Embedded monitoring and management software unit for distributed generation system in electric networks Karoly Ronay Doctoral School UTCN
Dorin Bică, Lucian Ioan Dulău Department of Electrical and Computer Engineering, Faculty of Engineering “Petru Maior” University of Tîrgu-Mureş Tîrgu-Mureş, Romania
[email protected],
[email protected]
Dept. of Electrical Engineering Technical University of Cluj Napoca Cluj-Napoca, Romania
[email protected] Abstract—This paper presents a design approach to an embedded monitoring system found in distributed renewable energy power systems. The grid connected distributed power system, delivers energy to the consumers located on the microgrid, operating at distribution voltage levels, and also delivering the remaining surplus energy to the main distribution grid. The aim of this paper is to present the development process of a monitoring system for an energy consumer/producer unit in a local microgrid. With the help of this monitoring system, the user can view in a dedicated management software, the amount of energy produced from the distributed generation unit and the energy consumed by the local consumer. The software presents in almost real time the power and energy parameters, and the measuring data are saved in a database. This is an economic and implementable method, which helps the user, to keep up to date of the energy balance sheet of the energy consumer/producer unit. Index Terms— Computerized monitoring, Distributed power generation, Graphical user interfaces, Energy measurement, Power measurement (2017 IEEE Taxonomy)
I.
INTRODUCTION
This article presents the development and implementation of the embedded monitoring system and the dedicated management unit software for this application. In this introduction section, the major terms of the paper and title are defined followed by the presentation of the other section where the main ideas and concepts are elaborated. An embedded monitoring unit is a computer system with a dedicated function, found within a larger electrical system, in this case an electrical power network, working in almost realtime computing. It is embedded as part of a larger system or a complete and existing device, including peripherals of hardware, which can be static or dynamic parts. This embedded monitoring system is based on a microcontroller development board, connected to external peripheral devices. The advantages of an embedded systems when compared with a general-purpose devices in that same class are: programmability, low power consumption, small size,
extended operating domain, accessible development and lower cost. An embedded system [1], [2] is a microcontroller based system that is built to control a specific function or range of multiple functions, and is not designed to be programmed by the end user, only to make choices concerning functionality, but without changing the running software/firmware of the system. The monitoring process is defined as the regular observation and recording of activities taking place in a system or a project. It is a process of routinely gathering information on all aspects of the system, as in [3]. To monitor a process, is to check on how system activities are progressing. It is automated observation, systematic and purposeful monitoring. This also involves giving feedback about the progress of the system to the users, implementers and beneficiaries of the system. Reporting enables the gathered information to be used in making decisions for improving system performance, efficiency and energy-saving performances. The purpose of the monitoring system and management software is very important in system planning and implementation. The monitoring process provides information that will be useful in: •
Analyzing the current situation in the system, determining whether the inputs or resources in the system are well utilized
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Identifying problems facing in the system or project and finding the optimal solutions from the gathered information
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Ensuring that all activities in the system are carried out properly and in time
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User experience from one system experience on to another
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Determining also from monitoring data, whether the project was planned is the most efficient way, and solving problems if occurred.
978-1-5386-3943-6/17/$31.00 ©2017 IEEE
II.
DISTRIBUTED GENERATION. SYSTEM OVERVIEW
A distributed photovoltaic (PV) power system is presented in Fig. 1, connected to the local distribution low-voltage electric network. The main PV array is connected to the grid tie inverter. The first connection is to the direct current input section of the inverter, where the DC connections are made (junction box) and the short circuit protection exists. From there the PV array is connected to the inverter’s DC/DC converter, and in function of the string voltage can be a boost or a buck–boost converter. After the converter, the path is followed by the alternative current section of the inverter, which incorporates the power semiconductors, high frequency switching devices. This final stage of the bridge inverter can be formed from insulated-gate bipolar transistors (IGBT), or the metal–oxide–semiconductor field-effect transistors (MOSFET) [4]. The output is connected to a low pass filter and it takes care to smooth the transients generated by IGBT or MOSFET switching in the inverter [5] [6]. The output of the inverter is connected to a power AC switching device, and has the role of connecting and disconnecting the distributed generation PV system from the power network. The AC highvoltage switching devices (found on schematic as SPV, Sload, SLV) have the ability to be controlled from a low current source from a remote distance. The actual solution can be implemented with transistor driven relay switching devices, which is an optimal solution with the above mentioned embedded microcontroller system. Both the grid inverter and consumer, can be connected to the main power grid through the power switching devices (power contactors), Sload for the AC load, SPV for the grid tie inverter and the SLV, SHV for the transformer and main power grid side. In the Fig. 1. the given power switch configuration, illustrates a regular, grid connected operation mode, where the generated power is transmitted to the distribution network, feeding to the grid and to the local consumers also.
