A New Protection Scheme for Short Transmission Lines Using IEEE 802.11 Protocol M. M. Eissa (1), Senior Member, IEEE, A. S. Ali (2), M. E. Masoud (1) and K. M. Abdel-Latif (1) (1) Department of Electrical Machine and Power Engineering (2) Department of Electronics, Communication, and Computer Engineering Faculty of Engineering-Helwan University at Helwan Email:
[email protected]
Abstract- In this paper a new protection protocol for short transmission lines using wireless communication is introduced. When relays communicate with each other, they can exchange information which help the relay to take an accurate decision. The suggested protection system collects currents data from both ends of transmission line through Intelligent Electronic Device (IED) and communicate with each other to share the data. The IEEE 802.11, wireless protocol, is used as a communication protocol. Keywords: short lines, wireless communication, intelligent electronic devices, differential protection and IEEE 802.11 protocol. I. INTRODUCTION
T
he pilot wire differential protection is one of the most common method for protecting the short transmission line. Most differential relays are current differential relays in which vector difference between the currents at both ends of transmission line is used for relay operation. Another type of pilot wire differential protection is balanced voltage differential protection. The operation of this type is based on balanced voltage principle across two over current relays located at both ends of transmission line [1]. The disadvantages of using pilot wire protection are limited by specified line length (10 to 20km) because of the resistance and capacitance of the pilot wire, the lose of relay function due to line disconnection, the wire link needs to additional protection and high cost. Intelligent Electronic Devices (IED) is used for protection, data acquisition, metering, and control. IEDs are designed to perform conventional protection and control functions and may also serve as a source of metering and maintenance related data such as event, fault and disturbance recording information. Many different types of communication media can be used to conduct the data between IEDs. They include copper communication cable, Power Line Carrier (PLC), land line telephone, optical fiber cable and wireless communications. Wireless includes FM and microwave radio as well as cellular networks and satellite communications. The paper introduces a new methodology for protecting the short transmission line to satisfy the following features: Not stand alone decision Exchange information with the neighbors. Relays behave adaptively according to any change in system parameters Independent action according to the available received information. II. OVERALL STRUCTURE OF THE PROPOSED SYSTEM The suggested scheme consists of three main parts: Two Intelligent Electronic Devices (IEDs). Wireless Communication Network. Wireless Communication Protocol. Fig. 1 shows the main parts of the new technique used for protecting the short transmission lines. The system consists of current transducers which fed the IEDs by current signals. The current signals are exchanged between two IEDs through wireless communication network.
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Fig .1
Main parts of the suggested scheme.
Protection devices may receive or send data to controllers or other protection devices to form a system whose primary function is that of protecting the transmission line. The protection device (IED) contains the protection scheme which used to protect the short transmission lines. The details of the suggested system are illustrated in the following four sub-sections.
Protection scheme.
Wireless communication network.
Wireless communication protocol.
Case study. Protection Scheme
The protection scheme is based on current signals measured at both ends of transmission line and transmitted via communication network. The protection scheme is applied on the power system configuration shown in Fig. 2.
Fig. 2
Power system configuration.
