for the speed regulator associated to the steam turbine (if T4 = 0) or to the hydro turbine (if T4 ≠ 0).
III. SYSTEM MODELING
A. Voltage Regulator
Fig. 2. Model for the speed regulator for a thermal/hydro turbine.
A synchronous generator is used to represent the independent power producer; it is the type SM 59 with eight controls in the ATP model databank [1]. The voltage regulator
Fig. 3. Single line diagram for the electrical system in the case considered
2
TABLE I SYNCHRONOUS MACHINE PARAMETERS FOR THE INDEPENDENT GENERATOR DATA NEEDED FOR G2 (G2 = G3) Sn = 5MVA x0=0.046pu Un= 6.6kV T’d0=1.754s RA= 0.004pu T’q0=0s xL= 0.1pu T”d0=0.019s xd= 1.8pu T”q0=0.164s xq=1.793pu H2=1s ;H3=2s x’d=0.166pu P=4 poles x’q=0.98pu f=60 Hz x”d=0.119pu ωs = 188.5 rad/s x”q=0.17pu ---
IV. CASE STUDIES Beforehand must be emphasized that similar studies, with a different software, were made in [6]. Reference [7] shows the use of ATP for the modeling of a synchronous generator voltage regulator that is driven by a hydraulic turbine.
I. Loss of generating unit G3 Figure 4 shows the voltage behavior at the CCP point of connection (busbar 3) after the loss of generator G3 at the IPP.
C. Electrical System The independent power producer generators become part of the electrical system of a power authority distribution network, as illustrated in Figure 3. Such system is connected to the independent power producer through an interconnecting circuit breaker, following instructions established in [5]. Data depicted in Figure 3 refer to the system rated values, however, particularly for the independent power producer generator does not operate with a 0.8 lagging power factor, after performing a load flow. The source type representing the power authority was defined as a three-phase ideal source, being considered, therefore, as an infinite bus bar. To use such controllable model in ATP, it will be necessary to define the data listed in Table I. The rated parameters obtained for the machine voltage and speed regulators, as well as data referred to the independent power producer synchronous generator, were obtained directly from manufacturers.
Fig. 4. Voltage at bus bar 3 after the loss of generator G3 at the IPP.
It becomes evident, through Figure 4, a phenomenon that strongly affects energy quality [8]. It is a low frequency (around 1.5 Hz) transient oscillating mode. The voltage at the CCP (busbar 3), Figure 4, is reestablished to a permissible value under the point of view of references [9], [10] and [11]. Due to the loss of machine G3 a reduction on the generation of reactive power occurs. By this way, a voltage drop in the IPP installations occurs, taking to the actuation of the voltage regulator in G2 with the objective being the elevation of voltage at the independent generation bus bar, bus bar 4. Consequently, voltage at the CCP also increases, because bus bars 3 and 4 have a strong electrical connection [12, 13 and 14]. Figure 5 illustrates the voltage behavior at the IPP generation bus bar and Figure 6 shows the acting of voltage regulator at the generator G2 in the IPP premises.
D. Power Flow The energy independent generation provides a total power of 4 MVA to the interconnection with the power authority electrical system, through the coupling transformer, T2. Furthermore, the independent power producer provides energy to its internal demand, rated in 2.8 MVA. Active and reactive power produced by the power authority (G1) and the Independent Producer (G2 and G3) can be seen in Table II. TABLE II ACTIVE AND REACTIVE POWER GENERATED BY THE SYSTEM POWER SOURCES Source PG [MW] QG [MVAr] G1
21,182
4,087
G2 = G3
3,302
0,385
Before the presentation of the cases under investigation, it is necessary to highlight that depending upon the “penetration”, for the distributed generation, the obtained results will be affected in a different manner.
Fig. 5. Voltage at the independent generation bus bar after the loss of G3.
3
load rejection viability studies are necessary in cases in which the frequency at the IPP system are not able to return to 60 Hz. Therefore, the priority in industrial processes need to be taken in consideration, as well the modeling of industrial loads [3][4]. Due to the inertia in the electrical power system, G2 comes back to the synchronous speed even after oscillations. Therefore, from its speed regulator is not required to much action.
II. Three-phase short-circuit at the CCP. Studies are made according reference [11], concerning the fault clearing time. This reference advises that under the event of a fault in the network, the independent power producer generator must be taken out of the system in a maximum time of 6 cycles and the rearward relay must trip in 18 cycles (300 ms). Therefore, our goal in this case is to watch the behavior of the connection in the frame of the 6 cycles with the system under fault. Figure 9 illustrate the voltage behavior at the CCP with the assumption of this contingency.
Fig. 6. Voltage regulator response.
It becomes evident at Figure 6, that happened an over excitation of machine G2 of approximately 18%. Figure 7 depict the generator G3 speed increase at the IPP, due to the fact that when electrically disconnected the sudden loss of load causes the increase in speed. Can be observed that the speed will be stable in a speed above the synchronous speed with the reached maximum being ω = 217rad/s (2072.19 rpm).
