Power Quality Monitoring of Distributed Generation Units Using a Web ...

Power Quality Monitoring of Distributed Generation Units Using a Web-based Application S. Antila, K. Kivikko, P. Trygg, A. Mäkinen and P. Järventausta Tampere University of Technology, Institute of Power Engineering Abstract This paper presents power quality data management system developed in a research project at Tampere University of Technology (TUT), Finland. The system consists of a remote reading system, a database developed for managing measurement data, and a web-based application for power quality monitoring. The system is implemented by using application service provisioning (ASP) model. Also suitability of system for power quality monitoring of distributed generation and a case study are described in this paper. 1.

INTRODUCTION

In modern information society requirements and expectations associated with power quality have become increasingly important. Reasons for that are increased requirements for power quality by network utilities, customers and regulators. Many industrial and commercial customers have equipment that is sensitive to power disturbances. Therefore, it is more important to understand the quality of power being supplied. Also regulation models, laws, and guidelines relating to network business provide requirements for developing power quality management. Electricity is not anymore a luxury article like few decades ago, but it has become a necessity and a part of our everyday life. Even short interruptions and voltage sags can be harmful when the amount of computers, programmable logics etc. in industry and as well in households have increased rapidly. The standard EN 50160 gives the main characteristics of the supply voltage at the customer’s supply terminals under normal operating conditions with regard to frequency, magnitude, waveform and symmetry of the three phase voltages. So far continuous power quality monitoring in larger extent is not yet very common, but the situation is changing quite rapidly. Also integrating distributed generation into electricity distribution network settles new needs for power quality monitoring, because distributed generation units are usually connected to weak distribution network. In Finland these units (i.e. wind mills) are located mostly in the western coast or in Lapland in the fells, where the wind circumstances are most favourable. These are partly very sparsely populated areas where energy consumption is low, distribution network is weak and transmission distances are long. In these conditions quick changes in wind speed or in

power may produce remarkable voltage variations, which can lead to malfunction of devices and customers’ discomfort. That is why power quality should be monitored at distributed generation units to find and prevent possible problems. Web-based applications have many advantages compared to ordinary applications and they offer an easy way for power quality monitoring. In addition to usability and functionality, web-based applications offer possibility for electricity utilities to outsource their operations where the service is provided using application service provisioning (ASP) model. Power quality monitoring is one example of implementing outsourced service using ASP model where the service provider takes care of supply chain of the power quality information in total and allows customer to focus on core business and leave technological solutions into provider’s responsibility. Tampere University of Technology (TUT) has going on projects on power quality data management and integration of distributed generation into electricity distribution network. In case of power quality management a target is on one hand to define and demonstrate power quality monitoring functions as a part of integrated automation systems, and on the other hand power quality issues are considered from the point of view of network planning and operation including basic analyses, measurements, outage reporting, and also issues of distributed generation. A wider concept for power quality monitoring is presented e.g. in reference [1]. The project of distributed generation investigates the integration of distributed generation into an electricity distribution network from planning and operation point of view. The project includes development of network steady state and stability analysis tools and methods, consideration of distribution network protection, power quality and reliability, and investigation of

basis for connection fees and transfer charges of distributed generation. This paper introduces a power quality data management and web-based monitoring system, which is suitable for continuous power quality monitoring also in distributed generation. The developed system consists of power quality meters (Quality Guards), their remote reading software, database server and structure, web application for presenting and analysing data, web server, and client (i.e. web browser). In addition to power quality monitoring functions, functions for energy consumption and building automation (i.e. water and heat consumption, state information, and temperatures) can be implemented in the system. Developed web-based power quality monitoring system makes it also possible to gather together all measured power quality data from various distribution generation units of the same producer. Also new technologies, which make the development of web and mobile applications possible, are presented. Power quality monitoring has been studied and applications have been developed worldwide, too. Reference [3] describes a low-cost smart web sensor, which is designed and implemented to acquire, process and transmit data over 802.3 network and to construct dynamic HTML pages. Reference [4] presents a web-based multi-channel power quality monitoring system, which consists of PQ meters, data transmission and displaying in the Internet. Reference [5] depicts a concept of database management system to store power quality data and a web-based user interface of the system.