The suggested technique depends on the current deviation signal in its operation. To explain further, the inception of a fault in a power system will cause the post-fault current at a relaying point to deviate from the steady-state pre-fault current The technique can be explained through an analysis of two key components. The first component will be referred to as the starting element and the second is considered to be the differential element both are defined as follows: The starting element is considered to be the first step in initiating the relay operation. It can be initiated when the magnitude of post-fault current exceeds the magnitude of pre-fault current by a specific limit[5]. The differential element follows the starting element and recognizes the protected zone that distinguishes between internal and external faults. Firstly the differential element calculates the current deviation signals during ¼ cycle using equation. W
∆I = ∑ [I se ( j ) + I re ( j )]
(1)
j =1
where ∆I is the current deviation signal for phase a, b and c. w is the number of samples during ¼ cycle). Secondly, the current deviation signals will be compared after ¼ cycle with the same current deviation signals of the previous cycle. The comparison can be mathematically explained as follows: 2
For normal operation and external faults: │ ∆Inew │ ═ │ ∆I previous│
(2)
For internal faults: │ ∆Inew│ > │ ∆I previous│
(3)
Determining threshold values is essential in identifying the relay, especially in the case of external short circuit. The current which flow through the line may be equal both under normal operating conditions and in the event of an external short circuit. Even under these conditions, however, the differential protection scheme will receive certain inequality in the two sending and receiving ends currents. Therefore, the threshold should be selected based on a practical difference between the two sending and receiving end currents. It is noticed from the previous discursion that the two currents at both ends should be recognized. The procedures of transferring current data from the far end are discussed in the following sub sections Wireless Communication Network. Telemetry systems using radio waves or wireless offer several advantages over other transmission methods. Some of these advantages are: • • • • • •
No transmission lines to be cut or broken. Faster response time. Lower cost compared to leased lines. Ease of use in remote areas where it is not practical or possible to use wire or coaxial cables. Easy relocation. Functional over a wide range of operating conditions
Properly designed radio links can provide low cost, effective and flexible data gathering systems that operate for many years with very little maintenance. The components of a typical wireless telemetry system are shown in Fig. 3. The sensor is considered the data source. The output of the sensor is converted to digital data by a small computer device. The processor is interfaced to a modem device that converts the digital data into an analog signal that can be transmitted over the air. The radio transmitter then transmits the signal to the host site radio receiver. Now the process is reversed. The modem takes the analog signal received and converts it back to a digital form that can be processed by the data recovery equipment.
Fig. 3 Components of wireless telemetry system In a typical application, the host site requests data from the remote site(s). The host transmits a request to the remote unit telling it to send its data. The host reverts to a receive mode and waits the transmission from the 3
remote site. After the remote sends its data, it goes back to a receive mode waiting for further instructions to come from the host. Once the host receives the remote site information, it may send additional instructions to that site or continue on to request data from the next remote site. This polling process continues until all the remotes in the system have sent their data. Predictions of radio range can be made using a free space isotropic nondirective antenna model where the path loss is 22 dB for one wavelength of separation between antennas and increases 6 dB every time the distance is doubled. However, this model holds true only in free space. Under actual conditions, other factors must be considered Wireless Communication Protocol As mentioned earlier, the measured data via the sensor is processed and passed through an interface (e.g., RS 232) serial port to the Network Interface Card (NIC) of the wireless network (IEEE 802.11 NIC) and then it forward to a transceiver which transmit the data via a pre specified wireless channel according to the operating wireless protocol. In this paper, a wireless based protection system using the IEEE 802.11 wireless protocol is used. The IEEE 802.11 protocol can provide almost all the functionality and high data- transmission rates, as well as the wireless network can be connected to any wired network as an extension to the system. The IEEE 802.11 protocol operates by coordinating the access to the radio channels and sending packets of data between wireless access devices. Packets of data that are sent within the network must contain enough addresses and control information to allow them to reach their destination. The packets are sent directly between units (independent mode) or they may travel through a backbone network (distributed mode). The frame structure for an IEEE 802.11 data packet is shown in Fig.4. The frame structure consists of five fields. The data field can vary from 0 to 2312 bytes.
Fig. 4
IEEE 802.11 packet frame structure
The data packet also includes Frame Check Sequence (FCS) to check the packet for transmission errors. The Medium Access Control (MAC) header is divided into a frame control field or type of frame field contains up to 4 addresses. These addresses are used for source, distribution and destination packet routing. The MAC header also contains a sequence control to identify each frame in a sequence of frames. The MAC of the IEEE 802.11 involves sensing activity, timing, experimental back off and retry limits. Before stations can transmit, they must listen to radio activity around them to determine if they are allowed to transmit. The methods for controlling access to the wireless network systems may be distributed coordinated function (DCF) or point coordination function (PCF). DCF mode is peer-to-peer network where the temporarily wireless network that has no server or control access point, hub, or router. Since there is no central base station to monitor traffic or provide internet access, the various signals can collide with each other. PCF allows the coordinated operation of wireless data devices (stations). In PCF contention free system, communication devices wait until they receive a polling message before they transmit any information. Because a master host coordinates the transmission of all the devices within its networks, no device will transmit at the same time. It is possible to combine the benefits of the distributed (DCF) and control (PCF) access into one system. To control the overall flow of packets in the IEEE 802.11system control packets are used. These control packets include RTS, CTS, ACK, PS-Poll, CF-END and CF-END+ACK. The IEEE 802.11standard includes the original 802.11, 802.11a, 802.11b and 802.11g frequency bands and data transmission rates. The IEEE 802.11series of industry standards are base on 802.3 Ethernet technologies. The radio frequency systems can provide data transfer rates from 1Mbps to 54 Mbps operating on a frequency band of 2.4 GHz or 5.7GHz. Case study A digital simulation model for the power system network is applied using ATP program [6] and the MATLAB package [7]. Table 1 shows the parameters of the power system network.
