User PQ diagram as a tool in reactive power trade

2011 8th International Conference on the European Energy Market (EEM) • 25-27 May 2011 • Zagreb, Croatia User P-Q Diagram as a Tool in Reactive Power Trade Ivan Ilic #1, Alfredo Viskovic *2, Mario Vrazic #3 # Department of Electric Machines, Drives and Automation Faculty of Electrical Engineering and Computing, University of Zagreb Unska 3, HR-10000 Zagreb, Croatia 1 3 [email protected] [email protected] * HEP d.d. Ulica grada Vukovara 37, HR-10000 Zagreb, Croatia 2 [email protected] Abstract— Trading with reactive power will soon become a very important part of electric energy trade. However, while trading, voltage regulation in portable and distribution systems must not be neglected. In order to balance the technical and economic aspects it is necessary to be familiar with the electric power system, the source of electric energy most of all, meaning the synchronous generator and its possibilities. These possibilities are shown in the synchronous generator P-Q diagram. We nowadays use static P-Q diagrams, provided by generator producers, which in most cases refer only to generators. This article presents a dynamic user P-Q diagram containing changeable parameters and showing a real availability of a synchronous aggregate. I. INTRODUCTION A common P-Q diagram is static (Fig. 1) and does not show dynamic limits of a synchronous aggregate working range. Trading with electric energy requires a knowledge of total real production capacity. When using the standard P-Q diagram nobody knows the real possibilities of a synchronous aggregate, particularly in the reactive power domain. The dynamic user P-Q diagram [1], on the other hand, provides an insight into the real working range of a synchronous aggregate. By using this method a synchronous aggregate operating area can be increased by up to 30% in terms of capacity and up to 10% in terms of induction. Wider synchronous aggregate usage possibilities represent additional „assets“ in reactive power trade, which is crucial for the process of voltage regulation in the high-voltage grid. Another condition for creating a user P-Q diagram is a measurement system which initially measures all armature and excitation voltages and currents, as well as grid voltage. This article gives a description of all the conditions and systems necessary for the realisation of such a diagram. Fig. 1: P-Q Diagram of the Aggregate 978-1-61284-286-8/11/$26.00 ©2011 IEEE 580 2011 8th International Conference on the European Energy Market (EEM) • 25-27 May 2011 • Zagreb, Croatia II. DESCRIPTION OF THE P-Q DIAGRAM The P-Q diagram represents a diagram of possible use of a synchronous aggregate and, as such, has been used for a long time. Development of computer systems enabled a detailed modelling of the synchronous generator end region, at the same time giving a better insight in the conditions there, particularly in the capacity mode of generator operation. It also enabled development of measurement systems that can measure and process huge amounts of information in a relatively short time. Everything mentioned so far has made the classical (static) P-Q diagram out-dated and made introduction of dynamics in its use possible. The marked limits of the synchronous generator–aggregate authorised operating area in Fig. 1 indicate that the operating area is limited by unchangeable and changeable boundaries. The unchangeable ones are the minimum and maximum active powers which depend on the construction of the aggregate and its plant (turbine power, minimum boiler power, maximum water flow etc.). On the other hand, all the other boundaries are changeable, which means that they depend on the operating point (grid voltage, active and reactive power). It can be noticed that, if the warming of the rotor winding is monitored, it is possible to extend the operating area more to the right, i.e. deeper into the inductive area. It is also possible, if the warming of the stator winding is monitored, to increase the stator power, i.e. armature current. If the warming of the end region and the load angle are monitored, a widening of the operating area to the left, i.e. capacity area, is possible. All of the abovementioned requires permanent monitoring of the mentioned physical quantities and a complete knowledge of electric, magnetic and thermic conditions in the generator. Widening the operating area does not necessarily mean shortening the generator life span. The marked limits of the allowed load are mostly arbitrary and in most cases questionable. Cautious experts will move work limitations and protection boundaries to a safe distance if they do not know the real possibilities of the generator. The reason is not, of course, a lack of expertise, but higher security requirements caused by inability to get insight into the real possibilities and limitations of the generator/aggregate. By a detailed knowledge of the generator/aggregate drive properties and insight in its current condition achieved by a monitoring system it is possible to approach the real boundaries of the operating area and thus optimise its load in order to achieve a more effective voltage regulation of the high-voltage grid, as well as achieve a higher profit without shortening designed life span of the generator. It is possible, in controlled conditions, to cross some boundaries (for example the excitation current limit) if there is a justification for it, such as ensuring the stability of the electric power system at any cost. Such activities should not endanger functioning of the operation, which means they mustn't cause a malfunction (for example breakage of the winding due to a higher voltage), but one must be aware that such activities shorten the working life span of the machine. Apart from thermic limitations the limit of the generator operation stability is very important and much more demanding in terms of dynamics. This limit must not be crossed because it would mean entering