Application of 3G Technology in Power Quality Monitoring SystemYi Zhang, Honggeng Yang

School of Electrical Engineering and Information Sichuan University

Chengdu, Sichuan Province, China zhangyiscu@163.com

Gong Cheng Fujian Electric Power Research Institute

Fuzhou, Fujian Province, China 233616478@qq.com

Abstract—At the present time the dedicated wired network is used widely to transfer power quality monitoring data. This transmission mode has some shortcomings, for example, the accessing terminals scope and the bandwidth resources are limited; the investment of the construction and maintenance is larger and the period is long. In view of the advantages of 3G technology, this paper proposed a 3G-based power quality monitoring system communication network, uses the VPN technology based on the L2TP and IPSec combination to build a virtual private monitoring data transmission network channel. Currently the 3G wireless networks is successfully applied in Sichuan power grid unified power quality monitoring system in China as an important supplement to the wired network and the actual field test results prove the reliability and practicality of this solution.

Keywords- 3G technology; power quality monitoring system; communication network; VPN

I. INTRODUCTION Along with the development of power network, more

disturbance loads like power electronic devices, arc furnaces and electric railway loads accessing to the power network, the Power Quality (PQ) problems caused by these loads had drawn great attention of power supply departments and power users. The continuous PQ Monitoring (PQM) and analysis are the preconditions of disturbance loads control and power system’s PQ improvement. The traditional single-point measurement method can not meet the current PQM requirements of netlike, information and standardization [1]. Along with the developments of communication and information technology, provincial or regional PQM centers and particular monitoring systems aiming at electrified railway, new energy and energy-intensive industries had been built [2-6].

PQ Monitoring System (PQMS) includes monitoring terminals, communication network and monitoring center. The communication network is responsible for transferring the measuring data from the monitoring terminals to the monitoring center and sending the monitoring and control commands of the center to the terminals and it’s the important component of PQMS [7]. Early wired dial-up communication mode using the public telephone line is gradually replaced by Ethernet communication mode using fiber-optic. However, this dedicated wired communication mode has following shortcomings:

(1) Although some dedicated power communication networks had been built in some provinces, however in some remote substations which hadn’t been communication transformed the optical fiber cable connectivity is not yet available.

(2) In the process of checking the disturbance loads accessing to the power network and particular projects commissioning (like converter station commissioning) the portable PQ monitors are used for short-term PQM frequently. This kind of terminals can not access to the monitoring center directly with the dedicated wired network. The only method is bringing the measuring data back and importing to the database manually.

(3) As far as the substations have the available wired communication network, along with the increase of types and quantities of the front-end processors and data collection terminals which all using the Ethernet communication mode, the network resources like IP addresses, fiber ports and network bandwidth are limited.

(4) Due to the single dedicated wired communication mode, once the single network break down, all data transmission will be interrupted and it take long time to recover after the fiber is damaged, during that time the monitoring center can’t get the measuring data..

(5) For the steel, metallurgy, chemical and other industries which the dedicated communication network isn’t available, the PQM is also more and more important, but the high cost and long cycle of implementation of dedicated fiber-optic network; the larger operation and maintenance workload after the construction due to the impact of human and climate factors objectively hinder the promotion and application of PQM in these industries.

At the present time China has entered the third generation mobile communication (3G) era, compared with the previous communication technology the 3G technology has broad signal coverage, stable channel guarantee, high-speed bandwidth and successful engineering project applications. To solve the problems mentioned above the 3G technology provides a new solution. The Virtual Private Network (VPN) technology is used to plan and construct a secure and convenient dedicated channel in the high-speed communications platform provided by 3G public networks. In this paper, the 3G-based communication network architecture of PQMS is researched and the implementation solution is proposed. This solution is successfully applied in the unified PQMS in Sichuan power grid, China. By testing the rate and stability of field data transmission, the reliability and practicability of the solution is verified. As an indispensable complement to the wired network, the 3G wireless network has played a crucial role in PQM of certain particular occasions or as the spare communications network.