Fig. 7. Speed response of the independent power producer synchronous machine G3. Fig. 9. Voltage at busbar 3 in the event of a fault.
Figure 8 portray the speed response for the IPP synchronous machine G2 with the loss of generating unit G3. Must be observed that immediately after the loss of G3, the first swing of the generator G2 takes the speed to ω = 187.1rad/s (1786.67 rpm), equivalent to a frequency of 59.55 Hz.
It can be observed a voltage interruption at the CCP, however this interruption lasts for only 6 cycles, corresponding to the period of time allowed for the fault in the system. After the fault removal, voltage comes back to 1.0 pu.
Fig. 10. Voltage at the IPP generation busbar. Fig. 8. Speed response of the IPP synchronous machine G2.
With the independent generation busbar close to the CCP, it will experience a voltage dip. In this case the voltage regulator of both machines at the IPP will act to increase their field excitation, the aim being the increase in voltage level at the generation busbar 4. Figures 9 and 10 illustrate the behavior of
Therefore, the machine shows a deceleration. However, this under frequency if compared with the under frequency limit established by standards [11], is not enough to activate the under frequency protection. It must be emphasized that the
4
voltage at busbars 3 and 4 respectively; busbar 4 is at the IPP premises and portray the voltage regulator response for such machines. The voltage regulator responses of machines at the IPP are identical, so it is depicted in Figure 11 only the regulator response for one of the machines. From the information in Figure 10, can be verified that the IPP generation busbar experiences a voltage dip of approximately 55%, consequently, if the automation electronic equipment at the IPP plant, including computers are not suited to endure such kind of Short-Duration Voltage Variation (SDVV), all those equipment will be restarted.
Fig. 13. Voltage at the CCP with the outage of LD 2 without the IPP.
In both figures, with the outage of LD 2 (distribution line), a voltage drop comes into picture; this is related to the increase in the electrical losses caused by the increase in the distribution line impedance, when removing LD 2. In Figure 14, after the voltage drop to 0.957 pu, in a short time period voltage starts to increase again in a transient fashion, and becomes stable around 0.975 pu. This is due to the fact that this bus bar is electrically connected to the independent power producer bus bar.
Fig. 11. Voltage regulator response for one of the IPP machines.
Due to the short-circuit application, the IPP synchronous machines present a damped oscillating transient in their speed variation. Figure 12 illustrates how the speed of machines G2 and G3 behave in the presence of the applied disturbance. Immediately after the fault application, synchronous machines have the tendency to increase their speed. Furthermore, it is evident that generator G2 that has lower inertia than G3, presents higher oscillation damping.
Fig. 14. Voltage at the CCP with the outage of LD 2 with the IPP
Figure 15 depicts the behavior of the voltage at the independent power producer busbar and Figure 18 reveals the behavior of the voltages regulators for the machines at the independent power producer. A careful analysis of Figures 15 and 16 shows that when the busbar 4 shows under-voltage, the voltages regulators increase the excitation in order to the established voltage at the generator bus bar to come back to 1.0 pu. As can be seen in Figure 16 machines are with 11% over excitation at the end of the transient.
Fig. 12. Speed response for both generators at the IPP.
At Figure 12 can be observed that the machines didn’t oscillate for a long time span, the applied short-circuit lasted for only 6 cycles. Therefore the under or over frequency protection doesn’t have enough time to act.
III. Outage of the LD 2 distribution line. Figures 13 and 14 depicts the behavior of the voltage at bus bar 3 (CCP), respectively without and with the presence of the independent power producer (IPP).
Fig. 15. Voltage at bus bar 4 with the outage of LD 2.
5
[5] [6]
[7]
[8] [9] Fig. 16. Voltage regulator response for both machines. [10]
V. CONCLUSIONS [11]
This work shows the main changes in voltage profile, at an electrical power authority system, more specifically in its distribution level network, due to the presence of an independent power producer with two generators. The loss of the generating unit G3 takes to a voltage fluctuation at the CCP, consequently the IPP generation busbar will be also the subject of a voltage fluctuation. Such fluctuations can give birth to flicker effects at the loads connected to the CCP, as well power and torque oscillations in electrical motors in the IPP installations, with interference in data processing equipment and industrial process control systems. For short-circuit situations at the CCP, the IPP is subject to substantial voltage sag. Such dip lead to losses in their industrial process, with consequent waste of raw material depending on the process involved. Therefore, it is highlighted the need of a better specification of equipments concerning their voltage variation support ability. Protection must be accurately designed to open the interconnection circuit breaker in order to avoid such disturbance in the IPP. It must be highlighted that during short-circuit attention must be turned to the over voltage produced by the fault removal in the steadystate reestablishment. With the occurrence of under voltage at the CCP (with the example of what occurs in the last case of section IV) the presence of the independent power producer is beneficial to the system, since the action of the excitation regulator is a contribution in the sense to improve the voltage at the generation busbar, due to its proximity to the CCP. Therefore, it can be inferred that the quality of protection and machine controls can be a major influence in the system adequate operation.
