FIELD OF THE INVENTION
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The present invention relates to a power line communication system and method. More particularly, the present invention relates to a power line communications system wherein the power frequency in an inductive electric load (IEL) is used as a signal carrier. The invention is particularly suitable for use where the IEL is remote from the power supply such as, for example, in down hole oil production situations where very long power cables are used.
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OF THE INVENTION
Power line communication systems are generally well known. One known system modulates the current from a main power supply to signal between units located on a common supply. The power supply is also used to power the devices and any other electrical load on the line, for example, an electric motor. Their mode of operation is to draw a modulated current in addition to the load current drawn and by placing a transmitter across the supply in parallel to other devices/loads. Such systems are therefore suitable for fitting to existing power systems where such devices may be used as a transmitter/receiver. Such systems do not convey information about the load but use the communications capabilities of the existing power system. Systems where a receiver can detect information by looking at current flow on the cable are disclosed in U.S. Pat. No. 5,581,229, U.S. Pat. No. 5,589,813, U.S. Pat. No. 5,621,344 and U.S. Pat. No. 5,491,463.
Other systems which are known and which are suitable for use in downhole applications having a single power supply line pass information between units by coupling an additional carrier onto the power supply line. Information is passed via the current flow between each transmitter/receiver pair and the receiver is a load for the transmitted current. Systems using this principle are disclosed in U.S. Pat. No. 5,539,375, U.S. Pat. No. 4,620,189, U.S. Pat. No. 4,631,535, U.S. Pat. No. 4,631,536, U.S. Pat. No. 4,523,194, U.S. Pat. No. 4,157,535, U.S. Pat. No. 4,365,506, U.S. Pat. No. 4,178,579 and U.S. Pat. No. 3,340,500.
It is known that faults within a motor or other electrical load may change the time or frequency domain characteristics of a current drawn from a supply. This is due to the characteristics of the motor being altered due to the presence of faults. A source of such faults is short circuits between windings in an inductive (magnetically coupled) load. Such short circuits cause amplitude changes in the current and changes in the phase current cause imbalance in poly-phase machines. It is also known that mechanical faults in a rotor or bearing faults within a motor will cause changes in the frequency content of the current drawn by the motor. It is therefore possible to induce or mimic such faults to an extent that is not detrimental to the performance of the IEL to modulate a signal and thereby communicate information along a power line.
Previous methods used to provide communication devices are, for example, earthloop based communications. These types of devices ground a phase or neutral point through an impedance and communicate by passing a current around the formed loop, for example, down the phase cable and returned via earth. Such systems are susceptible to phase-to-earth faults. This can create a loss of communications and therefore cannot be used with earthed electrical systems.
IELs are commonly used where multiple loads are supplied from one power supply and each IEL's feed is not galvanically isolated from other IEL feeds. A disadvantage of this method is that all ground loop communications will fail if any of the IELs or their power feeds become grounded.
Other previously known methods require injection of a signal at the motor terminals using either direct, capacitive or inductive coupling techniques. A disadvantage of these types of methods is that if applied to a signal between phases rather than phase-to-earth, coupling is required between two motor terminals of potentially high voltage.
Previous communications systems have also been dependent as to whether the communication means is time-domain, digital or analogue, frequency keying, or amplitude or frequency or phase modulation.
Moreover, previous designs using earth loop communications have used various means of permitting insulation testing of the IEL whilst any communications device is fitted and rely on a high voltage switching relay or a negative biased high voltage diode. This is therefore a disadvantage as a special design is required to allow insulation testing.
Previous communications systems also require special high voltage filter or coupling devices to remove power frequencies which not only adds to the complexity and cost of the device, but also adds to its relative size.
Devices are well-known in the art which can be added to a power system, for example, in the home or office, wherein the devices are independent of the loads attached. These types of devices all modulate the current from a common main power supply. These devices draw a modulated current in addition to the load current being drawn by placing the transmitter across the electrical supply in parallel to any other load. It should be noted that these systems are not designed to convey information from the load, but to use the communications capabilities of the power system already in place and are of background relevance only.
