Keywords BTS, Current measurement, DSP controller, Current Shunts, TMS320F28031.

Transcription

1 Volume 3, Issue 11, November 2013 ISSN: X International Journal of Advanced Research in Computer Science and Software Engineering Research Paper Available online at: Design of Power Management Controller for BTS Power Plant Monitoring based on TMS320F28031 DSP Ashish Kashyap * Dr. Neelu Jain ECE Department, E & EC Department, Chandigarh Engineering College, Landran, India PEC University of Technology, Chandigarh, India Abstract In this paper design of a Power Management Controller for BTS power plant using a TMS320F28031 DSP chip is presented. BTS plays a very important in mobile communication as it houses the radio transceivers and antenna that define a cell and handles the radio link protocols with the Mobile station (MS) and it is necessary to provide Un-interruptible Power Supply (UPS) to the BTS. The main features of this controller are to measure & calculate various metering parameters such as voltage, current, power, temperature for Load and Battery terminals of BTS power supply and take decision regarding switching of LVD and Contactor relay drives such that BTS site always has power backup. The metering parameters are calculated using 12-bit ADC and experimental results of the calculated parameters are presented to confirm the performance of controller. Keywords BTS, Current measurement, DSP controller, Current Shunts, TMS320F I. INTRODUCTION Base Transceiver Station (BTS) is the radio part which forms the cell structure in mobile communication systems. The BTS consists of a transmitter and receiver equipment apart from antenna for a radio cell. A single Base Station Controller (BSC) administers many BTS s. The BSC in turn is controlled by the Mobile Switching Centre (MSC). The BTS collects digital information sent by the base station control systems, converts it to an analog RF signal, and passes it to the cell site antenna for transmission onto the cellular network. The BTS also takes analog RF signals received from the antenna, converts them to a digital format, and sends them to the base station controller thereby enabling user to talk over the mobile phone. The BTS is an important transmission device between the mobile phone and the public network. The radio waves transmitted from a BTS are received by a mobile phone in a designated area called cell. When the phone moves to a new area, the call is transferred to the BTS in the new cell and continued without any breaks. A BTS is usually placed in the centre of a cell. Each BTS serves a single cell. Fig. 1 BTS communicating with mobile stations The cell size of the BTS is determined by the antenna transmitted power which gives the coverage area for communication. An Un-interruptible Power Supply (UPS) is planned to provide an uninterrupted operation of BTS equipment [1]. The power supply system at the BTS has AC mains as the main source of power, whenever the mains failure occurs or voltage drops beyond a threshold level then the power supply is shifted to battery source so that an uninterrupted supply can be provided to BTS. Whenever battery source voltage falls beyond battery undervoltage level the battery supply is turned off to save it from full discharging and BTS power failure occurs. To find out when to switch Mains Supply and Battery source on/off it is required to continuously monitor voltage and current readings and take decision accordingly. A common solution for current and voltage measurements in power device is based on current 2013, IJARCSSE All Rights Reserved Page 309

2 shunts and voltage transducers. The analog signals are transmitted to the DSP board and are then converted to digital format using Nyquist A/D converters with either 10 or 12 bit resolution [2]. Energy metering is also performed by calculating the power using current and voltage readings. Temperature inside the BTS power plant is also monitored and cooling fan is turned off/on according to the reading. The power management controller presented in this paper is designed to do all these operations. II. MEASUREMENT METHODOLOGY A. Current Measurement Precise and robust current sensors are necessary for determination of the current state and for current control [3].The most basic and intuitive method of current measurement is to run the current through a high precision electrically resistive component known as current shunt over which the voltage is measured. Shunts are differentiated from resistors by the fact that they are exclusively designed to measure currents [4]. This type of sensing is based on ohm s law. By placing a shunt in series with the system load, a voltage is generated across the shunt that is proportional to the system load current. The voltage across the shunt can be measured by differential amplifiers such as operational amplifiers (op amps) and the current can be found out. The shunt used for the design is Simpson 50mV/100A which provides 50mV output voltage at input current of 100A. B. Voltage Measurement Precise measurement of voltage is essential for numerous highly sensitive and valuable equipment in electric power systems [5]. A resistive potential divider is usually employed for the measurement of direct voltages. In this method, a high resistance potential divider is connected across the high-voltage winding, and a definite fraction of the total voltage is measured across resistor of small value. Fig.2 shows a simplified potential divider where R1 is selected to be higher than R2. V1 is the high input voltage and the measured voltage is the voltage across R2. The voltage measured can be found using the formula V2 = V1 R1 R1 + R2 HV=V 1 V 2 Fig.2 Simplified potential divider C. Power Calculation Power can be calculated by the formula given by P=VI. The current is calculated using current shunt method and voltage is calculated with help of voltage divider method. D. Temperature Measurement The security operation of electrical devices is a question of vital importance for the power system stability. Especially for the high voltage equipment, temperature has a direct effect on the operation of these devices. It is important to make sure that the equipment work within the scope of its temperature admission [6]. For calculation of temperature, LM35 sensor has been used which are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (centigrade) temperature. It provides an output voltage of mv/ C with typical accuracies of ±1 4 C at room temperature and ±3 4 C over a full 55 to +150 C temperature range. III. SYSTEM HARDWARE DESIGN The block diagram of the system has been shown in Fig.3. The DSP microcontroller ADC channels have input from three sections current measurement section, voltage measurement section and temperature measurement section. The current measurement section receives input from current shunt sensor which is fitted in the bus voltage line of BTS power supply; voltage measurement section receives input from bus voltage of the BTS power supply and temperature measurement section receives input from the LM35 sensor which is placed inside the BTS power supply chamber. The microcontroller processes the input from these three sections and controls the LVD and contactor relay drives at the output. From Current Shunt sensor Current Measurement Section Contactor Relay Drive To contactor Voltage input Voltage Measuement Section TMS320F28031 From LM35 sensor Temperature Measuement Section LVD Relay Drive To LVD Fig.3 Block diagram of the system 2013, IJARCSSE All Rights Reserved Page 310