2.

THE CONCEPT OF POWER QUALITY DATA MANAGEMENT AND MONITORING SYSTEM

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 and current over an extended period. The process of analyzing and interpretation has been traditionally performed manually, but recent advances in signal processing and artificial intelligence fields have made it possible to design and implement intelligent systems to automatically analyze and interpret raw data into useful information with minimum human intervention. [6] The power quality data management and monitoring system presented in this paper consists of power quality meters (Quality Guards), their remote reading software, database server, web application, web server and client (i.e. web browser). Purpose of the system development has been to illustrate and demonstrate outsourced service relating to power quality. Figure 1 illustrates the system architecture of the system. Quality Guard is a commercial product of MX Electrix Oy, which is a Finnish energy and power quality meter manufacturer. Quality Guard is a fairly cheap smart kWh-meter with power quality monitoring functions. Quality Guard is remotely read by using GSM modem. Measuring functions are implemented by using sparse sampling methods described in reference [7]. Most of the power quality quantities described in standard EN 50160 can be measured with Quality Guard. In addition to this, data on powers and currents can be measured.

Customer

Internet Web-based application

Quality Guard

Web browser

Quality Guard

Transmit DB Quality Guard

Quality Guard

Modem reading

Transmit Database

PQDB SQL procedures

Power Quality Database

Web server

Application Service Provider

Figure 1: System architecture of the web-based power quality monitoring system.

Quantities measured by Quality Guard are: • Voltage level, current, real power, total reactive power, fundamental frequency reactive power, apparent power, DC offset, total harmonic distortion (THD) of the supply voltage, and power factor for each of the three phases • Total real power into both directions • Total capacitive and inductive reactive power • Total apparent power • Frequency of the supply voltage • The ratio of the negative- and zero-sequence components to the positive-sequence component • Harmonics of voltage (3., 5., 7., 9., 11., and 13.) • Maximum values of total harmonic distortion (THD) of voltage, DC offset and power factor Quality Guard records also timestamp, length minimum/maximum value and rms value of voltage dips and swells, and starting and ending times of interruptions. In addition to this, quantities for energy consumption and building automation (i.e. water and heat consumption, state information, and temperatures) can be delivered with Quality Guard. The recording density can be set as some seconds, minutes or hours. Maximum of 24 quantities can be measured at a time and measured values are stored in memory of Quality Guard. With recording density of 10 minutes and 24 measured quantities the memory of Quality Guard can store data of approximately three days period. Setup and remote reading of Quality Guard are carried out by GSM modems or serial port of the meter using Transmit-system developed by Enersoft Oy or EQL Online application developed by Enease Oy. Transmit-system is a remote reading system of Quality Guards consisting of reading applications, timer applications and Transmit database. With timer applications, remote reading can be set to launch automatically, e.g., once a day. Measurements are stored into Transmit database, which is a relational database requiring MS SQL Server. Each measured value generates one row in the Transmit database consisting of name of measuring point, quantity, recording density, time stamp, measured value and profile, which means that each day 24*6*24=3456 rows are generated in the Transmit database by each measuring point with recording density of 10 minutes and 24 quantities. It follows that ten measuring points generate over 1000000 rows in one table in the Transmit database in time period of one month. Consequently, queries to Transmit database become slow and ineffective, which means that the structure

