GB2591483A

GB2591483A - A high voltage AC transmission system and a method for preventing/reducing overvoltage at an offshore plant

Info

Publication number: GB2591483A
Application number: GB2001259.7A
Authority: GB
Inventors: Forsell Ohma Hãvar; Johan Bjerknes Ole
Original assignee: Aker Solutions AS
Current assignee: Aker Solutions AS
Priority date: 2020-01-30
Filing date: 2020-01-30
Publication date: 2021-08-04
Also published as: GB202001259D0

Abstract

An AC transmission system (1) for power transmission to an offshore plant, such as an offshore platform for oil/gas extraction, comprises an onshore power station (2) to transmit electric power, an offshore plant (3) to receive electric power, and a submarine cable (4) connecting the onshore power station (2) to the offshore plant (3). The system further comprises at least one pipeline (5) provided with an electrical heating system (6), where the electrical heating system (6) is adapted to be connected to the onshore power station (2) through the submarine cable (4) when a significant power load drop or a significant overvoltage is detected or anticipated. In this way, induced overvoltage at the offshore platform may be prevented or reduced when there is a sudden load drop at the offshore platform.

Description

A HIGH VOLTAGE AC TRANSMISSION SYSTEM AND A METHOD FOR PREVENTING/REDUCING OVERVOLTAGE AT AN OFFSHORE PLANT

TECHNICAL FIELD

The present invention relates to a high-voltage AC transmission system for power transmission to an offshore plant from an onshore plant. The system is adapted to prevent/reduce overvoltage when a sudden load drop occurs at the offshore plant. The present invention further relates to a method for preventing/reducing overvoltage at sudden load drops in a high-voltage AC transmission system through a long submarine cable.

BACKGROUND ART

In high-voltage AC transmission systems transmitting a high load to remotely located users through long cables, there is a problem with a high voltage rise when the load suddenly drops with a high extent. This may e.g. be the case when the load requirements of the user are suddenly changed, e.g. when a large machine or other power user is shut-off. Such users may e.g. be offshore platforms, desalination plants, fish farms, subsea factories, etc. This effect is further increased by the Ferranti effect due to high voltage and capacitive charging currents in the transmission cable.

One common method for reducing overvoltage at sudden load drops at an offshore plant is to use reactive compensation methods, e.g. Static Var Compensators (SVC) at the onshore power station. This method may work in some cases, but for longer transmission cables and/or high load drops, the load drop may lead to a voltage rise of such a magnitude that the SVC cannot reduce it to a low enough value at the remote offshore plant. One solution may be to install a higher number of compensating components that will increase the cost of the onshore power station. For longer transmission cables, SVCs at the onshore power station will further have a reduced effect on the voltage reduction when compared to an offshore compensation method. However, the use of offshore topside reactors for compensation is generally avoided due to weight and space constraints. In addition, such reactors on the topside could also be required to be shut down (depending on its ATEX certification or Ex-rating for zone 2 or zone 1 typically) in case of a situation where there is a gas leak on the platform or other hazardous situations.

Even if this known method works well for some applications, there is still room for an improved system and method for preventing/reducing overvoltage at load drops in a high-voltage AC transmission system.

DISCLOSURE OF INVENTION

An object of the present invention is therefore to provide an improved high-voltage AC transmission system. A further object of the invention is to provide a method for preventing/reducing overvoltage in a high-voltage AC transmission system.

The solution to the problem according to the invention is described in the characterizing part of claim 1 with regard to the AC transmission system and in claim 9 with regard to the method. The other claims contain advantageous embodiments and further developments of the AC transmission system and the method.

In an AC transmission system for power transmission to an offshore plant, comprising an onshore power station adapted to transmit electric power, an offshore plant adapted to receive electric power, a submarine cable connecting the onshore power station to the offshore plant, the object of the invention is achieved in that the system further comprises at least one pipeline provided with an electrical heating system which is adapted to be connected to the onshore plant through the submarine cable when an overvoltage is detected.

By this first embodiment of the AC transmission system according to the invention, the power factor of the AC transmission system can be compensated by the use of an electrical heating system of a pipeline, where the electrical heating system is connected to the AC transmission system when an overvoltage is detected. By connecting an electrical heating system to the power receiving point of the AC transmission system, the reactive current of the system can be compensated for by the use of the electrical heating system, which mainly loads the system with an inductive component. The submarine cable on the other hand mainly gives rise to a capacitive component. In this way, the power factor is compensated for at the offshore plant, where the overvoltage will occur, and there may not be a need to install SVC equipment at the offshore plant.

The AC transmission system is a high load, high voltage transmission system comprising a long submarine cable. The AC transmission system is in one example able to transmit at least 1 MW at a voltage of over 76 kV, and the cable is at least 100 km long. The electrical heating system may comprise one or more tie-in pipelines, which leads oil and/or gas from a subsea well to the offshore platform. An electrical heating system on a pipeline is used to prevent hydrate and wax formation in subsea transport flow lines that will cause undesired fluid properties and may block the flow in the pipeline. The electrical heating system of one or more pipelines may be connected to the submarine cable when an overvoltage is detected. The number of connected pipelines may depend on the level of overvoltage.

