A Proposed Redundant System for Power Quality Monitoring in Distribution Systems with DG Units
A. E. Hassan, S. A. Farghal, M. M. El-Saadawi / A. Abd El-AleemDept. of Electrical Engineering / Department of Electrical and Computer Engineering
Faculty of Engineering / Faculty of Engineering
University of Mansoura, 35516, Egypt / Delta University, Egypt
m_saadawi@ mans.edu.eg /
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[1]Abstract— As the number and diversity of Distributed Generation (DG) connected to the grid increases, the power quality becomes a major problem that requires efficient and reliable methods for monitoring. Traditional Supervisory Control and Data Acquisition (SCADA) systems with centralized control rooms, dedicated communication lines, and specialized operators are not effective to handle a large number of DG resources spread over the distribution networks. A new monitoring system is required to ensure a proper interface between the DGs and the electric grids. A real time monitoring will help to take corrective actions. This paper presents a proposed Redundant Remote Monitoring System (RRMS) based on embedded technology. A designed embedded data acquisition device is used to collect the required data from each DG unit. These data are sent to the monitoring center through redundant communication using GSM (Global System for Mobile Communications) as wireless communication and internet as a wired communication. The data are saved in database storage, using Human Machine Interface (HMI); and could be monitored in the control center and displayed in a website from anywhere. The proposed system is implemented and tested on a simulated DG system to monitor and analyze the power quality impacts of DG resources on a distribution system.
Index Terms— Power quality, Remote monitoring, Distributed power generation, Communication systems, Data acquisition.
I. Introduction
Integrating DGs with the distribution systems leads to problematic impacts such as power system operation impacts, power quality impacts, energy security impacts, financial cost impacts, and regulatory impacts [1-3]. These impacts could be avoided by monitoring those DG units twenty-four hours a day. Based on that, a reliable monitoring system is required. Such system has to communicate with the utility to transmit DGs data to control centers [4]. Since DG units may be installed in remote areas, a remote monitoring system will increase system availability and operational reliability [5].
The impact of integrating DGs on power quality (PQ) of the distribution system is one of the major DG impacts [6-10]. Monitoring the distribution system power quality with integrated DG units is a major concern of many researches [11-19].
L. Cristaldi et al in [11], proposed a methodology to monitor a number of power quality indices for every load connected to the point of common coupling. A distributed master-slave structure was adopted as the architecture of the proposed system. However, this architecture has some drawbacks. First, it is an expensive system, as it needs to process the data obtained by measurements performed simultaneously by all slave devices. Second, the system power consumption is relatively high. Finally, the connection between slave and master will fail if the connection media fails which may reduce the system reliability.
D. J. Won et al in [12] presented a proposed PQ monitoring system with central processing scheme. The proposed system is very economical especially for large-scale systems as the price of PQ meter can be dramatically lowered in that scheme. The main drawbacks of that system lie in: highly power consumption and heavily communication burden.
Ringo et al in [13] presented a web based multi-channel PQ monitoring system for a large network. The system consists of few basic hardware components including PQ meters to measure and store the data. The data are then transmitted to the central system server through leased, dial-up lines, customer intranet Ethernet network or DSL modem.
Authors in [14] developed a data acquisition system for remote monitoring and control of renewable energy sources. The proposed system is based on the Client/Server architecture. The measured parameters are available on-line over the Internet. The main drawback of that system is that it depends on a unique communication system to monitor the measured data.
Kushare et al in [15] presented a proposed real time web-based PQ monitoring system. The system allows user to access the PQ information and provides auto email notification of the custom PQ report to specified users. The proposed system was applied to industrial customers, but has not been tested on DG applications.
B. Vural et al in [16] presented a remote PQ monitoring system for low voltage sub-networks using MATLAB Server Pages. The proposed system uses the MATLAB HTML tag library and the data captured by the data acquisition hardware to evaluate the PQ parameters. The end users can monitor the status of PQ only through the internet.
In [17] authors introduced a system for remote monitoring and control of complex stand-alone PV plants. The remote communications were based on the GSM network and, in particular, on the short text message service. In [18] authors presented a design of a microcontroller-based wireless data acquisition system. The system was used for reading, storing and analyzing information from several PV water pumping stations situated in remote areas. The remote communications were based on the GSM network.
H. Mao in [19] designed a scheme of remote wireless monitoring system based on General Packet Radio Services (GPRS) wireless network. The GPRS may not be available in all places of the remote units.