of Transmit database is not suitable for database applications. For the reason of mentioned shortcomings in the structure of Transmit database, it has been necessary to develop a database called Power Quality Database (PQDB) with a new database structure. The structure of PQDB is suitable for long-time data saving and for applications to use. Each measuring point has its own table where each measured value with same timestamp is stored in same row. In result of this, the number of stored rows in one day diminishes to 144 from 3456 and the number of rows stored in one table is independent of the number of measuring points. Data of voltage dips/swells and interruption is also stored in PQDB. Measurements are transferred from Transmit database to PQDB by automatically called stored procedures of MS SQL Server. Data stored in PQDB is analyzed and presented by using web-based application. Implementation of the application is carried out using Microsoft Visual Studio.NET application development tool and C# programming language, which are parts of the new Microsoft .NET architecture. Functionality of the web application consists of: • Presenting measurement data graphically in chart and grid mode. • Presenting voltage dips/swells in grid mode. • Presenting pulse measurements (e.g. water and heat consumption) and energy consumption calculated from power measurements. Graphical user interface of the web application is presented in figure 2. Authorization is required to access the solution, and authorized measuring points are concluded with the logging into solution. After logging in, the main page with basic information of the measuring point is presented.

Figure 2: Graphical user interface of the web application for presenting power quality data.

available on the server side. There is no need to have a system administrator to install the software to each PC in the company. The time-consuming software upgrade process is eliminated. • Maximization of system scalability: Web pages can be developed gradually. There is no need to recompile or relink the whole application to present updated information in the Internet. • Supporting multi-media, video conferencing, etc.: Internet browsers have the capability to show pictures, charts, graphics, audio sounds, and video clips. Therefore, interactive and effective two-way communication can be achieved. • Providing relational database access: Internet applications can be database-driven to present the latest information in the database all the time without any Web page changes. Data stored in a popular relational database can be dynamically displayed on Web pages and updated through the Internet.

User can select examined quantities from the list and time period from the calendars in the right hand side of the user interface. Time period can be specified from the drop lists below calendars. After selections user can examine trends of selected power quality measurements in chart mode or examine specified measured values in grid mode. Visible chart and grid are automatically updated when quantities or calendar selections are changed. Voltage dips/swells and interruptions are presented both in chart and grid mode. The screen is printable and charts can be saved to user’s computer for further study. Changing measurement point results presenting main page, changing today’s dates to calendar components, and updating list of quantities with ones measured from selected measuring point. 3.

ADVANTAGES OF WEB APPLICATION

The development of advanced Internet technologies has had influence on trends in software production. Using the Internet techniques in application development gains the following benefits: [5] • Fairness: Users do not need to purchase any expensive hardware equipment or software application to gain access to the Internet throughout the world. One can use various communication media, e.g., normal telephone line, ISDN line, cable modem, etc., to retrieve information or update data remotely. • Supporting cross-platform architecture: In standardized Internet browser environment with HTML and TCP/IP protocols, users will have the same look and feel for web displays in different hardware platforms. Hence, graphical user interface is developed once and can be presented in Windows and UNIX platforms. • Supporting open system architecture: Internet applications follow open system standards, e.g., HTML, HTTP, TCP/IP, FTP, PPP and SQL. Data exchange and system expansion are done with minimum efforts. Application and database interface are defined consistently according to industrial guideline, e.g., Open Database Connectivity (ODBC). • Providing user-friendly and consistent human-machine interface: Working on wellbehaved Internet browser, users do not need to learn difficult commands or mouse operations to surf any newly developed application. Hence, training efforts can be minimized. • Minimization of installation and maintenance efforts: New applications can be directly downloaded through Internet to client sides. Software upgrades can be done automatically when new versions are

4.

POWER QUALITY MONITORING AS APPLICATION SERVICE PROVISIONING (ASP)

Concentration on core businesses is at present a trend among Finnish distribution companies and same kind of development can be found in other countries, too. Companies are outsourcing their functions and only the most important business (i.e. managing the network assets) is left to the distribution company. For example, network planning and construction and maintenance are more often given to another company providing this kind of services. One example of future trends could be outsourcing power quality data management and monitoring where the advantages of web applications are undeniable. A company supplying software applications and/or software-related services over the Internet is called application service provider (ASP). An ASP owns and operates a software application. The ASP also owns, operates and maintains the servers that run the specific application. An ASP employs the people needed to maintain the application, and also makes the application available to customers everywhere via the Internet, either in a browser or through some sort of thin client. The ASP bills for the application either on a per-use basis or on a monthly/annual fee basis. The ASP model has evolved because it offers some significant advantages over traditional approaches. Especially for small businesses and start-ups, the biggest advantage of an ASP is the low cost of entry and short setup time. Furthermore, the ASP model, as with any outsourcing arrangement, eliminates head count. IT headcount tends to be very expensive and

