INTEGRATED POSITIONAL PROTECTION OF TRANSMISSION SYSTEM USING GPS

1. INTRODUCTION

It is not practical and economically feasible to design and manufacture electrical system that will never fail in service. System will fail, and there comes the role of protection. Power system protection is a technique employed in power engineering to detect the faults and failures produced in the power system and to isolate the faulty parts from the rest of the network. The history of protection begins with the very beginning of electric power transmission history.

A century has passed since the application of the first electro-mechanical overcurrent relays in power system protection. Now a days relay technology has enjoyed successful development based on the application of digital techniques for power system protection. It is the time to think about veracious modern protection techniques supported by latest communication facilities and digital technologies. This paper presents a new technique for the protection of power transmission systems by using the Global Positioning System (GPS) and fault generated transients-The Integrated Positional Protection technique.

In Integrated positional protection of transmission system using global positioning system we use a specially designed integrated positional protection relay, which contains a fault transient detection system together with a substation communication network. The relay detects the fault generated high frequency voltage transient signals and records the time instant corresponding to when the initial travelling wave generated by the fault arrives at that busbar. The relay uses one of the most efficient time tagging scheme, GPS time tagging on the received transients and the protection algorithm is based on the comparison of these time tagged data. The communication unit is used to transmit and receive coded digital signals of the local information to and from the associated relay(s) in the system. At each substation, the relays are capable of determining the location of the fault by comparing the GPS time stamps measured locally with those received from the adjacent substations. If the fault position is inside the protected zone coming under any relay, the corresponding relay will issue the required tripping signal and the faulty portion is successfully isolated from the grid.

2. Protection of Transmission System

Power system protection is a technique employed in power engineering to detect the faults and failures produced in the transmission system and to isolate the faulty parts from the rest of the network. The electric power transmission system plays a vital role in between the power generation and utilization. In order to maintain the continuous supply and to reduce losses, we must design and protect the transmission system very carefully. The main goals of electric power transmission systems are

· Continuity of supply

· Ensure the safety of personal

· Minimization of damage

· Safeguarding of entire system

In power generation distribution system the transmission system is the portion from the step-up side of generation transformer to the high voltage side of the distribution transformer. So the main levels of application of electric protection are

· Generating side

· Distribution side

· Transmission system protection

Among the three main levels of protection, our topic of interest comes under the transmission system protection. The transmission systems were introduced a century ago. Since its introduction various developments and changes were occurred in the electric protection techniques. In order to analyze the advantages of integrated positional protection technique we must consider the protection history.

3. History and developments in protection methods

The fig.1 shows the development graph of protection methods.

4. INTEGRATED POSITIONAL PROTECTION OF TRANSMISSION SYSTEM USING GPS

‘Integrated Protection’ is used to denote the integration of protective devices for multiple transmission lines within the grid into one protective relay in substation. This master relay will ensure protection to the different transmission lines connected to the particular substation. In the late 1960's, Rockefeller proposed the application of a centralized substation protection system based on a centralized computer system. The concept fits well with the concept of an overall integrated protection where the protection package would not only oversee individual units of plant but also a section of the network. However, the idea has not been put into practice thus far since the computer hardware/software and the communication technology were available to support such an idea. Since, relay technology has enjoyed successful development based on the application of digital techniques for power system protection; it is time to think about the integrated protection schemes. The basic principle and characteristics of Integrated Protection are

4.1 Transient Based Protection (TBP)

Protection of power transmission system based on the detection of fault generated high frequency transients' is a concept for a new generation of protection, in short, 'Transient Based Protection' (TBP). When ever the fault occurred in a transmission system different transients are produced. The transients are the additional current or voltage components with short time period (micro or nano second) and frequency variation from DC to very high value (KHz or MHz). When any kind of fault occurs in the transmission system, the transients produced will propagate in all available directions. These additional components contain extensive information about the fault. By conducting suitable analysis of these transients, it is possible to determine the nature and characteristics of the fault. Transient study can give us the idea about the type of fault, exact time of fault and the distance to the fault from the reference station.

Fig.2

In transient based protection basically we are entrapping the transients approaching at different end stations in the grid using relays and coupling arrangements. The information collecting devices are connected to the power line busbar using CVT. By comparing the arrival time of different transients the center control unit will distinguish whether the fault is in the protected zone or not. The given below equation can be used for the calculation of fault distance.

Where

tf1-arrival time of 1^st transient

tf2-arrival time of 2^nd transient

X-distance to fault from end station

u-velocity of traveling wave

In fig.2 fault location between two adjacent end stations is shown. Here a fault is occurring at a distance ‘x’ from the first reference station. At the time of fault the transients arise and travel towards two ends. At the end stations these transients are collected using coupling arrangements; here current and potential transformers are used. The received signals are fed to the GPS false system which is provided by GPS antenna for time tagging facility. They will transfer the time tagged digital codes to the central control unit where the fault location is taking place. The above given equation is the base used for the fault position calculation

4.2 Positional Protection Scheme

Fig.3

The fig.3 shows the schematic representation of positional protection scheme. In this protection scheme relays are installed at each busbar and are responsible for the protection of network. When a fault occurs on any part of the transmission network transients will arise and propagate in all directions. Different relays trap these transients through some kind of interfacing modules like CT/PT,CVT etc. Each relay records the arrival instant of different transient signal generated by the fault and time tag the digital information. These specially designed relays are capable of communicating with the neighboring relays in the network, and by comparing the information form all relays we determine whether the fault is with in its protected zone. Then trip signals are issued if needed.

