IE20070370U1 - A method of synchronising line-mounted sensors - Google Patents

Abstract

ABSTRACT This invention relates to a method of synchronising line-mounted sensors in a system for monitoring a medium voltage electricity supply network, the network comprising a plurality of power lines to be monitored. The system comprises a central controller and a plurality of remote line-mounted sensors in communication with the central controller. Each of the sensors has an internal clock. The method comprises the steps of the central controller transmitting a conducted communications signal to each of the line-mounted sensors in the system along the power lines and each sensor, on receipt of the conducted communications signal, setting their internal clock to a timing signal embedded in the received signal. In this way, highly accurate timing may be achieved allowing more accurate control of the network to be achieved. Ideally, a Broadband Power Line (BPL) conducted communications signal is used to transmit the signal to each of the line mounted sensors.

Description

A Method of synchronising Line-Mount Introduction This invention relates to a method of synchronising line mounted sensors in a system for monitoring an electricity supply network having a plurality of supply lines. the system comprising a central controller and a plurality of remote line mounted sensors. each having an internal clock.

Over the last number of years, it has become increasingly important for electricity suppliers to more carefully monitor their network and more particularly to more accurately determine the timing and location of faults in their electricity grid. In order to provide a high level of service to their customers and operate their network in a highly efficient manner, it is most important that faults may be recognised and corrected in as short a time as possible. To this end. many electricity suppliers have begun introducing advanced monitoring systems throughout their network. These monitoring systems typically comprise a central controller and a number of remote sensor units that monitor overhead and underground power lines. On the remote sensor units detecting a fault in the network, they send a signal to the central controller which alerts a responsible individual who may rectify the fault. Quite often, these systems further comprise a data acquisition and control unit (DAC) intermediate the central control unit and the remote sensors to relay information from nearby remote sensors onwards to the central control unit. In order to obtain useful information from the monitoring system, it is essential to have each of the sensors time synchronised with the remaining sensors in the network. This greatly facilitates fault location and can significantly speed up the time taken to correct a fault in the network. Furthermore, maintenance of accurate timing data on the remote sensors provides advanced signal analysis which may be of great value to the network operator.

Presently, there are two known methods used to synchronise remote sensors. Both of these methods synchronise the remote sensors to the sensor network gateway (SNG) which is the interface between the remote sensors and the DAC. These two methods are commonly referred to as GPS timing and radio timing. In GPS timing, lE070330 the remote sensors incorporate a GPS receiver, which makes available highly accurate timing data that is synchronised to coordinated universal time (UTC). The sensors are synchronised to this time and therefore to each other. A disadvantage of GPS timing is that each of the remote sensors requires a separate GPS receiver which. in certain circumstances, may be prohibitively expensive to implement, particularly in very large scale implementations. Furthermore, this complicates the circuitry of the sensor, thereby further adding to the expense of the sensor. The sensor will require an additional GPS module including a microcontroller. This increased complexity will inevitably reduce reliability due in part to an increased component count. The GPS module also requires power to maintain accurate timing lock which can represent up to between 25% and 40% of the total power required by the line unit. Finally, satellite acquisition and the time to acquire lock can be issues with the GPS modules that are undesirable.

The second method, namely, radio timing, consists of the SNG sending out a timing message to each of the sensors associated with that SNG. The sensors that receive the radio timing signal synchronise to this timing beacon and this ensures that the sensors are time synchronised to the SNG and each other. A disadvantage of this system is that frequent messages need to be transmitted on the radio. Although the radio normally used for communications may also be used for timing, due to the required frequency of the timing packets the radio has a high operational duty cycle which results in a relatively high power requirement. In any event, this further requires a system that must have a DAC and an associated SNG which may be undesirable in certain applications. Furthermore, by having such a method, each of the SNG’s must itself be synchronised and it is not uncommon to experience differences between the synchronisation between different SNG's and hence the synchronisation of different sensors in the network which does not allow for the highly accurate timing required.

It is an object therefore of the present invention to provide a method of synchronising line mounted sensors that overcomes at least some of the difficulties with the known methods, that is both relatively simple and inexpensive to implement.

IEUIOJIO Statements of Invention According to the invention, there is provided a method of synchronising line-mounted sensors in a system for monitoring an electricity supply network having a plurality of supply lines, the system comprising a central controller and a plurality of remote line- mounted sensors, each of the sensors having an internal clock, the method comprising the steps of the central controller transmitting a conducted communications signal to each of the line-mounted sensors in the system along the supply lines and each sensor. on receipt of the conducted communications signal along the supply line from the central controller, setting their internal clock to a timing signal embedded in the received conducted communications signal. By having such a method, the sensors will be synchronised to the timing signal embedded in the conducted communications signal. As the central controller communicates with each of the line units over the power lines using the same communications signal and method, there will not be a tendency for drift in time synchronisation between different sensors. Furthermore, this is seen as a particularly simple and inexpensive way of providing time synchronisation for each of the sensors that may be incorporated into a communications system of the sensors and central controller. Additional equipment such as GPS timers, radio transmitters and receivers will not have to be provided.