The electrical heating system is in one example a direct electrical heating (DEH) system. In such a system, the current is fed directly through the pipeline, which must be conductive. The single-phase AC power supply connects the phase at one end of the pipeline and the neutral at the other end, and the current through the pipeline will heat the pipeline. The pipeline is insulated and is provided with a conductor on the outside.

The electrical heating system may also be an induction heating, either with a standard 3-phase transformer or with a special 3-to-2-phase transformer interface to the voltage level of the long AC power cable. Additionally, both an electrical heating based on Pipe-in-Pipe installations with an inductive power factor and inductive skin effect heating or an induction tube heating system could be used to provide a power factor compensation.

A side effect is that the direct electrical heating system and/or the induction heating system will compensate some of the capacitance in the long AC power cable if it is connected in normal operation and it can in this case contribute to lower transmission losses.

In a method for preventing/reducing overvoltage in an AC transmission system adapted to transmit electric power from an onshore power station to an offshore plant through a submarine cable, the steps of; transmitting a high power to the offshore plant, monitoring the used power load at the offshore plant, connecting an electrical heating system arranged in a submarine pipeline when a significant power load drop is detected are disclosed. A significant power load drop is a load drop that will give a certain voltage increase and is e.g. dependent on the actual power consumption and on the dimensions of the submarine cable, e.g. the internal resistance of the submarine cable. A significant power load drop may in one example be at least a 20% load drop, and in another example at least a 30% or 50% load drop. A significant load drop may give rise to a voltage increase of at least 10%, which may be harmful to the installed equipment.

By this method, overvoltage caused by a large power load drop can be prevented or reduced to a safe level. By connecting an electrical heating system of a pipeline to the offshore plant, the inductive nature of the electrical heating system can be used to compensate the power factor of the transmission system, and can thus reduce the induced overvoltage, such that the overvoltage caused by the reactive power in the AC transmission system can be compensated for.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in greater detail in the following, with reference to the embodiments that are shown in the attached drawings, in which Fig. 1 shows a schematic view of an AC transmission system according to the invention, and Fig. 2 shows a flow chart of the method to prevent overvoltage according to the invention.

MODES FOR CARRYING OUT THE INVENTION

The embodiments of the invention with further developments described in the following are to be regarded only as examples and are in no way to limit the scope of the protection provided by the patent claims.

Fig. 1 shows a schematic view of an AC transmission system according to the invention. The AC transmission system 1 comprises an onshore power station 2 that is adapted to supply an offshore plant 3 with electricity. The offshore plant is a plant arranged offshore that requires a high amount of electric energy and may be e.g. an offshore platform adapted to drill for and/or produce, store or offload oil and/or gas, a desalination plant, a fish farm, a subsea factory, etc. The offshore plant is connected to the onshore power station through a submarine cable 4, which is adapted to transmit the required energy. In one example, the submarine cable is e.g. arranged to transmit several MW or more at a high voltage of e.g. 76 kV or more, and the submarine cable may e.g. be a 3-phase cable having a 300 mm2 core of cupper and having a length of e.g. 162 km. The offshore plant may also be a small island supplied with electricity from the mainland.

The onshore power station is mainly adapted to supply electric energy to the offshore plant, but may also receive electric energy from the offshore plant, e.g. if the offshore plant sometimes produces more energy than it consumes, or if the submarine cable is also connected to a wind power plant or a solar panel array.

The offshore plant, in the shown example an offshore drill platform, comprises several different machineries that consumes electric energy.

Some of these loads are relatively large and will thus consume a considerable amount of current. When such a load is suddenly shut off or disconnected, the reactive energy in the long submarine cable will induce an overvoltage in the transmission system, which may damage other equipment still connected to the transmission system. The amount of overvoltage depends e.g. on the amount of the load decrease, the time duration of the load decrease, the length and construction of the submarine cable. The system may also comprise step-up transformers. Some equipment may be shut off in a controlled slow and gentle way, but other equipment may be shut off instantaneously.

In order to prevent or at least reduce the induced overvoltage at a sudden load drop, the system further comprises a pipeline 5 provided with an electrical heating system 6. The pipeline is a flow line connecting a subsea well with the platform. The electrical heating system is arranged to prevent the formation of hydrate and wax in the flow line, which may reduce or prevent the flow inside the conduit. The electrical heating system is connected to the platform through a submarine cable 8 adapted to supply electrical power to the electrical heating system when required.