[12]
[13]
[14]
BIOGRAPHIES Fabrício Augusto Matheus Moura (MSc), was born in February 25, 1983. Received his BSc and MSc degrees respectively in 2005 and 2008 at the School of Electrical Engineering at Universidade Federal de Uberlândia (UFU) and at the Power Quality and Energy Rationalization Group. Currently he is pursuing a DSc degree, in the Rural Electricity and Alternative Energy Laboratory, with the conclusion estimated for the beginning of 2011. José Roberto Camacho (PhD) (IEEE M’1993, IEEE SM’ 2006) was born in Taquaritinga, SP, Brazil in November 03, 1954; he received his PhD degree in Electrical Engineering in the Electrical and Electronic Engineering Department at the University of Canterbury, Christchurch, New Zealand, in August 1993. He is a full professor at Universidade Federal de Uberlândia since 1994. His areas of interest are: Distributed Generation and Electricity for Rural Applications and Alternative Energy. Marcelo Lynce Ribeiro Chaves (Dr) was born in Ituiutaba, Brazil, on October 03, 1951. He received the BSc and MSc degrees from Universidade Federal de Uberlândia (UFU), Brazil, respectively in 1975 and 1985, and the DSc degree from Universidade Estadual de Campinas (UNICAMP), Brazil in 1995, all in electrical engineering. He is a Senior Lecturer with the School of Electrical Engineering at Universidade Federal de Uberlândia. His main interests are electromagnetic transients in power systems, insulation coordination, electromagnetic compatibility and motor drives.
REFERENCES [1] [2]
[3] [4]
IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Std. 1547, New York, USA, 2003. W. Freitas, A. M. França, J. C. M. Vieira Jr., L. C. P. da Silva, "Comparative Analysis Between Synchronous and Squirrel Cage Induction Generators for Distributed Generation Applications”. IEEE Trans. Power Systems, vol. 21,NO.1, FEBRUARY 2006. C. Saldaña, G. Calzolari, G. Cerecetto, "ATP modeling and field tests of the ac voltage regulator in the Palmar hydroelectric power plant", Electric Power Systems Research, Elsevier, 76 (2006), pp.681-687, doi:10.1016/j.epsr.2005.12.020 R. C. Dugan, M. F. McGranaghan, S. Santese, H. W. Beaty, Electrical Power Systems Quality, McGraw-Hill, New York, 2002. Agência Nacional de Energia Elétrica – ANEEL. Resolução N 505, 26 de Novembro de 2001. ONS-Operador Nacional do Sistema Elétrico, Sub module 2.2, The Basic Network Performance Standards, Brasília, DF, 2002, accessed at the Internet, 22/11/2007, at http://tinyurl.com/ypdyhc. (In Portuguese) LIPA-Long Island Power Authority, Control and Protection Requirements for Independent Power Producers, Transmission Interconnections, found at the internet in 22/11/2007, at http://tinyurl.com/33clq4. F. A. M. Moura, J. R. Camacho, J. W. Resende, and W. R. Mendes, “Synchronous Generator, Excitation and Speed Governor Modeling in ATP-EMTP for Interconnected DG Studies”, ICEM2008 – XVIII International Conference on Electrical Machines, Vilamoura, Portugal, September 2008, doi: 10.1109/ICELMACH.2008.4800246 F. A. M. Moura, J. R. Camacho, J. W. Resende, and W. R. Mendes, “ATP on the Impact Analysis of an Independent Power Producer in a Distribution Network”, ICHQP 2008 – XIII International Conference on Harmonics and Quality of Power, Wollongong, Australia, 2008, doi:10.1109/ICHQP.2008.4668777 F. A. M. Moura, J. R. Camacho, M. L. R. Chaves, and G. C. Guimarães, “Independent Power Producer Parallel Operation Modeling in Transient Network Simulations for Interconnected Distributed Generation Studies”, Electric Power Systems Research, Elsevier, Volume 80, Issue 2, February 2010, pp. 161-167,doi:10.1016/j.epsr.2009.08.016
ATP-EMTP – Alternative Transients Program, accessed in the internet in 22/11/2007, http://www.emtp.org. G. C. Guimarães, Computer Methods for Transient Stability Analysis of Isolated Power Generation Systems with Special Reference to Prime Mover and Induction Motor Modelling, PhD Thesis, University of Aberdeen, 1990. P. M. Anderson and A. A. Fouad, Power System Control and Stability, vol. I. Iowa, 1977. P. Kundur Power Systems Stability and Control, McGraw-Hill, EPRI Power Systems Engineering Series, New York, 1994.
Geraldo Caixeta Guimarães (PhD) was born in Patos de Minas–MG, Brazil, in 1954. He graduated in Electrical Engineering at Federal University of Uberlândia, Brazil. He received his MSc degree from Federal University of Santa Catarina, Brazil, and his PhD from University of Aberdeen, Scotland. He is presently a professor and a researcher at School of Electrical Engineering, Federal University of Uberlândia. His research interest areas are: Power System Dynamics, Distributed Generation, Renewable Energy and Applied Electromagnetism.
6