This paper proposes a reliable redundant monitoring system for remote monitoring of DG units. Detailed analysis, design and implementation of that system on PQ are presented. The proposed system could help in increasing the performance of the distribution systems including DG units, which leads to the use of DGs with cost effective way. The proposed system is implemented and tested on a simulated DG system to monitor the power quality impacts of DG resources on a distribution system.
II. The Proposed System
The following subsections explain overall framework, hardware and software design of the proposed RRMS. The authors have described a detailed system description in [20].
A. System framework
The proposed system is divided into two major phases. The first phase is “Gathering Data Module”, whereas, the second one is “Monitoring Data Module” which consists of many layers. Fig. 1 shows the architecture of the proposed system.
1) Gathering data module
This module is responsible for gathering the data from all DGs, sending it to the monitoring center and saving it in database. This module consists of the following layers:
1- DG system layer: consists of two sub-layers. The first is the DG units and the second is the DG parameters’ sensors, which sense and measure DGs parameters and send them to data acquisition layer.
2- Data acquisition layer: consists of three sub-layers. First, data acquisition hardware which is responsible for collecting DGs parameters from DGs sensors and sampling the analog parameters using analog to digital converter. The second sub-layer is the data acquisition system software, which manages hardware to collect DG parameters from its sensors and sends them to the communication sub-layer. The third is the communication software, which is responsible for managing communication with monitoring server using the master or standby communication media. The communication software detects the available communication method and sends data via available communication media. This software also sends data to local monitoring Human Machine Interface (HMI) which uses RS-232 serial port [21].
3- Communication alternative layer: consists of master, standby and RS-232 port as a data interface. These alternatives are used to carry DG parameters from data acquisition to remote monitoring server. To send data to monitoring server; the master method uses wire media whereas the standby method uses wireless media. For many reasons the internet is selected as the master connection whereas GSM is the standby one [20].
4- Server software layer: consists of server application and database. Server application is the software for listening to the data coming from master or standby media and saving them in database. Database receives data from server application and stores them in a database with specific rules.
2) Monitoring data module
After gathering, transmitting and saving DG data in database, it will be analyzed and interpreted so that they can be monitored using computational engine. The operator must create alarming constraints to monitor the DGs parameters’ limits. This monitoring module can be explained as follows:
Computational engine consists of two sub-layers: data analyzer and displaying methods. Data analyzer layer is responsible for analyzing data and interpreting them to be useful information. Data analysis means: creating constraints and conditions for the parameters received from gathering phase. This data are interpreted according to constraints added by the operator. Each parameter will be checked for all prescribed constraints.
Displaying methods are responsible for displaying alarms, graphs, and data of DG interpreted by data analyzer. Local SCADA displays and monitors any DG unit connected to data acquisition locally via communication module using RS-232 serial port. The DG parameters could be monitored from remote monitoring server using remote SCADA software connected to database and retrieve data. DG parameters could also be monitored at website from anywhere.
B. System hardware design
The hardware is divided to data acquisition, which is used to collect sensors values using analog channels and connected GSM module, which is used to connect the GSM receiver to monitoring server. Fig. 2 shows a block diagram of the proposed data acquisition and communication module. The microprocessor will collect the values of DG sensors data and send them to the communication module. The communication module will send data to monitoring center using master or standby connection. In this study the microprocessor used in data acquisition is PIC-16F877A [22].
Detailed elements and circuit design of data acquisition can explained as follows:
• Analog circuits
The proposed data acquisition uses analog channel in PIC-16F877A to read values of sensors connected to DGs. The PIC-16F877A reads the analog value by using analog to digital converter 12-bit resolution.
• Internet and GSM connection modules
Both the internet and GSM modules are connected to PIC-16F877A through selection module and MAX232 chip. In this study the used internet module is EB023 [23], while the used GSM module is WAVECOM [24].
• Server side GSM connection
The GSM receiver will be connected to monitoring server via serial RS-232 to receive incoming data from GSM transmitters over mobile network.
Figure 1: Architecture of proposed redundant remote monitoring system
Figure 2: Data acquisition and communication module block diagram
C. System software design
The overall system software consists of three modules: data acquisition software, remote monitoring server and displaying and monitoring software.