the asynchronous working mode which means that the generator falls out of pace. Such work is accompanied by heavy vibrations, a change of rotation speed and a big mechanical strain on the whole aggregate system. It is the reason why experts want to have good protection when adjusting protection and operating limits. We can prevent switching to asynchronous operation by monitoring load angle (the δ angle) as well as other relevant physical quantities such as: armature voltage and current, excitation voltage and current, grid voltage, and temperature in the cooling system of the active parts of the machine. Besides this information, we need to make a detailed model of the synchronous machine. III. MAKING A P-Q DIAGRAM In order to make an online P-Q diagram it is important to do the following: - Get all the needed information about the generator (blueprints, measurements, characteristics...) - Install a measurement system to measure the relevant physical quantities - Test no load operation and a short circuit - Measure several operating points for different grid voltages - Make a FEM model of the end region of the generator - Make a generator model for an online P-Q diagram - Continue to monitor the operating points (each new operating point makes the model more accurate) A. The Measurement System Prior to doing a measurement system project some basic requirements for its functioning need to be fulfilled regarding: 1. Possibility of Internet (Intranet) communication, 2. Possibility of calculating active, reactive and virtual generator power, 3. Possibility to process armature current and voltage, excitation current, and the δ load angle, 4. Possibility of measuring the stator and rotor temperature [2], 5. Connection with the signals of armature current and voltage, excitation current and voltage with galvanic separation and maximum protection to avoid any possibility of interference between the designed measurement system with the measuring and protection systems of the generator and the power plant, 6. Sufficient accuracy, 7. Possibility of magnet field signal processing using available maximum and effective values of the basic harmonic and total magnetic field signal, 581 2011 8th International Conference on the European Energy Market (EEM) • 25-27 May 2011 • Zagreb, Croatia 8. Possibility of processing vibration signal to obtain available values of acceleration, speed and vibration shift, and online FFT analysis, 9. Flexibility and possibility of enlarging the measurement system, i.e. possibility of its upgrade in case of extending the operating requirements from the P-Q diagram. To begin with, it is necessary to become familiar with the generator on which the measurement system is installed. Firstly, measuring points must be chosen very carefully to ensure the measuring to be representative. By that we mean places of installing the measuring probes into the generator with the purpose of measuring temperature, magnetic induction and vibrations. Technology of installing measuring probes should be carefully analysed according to the requirements of the operational security. For example, in case of installing measuring signal lines from hydrogen cooled turbo generator it is necessary to carry out, apart from a suitable construction of a signal line cable glands, a very carefully controlled testing of gas tightness in the cable glands, as well as in the final installation. A technically satisfying (concrete) solution of the gas tightness cable glands for Plomin 2 power plant turbo generator is shown in Fig.2. Furthermore, all these signals have to be protected from disturbances. It is understandable that installation of probes should not cause breaches which would weaken the winding insulation system. Measuring the load angle (the δ angle) requires a comparison of electrical and mechanical quantities, for example generator voltage and the mechanical position of the rotor. A phase shift of these two quantities is the load angle in which an always present inherent shift is calculated (the δ0 angle). This value is measured during the synchronisation process, just before the synchronisation itself. Later, this value is subtracted from all the measured values of the load angle giving the current load angle value. Fig. 3: Placement of the Pt 1000 probes in the end region Fig. 4 gives an example of the measurement system installed on the generator in the Plomin 2 power plant. Fig. 2: Installation of the cable glands into the hydrogen cooled turbo generator (250 MVA power) While designing and installing the probes (for measuring magnet induction and temperature) attention should be paid to their proper spatial distribution. It means that work should be done simultaneously on the FEM models in order to determine the installation place for the probes with certainty (Fig.3)[3]. Other physical quantities are measured by already installed probes (shunts, current and voltage measuring transformers, etc.). One has to be careful not to disturb functioning of the existing measuring and security systems when installing a new measurement system and measuring probes. It is best achieved by galvanic separation of the measurement systems. B. Model of the Synchronous Generator After installing all the above mentioned probes and their connection in the whole measurement system initial measurements can be carried out, i.e. testing the no load operation and a short circuit [4]. These tests determine the key parameters of the synchronous generator, such as saturated and non-saturated synchronous reactance, rated excitation current, etc. while modelling the generator in one of the FEM tools should be done simultaneously. The main parameters for the model are synchronous reactances (direct Xd and quadrature Xq for the hydro, i.e. Xs for turbo generators). Two kinds of synchronous reactances are used in practice: saturated for the inductive operating area and non-saturated for the capacitive operating area. This leads to a discontinuity in making the P-Q diagram when passing from one operating area into the other, which is a source of inaccuracy. We would like to make a P-Q diagram in which a change of synchronous reactances regarding the operating point is continuous, which it is in reality, and install it as such in the model and online user P-Q diagram. To determine synchronous reactances it is necessary to measure several operating points. 