Furthermore, it is no longer necessary to provide a DAC or an SNG as these are no longer required. in one embodiment of the invention, the step of transmitting a conducted communications signal to each of the line mounted sensors further comprises transmitting a Broadband Power Line (BPL) conducted communications signal. This is seen as a particularly simple and cost effective conducted communications signal to use that will allow time synchronisation in a highly efficient manner.

In one embodiment of the invention, the central controller synchronises the sensors at predetermined intervals throughout the day.

In a further embodiment of the invention, each time the remote line mounted sensor receives a signal from the central controller, it resets the internal clock to the timing signal embedded in the received conducted communications signal. In this way, the %l£u7o330 remote line mounted sensor will continuously update their clock as each communication they receive will contain a timing signal sent by the central controller thereby ensuring that the line mounted sensor is always synchronised with the central controller and hence the other line mounted sensors in the system. in another embodiment of the invention, the central controller transmits a specific timing signal instructing the remote line mounted sensors to reset their internal clocks to the timing signal embedded in the transmitted conducted communications signal. in one embodiment of the invention, the central controller, on receiving a conducted communications signal from a line mounted sensor, compares a timing signal contained in the conducted communications signal with the system timing signal and on detecting a discrepancy between the two timing signals, the central controller transmits a conducted communications time synchronisation signal to the line mounted sensor. In this way, a continuous check may be carried out to see if a sensor is synchronised to the remaining sensors in the network and if not, whether synchronisation is required of that sensor.

In a further embodiment of the invention, each line mounted sensor further comprises three line units, each of which is mounted on a different phase to the other line units, and in which the method further comprises the step of synchronising one of the line units by sending a conducted communications signal to that line unit and thereafter that line unit transmitting a synchronisation signal to the two other line units of the line mounted sensor. In this way, synchronisation with only one of the line units is necessary as this may then synchronise with the other line units which may reduce the overall cost if deemed necessary for a particular application.

Detailed Descrigtion of the Invention The invention will now be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawing, in which Fig. 1 is a block diagram of a system for monitoring an electricity supply network in which the method according to the invention is carried out. l£07o37o Referring to the drawing. there is shown a system, indicated generally by the reference numeral 1, for monitoring an electricity supply network 3 having a plurality of supply lines 5. The system comprises a central controller 7 and a plurality of remote line mounted sensors 9, each having an internal clock (not shown). Each of the sensors further comprises a triplet of line units 11(a), 11(b) and 11(c), each of which is mounted on a separate phase to the other line units. The system further comprises a conducted communications gateway 13 accessible by the central controller 7. The conducted communications signals may be sent between the central controller 7 and the remote sensors 9 via the conducted communications gateway.

In use, the central controller 7 transmits a conducted communications signal, in this case a BPL conducted communications signal, over the power lines 5, to each of the remote sensors 9. The remote sensors 9, on receiving the conducted communications signal 1 along the supply line from the central controller, set their internal clocks (not shown) to the timing signal embedded in the conduction communications signal. It is envisaged that the central controller may synchronise the signals at predetermined intervals throughout the day. Alternatively, the central controller may review each conducted communication received from a particular remote sensor and determine whether or not the timing signal embedded in the conducted communication from the remote sensor is synchronised with the system clock (not shown) governed by the controller. if the timing signals are different, the controller 7 may then transmit a synchronisation signal to that sensor in order to synchronise it with the remaining sensors.

In another alternative, the line-mounted sensor may simply reset its internal clock with the timing signal every time it receives a communication from the central controller.

There are at least two sources of timing signal that are available from conducted communications signals. The first is the framing of the conducted signal data and the second is the transmission of explicit timing data on the communications channel. At one level, synchronisation is achieved through several line mounted units looking at the same conducted communication packets. In addition, the conducted communications frames are transmitted in a synchronised manner. Further, with access to the lower layers of the conducted communications stack, specifically the framing layer, timing specific data may be inserted into the communications frame.

Furthermore, on the receiver side, incoming messages can be time stamped using the local clock. By comparing the received signals time stamp and time data (transmitter side) embedded in the data frame, an accurate measurement of time difference is achieved. This information is used to give highly accurate time synchronisation. With repeated application of this time stamping and transmission technique a highly accurate measure of frequency offset may also be achieved. With this information available, it is clear that time synchronisation may be maintained between timing updates.

