2007 Poznańskie Warsztaty Telekomunikacyjne Poznań 6-7 grudnia 2007 P OZNAN POZNAN UNIVERSIT UNIVERSITY Y OF OF TECHNOLOGY ACADEMIC ACADEMIC JOURNALS JOURNALS No 55 Electrical Engineering 2007 Przemysław LISOWSKI* MINIMIZING INRUSH CURRENT WHILE CHANGING UPS UNIT STATE USING A VOLTAGE REGULATION METHOD The paper provides a simplified functional model of a line-interactive uninterruptible power supply (UPS) equipped with a low frequency main transformer. The model served as the basis for proposing three methods of generating output voltage using a voltage regulation method, aimed at reducing the inrush current of the main transformer while changing the operation of a UPS unit incorporating the synchronous switching method. Further possibilities of minimizing disturbances while switching using digital signal microcontrollers to control the operation of a UPS unit are presented. Keywords: line-interactive UPS, inrush current, voltage regulation method 1. INTRODUCTION Line-interactive UPS units equipped with a low-frequency main transformer and a main circuit breaker based on an electromagnetic relay form the most popular type of backup UPS units used in distributed uninterruptible power supply systems [1][2]. They owe their popularity to a relatively low price and high resistance to environmental conditions, in particular the power supply [3], hence it is the main objective of the manufacturers to increase their reliability while maintaining the low price. One of the causes of dysfunctions of the UPS units under discussion is the inrush current of the main transformer occurring at random while changing the mode of operation of the UPS unit from basic to autonomous [4]. The occurrence of the inrush current of the main transformer while changing the mode of operation depends on the moment of blackout, the time of the power interruption and the nature and load quantity. It is thus reasonable to prevent such a situation by appropriate control of the UPS unit operation, and in particular generation of output voltage [5]. In order to specify an algorithm for controlling the operation of a UPS unit during a power failure, there was a need to develop its functional model which would be as simple as possible. The UPS model contains e.g. an equivalent circuit diagram of the main transformer. Knowing the components of the equivalent * Poznan University of Technology. PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 1/8
Przemysław Lisowski circuit diagram of the transformer makes it possible to take into consideration their impact on the UPS unit switching from basic to autonomous operation. Transformer modelling was preceded by a series of measurements of its parameters and calculations reflecting their impact on individual components of the equivalent circuit diagram. The paper further presents the results of measurements and calculations of the values of components in the equivalent circuit diagram of a typical main transformer used in UPS units of approx. 700 VA. Their importance is discussed and, subsequently, a simplified model of a transformer active in a UPS unit is proposed. A further proposal comprises methods of influencing the waveform of the UPS unit output voltage by regulating voltage, employing the synchronous switching method, aimed at minimizing the inrush current while changing the mode of operation. Benefits resulting from the use of modern digital signal microcontrollers to control the operation of line-interactive UPS units are discussed towards the end of the paper. 2. UPS UNIT MODEL The proposed functional model of a UPS unit (Fig. 1.) features an equivalent circuit diagram of the main transformer. In order to determine the values of components of the equivalent circuit diagram of the transformer, measurements of the Tr720/12 transformer were carried out both in the no-load state and under short-circuit test operation [6][7][8]. The results of tests in the no-load state with rated voltages served as the basis for calculating inductance of the primary winding L 1 (with z1 winding turns) and secondary winding L 2 (with z2 winding turns). R Fe resistance is determined to illustrate losses in the steel core. The results of measurements in the short-circuit test operation made it possible to determine short-circuit impedance Z z including its components: R z resistance and X z inductive reactance. Test results are shown in Table 1. Table 1. z1 L 1 [H] (X 1 [ ]) z2 L 2 [H] (X 2 [ ]) R Fe [ Z z [ R Z L Z [H] (X Z [ ]) 510 8.8 (2760 34 0.042 (13.2) 8700 19.6 19.3 0.01 (3.24) PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 2/8
Minimizing inrush current while changing UPS unit state using a voltage regulation method 3 Figure 1 illustrates the model of a UPS unit with the main transformer described by an equivalent circuit diagram with the secondary side referred to the primary side [7]. Fig. 1. Functional model of a UPS unit with elements of an equivalent circuit diagram of the main transformer The UPS unit input is connected to the supply described by u AC voltage and Z AC impedance. In the basic operation mode, i IN current flows into the UPS input. A load with Z O impedance, drawing i O current is placed at the UPS output. SM is used to designate the main circuit breaker which disconnects the UPS unit from the mains during a power failure. Processes accompanying a change in the operation of the main circuit breaker are related symbolically to the main circuit breaker, i.e. electronic disconnection of the battery charging system (marked with Z R impedance) and switching-on of an inverter (with the source voltage u I and impedance Z I ). TM transformer has been modelled using lossless inductance L T drawing all core magnetizing current (R Fe parallel resistance responsible for losses in the core has not been taking into consideration), as well as Z 1 and Z 2 ' impedance. Primary winding resistance R 1 and serial inductance L t1 related to the leakage flux of this winding form part of the Z 1 impedance. Likewise, in the secondary winding, impedance Z 2 is made up of resistance R 2 and inductance L t2. Individual parameters marked with the prime symbol can be determined on the basis of the following relations: R 2 = R 2, L t2 = L t2, i R = i R, i I = i I, etc., where z1, z2 number of turns of the primary and secondary winding, respectively. PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 3/8
