A. E. Hassan, M. M. El-Saadawi, S. A. Farghal A. Abd El-Aleem Dept. of Electrical Engineering Department of Communication and Computer Engineering

Faculty of Engineering Faculty of EngineeringMansoura University, 35516, Egypt Delta University, Egypt

Abstract— Developing the smart grid with distributed generation (DG) has many advantages but, integrating DGs with distribution system leads also to many problems. Due to those problems a proper interface between DG and electric utility system has to be done. Efficient interface system helps in instantaneously monitoring of DG units. Also it helps in providing information about the DG system twenty four hours a day. Utilities have to analyze this information to detect the problems of integrated DG and make best decisions. Most importantly, interface systems must be fast, smart and have full time availability to ensure the system runs efficiently and safely. In this research, authors proposed a design of a fully Redundant Remote Monitoring (RRM) interface with redundant communication and redundant display. An embedded system is designed to collect DG information through sensors and to use available communication media for sending this collected information to a control centre. The proposed system is designed using GSM as wireless communication, and internet (over telephone lines) as a wired communication to create redundant communication. The application server receives the data coming and stores it in SQL database. The redundant displaying methods which include the computers in control centers through human machine interface in the control center and an internet website, analyze the data and interpret it to useful information. The received data is used in studying many case studies, some examples is presented to validate the proposed system.

Index Terms — Remote monitoring, Distributed generation, Communication systems, Data acquisition, SCADA.

I. INTRODUCTION

Over the last two decades, many DG resources have been connected to distribution networks worldwide. DG systems may have a significant impact on the system and equipment operation in terms of steady-state operation, dynamic operation, reliability, power quality, stability and safety for both customers and electricity suppliers [1]. These impacts may manifest itself either positively or negatively, depending on the distribution system, distributed generator and load characteristics [2-11].

Power quality monitoring has an increasing role in the deregulated electricity supply market [2]. A real time power quality monitoring will help in taking the necessary corrective actions [12].

DG could be installed at remote locations on feeders and in close proximity to the loads. For efficient utilization and operation of DG units, operators and owners of DG sets have to know every aspect of their DGs [13]. Current remote monitoring systems use wired or wireless media to carry data from DGs to the monitoring center. One-way communication can become an ineffective way to exchange information whenever that media is unavailable in remote places where DG units installed. Current remote monitoring systems are designed as web-based applications, windows

applications or mobile monitor applications, where each system has some advantages over the others [14 -18].

With the increase in the web-based applications, user can monitor the DG units worldwide or through the WAN, but the web based application doesn't have the possibilities of SCADA software such as Human Machine Interface (HMI), alarms and graphs [18]. Although windows applications supports SCADA software but when it is used for monitoring DG units; the users can only monitor the DG locally.

A Smart monitoring system ensures that data is transmitted in an efficient and safe way. This smart system could be designed and implemented through a proper interface between the DGs and the control center. Such monitoring system needs to communicate with the control center to provide all data of the DGs. Most importantly, it must be fast and reliable to ensure that the system runs safely [19].

In this research, authors proposed a monitoring system which can collect all needed data of DG with redundant communication and redundant display. According to IEEE Standards: data monitoring could be locally or remotely [20]. The proposed system is designed using GSM as wireless communication, and internet over telephone line as a wired communication to create redundant communication. The proposed system will use web-based application which is built based on TCP/IP communication protocol to monitor the DG remotely and locally. SCADA software is designed and implemented in the monitoring center to support the possibility of redundant DG parameters display. A review of previous work of DGs monitoring is introduced in the following section.

II. REVIEWMonitoring of power quality for DG units using a web-

based application was developed in a research project at Tampere University of Technology in Finland. The system consists of a remote reading system, a database for managing measured data, and a web-based application for power quality monitoring. The system is implemented by using application service provisioning model [19].

In [21] a remote monitoring system for prefabricated substation in China was proposed. The system can realize wireless data transmission via both GPRS network and Internet. The main function of the system is to remotely monitor and control the operational status of the substations by mobile monitors and control center. Engineers with mobile phones (mobile monitors) can use the real-time data for accurate examination and repair of the substation.

