IX SIMPÓSIO DE ESPECIALISTAS EM PLANEJAMENTO DA OPERAÇÃO E EXPANSÃO ELÉTRICA

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1 IX SEPOPE 23 a 27 de maio de 2004 May, 23 th to 27 rd 2004 Rio de Janeiro (RJ) Brasil IX SIMPÓSIO DE ESPECIALISTAS EM PLANEJAMENTO DA OPERAÇÃO E EXPANSÃO ELÉTRICA IX SYMPOSIUM OF SPECIALISTS IN ELECTRIC OPERATIONAL AND EXPANSION PLANNING SP 163 The FURNAS Experience on Real Time Simulation Victor Alexandre Belo França (*) Sergio do Espírito Santo Helio Hayashi de Almeida Carla Heloisa Bentes do Prado Damião FURNAS Centrais Elétricas S.A. ABSTRACT FURNAS Real Time Power System Simulator has been working with real time power system simulation in order to help the operation planning and the system operation itself. During the years, the Laboratory has passed through Analog to Digital technology. This paper presents FURNAS knowledge and experience in this area. KEYWORDS: Real Time, Analogue Simulation, Modeling, Real-Time Digital Simulation, Equipment Testing, Electrical System Operation. 1.0 INTRODUCTION In the modern world in which we live in, where the evolution of technology brings in a high complexity in the analysis of systems, simulation has been playing an important role either during the project period or for a future analysis or evaluation. The Electrical Power System is very complex and dynamic and has been using simulation more and more as a tool to improve its performance. One of the simulation techniques is real time simulation. In this case, the simulation must be performed in a way that allows the simulated system to respond at the same speed, neither slower nor faster, as the real world. This ability is particularly interesting when we need to evaluate the performance of control or protection systems because it allows the devices to interact with the simulation, creating a closed loop testing. This is possible due to the fact that both (real device and simulated system) run at the same time rate. The quality of the simulation is highly dependent on the modeling of the system and it must be carried out carefully. The more precise the modeling is the more reliable the simulation becomes. When the modeling for real time simulation is performed using reduced physical models or a hardware implementation for each component of the system, the simulation is said to be analog and the set of devices used to perform it is usually known as a TNA (Transient Network Analyzer). On the other hand, when the modeling for the simulation is performed using a mathematical representation of the system, implemented by software, it is said to be a real time digital simulation. 2.0 THE HVDC SIMULATOR FURNAS experience in real time simulation started during its HVDC Transmission System project in the 80 s. The manufacturer realized that such a huge project would demand a tool to help in the project and in the performance studies and analysis of control and protection systems. Then it was decided to build a simulator to supply this lack. The Main Circuit (Generation and transmission systems) was planned as a scaled down HVDC converter station interconnected to the real controls and protections. The measurements from the system would be sent to the controllers in per unit scale and the controllers would send the firing pulses to the valves. For that reason the controllers (*) Rua Real Grandeza, 219- Bl. E sala CEP Rio de Janeiro-RJ Telefone : (021) Fax (021) vfranca@furnas.com.br 1

2 Figure 1 The Main Circuit Simulator of the FURNAS HVDC Link act as they were connected to the real system. Figure 1 shows the configuration of the Main Circuit that represents a bipole of DC transmission. It is composed of AC generators, transmission systems and filters, transformers, DC converters and transmission System and synchronous machines, all of then mounted in 19 inch panels. Actually that is only half part of the Simulator. Each part is identical, meaning that it is able to model two bipoles or perform two different studies at the same time. These models were built using analog components, such as resistors, inductors, capacitors, transformers and thyristors and amplifiers. The concern about the quality of the modeling compelled the designers to include special circuitry in order to minimize differences between the models and the real components. For example, we can point out the loss compensation of the transformer models to increase the quality factor and the loss compensation of the DC line connections. In such a case, the simulator is normally referred to as an analogue HVDC simulator rather than a TNA in order to indicate its enhanced capabilities. Several tests were carried out during the design and commissioning tests to improve the behavior of the control systems, mainly concerning the recovery time and stability of AC system under disturbance. After the HVDC link commissioning, the goal was not to use it as a development tool but as an operation analysis tool as well as for training purposes and supply spare parts. Since that FURNAS has been using the HVDC simulator to reproduce and analyze the behavior of the system when facing any kind of disturbance, test new software versions for the control system, and carry out studies to evaluate the impact of digitalizing analog functions. As a security strategy, one part of the Simulator has remained with the same configuration implemented in the plant while the other part has been used to attempt for different implementations THE SVC SIMULATORS In spite of the fact that the HVDC simulator was planned and trimmed for a specific task, some elements can be used to perform studies of other systems with a reasonable precision. This flexibility together with the good results presented encouraged FURNAS to keep improving