4.3 GPS

Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbits. It is also possible to cover entire earth with 3 satellites alone. But as the no. of satellite increases the tracking efficiency also increases. These GPS satellites circles around the earth twice in a day. They are capable of sending and receiving signals from different GPS modules in the earth. This allows precise determination of location, velocity, direction and time. The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

4.5 GPS aided Integrated Protection

This is a combined protection technique using both the transient based protection (TBP) and the GPS. In this method the transients generated by the fault is received by some specially designed relays and the end stations. Here these signals are time tagged at the received instant using the GPS with its ability to provide time synchronization with an accuracy to 1us over the wide area. The communication gateway is used to transmit and receive coded digital signals of the local information to and from the associated relay(s) in the system. Usually satellite communication is used. Then at each substation, the relays determine the location of the fault by comparing the GPS time stamps measured locally with those received from the adjacent substations. By considering the data from all integrated relays each relay will decide whether the fault is in the protected zone coming under that particular relay and it issues trip command if needed. Now the one relay integrated on a substation is enough to provide protection to all transmission lines connected to it. In practical case backup protection schemes are also included.

The fig.5 shows the schematic representation of integrated positional protection using GPS. Here three substations are shown. B1, B2, B3 and B4 are busbars at substations. The line-1, line-2 and line-3 are the interconnecting transmission lines. Four integrated relays are connected to the transmission line through VTs and CTs. Different circuit breakers associated with different transmission lines are connected to the integrated relays. These special relays are capable of communicating with each others through the global communication gateway. Protection algorithm is developed by these integrating relays based on the transient based positional protection. Here we are also describing the proto type designing of one integrated positional protection relay.

5. INTEGRATED POSITIONAL PROTECTION RELAY

Fig.6

The fig.6 shows the proposed integrated relaying system, which can be divided into three major blocks: (1) Transducer interface unit, (2) the communication network and (3) integrated relay unit.

(1) Transducer interface unit

The transducer interface unit consists of an analogue to digital interface module (ADI) and a Merging unit. The Merging Unit (MU) is a measurement and control unit, which interfaces to apparatus through different types of transducer, such as conventional CT's and VT's, optical combined sensors and electronic combined sensors. The analogue and digital signals measured are converted into optical format and sent to the central protection relay through the optical network. The MU unit also receives and executes control signals from the relay through the control circuit.

In this application, the Interface unit is connected to the transmission line using the coupling capacitor of the conventional CVTs, which are able to detect the fault generated high frequency voltage signals of the substation busbar. The ADI is responsible for sampling and converting the analogue measurement into digital format and send to the MU unit. The MU will transfer the time tag signals obtained from the GPS clock into the data format and send to substation Ethernet based on communication standard. A sampling frequency of 1 MHz is used in this device. The digital filters are used to detect the high frequency signals generated by the fault and the accuracy of fault location was a function of the sampling rate used to digitize the measured signals. This accuracy was directly related to the sampling rate and the higher the sampling rate, the more accurate the measurement.

(2) Communication network

The optical Ethernet is used as the substation communication network for the integrated relay, which is a communication network interfaces not only to critical equipment in MU and the relay, but also to a number of other equipment, such as communication gateway, Human machine interface and GPS clock, etc. The transducers are not connected directly to the protection relay as in the conventional approach but through the Ethernet. The Ethernet channel also receives trip command from the relay and sends to associated CBs. The new method of collecting measurement data and controlling switchgear brings changes to the design of substation protection and control equipments.

(3) Relay Unit

The central protection relay unit receives measurement from all locations in the substation through the network and information from other related substations through the communication gateway. The relay then performs all calculation to determine whether there is a fault within the substation or on its associated line section. The relay will issue a trip command to open the associated circuit breaker through the network if a fault is detected. The relay unit mainly consists of a communication interface, transient filter, multi-channel time comparison algorithm and trip decision unit. As shown in Fig.2, the relay receives voltage signals from MU through the Ethernet, a threshold is set to determine whether a fault is occurring on the transmission line system. The relay will start to act once the threshold is met. The transient filter receives the voltage signal from the communication interface and extracts a desire band of high frequency transient, from which the arrival time of the fault generated transient can be derived based on the GSP tagging.

The multi-channel time comparison algorithm receives the time information from neighbouring substation(s) and compares with that derived from local measurement to determine whether a fault is within its protected region. The decision to trip the local breaker depends on the comparison between the times measured by the GPS systems at that location and those measured by other relays. Unlike conventional protection scheme, where each relay is dedicated to the protection of one line section and only associates with one circuit breaker on that line section, the proposed relaying scheme will be responsible for protection of several lines connected to the busbar where it is installed.

6.CONCLUSION

The paper reviews the concept and development of integrated protection. The technique has features of integrated protection, in which one relay alone is responsible for the protection of related line sections to its substation. A specially designed integrated positional protection relay, which contains a fault transient detection system together with a substation communication network, is used to extract the fault generated high frequency voltage transient signals and a GPS system to time tag these signals. The travelling time of the transient high frequency signal from the point of fault to the adjacent substation is used to determine the fault position. The proposed technique offers a very fast relay response and high accuracy in fault location.

The new protection principles and schemes that have been introduced are at an early stage of development so there could still be many practical problems. However, it is sure that the integrated positional protection will be an important new development. Now a days our digital and communication technologies developed enough to think about the adoption of integrated protection techniques.



By
Tony Tomson
tonytomson@gmail.com
tonytomson@hushmail.com