Przemysław Lisowski Using the results presented in Table 1, individual values of components in the equivalent circuit diagram of the transformer have been calculated according to the following relations [7]: L T = 0.5 (L 1 + L 2 L Z ), L 2 =L 2, R 1 = R 2 = 0.5 R Z Since the short-circuit reactance X Z is considerably lower than the R Z short-circuit resistance, in order to find out whether it can be omitted in subsequent calculations, additional dynamic measurements of the transformer in the no-load state have been carried out. The test consisted in measuring the voltage in the primary and secondary winding and the primary winding current during a 5-ms interruption in the power supply in one of the semi-periods of the supply voltage wave. An interruption in the power supply to the primary winding guaranteed that an inrush current occurred with a high first pulse amplitude significantly exceeding the amplitude of the transformer current under rated operation conditions. The voltage wave obtained in the unloaded secondary winding served as the basis for calculating the excitation flux. Similar calculations were performed on the basis of the excitation voltage in the primary winding with and without taking into consideration R 1 serial resistance. The results of measurements and calculations are shown in Fig. 2. Fig. 2. Voltage, current and flux waves calculated using different methods. In order to illustrate all measured and calculated values of waves in one chart, which makes it possible to show temporal relations, the voltage wave is referenced to its amplitude, the current wave to the first pulse inrush current amplitude and the flux referenced to its maximum value in the steady state of the transformer operation. PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 4/8
Minimizing inrush current while changing UPS unit state using a voltage regulation method 5 On the basis of Figure 2 chart, an assumption can be made, that if only R 1 resistance is taken into consideration while calculating the flux from the voltage wave of the primary side, it serves as a good approximation, even with large currents, of the flux calculated from the voltage wave of the secondary side. In the case of operation in the no-load state or with small load currents, also the primary winding resistance can be omitted, in order to simplify calculations. Any simplifications in calculations are beneficial when cheap microcontrollers are used to generate output voltage of UPS units operating in the autonomous state. Eventually, a model of the main transformer was used in the further analysis, with L T inductance through which the i LT current flows which is responsible for generating the main flux in the core, as well as serial resistances R 1 and R 2. 3. VOLTAGE REGULATION METHOD The main aim of voltage regulation is to influence the voltage wave (deformed as a result of the power failure) in excitation windings of the main transformer, and in particular in L T inductance (Fig. 1) so as to prevent saturation of the core which leads to the inrush current (Fig. 2). In the model concerned, the flux wave in the transformer is the integral of the voltage wave in L T inductance. In the steady state, at rated power conditions, the flux waveform in the transformer core is symmetrical to zero. The flux reaches its maximum value, proportional to the mean voltage value in a semi-period, for the voltage u LT = 0. If the mean voltage value u LT is reset to zero in a period consisting of a blackout semi-period and a semiperiod directly following it, saturation of the core and occurrence of the inrush current are thus prevented. UPS units under discussion are usually equipped with a system of measuring output voltage and load current [1][2] and only information from these systems may be used to modify the output voltage wave in the autonomous state generated by the microcontroller. In the basic operation, the power is transferred from the mains directly to the load (Z O )(Fig. 1) and to the battery charging system (Z R ). In the case under analysis, the total transformer current i LT + i R causes a negligible voltage drop in the R 1 resistance of the transformer. Hence, for the power supply voltage of 230V, it is assumed that u LT u O. In this operation mode, the measurement of output voltage will enable the mean voltage value u LT to be calculated for the period. In the autonomous operation, when the power is transferred from the inverter via the transformer to the load, it might be necessary to take serial resistance of the transformer R 1, through which all i O load current flows, into consideration. In this case, u LT = u O + i O R 1. PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 5/8