582 2011 8th International Conference on the European Energy Market (EEM) • 25-27 May 2011 • Zagreb, Croatia It is also necessary to measure the leakage reactance of the generator. According to the IEC norms the Xσ leakage reactance is determined by charging the armature winding by a three-phase voltage system when the rotor is extracted. Reactance measured in such a way contains the part belonging to the magnetic current which encloses the space where the rotor is otherwise situated. This part of reactance does not represent the leakage reactance and it should be measured and subtracted from the total reactance of one-phase of the winding. The leakage reactance can be measured on already operating generators by measuring the induction in the air gap. It can be measured as a ratio of the measured induction values in the air gap for Un in the no load test and for In in the short circuit test. Synchronous reactance of the generator can then be worked out in an analytical and graphic way for any operating point of the P-Q diagram from the characteristics of the no load operation and short circuit, measured value of the leakage synchronous reactance, voltage, generator, active and reactive power, and excitation current. Of course, if more different operating points are measured, particularly for different grid voltages, the model is more accurate. The idea is, after installing the measurement system and after initial measurements, to set the measurement system in such a way that it automatically measures the operating points not yet read, thus improving the accuracy of synchronous reactance for more operating points. The accuracy of the model is also improved in this way. Fig. 4. Example of the Measurement System A software subsystem controls the implementation of the described idea by analysing the existing readings and deciding, according to different criteria, if a current operating point needs to be recorded. After recording the current operating point, the information is loaded into the model with the purpose of entering the unknown value of synchronous reactance or its correction in case it has previously been entered. Once synchronous reactance is known and the δ load angle is measured the user P-Q diagram can be plotted (Fig. 5). IV. ADVANTAGES OF A REAL TIME P-Q DIAGRAM When a user P-Q diagram is plotted, it shows the operating area or the area that can be used. What is important is that the operating area changes depending on the operating point, so that the power plant staff, the production operator and the distribution operator can see what is at their disposal, which is important not only from the point of view of the electric energy trade, but also because of keeping the electrical power system stable. 583 2011 8th International Conference on the European Energy Market (EEM) • 25-27 May 2011 • Zagreb, Croatia So, besides the technical advantages (a more effective use of the aggregate and better possibilities of keeping the electrical power system stable) this technology provides possibilities for a considerate additional income quickly creating profit which is much higher than investments. REFERENCES Fig. 5. User P-Q Diagram screenshot However, these are all technical advantages. The real reason and justification of the whole research is a financial profit which the implementation of this technology makes possible. It is estimated that there is a possible increase of profit 15% in the production of active power achieved only by a more economical use. By freeing hidden reserves in the operating area of the P-Q diagram in reactive power trade further 1-3% income increase is possible. If this, very conservative estimate, is applied in one production unit of, for example, 250 MVA, it would lead to the following calculation: - If everything is calculated in active power, with cosφ=0,8, the active power of 200 MW is produced - With the assumption that the price of 1 MWh is € 50 (which is a low price), and that the generator is in operation 8,000 hours a year - A total annual production can be calculated in euros, which is 200*50*8,000 = €80,000,000 - Which, with the income increase of 1% for active and 1% for reactive power, makes €1,600,000 - And that is far above the price of investment which is between €300,000 and €450,000 for such a production unit It is visible from the above calculation that the investment pays off in only two to three months, even with a very conservative estimate. V. CONCLUSION The user P-Q diagram in real time provides: • A more economical production, • Conditions for more advantageous trade and, thus, higher profit, • Better possibilities of keeping the electrical power system stable. 584 [1] I. Ili, Z. Maljkovi, I. Gašparac et. al., Methodology for Determining the Actual P-Q Diagram of a Hydrogenerators, Journal of Energy, Vol.56 No.2 pp. 141 - 181, February [2] Zhang, Yu; Wang, ZhiShan; Li, JinHua, Design a Wireless Temperature Measurement System Based on NRF9E5 and DS18B20, Measuring Technology and Mechatronics Automation (ICMTMA), 2010 International Conference on , vol.1, no., pp.910-913, 13-14 March 2010 [3] W. Li, D. Shuye, J. Huiyong, L. Yingli, Numerical Calculation of Multicoupled Fields in Large Salient Synchronous Generator, Magnetics, IEEE Transactions on , vol.43, no.4, pp.1449-1452, April 2007 [4] Simond, J.-J.; Ramirez, C.; Tu Xuan, M.; Stephan, C.-E., A numerical test platform for large synchronous machines also useful as a design optimization tool, Power Engineering Society General Meeting, 2006. IEEE, 18-22 June 2006, Page(s): 7 pp.-