It is therefore common for devices to require a dedicated source for communication between different units. The modulated AC or DC signal current is drawn from a coupled source voltage and not the general power supply voltage source. These previous types of devices therefore operate by filtering the power supply and passing information between units by coupling an additional carrier onto the power supply lines. Information is passed via a current flow between each transmitter/receiver pair on the bus.
It is an object of at least one aspect of the present invention to obviate/mitigate one or more of the aforementioned problems and disadvantages in the prior art.
It is a further object of the present invention to provide a remote IEL wherein the IEL is a transformer, inductor, motor or other such device of single phase or poly-phase construction.
It is yet a further object of the present invention to provide a remote IEL which overcomes the cost and complexity of an additional cable for communication purposes.
It is also an object of the present invention to provide an IEL device which due to the lack of an independent cable is suitable for submersible pumps used for mine, sump and quarry de-watering and downhole oil production situations where previously the length of cable to the pump and inaccessibility would incur a large cost of installation and maintenance.
It is yet a further object of the present invention to provide an IEL device which is independent of earthing and will continue to transmit whilst power is applied to the IEL.
Another object of the present invention is to provide a potential between two windings in an IEL, wherein the potential between the windings is lower than the potential applied to the IEL.
A further object of the present invention is to provide a point in an IEL which has a low potential to earth (the neutral point of an earth neutral system or the neutral of an unearthed, fault-free poly-phase system) wherein there is a lack of high electrical stress from any of the components to earth potential under normal operating conditions.
Furthermore, it is an object of the present invention to provide information as a constituent of a power flow to an EL. Information can therefore be communicated at all times that the IEL is powered.
It is a further object of the present invention to provide a secondary supply of lower potential wherein the secondary supply continues to modulate a powerflow, enabling information to be carried even though the secondary supply is incapable of powering the IEL.
Moreover, it is an object of the present invention to provide a system of transferring information which is a constituent of a power flow to an IEL which is unaffected by ground faults.
A yet further object of the present invention is to provide a system for transferring information which is a constituent of a power flow to an IEL wherein a receiver located on a feed to each IEL can easily detect a signal from each transmitter. Alternatively, it is a further object to provide a single receiver located on a common feed to all IELs, wherein the receiver filters out each of the signals being transmitted.
A further object of the present invention is to provide a method of transferring information as a constituent of a power flow which is independent of the modulation method. Moreover, it is an object that a plurality of signals are transmitted from one modulating transmitter and that more than one transmitter communicates per current drawn from the source.
It is a further object of the present invention to provide a method of transferring information as a constituent of a power flow wherein the system is independent of any galvanic isolation/connection of the electrical system or IEL to earth and as such needs no special design to permit insulation testing.
A further object of the present invention is to provide a method of transferring information as a constituent of a power flow wherein the system is capable of being installed in voids within an IEL as long as access to two windings of the IEL are available.
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OF THE INVENTION
According to one aspect of the present invention there is provided a power line communications system including a power supply, a power line, a transmitter means, an IEL and a receiver means, wherein in use, information is transmitted from the transmitter means to the receiver means via the IEL on a power signal by the transmitter means modulating at least one characteristic of the IEL.
Preferably, the characteristics of the IEL which are altered include resistance, inductance and/or capacitance of any primary and/or secondary transformers, or a motor stator or rotor winding.
Preferably, multiple transmitter means are used with each transmitter means using a different modulation technique.
Advantageously, multiple transmitter means are used with each transmitter means using the same modulation technique, but each having different modulation modes within that technique.
It is preferred that the modulation is effected by an amplitude modulation technique.
Preferably, the amplitude modulation technique modulates the characteristics of the IEL such that changes in amplitude of the current or voltage in the power line can be detected.
Preferably, said transmitter means for modulating the characteristics of the IEL comprises a modulation frequency generator. Desirably said transmitter means is connected directly to the IEL.
It is preferred that the receiver means is able to filter out and recover the transmitted information.
Preferably, the receiver means is placed anywhere on the power line supplying power to the IEL and measures the power flow to the IEL and, with a de-modulator, measures the level of modulation of the power flow to the IEL created by the transmitter means wherein the de-modulated signal is the information being transmitted.