In the event that the conducted communications bridge cannot provide timing information to the central controller, the web controller, UTC based timing information will be provided by any of the following units with built in GPS timing source: SNG or line unit. Depending on the design of the BPL bridge. the central controller may be synchronised to it by utilising NTP (network time protocol). Due to the variability of the interconnecting network, the internet, this synchronisation will only have moderate accuracy, tens of milliseconds. In this case, highly accurate timing accuracy may be restored by using the method outline in the above paragraph. It will be appreciated that if in practice “conducted communication islands" should occur, that overall system timing integrity can be restored by the application of a GPS enabled SNG or line unit.

In this specification the term accurate has been used in relation to the timing of the sensors. By "accurate" what is meant is that timing control of +/-5 uS on 50Hz system may be achieved. 0.1 degree of arc is equivalent to 5.5uS (5.5x10"-6) or 4.63uS on the 60Hz system. At this level of accuracy we get excellent measurement of current or voltage line parameters. We can make useful measurement of out-of-balance current 3lz (3 times the zero sequence current) as well as positive and negative sequence currents. We can work with 1 degree of phase measurement accuracy with some system performance degradation.

Furthermore, the present invention allows for advanced signal analysis. Many field line monitoring units measure the gross amplitude of the line current (summation of '5 070330 the overall magnetic field in the vicinity of the power line: typically on a three phase line), while others measure the amplitude of the current on a single phase. By advanced, what is meant is that the present invention has means for measuring both phase and amplitude of line signals and combining these signals with others, open- delta-voltage for example, processing the combination to give useful information on fault location, line condition and other parameters.

In the specification, two major concerns are providing low cost and low power consumption. If for example line borne communications were used (and these communications schemes tend to require synchronization of transmitter and receiver) then the argument for including a relatively expensive GPS module ($20- ) is quite weak. In addition, as the present invention already has to allow a power budget for the communications method the present invention practically speaking allows the user to get the timing for "free" in both terms of power budget (size and weight of powering circuits) and materials costs. This is highly advantageous.

Finally, it is possible for the present invention to facilitate synchronization with multiple systems. Each system will need to synchronize to the BPL carrier to function and once carrier acquisition is achieved then it is possible to use this for overall system timing. The precise method of doing this is to somewhat chip dependent. There are numerous reasons why one would not want to use conducted communications to synchronize the sensors such as fear that the line itself my break results in loss of communications and timing and all timing becomes reliant on a single source or component which could be a source of unreliability.

The present invention has found that these concerns are in fact unfounded and the benefits of cost and efficiency outweigh the pitfalls mentioned above. There are numerous advantages to using the conducted communications method described for synchronization. By using the conducted communications for time synchronization, the synchronization method is low cost and power efficient.

Furthermore, conducted communications will travel with the line and will “go around" corners. The use of conducted communications takes advantage of an installed infrastructure thereby providing a reduced initial outlay. Finally, the utilities operating the network may have access to a reasonable amount of the communications channel at very low cost. GPRS data might cost in the order of €1 l£u7o37o per Megabyte and the present invention will allow for a very low operating cost. It is envisaged that instead of using the invention with the sensors comprising triplets as shown, the method could operate well with single sensors in isolation.

In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms “include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiment hereinbefore described, but may be varied in both construction and detail.

Claims (1)

1. CLAIMS A method of synchronising line-mounted sensors in a system for monitoring an electricity supply network having a plurality of supply lines, the system comprising a central controller and a plurality of remote line-mounted sensors, each of the sensors having an internal clock, the method comprising the steps of the central controller transmitting a conducted communications signal to each of the line-mounted sensors in the system along the supply lines and each sensor, on receipt of the conducted communications signal along the supply line from the central controller, setting their internal clock to a timing signal embedded in the received conducted communications signal. A method as claimed in claim 1, in which the step of transmitting a conducted communications signal to each of the line mounted sensors further comprises transmitting a Broadband Power Line (BPL) conducted communications signal. A method as claimed in claim 1 or 2, in which the method comprises the additional steps of one of each time the remote line mounted sensor receives a signal from the central controller it resets the internal clock to the timing signal embedded in the received conducted communications signal; and the other of the central controller transmitting a specific timing signal instructing the remote line mounted sensors to reset their internal clocks to the timing signal embedded in the transmitted conducted communications signal. A method as claimed in any preceding claim, in which the central controller, on receiving a conducted communications signal from a line mounted sensor, compares a timing signal contained in the conducted communications signal with the system timing signal and on detecting a discrepancy between the two timing signals, the central controller transmits a conducted communications time synchronisation signal to the line mounted sensor. A method substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.