3 DSP microcontroller TMS320F28031 has been used. This microcontroller has highly efficient 32 bit CPU and can operate at a frequency of 60 MHz. This microcontroller works on 3.3 V supply and has inbuilt 12 bit ADC. This internal ADC is used to carry out sampling of the input analog signals that are provided by current shunt sensor, voltage divider and LM35 sensor. The reference voltage for the ADC is set to be 3.3V; the ADC output count comes out to Digital Value = V In the current measurement section (Fig. 4) the header connector has input fed from the shunt. This input signal is passed through an RC filter followed by OP2177 which is used in differential amplifier configuration and provides gain of 82. The output of shunt is in range of 0-50mV and as the 1st op-amp provides gain of 82 so maximum output at C1 will be 4.1V. As the microcontroller pins have 3.3V tolerance level so to step down this voltage 2nd and 3rd op-amps have been used. These op-amps provide a gain of 16/20 and maximum output voltage at IOUT is 3.3V. IOUT is connected with ADC channel1 of microcontroller. So it can be said that the op-amps are configured to provide a gain of 66 such that the output of shunt in range 0-50mV can be converted into 0-3.3V and ADC reading can be achieved for full range. Fig.4 Schematic for Current measurement In the voltage measurement section (Fig. 5) the input voltage is provided by connecting positive terminal to SHUNTCOMMON and negative terminal to BUSPOSITIVE. The input voltage ranges from 0-72V, to convert it range the range of 0-3.3V a voltage divider circuitry is formed such that analog signal gets a gain of 1/27 before reaching ADC channel input. VOUT is connected with ADC channel2 of the microcontroller. Fig.5 Schematic for voltage measurement In the temperature measurement section TEMP (Fig. 6) gets input from the LM35 sensor and a gain of 2 has been provided using LF353 in non-inverting amplifier configuration. TEMPOUT is then connected with ADC channel2 of the microcontroller. Fig.6 Schematic for temperature measurement 2013, IJARCSSE All Rights Reserved Page 311

4 It can be seen in Fig. 7 that ADC channel1, channel2 and channel3 are receiving input analog signals from current (IOUT), voltage (VOUT) and temperature (TEMPOUT) sections respectively. Sampling rate selected for all the channels is 20µs. channel 1 is sampled first and the output ADC count is stored in the buffer, then channel 2 is sampled in the same way and then channel 3. This process goes on till 100ms is complete for individual channels. When 100 ms time is complete, average of the ADC counts value stored in the buffer is calculated to get accurate ADC count for that duration. Fig.7 Schematic design for controller and relay portion IV. SYSTEM SOFTWARE DESIGN The flow chart of software design has been shown in Fig. 8. The software is designed using Code Composer Studio (CCS). At the start up the microcontroller will initialize system control register. Internal oscillator 1is selected as clock Source and all unused clocks are turned off to conserve power. The oscillator frequency is selected to be 60 Mhz. Peripheral clocks are enabled for ADC module and disabled for rest of the modules. In the next step PIE (Peripheral interrupt enable) control registers are cleared to disable all the peripheral interrupts. Flash memory is enabled in the Pipeline mode for improving performance of the Microcontroller. All the variable parameters like voltage gain, current gain, temperature gain and different flags used have been initialized in this section. ADC module is initialized such that ADC uses external reference voltage, ADC is powered up and enabled. All the ADC interrupts have been disabled and ADC is set to work in sequential running mode. Channel1, 2 and 3 being used for voltage, current and temperature inputs have been initialized and their trigger is selected to be software trigger. The sample windows for all the three channels have been set to 64 cycles, which is maximum for this microcontroller. The conversion time that ADC takes is 13 clock cycles. Controller Frequency = 60 MHz so 1 clock cycle = nsec ADC Conversion time= 13 x = nsec Sample window= 64 x = 1.066µ sec Total time to process one sample = Conversion time + Sample Window = 1.283µsec So, the ADC takes µsec to give digital output. Timer 0 interrupt is enabled and is configured to provide interrupt after every 20 µsec. For all the three channels ADC count results are stored are stored in their respective buffers and sample count is incremented. Whenever the sample count goes beyond 5000, current, voltage and temperature are calculated using gain values. Microcontroller calculates the current, voltage and temperature values from the respective ADC counts. The power calculations are performed using the current and voltage values. According to the calculated values decision regarding switching of RELAY1 and RELAY2 is being performed. If the power source is AC mains and the voltage falls below a level of 42 Volts then contactor relay is turned on to remove AC mains as power source and battery source is connected as power source. When the battery voltage falls below 41 V and battery current falls below 15% level of its current capacity then LVD relay is turned on to disconnect the battery and BTS power failure occurs. When AC main supply returns back the contactor relay and LVD relay are both switched off. 2013, IJARCSSE All Rights Reserved Page 312