978-1-4577-0547-2/12/$31.00 ©2012 IEEE

II. THE 3G-BASED COMMUNICATIONS NETWORK OF PQMS

A. PQM VPN VPN is a technology which can establish a private network

in the open public network; it adopts the tunneling technique, encryption and decryption technology, authentication technology, key management technology and access control technology to establish a temporary and secure channel in public network [9]. Currently the 3G router, firewall and the operating systems like Windows, Linux can all support VPN function.

Tunneling protocol is the core technology of VPN. According to the protocol type it can be divided into two kinds: link layer protocols and network layer protocols. The former includes Point-to-Point Tunneling Protocol (PPTP), Level 2 Forwarding Protocol (L2F) and Level 2 Tunneling Protocol (L2TP). The first two tunneling protocols have been eliminated practically. L2TP is an extension of Point to Point Protocol (PPP) and it combines the advantages of PPTP and L2F and supports multiple networks protocols, WAN and Ethernet technology, but the L2TP itself does not provide any data security. Network layer protocols include Generic Routing Encapsulation protocol (GRE) and Internet Protocol Security (IPSec). IPSec is a series of open standards for IP security drawn up officially by the Internet Engineering Task Force (IETF). It provides security for data transmission in the network layer and it’s the most secure IP security protocol [10]. It has been successfully applied in power dispatching data network and it’s suitable for PQMS application which requires high safety performance.

In this paper, the L2TP and IPSec protocols are combined to build a fast, safe and stable 3G-based PQM virtual private communications network. IPSec can take advantage of the L2TP’s perfect, centralized and unified user-level authentication and authorization mechanisms and make up the security shortfall of L2TP.

B. Communication network architecture The 3G-based PQM communication network architecture

is shown in Figure 1.

PQM on-line terminals or mobile portable monitors can access to the wireless network through 3G routers, multiple terminals in the same substation can share one 3G router. The communication network using L2TP tunnel includes L2TP Access Concentrator (LAC) and L2TP Network Server (LNS). LAC is a Network Access Server (NAS) close to the PPP user side and it uses the tunnel to send any network layer protocol data encapsulated in PPP. It is the sender of the incoming call and the receiver of the outbound call and it uses APN authentication mode to prevent unauthorized users from dialing in PQ communication network. LNS is the server side of PPP for handling the L2TP. It is responsible for establishing, maintaining, releasing tunnel. A dedicated line is used to connect LAC and LNS. The firewall blockades any port except the ones VPN used to isolate the 3G public network from the

center network and ensure security of inside network. The AAA server in PQM center saves the user names and passwords of the monitoring terminals which are used when the connection is established. It supports the Remote Authentication Dial-In User Service (RADIUS) and executes the L2TP dial-in authentication for the accessing terminals. The communication server uses two methods to get PQ measuring data from terminals: the real-time communication transmission services mode based on Manufacturing Message Specification (MMS) which is defined in IEC61850 [13] and the PQDIF file transmission mode based on FTP. The former is for the real-time monitoring and the latter is used to transfer historical data over time. The database server is used to store real-time and historical PQ measuring data and indicators obtained by statistics and analysis. The clients in monitoring center or the mobile inquiry terminals (like notebook computer, smart mobile phone and Tablet PC etc.) accessing to the 3G network can using the browser to get the PQ status through the WEB server.

Figure 1. The hardware chart of 3G-based power quality monitoring communication network

C. The data transmission process of PQM The measuring data transmission process based on 3G

technology is divided into the following two phases:

(1) Establishing the VPN tunnel between the monitoring terminal and the communication server in monitoring center.

(2) The communications server takes the initiative to call and transport real-time and historical measurement data from monitoring terminals through the VPN tunnel.

The establishment processes of L2TP and IPSec based VPN tunnel is shown in Figure 2.

Figure 2. The establishment processes of L2TP and IPSec based VPN tunnel

After the VPN tunnel is established, the communication server can access the terminal via an IP address. PQM terminal has implemented MMS and FTP server at the same time, the client (communication server) can get the real-time PQM data reports through MMS protocol or call the historical data PQDIF file which is generated by terminal regularly through FTP protocol.