its real time tools. So the planning and the operation areas decided to include in future purchases of important control systems a request to the supplier to provide an extra control system, equal to the real one that would be installed in the plant, to be used in real time studies. Therefore in the beginning of the 90 s FURNAS received two Static Var Compensation (SVC) Simulators, one for the substation of BANDEIRANTES and the other for the substation of BARRO ALTO. The SVC simulators included the real controls for firing pulses and a scaled down model from the real thyristor valves. The AC System from the HVDC simulator must be set up in a different configuration in order to model an equivalent system for the SVC tests. Figure 2 shows the interface used to connect the SVC simulator from BANDEIRANTES to the AC system model. Figure 2 The BANDEIRANTES SVC Simulator After the commissioning tests, both models were used to help in training, maintenance and operation. 2

3 4.0 THE DIGITAL MACHINE MODEL In the beginning of the 90 s, processing capacity of microprocessors increased dramatically. This allowed for the project of new real time digital models. The main idea was to model the behavior of a device by using mathematical equations solved in real time. Therefore, the behavior of a specific device, modeled digitally, could be converted into analog by using a digital to analog converter and then interconnected to a TNA through an interface. The interface must satisfy the level requireme nts imposed by the analog models so power amplifiers must be used. In order to perform a closed loop system, information from the analog system must come as a feedback to the digital model. Therefore this information must be converted to digital using an a nalog to digital converter before interacting with the mathematical equations. When the simulation environment is composed of analog and digital models interacting with one another, it is usually said to be a hybrid simulator. There are several advantages of the digital models over the analog ones such as flexibility to change parameters, size reduction, improved detailing of the model and it is cost-effectiveness compared to a similar analog model. The disadvantage is the interfacing whose imperfections could affect the quality of input and output information. FURNAS acquired digital models of synchronous machine in order to expand, to improve and to facilitate the representation of generators and synchronous compensators, including the speed and voltage regulators and PSS models. The digital model of a synchronous machine has been used to model the 50Hz power plant from the rectifier side and their regulators, the synchronous compensators connected to the inverter bus and the infinity bus used with the analog AC network. Figure 3 shows how a digital model of a synchronous machine is connected to the HVDC simulator set up as a synchronous compensator. Figure 3 The Digital Model of a Synchronous Machine Regarding this specific digital model, the interface uses a D/A converter to convert the voltages of the machines to analog and then the signals are connected to the analog models by a voltage amplifier used to satisfy the voltage levels and power requirements. The current fed by the amplifier to the system is then transmitted to the digital environment using a transducer and an A/D converter. Therefore, the digital value of the current allows the software model to calculate the reactive power consumed or provided and then update the output voltage, therefore creating a closed loop. The experience in using digital models has shown the direct benefit of using such kind of technology. We can point out the following ones: Reduced time and convenience to set up or modify a new configuration due to software environment Fast upload of new models such as different regulators once they are saved in software libraries Low drift of values and little trimming required (only in the power amplifiers and the transducers) 5.0 THE SIMULATOR EXPANSION In the middle of the 90 s, aiming at expanding and improving the capacity of real time simulator tools, a proposal to upgrade the simulator was made. The main purpose of the expansion was to improve the representation of the AC system at the inverter bus of the IPAIPU HVDC simulator. At that time the representation was limited due to the fact that the analog model uses resistors, inductors and capacitors to model the system and the number of components was limited. Besides, a great number of connections would increase the losses and consequently the quality of the modeling. Therefore, some studies started to be carried out in order to reach the best solution. The first question was: Analog or digital models? The analog technology had the advantage to be an approved technology that had been used for decades. Although it was a traditional technology, the costs were too high due to the high precision and low losses requirements. Another point was that at that time the digital technology was already a trend. The next proposal was to purchase single digital models for each important part of an electrical system and then interconnect it, which would depend on the necessity of the configuration with the analog models, too. The advantage lay on the fact that the digital technology proved to be reliable and easy to use. The disadvantages would be the interconnection itself of the models and the utilization of each single model depending on the proposal being made. The interface between the digital and the analog environment and vice-versa could propagate errors imposed by the processes of conversion, time delays and the physical connections such as loss and differences due to ground 3