As shown in Fig. 3, Data acquisition software will initiate at the DG by collecting the DG parameters from sensors using designed data acquisition hardware. These values are sent to local SCADA. Software on data acquisition checks the available connection (internet or GSM). This software initiates the connection with the available connection (internet or GSM) and sends the data to remote monitoring server by using the available connection. After data are sent to server, software of data acquisition will repeat this operation again. Remote monitoring server software listens to incoming data from internet or GSM connection. After data arrived, server application will save them in SQL (Structured Query Language) database in case that it is from a registered DG. On other hands if these data come from unregistered DG units, server application will not save them in the database. Finally, remote SCADA and web site will connect to the database to display the incoming newer data and apply constraints defined by the user. Detailed software analysis and components functionality is described as follows:
• Data acquisition software
The program of the microprocessor is responsible for managing data acquisition. The microprocessor is programmed to execute the following functions:
a-Collecting the analog and digital data from sensors,
b-Detecting the available connection,
c-Sending ok_signal to selection module,
d-Sending data over internet connection if the internet is available, and
e-Sending data over GSM connection if internet is not available.
The microprocessor program is written with C++ language [25, 26]. The output of the compilation is downloading on microcontroller to be executed.
• Remote monitoring server software
The server application is designed using Visual Basic. NET (VB.NET) [27]. The server application listens to incoming data from internet connection and split it to get the DG unit information (IP, SIM_NO, address...). If no data coming from internet connection it listens to data coming from GSM connection and splits it to get the DG unit information. Data coming from internet or GSM connection will be saved to database in specific tables.
• HMI software for SCADA
Human-Machine Interface (HMI) is quite literally where the human and the machine meet. It is the area where the human and machine interact during a given task. HMI is the first display method for displaying the DG data. The HMI retrieves data from database and interprets it according to constraints previously determined by users. The user selects the location of the DG unit and then clicks on monitor button to display data by graphs or tables.
There are two types of the HMI. The first interface is the HMI for monitoring the DG locally through its interactions with the application using DDE (Dynamic Data Exchange) protocol [28]. The HMI application will communicate with microprocessor through RS-232 data interface. The second one is HMI for monitoring the DG remotely using interaction with SQL database. Data in SQL database are coming from server application designed by VB.NET, which receives them from either the master or the standby communication system. The HMI used in this work is designed using INTOUCH from wonderware [29]. Both local and remote HMI are designed with the following abilities:
1-Adding a new DG to the monitored system,
2-Deleting any DG from the system,
3-Detecting alarms from any DG,
4-Monitoring all parameters of any integrated DG unit,
5-Representing historical and real data for any integrated DG unit.
• Web site
As explained in the previous items, the redundant technique will be used in the proposed system in two ways. Firstly, it is used for communication media (master and standby). Secondly, it is used for displaying methods (locally with SCADA and, remotely through a website). The website server will communicate with the database SQL server to retrieve the DG data. Authorized users only can monitor the DG from the website. The web site is programmed using PHP programming language [30], and it will be responsible for: Managing the authorized account, displaying the selected DG data, and connecting the database SQL server.
Figure 3: Flow chart for overall system software
D. System setting
This feature allows the operator, in both local and remote control center, to get useful information from the raw data collected and to take the right decision about each DG unit. For a good analysis, the operator has to put some constraints on different parameters of each DG unit such as the constraints put to detect whether the DG parameter is lesser or greater than the prescribed limits. Some constraints require more than one boundary. The user is able to add constraints for parameters containing more than one boundary. The constraints are put in the following form:
• LOLO (LOW LOW): this constrain used when the user need to determine whether the value of any parameter is lesser than a setting value.
• LO: this constrain used to detect whether the value of any parameter is within two values (Low Low and Low value).
• HIHI (HIGH HIGH): similar to LOLO but for higher values.
• HI: similar to LO but for higher values.
The screen for defining a new parameter with constrains is shown in Fig. 4. This screen contains the following options:
• Tag name text: to write the parameter name
• Button Type: to select parameter type (digital, analog, string...).
• Comments: to write a comment that describes the parameter.
• Log Data option: to save the data for historical trend.
• Initial Value: to set the parameter initial value.
• LOLO, LO, HI, HIHI: to write constraints values.
• Rate of change: to write values for rating alarm changes.
In another publication [20] there is a detailed description and screen shoots of the proposed system.
Figure 4: A Screen for defining a new parameter and its alarm constraints
III. Power Quality Case Study
Power quality monitoring is a process of gathering, analyzing and interpreting raw measurement data into useful information. The process of gathering data is usually carried out by continuous measurement of voltage, current and other parameters over an extended period. The process of analysis and interpretation has been traditionally performed manually, but recent advances in signal processing have made it possible to design and implement intelligent systems to automatically analyze and interpret raw data into useful information with minimum human intervention [31]. A major issue related to interconnection of DGs onto the power grid is the impacts on the quality of power provided to other customers connected to the grid. The main indices which define power quality include [32]:
Voltage regulation: the maintenance of the voltage at the point of delivery to each customer within an acceptable range.