very specialized, so this is frequently advantageous. Moreover, the ASP model also eliminates specialized IT infrastructure for the application as well as supporting applications and licences. Another important factor leading to the development of ASPs has been the growing complexity of software and software upgrades. Distributing huge, complex applications to the end user has become extremely expensive from a customer service standpoint, and upgrades make the problem even worse. The ASP model eliminates most of these headaches with short implementation time and updates. When a distribution company is buying the power quality monitoring services from an ASP and power quality data is collected to ASP-company’s database server. The data is accessed through the Internet and power quality reports can be made with the web application in the user interface of a web browser. The distribution company benefits from easy-to-use power quality monitoring applications, which do not spend the company’s human resources like ordinary applications with all the maintenance and updating tasks do. On the other hand, the ASP-company benefits from fewer application and database updates when all the applications and databases are located in the same place. Case of distributed generation In case of distributed generation it is also possible to gather together all measured power quality data from various distribution generation units of the same producer. In addition to power quality monitoring, e.g., energy production of each distribution generation unit can be examined easily at the same time. ASP model allows also presenting data to every party needing the information, e.g., network company, customer, consult, electricity contactor and researchers.

5. IMPLEMENTATION AND EXPERIENCES OF A CASE STUDY Power quality monitoring of distributed generation unit has been demonstrated in 750 kW wind mill in Närpiö. Närpiö is located in the western coast of Finland and the power quality monitoring has been in use there since January 2003 with Quality Guard. The Quality Guard is read remotely to Tampere University of Technology (TUT) and the power quality data is served to all parties that need it with a web application in a web server that is located in TUT. The parties having access to the power quality data in this case are the network company (who in this case also owns the unit) and the researchers of the project. First two months power quality was monitored with recording density of ten minutes, which is also the recording density described for the most of the

quantities in the standard EN 50160. Nevertheless, to get more accurate measurements of the power quality the recording density was reduced to one minute. Because of this, the meter has to be read remotely in every eight hours. Measurement data is transferred into power quality database (PQDB) once a day. Figure 3 illustrates trends of real powers over one week period with recording density of one minute.

Figure 3: Trends of real powers over one week period in Närpiö wind mill. With demonstrated system, the owner of the wind mill can easily check the power quality level of the generation unit. The fact that Quality Guard is basically a kWh meter offers automatically the amount of energy produced by the generation unit. The owner can also check if there have been interruptions or voltage dips/swells. There should be some logic to conclude the source of voltage dip/swell and in case of small dip the distributed generation unit has to support the voltage level. In case of autoreclosing, the distributed generation unit has to be automatically disconnected from to network. Power quality monitoring offers the owner statistics of these disconnections. Measured data allows also researchers to study and understand the behavior of the distributed generation unit and investigates the effects of integration of distributed generation into an electricity distribution network from planning and operation point of view. In the future, one Quality Guard will be installed also to the feeding primary substation connected to the wind mill. This allows the network company and other parties of the research project to analyze the effects of the distributed generation unit on power quality more widely. 6.

CONCLUSION

In this paper solutions and service models that distribution companies can use in power quality data management and monitoring were presented. One goal in our research was to develop a system

consisting of remote data reading, database structure for data storage and management, and a web application for presenting and analysing power quality data. One big challenge in our research has been to test this system in real working environment. The real working environment usually brings out new problems that were not considered in the research environment and that is why these kinds of applications should always be tested in an environment they will actually be used in the future. Concentration on core businesses is at present a trend among distribution companies. Application service provisioning (ASP) model gives new possibilities for distribution companies to monitor power quality continuously using outsourced service provider. The development and commercialising of these applications is about to start by a company founded by the researchers. 7.

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