When PQM data are transferred on L2TP and IPSec-based VPN tunnel, first the data need to be encapsulated by L2TP; subsequently be the encapsulated by IPSec. The two-stage data encapsulation diagram and corresponding data packet formats are shown in Figure 3. The IPSec encapsulation uses two basic protocols successively: the Encapsulating Security Payload protocol (ESP) provides data encryption guarantee and the Authentication Header protocol (AH) provides data source authentication and integrity guarantee.

Figure 3. The two-stage data transmission encapsulation diagram in the L2TP and IPSec-based VPN tunnel and corresponding data packet formats

At the receiving side of the tunnel, the analysis process is opposite: firstly, the IPSec decryption and authentication process; secondly, remove the L2TP header and PPP header in turn; finally, the data is sent to the internal server in monitoring center according to the destination address of internal IP header.

III. SPOT TEST RESULTS In the building processes of China Sichuan power grid

unified PQMS, some terminals installed in the remote areas’ substation which don’t have fiber-optic network and some portable monitor used for mobile measurement access to the monitoring center through the 3G wireless network. For some monitoring points which the wired network is often broken down, we retain the existing communication mode and make the 3G wireless networks as a backup transmission channel. Once the wired network failed, the center switch to the 3G wireless network automatically and immediately to make sure the normal and continuous operation of PQMS.

During the one year’s pre-operation time, the transmission rates, stability and security of the 3G wireless network have been tested. The test time is from September 1, 2010 to September 1, 2011; the test results are shown in Table 1, 2, 3. As space is limited only parts of the monitoring points’ test results are listed.

TABLE I. THE TEST RESULT OF FIELD PQDIF FILES TRANSMISSION RATE

Monitoring point name

Average file size (MB)

Transmission rate (Kbps)

Average transmission

time（s） Max Avg

220kV Huidong substation 1.3 405.9 251.5 42.3

220kV Lugu substation 1.2 431.6 240.5 40.9

110kV Songlin substation 1.1 295.8 208.9 43.1

110kV Ganluo substation 1.2 382.3 224.7 43.7

10kV Zhongya technology

Subscriber line 0.8 392.7 216.4 30.3

Every day the monitoring center on schedule (early in the morning, when the network load is low) calls the last day PQDIF files generated by the monitoring terminals. The PQDIF file size in the Table 1 is the average size of daily files of the year. We can see from the Table 1 the average data transmission time for each monitoring point of one day’s data is about 40 seconds which can meet the application requirements.

TABLE II. THE TEST RESULT OF ON-THE-SPOT REAL-TIME DATA TRANSMISSION RATE

Monitoring point name

Average real-time

data (MB)

Transmission rate (Kbps)

Average transmission

time（s） Max Avg

220kV Huidong substation 34.5 235.1 61.3 9.4

220kV Lugu substation 32.8 231.3 50.1 9.0

110kV Songlin substation 35.9 225.2 48.9 11.7

110kV Ganluo substation 33.1 222.2 45.9 15.8

10kV Zhongya technology

Subscriber line 34.4 232.9 46.7 14.3

The monitoring terminal uploads real-time data at one minute intervals. The real-time data size in Table 2 is the average size of transmission data sizes in one year. Based on the MMS protocol according to the data set configuration the terminal forms data reports of about 3K and sends them periodically. Because the lower transmission power, the real-time data transmission rate is lower than PQDIF file transmission rate. We can see from the Table 2 the average transmission time is much shorter than interval time of real-time data transmission and it can meet the application requirements.

TABLE III. THE TEST RESULT OF ON-THE-SPOT COMMUNICATION NETWORK STABILITY

Monitoring point name

The days of missing data

Miss rate（%）

220kV Huidong substation 11 3.01

220kV Lugu substation 0 0

110kV Songlin substation 0 0

110kV Ganluo substation 3 0.82

10kV Zhongya technology Subscriber

line 2 0.55

In the table 3 the number of days the real-time data is missing is shown (the historical data can be completed after the network is restored), the lost is because of the communication network problems. The main reasons for 3G network outages includes 3G router failure, VPN server system failure and the 3G operator’s network maintenance.