Figure 1. Schematic of a distributed photovoltaic power system connected to the main electric network
III.
SYSTEM OPERATION MANAGEMENT
A. Structuring management levels The operation management of the give system can be structured on multiple levels, as in [7] [8], only this approach is implemented for this specific system: •
Level 0. – Field level
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Level 1. – Direct control
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Level 2. – Process supervisory
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Level 3. – Production control level
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Level 4. – Production scheduling
B. Application of the management levels The entire structure of the management levels are applied on five functional levels. This application, for the proposed system is described on the first three functional levels. Level 0 - Field level, contains the field equipment and the peripherals of the hardware configuration. This components are the inputs-outputs from the microcontroller unit. In this application the field equipment are the current and voltage sensors, relay controlled power switches and circuit breakers. The monitoring [9] of the electrical parameters are done with non-invasive current transformers and voltage sensors. The operation of the switching devices are done by low current relay controlled power switches. Level 1 - Direct control, it is formed by the microcontroller unit, where the peripheral inputs-outputs are connected, data acquisition and processing is located and the communication is established for higher levels. For this application a microcontroller development board is developed and it is considered as the main control unit. The chosen microcontroller is an Atmel 8-bit AVR type of microcontroller, the ATmega328 model, providing 6 analog inputs, 14 digital input-output ports and serial-USB communication with the computer. In this case the analog inputs have an analog-digital converter of 10-bit resolution, and measures the level-adapted voltage un(t) and current in(t) from the sensors and transducers. The obtained digital numeric values are processed in the controller's main program cycle void loop(). With the help mathematical operators, software functions and programing statements like the: for loop, used for measurement iteration and timer, used for real time reference, measurement operation can be programed. One of them is the instantaneous power and the energy measurement, found in different points of connection, on the presented schematic of the system. The digital outputs are for driving the peripheral switching equipment. The digital high or low (1-0) logical statements can be used for the switching application. The computed data is sent by means of serialUSB connection using a dedicated communication protocol between the microcontroller development board and the workstation PC. Level 2 - Process supervisory, is the level where the supervisory workstations exists, collecting information from the microcontroller unit and providing the system operator with a graphical user interface. In this application on the
workstation PC, a dedicated management software is running, developed for this specific application, in Visual Studio, a Visual Basic form application. In this case the graphical user interface can communicate with the microcontroller, displays data in almost real-time in numeric and graphical form, and saves the data into a database. IV.
EMBEDDED MONITORING SYSTEM DEVELOPMENT
A. Hardware design The hardware design of this embedded monitoring system consists of: peripheral analog inputs (sensors and transducers), the microcontroller development board, based on the ATmega328 IC, and the optional peripheral digital outputs, as seen in Fig. 2. This hardware solution is found with the Level 0 and Level 1 of the application management structure.
consumers and power lines. For DC consumers found on the DC supply bus, a MOSFET power transistor based, low side switching application is proposed (for consumers with no common ground). The digital outputs are based on logic level signals. The low powered, high switching current relay can be used for both AC and DC consumers. The microcontroller communicates with the workstation computer through the serial/USB connection. A communication is established between the two devices (microcontroller development board and the host’s workstation) after the connection and a communication port is assigned (e.g. COM3) in the operating system of the workstation. In this application, the computed data from the microcontroller is sent to the host’s workstation through serial communication as a string of data, presented in the following chapter. This hardware design can be implemented on real power system equipment, in the distribution or junction boxes, and installing with regard of the norms and laws in the power system field. The presented design and test circuits where done in laboratory conditions, on phase voltage and low load current measurements. B. Software design The main software of the monitoring application can be illustrated as a block diagram, which is implemented on the hardware controller, and it is presented in Fig. 3.