EPRI PEAC Corporation [15] applies the latest

Proper Efficient Interface between DG Units and Electric Utility Grid

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technology in power monitoring and software that can offer remote monitoring of DG equipments. EPRI system offers customers the flexibility to view power quality, power flow, and DG status information via the World Wide Web (WWW). Data is made available to customers via a password protected web site. The system allows the operator to remotely switch a DG unit in and out of service when the need arises.

Ref. [16] presented the design and implementation of a platform (hardware-software) for monitoring and control the physical variables of a greenhouse such as temperature and luminosity. A new type of microcontroller, TINI (Tiny Internet Interfaces) was used. That microcontroller was used as a small server with additional advantages as being able to be programmed with Java language and to support a lot of communication protocols.

A real-time remote monitoring system that acquires data from any kind of sensors to be transmitted by radio frequency to a computer with an interface module situated within a 900 m radius was proposed in [17]. The system was tested in sensing a large area fields and was capable of monitoring crop local environmental and physiological status. The data transmission and storage in the computer was made in real-time.

The authors in [12] presented a web based power quality monitoring system. The entire system has been developed using Java technology. That proposed system allows user to access the power quality information and provides auto email notification of a custom power quality report to a specified user.

The development of a data acquisition system for remote monitoring and control of renewable energy resources (RES) plants was presented in ref. [18]. The system is based on the Client/Server architecture and there is no physical connection between monitored systems and the data collection server. This feature is essential in RES plants since they are usually installed in inaccessible areas. The measured data are distributed over the Internet to any remote user. The measurements can be retrieved by means of an Applet interface.

The author in [22] presented a proposed communication system based on the IEC 61400-25 standard for monitoring DG sources. Several power sources were integrated with the system using modbus RTU and TCP protocols.

In [14] the authors introduced a proposed remote monitoring system based on SMS of GSM as a medium for transmitting the remote signal. The system can monitor and control the remote communication between the monitoring center and the remote monitoring station. The system includes two parts which are the local monitoring center and the remote monitoring station. The local monitoring center consists of a computer and a communication module of GSM. The remote monitoring station includes a TC35 communication module of GSM, a display unit, various sensors, data gathering and processing unit. The software of the monitoring center and the remote monitoring station is designed using Visual Basic.

All previous systems lack reliability, because they use one way of communication and/or one way for displaying the data from the DG. Analyzing the merits and drawbacks of the surveyed systems show that a good and efficient

monitoring system should be:• Remote and real time monitoring,• Available all the time,• Reliable and accurate,• Redundant (has wire and wireless communication),• Low power consumption at DG side,• Quick at start up,• Low cost,• Containing historical storage possibility, and• Able to monitor DG from anywhere.

In this paper an RRM system is designed and implemented. The system is designed to fix the previously mentioned problems and achieve the main features required for a good monitoring system.

III. PPROPOSED SYSTEM ARCHITECTUREThe basic concept behind the proposed system is to

combine two communication media and two data displaying methods. This redundancy leads to an efficient and, highly reliable connection between DG units and monitoring centers. The redundant communication media are considered as main and standby. An embedded solution is used to reduce power consumption at DG side. The proposed system also contains historical storage for diagnostic. An analyzing technique is used in the proposed system to monitor and analyze the DG gathered parameters.

The proposed system architecture is presented in Figure 1. The system is divided into two major subsystems; the “data gathering” subsystem and the “monitoring” subsystem. The two subsystems are explained in the following section.

A. Data gathering subsystem This subsystem is responsible for gathering the data from

DG units, sending it to the monitoring center and saving it in a database. By using this subsystem the data acquisition and communication microprocessor are programmed to collect analogue and digital data, and then send it to the monitoring centre using master connection (internet over telephone line). If the internet is not available the communication module will select the standby communication (GSM). At the monitoring centre the application program listens to the incoming data from internet or from TC35 communication module of GSM [23] and saves the data in proper tables in a database. The whole system is implemented using Microsoft technology (C#, SQL database) [24]. The redundant displaying methods (HMI, website) analyze the data and interpret it to useful information. In this study, InTouch software is used to design HMI SCADA [25] and PHP is used for programming the website [26].