In the ID authentication test, we can’t establish the connection through a variety of unauthorized accesses. In the encryption test, by analyzing network packets using Sniffer software we can see after encrypted the contents transferred in VPN tunnel is cryptograph. We simulate the FTP attack, DoS denial service attack and RPC leak attack, and the results prove that the network has the capability of counteracting network attacks.

We can see from above test results that communication network based on 3G technology has high security and stability and data transfer rate can meet the requirements of PQM.

IV. CONCLUSION In view of various shortcomings of single wired

transmission mode in existing PQMS, this paper proposed a solution that build PQM communication network by using 3G-wireless technology, it has successfully used in China Sichuan power grid unified PQMS as an indispensable complement to the wired transmission mode. Compared with the traditional transmission mode, the communications network using 3G technology has following advantages: the access of monitoring terminals is more convenient and flexible and at the same time the portable monitors and mobile inquire terminals can access to the PQMS at any place and anytime. The L2TP and IPSec- based VPN technology ensures the security of PQM communication networks. The statistical results of actual field test prove the availability and reliability of the proposed solution.

REFERENCES [1] YANG Jin，XIAO Xiang-ning. “New development tendency of power

quality supervision technology -netlike, informization and standardization”. Electric Power Automation Equipment, 2004,24(11):82-87

[2] WANG Bin,PAN Zhen-cun,ZHANG Hui-feng. “Design of Distributed Power Quality Monitoring System Hardware”. Journal of Electronic Measurement and Instrument, 2004,24(2) :1-4

[3] WANG Ming-yu，ZHOU Yan-jing，ZHAO Jun-hui，et al. “Research of the networked integrated monitoring system for power quality”. Power System Protection and Control, 2010, 38(1):87-91

[4] FAN Rui-xiang ， XIN Jian-bo ， SHANGGUAN Tie ， et al. “Development and Application of Open-Type Power Quality Monitoring System for Jiangxi Section of Zhejiang-Jiangxi Electrified Railway”. Power System Technology, 2008, 32(21):68-71

[5] SUN Shou-jiang，YAO Guo-xing. “Design of Wind Power Quality Monitoring System”. M&E Engineering Technology，2007,36(5):32-33,49

[6] CHEN Mei-hua，ZHAO Jian-hong，HUANG Shiguan. “Research on Power Quality Supervision System In GISE’s Electric Network”. Metallurgical Collections，2004,153(5):13-14,38

[7] LIU Jun-yong，YANG Hong-geng，XIAO Xian-yong. “Issues and technology assessment on power quality Part 5:Power quality monitoring and data management”. Electric Power Automation Equipment, 2004,24(2) :1-4

[8] SHI Xiang. “Virtual private monitoring network of SCADA system based on 3G”. Computer Engineering and Design，2011,32(5):1158-1162

[9] Yuan Ruixi, Strayer W T. “Virtual Private Networks: Technologies and Solutions”. Boston: Addison Wesley Publishing Company, 2002.

[10] LI Pin. “Analysis of Security Property about VPN’s Principal Tunnel Protocols”. Computer Engineering，2006,32(13):164-165,169

[11] LUO Han-wu，LI Fang. “Solution of secure power dispatching data network based on IPSec”. Telecommunications for Electric Power System，2006,27(159):1-3

[12] PI Jian-yong，LIU Xin-song，LIAO Dong-ying，et al. “A Security Scheme for Power Dispatching Data Network Based on VPN”. Automation of Electric Power Systems，2007,31(14):94-97

[13] IEC 61850-8-1 Communication Networks and Systems in Substations-Specific Communication Service Mapping (SCSM)-Mapping to MMS(IS0/IEC 9506 Part 1 and Part 2) and to ISO/IEC 8802-3.