4 connections. Therefore, a careful evaluation of the interface as a whole would be a must. Figure 4 shows a possible configuration using several digital models and their handicaps. Figure 4 Digital Models Interconnected on a Hybrid Configuration During the evaluations for the best solution to be reached, a new proposal for a different way of real time digital simulator was presented. Instead of splitting the models required by the system into single digital environments and then interconnecting all of them, the idea was model the entire system in an only powerful digital system creating a fully digital real time simulator. The topology of the digital processing of models together in the same digital environment would shun away the concern of numerous interfacing connections and jumps from digital to analog and analog to digital of the signals from the simulation. The digital models would only interact in the digital simulation and the number of interfaces with the outside world would be done only with the signals required for the controllers or protection devices. FURNAS contacted the manufacturer in order to assure that its simulator would cover the main purpose of the upgrading, i.e., the expansion of the AC representation. As the Electrical sector is very concerned about new technologies, at that time many companies in the simulation area decided to upgrade their simulators purchasing analog or hybrid devices. In an audacious but right move, FURNAS decided to invest into a fully digital real time simulator, being one of the first companies to use this new tool. 6.0 THE REAL TIME DIGITAL SIMULATOR The Real Time Digital Simulator (RTDS) is a fully digital transient power system simulator used to perform great variety of closed-loop testing of control and protection devices. It allows the user to analyze the effects of a large variety of disturbances in the electrical system and its equipment in order to prevent undesired failures. The RTDS Simulator takes advantage of a custom parallel processing hardware architecture assembled in modular units and powerful software simulation. This great combination allows the RTDS to perform a continuous real time digital simulation. It can solve the power system equations fast enough to continuously produce output conditions that realistically represent conditions in the real network. Because the solution is in real time, the simulator can be connected directly to the power system control and protective relay equipment. For that reason it has been widely used to perform analytical power system simulation, test control and protective relay system, education and training. The power system network is arranged in the computer screen using a graphical user interface that loads electrical power system models from libraries and interconnects it in the topology of the system being simulated. When the system is fully represented, the compiler generates the code to be downloaded in the hardware that will be in charge of processing the mathematical modeling in real time. The RTDS simulator software is based on the Dommel Algorithm[1], which is the most used solution in the electromagnetic transient power system simulator algorithms, using the trapezoidal rule of integration to generate algebraic equations. Such kind of technique requires that a solution be calculated in distinct moments of time commonly referred to as time step. Typical values of time step are about 50 to 70 microseconds. Therefore, the solution of the equation that rules the power system behavior must be computed in a time period equal or less than one time step. Also the inputs and outputs that connect the RTDS with other equipment must be updated THE INTERFACE WITH THE ANALOGUE HVDC SIMULATOR As mentioned before, the main idea to purchase the RTDS was to enhance the representation of the AC network of the inverter system. Suitable interfacing points[2] require a stable voltage at the interface node and a stable current out of the node. The point of interface chosen was the inverter AC bus due to the presence of harmonic filters at the respective commutating buses and the converter transformer reactance that suits the stable voltage requirements. The next step was to adjust the modeling of the AC system with the node numbers capacity of processing from the RTDS. Two configurations were considered: One using the digital machine model to represent the synchronous compensator and the other one the modeling of the synchronous compensator in the RTDS. The first option was chosen due to the fact that the digital machine model provides a very good representation and saves processing capacity in the RTDS allowing the modeling to be more comprehensible. 4

5 Figure 5 shows the analogue versus the digital modeling of the AC system. Although the RTDS had been purchased specifically to enhance the HVDC simulator, its versatility permits a great variety of tests in the control and protection system areas. In short, the HVDC Simulator interfacing was only a part of the RTDS possibilities that were already available with the RTDS configuration then purchased THE SYNCRONIZER DEVICE TESTING Figure 5 The Analog versus Digital Representation of the Inverter AC System It can be seen that the digital modeling provides for a much more realistic representation and gives flexibility regarding the sort of fault and system arrangements to be simulated. The interface circuitry used was similar to that described before for the digital machine model. The voltage is sent to the Analogue HVDC Simulator using amplifiers and the current is fed back to the RTDS simulation. The overall system currently set up is show in the Figure 6. The Simulation is fully controlled by the RTDS which is in charge of simulating the AC inverter side applying