All data concerning parameters of each DG and input/output data of every DG are collected and saved in SQL database. The system is able to monitor all DG alarms at a time. After reading any data received from the SCADA software it will be deleted from the data Que. The sub-system consists of the following modules:1. DG module: is the Distributed Generator unit/units

whose parameters should be monitored.2. DG parameters module: represents the sensors which

measure the DG parameters and send them to the data acquisition system. The previous two modules represent the DG system.

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3. Data Acquisition System hardware module: this module is responsible for collecting DG parameters from DG sensors and sampling the analogue parameters using analogue to digital (A/D) converter. The hardware is constructed using embedded circuits.

4. Data Acquisition System software module: this module represents the software of the data acquisition system which manages hardware to collect DG parameters from its sensors and send them to the communication sub-layer. The data acquisition software is programmed to execute multiple tasks at a time. This software increases the system speed and makes it quick at start-up.

5. Communication software module: is the software used to manage communication with the server using the main (internet) or standby (GSM) communication media. This module formulates the packets for DG parameters data and uses the available communication method to send these data. The communication software detects the available communication media (internet or GSM) and sends data through it. The communication software also sends data to local monitoring SCADA using RS232 serial port. Figure 2 shows the proposed data acquisition and communication block diagram

6. GSM and Internet modules: these modules composed of communication method used to carry DG parameters from data acquisition to remote monitoring server. The internet module uses internet technology to send data and the GSM module uses GSM technology for the same purpose. The communication software determines which communication method could be used.

7. C# application module: is the software which listens to the data coming from internet or GSM and saves them in SQL database.

8. Database SQL module: is a database for saving DG parameters. Data base SQL receives data from C# and stores it according to specific rules. The database contains three types of data. The first is the DG information, the second is the DG parameters, and the third is a queue data for all DG parameters. The queue data is the data coming from any DG so user can monitor all DG alarms at a time.

Figure 1: Architecture of the proposed system

Figure 2: Block diagram of the proposed data acquisition and communication module

B. Monitoring subsystemAfter gathering DG data and saving them in the database,

analyzing and interpreting operations are required to monitor the DGs parameters. The operator must create alarming constraints to monitor the DGs parameters’ limits. The monitoring subsystem can be explained through some modules as follows:1. Display module: (Web site, Remote SCADA, and Local

SCADA): the communication software sends DG data to local computer via RS232 serial port and to remote monitoring server via internet or GSM. The user can monitor the DG parameters using local computer at the DG location via local SCADA software. He can also monitor DG remotely through monitoring server via remote SCADA software. Operator can access DG data also from a web site connected to the monitoring server.

2. Data analysis module: this module allows the user to create constraints and conditions for the parameters received from the gathering system.

3. Data interpreting module: this module is responsible for interpreting the received data according to constraints added by user. Each parameter is checked to all prescribed constraints.

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IV. SYSTEM REQUIREMENTS, SPECIFICATIONS AND MODELING

The proposed monitoring system is designed using SysML language [27]. The SysML model is able to capture all requirements of the proposed RRM system in a requirements diagram.A. System requirements diagram

The requirements Diagram for that system is shown in Figure 3. The diagram illustrates the requirements that are typically captured in a text specification. The requirements are shown in a containment hierarchy to depict the hierarchical relationship among them. The requirements diagram is in a top-level requirement that contains the lower level requirements. The proposed system requirements are explained as follows:1. Real time Monitoring Requirements: the monitoring

center or monitoring user must view the status of DGs in time to analyze the problem and solve it. The real time monitoring requirements are the communication layer between DG unit and the remote server.

2. Availability Requirements: the monitoring parameters must be available at all times without failure in communication, since these parameters are important to the monitoring user.