Przemysław Lisowski 4. CONTROL IMPLEMENTATION METHODS In order to ensure that the mean voltage value u LT, for a period equal to zero is reached during a power failure, the following methods of influencing the output voltage wave are possible. In the first case, in the steady state, a mean voltage value u LT is calculated in each semi-period, using a simple summation of u O voltage samples. A similar procedure is applied to the blackout semi-period, however, when the waveform generation is started through internal inverter, also the primary winding resistance R 1 of the transformer is taken into consideration. In order to reduce the impact of blackout on the mean voltage value u LT in the blackout semi-period to the maximum, a supplementary rectangular voltage pulse is generated with adjustable width (Fig. 3a). Fig. 3. Output voltage wave during power failure - first method of adjustment The rectangular voltage pulse amplitude may fall within the range ± 10% of the rated voltage 230 V amplitude. If a pulse thus generated fails to ensure that the mean voltage value from the previous semi-period is obtained, it will be necessary to generate a sine wave with decreased amplitude in the next semi-period, which would guarantee the mean voltage value at the level obtained in the blackout semi-period (Fig. 3b). Such a control option has an advantage in the form of limiting deformation of the voltage wave to one semi-period, in the semi-period following the blackout, only a decrease in the output voltage amplitude may occur. A drawback of such a solution is the need for additional calculations of the control ratio for a semi-period following the blackout on the basis of the mean value obtained in the blackout semi-period. A different approach to the control of output voltage of a UPS unit is also possible and is shown in Figure 4a. The rectangular pulse is generated as in the first case, however, the moment in which the total of mean values from neighbouring semi-periods (of the blackout, and immediately following the blackout) is reset is the moment when the inverter ceases to generate output voltage. Such an approach PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 6/8
Minimizing inrush current while changing UPS unit state using a voltage regulation method 7 facilitates controlling the output voltage wave, however, the consequence is that it is deformed in two neighbouring semi-periods. Fig. 4. Output voltage wave during power failure a) second adjustment method, b) third adjustment method In those UPS units in which the voltage wave generated by the inverter cannot be influenced and only the signal from the generator supplying the power stage of the inverter can be gated, it is possible to control the output voltage wave using the method presented in Figure 4b. As in the previous case, in the semi-period following a power failure, once the voltage means are balanced, the output voltage generation is blocked. Such an approach ensures a balance of mean voltage values u LT in neighbouring semi-periods, however, if the interruption caused by a power failure is too long, there may be a problem with ensuring an appropriate rms value of voltage in the analysed period. While implementing the aforementioned methods of control, it would be beneficial not to exceed the permissible amplitude and rms value of the output voltage in the transient period. 5. SUMMARY The presented method of minimizing the inrush current of the main transformer while changing operation by regulating the output voltage using the three possibilities described above is an excellent solution for cost-free modification of line-interactive UPS units equipped with cheap microcontrollers. The simplified electric model of the line-interactive UPS units under discussion, with a simultaneous use of results from the proposed static and dynamic transformer measurements, enables the operation of a UPS unit to be simulated both in the steady and transient states. The results of such simulations may be used both for designing new UPS units as well as transformers designated to be used in such units. PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 7/8
Przemysław Lisowski Algorithms developed on the basis of simulations may be of particular importance while being implemented in modern digital signal microcontrollers which are increasingly used in more expensive UPS double conversion units. Relatively low prices of the aforementioned digital signal microcontrollers encourage their application in the discussed category of cheap line-interactive UPS units, which are at present the subject of the author s research. An increased accuracy of analogue-to-digital converters implemented in digital signal microcontrollers will make it possible to measure voltages and currents in a system in a more accuracy, and the possibility to run a large number of complicated real time calculations will enable a larger number of factors to be taken into consideration when generating output voltage in the steady state and in transient states, ensuring that disturbances are minimized. REFERENCES [1] Lisowski P.: Zasilacze awaryjne UPS małej mocy część I. Przegląd Telekomunikacyjny, nr 12/2005 [2] Lisowski P.: Zasilacze awaryjne UPS małej mocy część II. Przegląd Telekomunikacyjny, nr 1/2006 [3] Lisowski P.: Kryteria wyboru zasilaczy UPS. Fachowy Elektryk, nr 2/2007 [4] Lisowski P.: Przerwy w zasilaniu odbiorników w instalacjach z zasilaczami UPS małej mocy. Elektronika, nr 6/2007 [5] Lisowski P.: Metody minimalizacji prądu włączenia podczas zmiany stanu zasilacza UPS. Materiały konferencyjne KSTiT 2007, 12-14 września 2007, str. 699-706 [6] Jezierski E.: Transformatory. WNT, Warszawa 1983 [7] Mizia W.: Transformatory. Wydawnictwo Politechniki Śląskiej, Gliwice 1998 [8] Kulkarni S.V., Khaparde S.A.: Transformer Engineering Design and Practice. Marcel Dekker, Inc., New York 2004 PWT 2007 - POZNAŃ 6-7 GRUDNIA 2007 8/8