The present invention regards a system for communicating over a power cable.

There is in many situations a need for communication over large distances. Often, such communication needs coincides with a need for transferring power.

For example in petroleum wells there is a need for communication from topside to downhole for transferring control signals to downhole equipment and from downhole to topside, for example for transferring monitoring data from downhole equipment. In petroleum wells the distances are large and the power source are often high voltage power sources.

Other examples of areas having communication needs are drilling operations for tunnels or wells, monitoring galleries in mines, etc.

Traditionally such communication has been based on dedicated cables or traditional modem technology. As there often are long distances through a rough environment and the power cable is radiating a lot of ripple noise, traditional modem technology often get problems. In hot environments optical fiber communication is not an issue due to limited performance of lasers at high temperature.

US 5995020 regards a downhole power and communication system for operating a tool string in a well. The system comprises an apparatus for transmitting electricity between a downhole well tool and the well surface. A controller is located at the well surface, a receiver module is engaged with the well tool, and a conductor is connected to the controller and the receiver module. The controller may include a communication transmitter and a modulator operable to generate an electric signal representing at least two states and/or a data bit representing an address. The address is used to connect power to a desired tool.

US 2007/0202839 regards a method of transmitting data over a regular power line of a switch mode power supply system. The data are transmitted by frequency modulating the power signal. This method is not adapted for use in petroleum wells and the communication is from the power source to the power user only.

US2007/0286305 regards a transmitting apparatus of digital signals to a user on a line in which a supply signal of electronic devices is present. One embodiment comprises a switching converter and means for varying the switching frequency of the converter on the bases of a signal to be transmitted.

The object of the invention is to provide a system for communicating from underground equipment over a power cable/line which utilizes the power transmission to the equipment to communicate from the equipment downhole to topside. The object of the invention is achieved by means of the features of the patent claims.

In one embodiment the system for communicating over a power cable comprises a power line connected to a power source in one end, a switch mode transformer connected to the power line at another end for transforming the voltage received from the power source through the power line, means for modulating the switch mode transformer, and a receiver adapted to receive and process signals from the switch mode transformer.

The power source is usually a high voltage power DC source in order to be able to transfer the power over large distances with as low loss as possible. The power source is in one embodiment a dc power source.

The switch mode transformer is connected to the power line and transforms the voltage supplied by the power source to a more suitable voltage level for the equipment. The switch mode transformer is for example a dc-dc converter. In one embodiment the switch mode transformer is a switch mode inverter.

The switch mode transformer may be chosen and/or designed to optimize the switching frequency. Faster switching/higher frequency improves the maximum data rate and leads to less need of filtering of the signal. In one embodiment the switch mode transformer comprises silicon carbide transistors. Silicon carbide transistors are suited for operating at high power and high temperature levels. They have low resistance and thus lower power loss and permits fast switching.

The operation of the switch mode transformer will normally generate ripple on the power line which will propagate through the power line. The switching mode is controlled by the modulating means in order to use the ripple as carrier for data transfer. The switching is in one embodiment controlled to generate ripple time series which contain data on a series format.

In one embodiment, the system comprises a means for reducing ripple from the switch mode transformer, for example a filter. The filter may be connected to or integrated in the switch mode transformer. The filter may shape the ripple to a suitable transmission level, leaving a dc voltage having a certain amount of ripple.

The means for modulating the switch mode transformer may be any modulator suitable for controlling the switching of the switch mode transformer. The means for modulating the switch mode transformer can implement a number of modulation methods. In one embodiment, the dc-component of the modulation signal is zero. This will ensure that the low voltage dc output from the switch mode transformer is not changed by the modulation. Examples of modulation methods that may be implemented by the modulating means are bi-phase modulation and frequency shift modulation. Biphase modulation provides larger signal bandwidth and a switching duty factor of 50%. Frequency shift modulation can provide duty factors different from 50%, but has narrower signal bandwidth. Other modulation methods may also be used, such as for example two-tone frequency modulation. Two-tone frequency modulation, however allows lower bit rate.

The signals generated by the modulation of the switch mode transformer will propagate through the power line and be received by the receiver. The receiver comprises or is connected to means to process the signals. The processing may comprise transforming, demodulating and outputting the signal. The demodulation method will correspond to the modulation method.

In one embodiment the receiver comprises or is connected to a pulse transformer for isolating the signal of interest from the high voltage signal. The pulse transformer is in this case arranged prior to the demodulation means.

The invention will now be described in more detail by means of examples and with reference to the accompanying figures.

Figure 1 shows a block diagram of the principle of the system according to the invention.

Figure 2 shows an embodiment of the invention with more details.

Figure 3 shows an example of a modulation sequence.

Figure 1 shows a block diagram of a system 10 according to the invention

A power source, in this case a high voltage DC power supply (HVDC) 1 1 is provided at the top side of a petroleum well. The HVDC is connected to a power line, the cable 14, leading down into the well to equipment/tools located downhole (not shown). Downhole the cable 14 is connected to a switch mode

trans former/inverter 15 for converting/transforming the high voltage power to a lower voltage 18 which may be used by the downhole equipment. The switch mode inverter 15 is connected to or comprises modulating means for modulating the switching based on the data to be communicated. In this example the modulating means are integrated in the switch mode inverter 15 and are bi-phase modulation means. Data from equipment downhole that may be communicated to the top side are for example; logging data, status of the equipment such as temperature, mode of operation, any failure states/alarm signals, etc. The data are input to the modulating means which modulate the switch mode inverter according to the data to be transferred. The switching action of the DC/DC transformer causes a ripple current to propagate along the cable 14. The modulation of the switch mode inverter causes a modulation of the ripple current which propagates through the cable and can be received by a (pulse) receiver 13. In this embodiment a pulse transformer 12 is arranged between the cable 14 and the receiver 13. The receiver 13 demodulates the signal and output the demodulated signal.

Figure 2 shows an embodiment of the invention with more details. The power cable 20 is typically a high capacitance coaxial cable with low impedance. It is fed with high voltage DC via a pulse transformer 25 being a part of the receiver 22 located topside.

Downhole the cable 20 feeds power to the switch mode inverter 21 which transforms the input power to a bus power more suited for feeding motors and other downhole equipment. The inverter 21 comprises a power switch transistor 24, for example a high speed SiC (Silicon carbide) transistor. The switching sequence of the transistor 24 is controlled by a bi-phase modulator 25. A capacitor 23 reduces the ripple on the line and thus improves the power transfer. The power cable 20 will in itself provide most of the input capacitance needed by the inverter.

Figure 3 shows a typical biphase modulation sequence (Manchester coded) and the corresponding ripple signal received at topside. Note the time delay related to the propagation over a 3km long power transmission cable.

With this modulation technique, having no DC-component, the power output voltage will typically stay close to half of the line input voltage which is well suited for motor designs. A load dependent regulation can be performed at topside based on measurement of the load current and the downhole voltage. If a downhole regulation is wanted, the use of two-tone modulation can be combined with duty factor control of the power output voltage from the inverter.