faults and rearranging the topology of the system as the tests demand (take a transmission line out for example), blocking and deblocking the converters and control the faults in the rectifier side. If necessary the RTDS can be used as a data acquisition system to analyze and store the simulation results. Nowadays, an independent data acquisition system is being used. With these possibilities in mind, it was advisable to wait for an opportunity to use them. That opportunity came up in an evaluation of a synchronizer device to be applied in a capacitor bank breaker[3]. The evaluation would consist of a check of performance of a synchronizer device developed by FURNAS and whether or not it would decrease the overvoltages and overcurrent applied in the system that were damaging pre-insertion resistors. Therefore, the AC system modeling, including the substation model, the breaker and the capacitor banks, were modeled in the RTDS, which send the voltage to the synchronizer device, which in turn sends back the order for closing the breaker. The breaker modeling was carefully carried out because it could affect the final results. In order to make it easier to run the simulations in batch mode, the breaker model in the RTDS was modified by FURNAS to provide for the breaker closing time to be loaded from an information file. The curve used to represent the breaker delay was the normal curve, based on an average and the standard deviation information. Three configurations were evaluated: The synchronizer device controlling the breaker, the synchronizer controlling a pre-insertion resistor, and the pre-insertion resistor randomly switched. So thousands of trials were performed randomly modifying the breaker closing time delay for each phase. The values of voltages and currents were stored and further analyzed. Similar procedures can be applied for transformers switching in and reactors switching off tests. After this internal study, FURNAS was hired twice to perform similar tests for different manufacturers PROTECTION SYSTEM TESTING Figure 6 - Actual configuration of the HVDC Simulator 6.2- CONTROLS AND PROTECTION SYSTEM TESTING Before the RTDS arrived, FURNAS had already performed tests in protection devices using results from ATP simulation file converted into analog signals and sent to the relay using amplifiers. With the acquisition of the RTDS this test scheme has become totally obsolete because the RTDS could run such kind of test in a closed loop, allowing the protection to interact with the simulation. The main idea for protection system testing[4,5] is to send voltage and current for the relays and to received the trip outputs. The trip signals are then used 5

6 to control the opening of the breakers. By using a facility from RTDS that permits the user to control the simulation events, a special script[6] was developed by FURNAS to perform the tests automatically, without interaction with the user. The script loads the cases previously compiled, sets up the type of fault, the localization, resistance of fault and so on. After the simulation has been made, the results are analyzed and saved. Besides, several special reports including the sequence events are generated. The procedure has been used to run cases during the day or the night with no necessity of supervision. During the last five years, a lot of studies have been performed, including internal studies and studies at the request of other companies and manufacturers. The great majority of the studies performed were those related to transmission line protection systems. However, special protection configurations also demand for real time digital simulation. Regarding this special schemes we can point out a bus bar protection and a phase shift transformer protection that were tested in FURNAS laboratory. Figure 7 shows a configuration of a transmission line protection system example, tested in the RTDS. Figure 7 Transmission Line Protection System Testing Example 7.0 CONCLUSIONS This article presents a synthesis of the evolution of real time simulation tools used by FURNAS. This evolution reflects the concern to keep high rates of confidence and the importance of real time simulation applied in the incoming of new projects and the support of the old ones. The set of real time tools and the skilled laboratory staff enable FURNAS to perform a great variety of tests of control or protection devices of its own interest or at the request of companies that hire FURNAS to provide simulation services BIBLIOGRAPHY [01]- H.W. Dommel, Digital Computer Solution of Electromagnetic Transients in Single and Multiphase Networks, IEEE PAS-88, Transactions on Power Apparatus and Systems, N o 4, April 1969, pp [02]- X.Wang, J.Giesbrecht, D. Woodford, L.Arendt, R. Wierckx, R. Kuffel Enhanced Performance of a Conventional HVDC Analogue Simulator with a Real-Time Digital Simulator Conference Proceedings of Power Systems Computation Conference (PSCC11), Avignon France, August 1993, Vol. 1, pp [03]- V.A.B. França, S.E. Santo, H.H. Almeida Using a Real-Time Digital Simulator to Test a Circuit Breaker Synchronizer Device Presented at ICDS '99, Vasteras, Sweden, May 1999 [04]- P.G. McLaren, R. Kuffel, R. Wierckx, J. Giesbrecht, L. Arendt A Real Time Digital Simulator for Testing Relays, IEEE Paper N o 915M319 pwrd, Summer Meting San Diego, July 1991 [05]- V.A.B França, Utilização de Simulador Digital em Tempo Real na Execução de Testes de Proteção de Ondas Trafegantes XV SNPTEE, Foz do Iguaçu,PR, Brazil, Oct 1999 (in portuguese) [06]- S.E. Santo, V.A.B. França, H.H. Almeida Testing a FURNAS Protection System using the RTDS Batch Mode Facility Proceedings of the 2001 International Power System Transients, June 24-28, Rio de Janeiro, Brazil 6