5 Start Initialize System Control Timer 0 Interrupt Subroutione Initialize PIE Control Storing ADC results Initialize Flash Initialize Variables Channel Number Initialize I/O Ports Initialize ADC Initialize CPU Timers Enable Timer 0 Interrupt == 1 Storing Current ADC count in buffer ADC sample count++ == 2 == 3 Storing Voltage ADC count in buffer ADC sample count++ Storing Temperatre ADC count in buffer ADC sample count++ While(1) Control LVD and PFC relay Drives End No If ADC Sample count > 5000 Yes Calculation of Current according to gain value Channel No ++ Switch Mux Address If ADC Sample count > 5000 Yes Calculation of Voltage according to gain value Channel No ++ Switch Mux Address No No If ADC Sample count > 5000 Yes Calculation of Temperature according to gain value Channel No ++ Switch Mux Address ADC Trigger Return Fig.8 Flowchart of software design V. RESULTS The test results have been shown in Table 1. The input voltage is varied from 42V to 60 V and the error in the actual voltage and calculated voltage by the controller has been shown in the Table. The maximum positive error and maximum negative error comes out to be 0.02 % and -0.40%. For current reading the shunt is connected with source channel and the load is varied using an electronic load to vary current in the range of 10A to 96A. The maximum and minimum error comes out to be 2.60 % and 1.20%. The actual and calculated power is calculated from the current and voltage readings and maximum and minimum error comes out to be 2.40 % and 0.90 %. For temperature readings the Power management Controller is placed in the chamber and the temperature of chamber is varied from 10 C to 100 C and maximum and minimum error comes out to be 3.00 % and 0.00 %. TABLE I TEST RESULTS (A) Voltage measurement result (B) Current measurement result S.NO Actual I/P (V) Calculated Voltage (V) Error Shunt I/P (mv) Actual Current (A) Calculated Current (A) Error % % % % % % % % % % % % % % % % % % % % % % 2013, IJARCSSE All Rights Reserved Page 313

6 (C) Power measurement result (D) Temperature measurement result S.NO Actual Actual Calculated Calculated Power Error Temperature Temperature Power (W) (W) ( C) ( C) Error % % % % % % % % % % % % % % % % % % % % % % VI. CONCLUSIONS The proposed Power Management Controller is capable of providing current, voltage, power and temperature calculations with maximum error of 0.02%, 2.60%, 2.40% and 3.00% respectively. The average error coming out for current, voltage, power and temperature readings is -0.18%, 1.65%, 1.47% and 0.81% respectively. So it can be concluded that this Power Management Controller can work within + 2% accuracy limits. ACKNOWLEDGMENT The authors would like to thank Exicom Tele-systems Private Limited for providing the experimental setup for the research work. The authors would also thank Vimal Sethi, Kuldeep Singh, Ramesh Gupta and Sunit Singh for their valued assistance. REFERENCES [1] Ajosh.K, P.Sujit,Aravind Rajan, Aravind V, and Raveendranathan K.C., A Smart BTS Power Management System, International Conference on Computational Intelligence and Communication Systems, 2010, [2] Manoel Eustáquio dos Santos, Braz de J. Cardoso Filho, Flavio H.Vasconcelos, Voltage and Current Measurement System for Medium Voltage Inverters, Conference Record of IEEE Industry Applications Conference vol.2, 2002, [3] Richter and Florian, Precise Current Sensor for Power Electronic Devices, Power Electronics Specialists Conference, PESC IEEE, pp [4] Petter, J.; McCarthy, Jack; Pollak, P. and Smith, Christopher C, Survey of DC current measurement techniques for high current precision power supplies, Nuclear Science of Symposium and Medical Imaging Conference, 1991, Conference Record of the 1991 IEEE vol.2, [5] Josemir Coelho Santos, M. Cengiz Taplamacioglu, and Kunihiko Hidaka, Pockels Voltage Measurement System, IEEE transactions on power delivery, vol. 15, no. 1, January 2000, [6] Yaguang Guo, B.X. Du, Y. Gao, Xiaolong Li and H.B. Li, On-line Monitoring System Based on MODBUS for Temperature Measurement in Smart Grid, Innovative Smart Grid Technologies - Asia (ISGT Asia), 2012 IEEE Conference, , IJARCSSE All Rights Reserved Page 314