3. Reliability Requirements: data send from DGs to monitoring user must be reliable by using reliable

communication media (telephone line, GSM wireless) and protocols (TCP/IP).

4. Accuracy Requirements: the system must collect data from DG and send it to the user with high accuracy.

5. Power Consumption Requirements: the power consumption of the system used for gathering and sending data back to the server must be minimized. The power consumption is reduced by using embedded system requirements.

6. Redundancy Requirements: the system must be redundant as far as communication and displaying are considered because the redundancy in system raises the availability and reliability. The redundancy in communication is designed using two communication ways (internet over telephone lines, and GSM). The redundancy in displaying is designed to use three displaying locations (local, remote at remote server, remote on web site)

7. Start-up Requirements: the system has to rapidly restart (after any failure or hang-ups) to monitor the DGs all times. The start-up should be fast by using embedded system requirements.

8. Cost Requirements: the cost in any system is one of the important parameters and the proposed system is designed to be more economic than other monitoring systems.

Figure 3: Requirement diagram showing the system requirements for DG remote monitoring system

B. System SoftwareHardware and software are used to construct the proposed

system. The hardware includes data acquisition, communication module, communication transmitters, and communication receivers. On other hand, the software is

used to manage the hardware to accomplish monitoring operation.

Figure 4 shows a flow chart for the overall system software. As shown in the figure, Data acquisition software is initialized at the DG side by collecting the DG parameters

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from sensors via designed data acquisition hardware. These values are sent to local SCADA. Data acquisition software checks the available connection (internet or GSM) to send the data to remote monitoring server. After data is sent to the remote server, the data acquisition software will repeat this operation again. Remote monitoring server software listens to incoming data from internet or GSM connection. When these data is received, the server application will save it in SQL database only if these date come from a registered DG. On other hands, the server application software will not save any data come from unregistered DG units. Finally, remote SCADA and web site are connected to the database as displaying software to display the incoming new data and apply the constraints defined by the user.

V. SYSTEM IMPLEMENTATION The implementation of the proposed RRM system is

divided to the following stages:1- Hardware implementation

a. Data acquisition circuit design.b. Connect selection module with the data acquisition

circuit.c. Test internet connection circuit.d. Test the GSM connection circuit.

2- Software implementationa. Download and execute microprocessor application on

data acquisition.b. Send data over GSM connection using AT commands

programming.c. Receive coming data to server application.d. Save incoming data in database.e. Display and monitor incoming data using SCADA

software.f. Display data on website software.g. Gathering DG data.

The monitored data saved in the database are used in studying many case studies, two of them are summarized in the following section (Voltage stability, and Voltage regulation). Other studied cases are published in [28].

Figure 4: A flow chart for the overall system software

VI. MOINTORING OF VOLTAGE STABILITY AND VOLTAGE REGULATION

A. Voltage stabilityVoltage stability is the ability of power system to

maintain steady voltages at all buses in the system after being subjected to a disturbance from a given initial operating point [29]. Under normal operating conditions, the bus voltage magnitude (V) increases as reactive power (Q) injected at the same bus is increased. Integrating DG units into distribution systems can have an impact on different practices such as voltage profile, power flow, power quality, stability, reliability, and protection [30]. The system is said to be unstable when the voltage magnitude of any bus decreases with the increase in reactive power for that bus. Although the voltage instability is a localized problem, its impact on the system can be wide spread as it depends on the relationship between transmitted power (P), injected reactive power and receiving end voltage [29].

The voltage stability for a simple power system (shown in Figure 5) can be represented by an index L. Equation (1) is used to detect the voltage stability of that system [31]. For a stable system: L must be less than 1.0, and when L approaches 1.0, the system reaches instability condition:

L = 4(X Pj - R Qj)2 + X Qj + R Pj (1)

Figure 5: Equivalent circuit of grid-connected DG

To define the voltage stability index in RRM system, the operator has to create the parameters: R, X, P J, QJ and L in HMI as shown in Figure 6. The RRM proposed system will read these parameters and calculate the L value, compare it with the value of 1.0 and give an alarm if L equal one with cyclic execution 10 ms. Figure 7 shows a screen shot for

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calculating the equation of index L in application script.

a) R - parameter

b) X - parameter Figure 6: Screen Shots for creating parameters R, X, PJ, QJ and L.