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Table 1 Power network data Power system Parameters element Transmission Length = 10 km Line R1= 0.02083 Ω/km L1= 0.8984 mH/km C1= 0.0129 µF/km R0= 0.1148 Ω/km L0= 2.2886 mH/km C0= 0.00523 µF/km Source-1 Rs= 1.0515 Ω Ls= 137.5 mH Source-2 Rs= 1.0515 Ω Ls= 137.5 mH Fig.5 (a, b&c) shows the current signals measured at the sending and receiving ends when single line to ground fault through 10Ω on phase A at 2km from source1. Fig.5 (d) shows the deviation signals for phase A, B and C. The deviation signal of phase A is greater than the threshold value while the deviation signals for phase B and C are less than the threshold value. This means that the fault is internal single line to ground phase A. Fig.6 (a,b&c) shows the current signals measured at the sending and receiving when single line to ground fault through high impedance equal 100Ω phase A at 2km from source1. Fig.6 (d) shows the deviation signals for phase A, B and C. The deviation signal for phase A is greater than the threshold value while the deviation signals for phase B and C are less than the threshold value. This means that the fault is internal single line to ground phase A. The threshold is selected based on deviation signals for phase A, B and C. In this case study the threshold value is selected to cover a wide range of fault resistances up to 100 Ω as shown in Fig.5 (d) and Fig.6 (d). Fig.7 (a,b&c) shows the current signals measured at the sending and receiving ends when line to line fault on phase B and C at 4km from source1. The Fig.7 (d) shows the deviation signals for phase A, B and C. The deviation signals of phase B and C are greater than the threshold value while the deviation signals for phase A is less than the threshold value. This means that the fault is internal double line fault phase B and C. Fig.8 (a,b&c) shows the current signals measured at the sending and receiving ends when external three phase short circuit. Fig.8 (d) shows the deviation signals for phase A, B and C. The deviation signals for phase A, B and C are less than threshold value. This means that the fault is external. III. CONCLUSION The paper introduced a new protection technique for short length transmission line using IEEE802.11 protocol. The suggested protection system collects the current data from both ends of transmission line. The technique has the feature of pilot wire protection. The limited length of transmission line because of resistance and capacitance is avoided. A reliable and accurate decision of transmission line protection is obtained using the technique with two current information available at both ends. A wireless protocol is suggested for data handling. In this paper we apply a wireless communication system in a simple case (two ends of transmission line) but in our future work we will use the wireless communication system in multi-terminal power system network. IV. REFERENCES [1] S. H. Horowitz, A. G. Phadke, "Power system relaying", Research studies press, Taunton, Somerest, England, 1992. [2] Relaying comity group, “Application of a Peer-to-Peer communication for protective relaying “, IEEE trans on power delivery, Vol:17, No:2, April 2002. [3] X. R. Wang, K. M. Hopkinson, J. S. Thorp, R. Giovanini, K. Birman and D. Coury "Developing an agentbased backup protection system for transmission networks", Power Systems and Communications Infrastructures for the Future, Beijing, September 2002 [4] M. M. Eissa, “Local area supervisory protection system at transformer taps “Electronic power systems research, 62 (2002), pp 105-110. [5] M. M. Eissa and O. P. Malik "A new digital directional transverse differential current protection technique", IEEE Trans, on Power Delivery, vol. 11, No.3, pp. 1285-1291, July 1996. [6] Leuven EMTP center, ATP Rule book/User's Manual, July 1987. And, ATP-EMTP Can. EMTP Users Group. [7] A. Biran and M. Breiner "Matlab6 for engineers" printed in Great Britan by Henery Ling Limited, Dorset Press. 5
Fig. 5
Relay performance for single line to ground fault through 10Ω at 2km from source-1
Fig. 6 Relay performance for single line to ground fault through 100Ω resistance at 2km from source-1 6
Fig. 7
Relay performance for line to line fault at 4km from source-1
Fig. 8
7
Relay performance external three phase short circuit