Figure 2. The proposed hardware design schematic of the embedded monitoring system
The input filed devices are the voltage and current sensors, and their parameters are measured in the analog-digital converter port (ADC ports). The method of the measurements are based on voltage sensor, formed by the AC transformers, noninvasive current sensors, with split core, for non-invasive access in the distribution or junction box. To measure DC parameters: the voltage is measured with a resistive voltage divider, and for the current measurements, a Hall-effect based sensor is considered (e.g. ACS712) or a shunt resistor. The output values of the sensors must be in the range of 0-5V, this is the input voltage limit to the A/D converter. The input parameters presented in the Fig. 2. are: PV system on the DC bus: us_dc(t), is_dc(t); PV system inverter output: us_ac(t), is_ac(t); The load (local consumer): uc_ac(t), ic_ac(t); The proposed output peripheral, in this case is, a transistor driven relay based AC high voltage switching device, for AC
Figure 3. Block diagram of the software design
The program starts with the definitions of the ports and pins used by the microcontroller. After the initialization of the communication, the data acquisition starts. The analog.read function reads the ADC port and returns a numeric value. One read of the converter, has a sample rate of about 10 kHz, and this reading speed is decreasing as multiple analog values are read in or if data computing time is increasing. The data
computing is the main loop, where from the gathered information (the primary electrical parameters) the power and energy values are calculated. First the power calculation [10] in made in sampling loops, and after the power calculation, the energy calculations are done with a timer, started in the beginning of the loop, and it is always incrementing continuously. The communication with the GUI application and the display of the parameters are presented in the following section. The software solution is linked with the Level 2 of the application management structure, the represents the process supervisory unit. C. Graphical user interface. Management software The graphical user interface, presented in Fig. 4, has purpose of visualizing and processing the obtained data from the microcontroller. This software application was developed in Visual Studio 2015 Community, and it is based on a Visual Basic form application with additional gauges for enhance the virtual instrumentation part of the interface.
association of the label n, and sorts the number value what comes after the character as the values of that specific label. V.
CONCLUSIONS
The monitoring process is an important element in the electrical power network, because of the regular observation and recording of the parameters, which are taking place in the system. This embedded monitoring and management software unit it is designed to can carry out this tasks of power and energy monitoring, by using microcontroller based measurement techniques and a dedicated software user interface. In comparison to other power meters used in the electrical networks, which gives to the user direct, on display information, or longer periodical reads (e.g. GSM telemetry system) with monthly balances in one measurement point, this embedded monitoring and management tool what was developed and presented in this article, can provide multiple measurement points (energy consumer/producer), a real-time chart of the measured values and saving the data for future analysis. Also, command can be sent from the software to open or close power switches remotely. The hardware part can be implemented in the existing network with minimal invasive aspect. The developed hardware-software communication protocol is stable for the presented version, but future development is considered for stability. The GUI management software is essential element for the user, where it can check on how system activities are progressing giving real-time feedback about the state of the system. The overall management unit can be great tool in the future of the micro grids with the distributed energy generations. REFERENCES [1] [2]
Figure 4. Graphical user interface of the application, developed in a Visual Basic form application (Visual Studio 2015), the main component of the monitoring and management software (windows screen capture)
The interface displays the test values of the power generated by the PV system and the power consumed by the local consumer. The values are shown on virtual instrumentation panels, than plotted on a real-time chart and saved in a database, with in an interval of one second. The chart displays a time frame window of an hourly measured values. With this, the user can manage the energy balance sheet after the obtained information in the database, can track the energy production of the distributed generation system, and also can observe the energy consumption of the monitored household energy consumption. The information about the measured parameters are sent to the GUI application through a dedicated communication protocol. Each parameter has a specific label, formed by a specific character that is attached to that label in the background program of the GUI application. If in the data string, what comes from the microcontroller, that specific character is found, then the followed information is sent to that specific label in the GUI. Example: a string of data is sent to the GUI application in a form like this: “T123.00”, the background program identifies the character “T” as the
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