Figure 7: Screen Shot for calculating equation of L factor

Plotting the L factor with time is an available option in RRM system. Table 1, shows the L values for different simulation data, whereas Figure 8 shows a graph of the L factor with time for the same data.

Table 1: Active and reactive power with fixed R, X (R= 0.1196, X= 0.1263)

PJ (MW) QJ (MVAR) L0.39 0.14 0.0685540.48 0.19 0.0871510.56 0.23 0.1034970.64 0.28 0.1208740.73 0.33 0.1401090.85 0.38 0.1649840.92 0.43 0.181121

Figure 8: L-factor versus time at fixed R and X (R= 0.1196, X= 0.1263)

B. Voltage regulationA primary objective of distribution system design is to

supply customers at a voltage within a prescribed range. There are two voltage ranges: Range A, covering normal operation, whereas Range B for infrequently occurring circumstances. Normal variations in load, and DG operations, fall into the category covered by Range A. The service voltages for Range A: are between 209 V and 231 V, on a 220 V basis. Thus, the lower limit of the primary feeder voltage must be maintained above 95% and less 105% of nominal to account for transformer and secondary service cable voltage drops under full-load conditions. Generally, this requires that the minimum primary feeder voltage be maintained above at least 98% and 102% of nominal [32].

The proposed RRM system is able to monitor the voltage regulation as an attribute of the power quality by monitoring voltage level and configured constraints. Constraints for the voltage regulation are configured to keep the voltage level at a range between 209 V to 231 V.

Adding the voltage level to the real graph and alarm list will help the operator to get useful information about the voltage regulation. Figure 9 shows the screen of creating the parameter VOLT_LVL (voltage level) and its constraints.

Figure 9: Screen Shots for adding voltage levels and their constraintsThe screen shown in Figure 10 explains how to add alarm

list to display the voltage parameter constraint when energized. The screen shown in Figure 11 shows how to add real graph and voltage level parameter to this graph.

The operator can monitor and analyze the voltage regulation online using these constraints. If the value of the voltage is above 231 V or less than 209 V, an alarm will appear on the screen to alert the operator

Figure 10: Screen Shots for adding alarm list to display alarm

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a. Screen of adding real graph

b. Adding parameter voltage levels to real graph

Figure 10: Screen shots for adding real graph and voltage levels to the graphIn this case study, the values of recorded voltages in 60 seconds are given in Table 2. When these data are processed by the RRM system, a graph will plot for the voltage level as shown in Figure 12. On other hand, since the last recorded value is 233 V, the high constraint (HIHI) for VOLT_LVL parameter is energized and an alarm is displayed in alarm list as shown in Figure 13.

Table 2: Measured Voltage in 60 secondsTime Voltage (V) Time Voltage (V)

12:13:30 217 12:14:00 22412:13:36 226 12:14:06 21512:13:42 219 12:14:12 22512:13:48 225 12:14:18 21812:13:54 214 12:14:24 233

Figure 12: Graph for voltage level constraint at 233 with limit 231

Figure 13: Alarm for voltage level constraint at 233 with limit 231

VII. CONCLUSION

An RRM system for monitoring DG parameters and information is proposed. The design and implementation of the system is presented. The system keeps track of the DG health which gives the utility engineer the ability to take critical decisions in emergency situations. The proposed system software is layered software which is implemented in C#, SQL and InTouch. The proposed system is analyzed and designed using SysML modeling language which is used for that purpose of embedded systems. The system is reliable enough to make sure that DGs are monitored twenty-four hours a day. The system uses two ways of communications media. Users and utility engineer can access the system using HMI locally or remotely. Two case studies are presented to prove that the monitored data are valuable and to validate the proposed system.

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