A production pipeline provided with an electrical heating system of the direct electrical heating (DEH) type is normally supplied from a symmetry-providing system of transformers, capacitors and reactors on the topside of the offshore platform. By supplying the D EH circuit directly by using a 3-to2-phase transformer, it will function in approximately the same way as an offshore reactor. By arranging both the pipeline-interfaces and the transforming equipment on the seabed, away from the topside of the offshore platform, this equipment must not be shut down when there is a hazardous situation such as a gas leakage on the platform topside. In this way, the reactive compensation can be used at all times, which topside reactors cannot. Thus, the electrical heating system can be used as an offshore/subsea reactor and can be energized during a sudden load drop.

In this way, it can contribute to a reduced voltage rise since its active and reactive power consumption increases with the square of the voltage applied. With respect to voltage rise reduction, the reactive (inductive) power consumption of the electrical heating system is especially effective to counteract the capacitive effects in the remote end of a long subsea AC power cable.

Other flow line heating solutions with an inductive power-factor will typically provide similar dampening of capacitive driven voltage rise effects, e.g. induction heating of flow lines would be a relevant example in this respect both with standard 3-phase transformer and more special 3-to-2-phase transformer interfaces to the voltage level of the long AC power cable.

Additionally, both electrical flow line heating based on Pipe-in-Pipe installations with an inductive power factor and inductive skin effect heating or induction tube heating systems could be used to provide similar dampening of capacitive driven voltage rise effects at a scale comparable to their typical normal power demand. A side effect is that the DEH and/or induction heating in normal operation will compensate some of the capacitance in the long AC power cable and can therefore contribute to lower transmission losses in the transmission system.

With the inventive AC transmission system, the voltage stability can be increased and the reactive power flow can be reduced by using the electrical heating system of the pipeline to control the power factor of the transmission system. In this way, overvoltage due to random shutdowns or load drops at the remote end of the submarine cable, where an onshore power plant is connected to e.g. an offshore platform supplied with power from the onshore power plant, can be prevented or reduced significantly, which will help to protect the installation against transient overvoltage and surges.

In the method for preventing overvoltage in an AC transmission system 1 adapted to transmit electric power from an onshore power station 2 to an offshore plant 3 through a submarine cable 4, the following steps are comprised.

In step 100, high power is transmitted to the offshore plant through the submarine cable. The power may e.g. be 1 MW or more, at a voltage of e.g. 76 kV or more. The length of the cable may be e.g. 100 km or more. With a short cable or a low power transmission, a problem with induced overvoltage will not occur or will be relatively low, such that the installed equipment will not be broken or destroyed.

In step 110, the used power and/or the actual voltage is monitored at the offshore plant. The used power may be determined by measuring the actual 15 current and voltage, or only the voltage may be measured.

In step 120, a significant power load drop is determined or anticipated. This load drop may be estimated by measuring the actual current and voltage, or by measuring the voltage increase. When the power load drop is determined, an electrical heating system of a pipeline is connected to the submarine cable, such that the induced overvoltage is reduced or prevented. It is also possible to measure a voltage increase, and when a significant voltage increase, which may be e.g. more than 10% or more is determined, the electrical heating system is connected to the submarine cable.

It is also possible to that a central control system initiates a power load drop for some reason. In this case, the control system can give a pre-warning to the control system of the offshore plant. If the anticipated power load drop is significant, the power factor of the transmission system may be adjusted in advance.

A significant power load drop is a load drop that will give a certain voltage increase and is e.g. dependent on the actual power consumption and on the dimensions of the submarine cable, e.g. the internal resistance of the submarine cable. A significant power load drop may in one example be at least a 20% load drop, and in another example at least a 30% or 50% load drop. A significant load drop may give rise to a voltage increase of at least 10% or 20% or more, which may be harmful to the installed equipment. Depending on the installed equipment, the allowed significant load drop or voltage increase can be determined.

The system and method is mainly adapted to prevent significant power load drops to cause a high voltage increase that will damage installed equipment on the offshore plant. It is also suitable for compensating for smaller voltage increases, in the range between 5 -10%, which will not damage the installed equipment. Such power factor compensation will e.g. reduce the transmission losses through the submarine cable.

The invention is not to be regarded as being limited to the embodiments described above, a number of additional variants and modifications being possible within the scope of the subsequent patent claims. The AC transmission system may have any size and configuration, and may comprises one or more offshore platforms.

REFERENCE SIGNS

1: High-voltage AC transmission system 2: Onshore power station 3: Offshore plant 4: Submarine cable 5: Pipeline 6: Electrical heating system 7: 3-to-2 phase transformer 8: Submarine cable

Claims

CLAIMS1 An AC transmission system (1) for power transmission to an offshore plant, comprising an onshore power station (2) adapted to transmit electric power, an offshore plant (3) adapted to receive electric power, a submarine cable (4) connecting the onshore power station (2) to the offshore plant (3), characterized in that the system further comprises at least one pipeline (5) provided with an electrical heating system (6), where the electrical heating system (6) is adapted to be connected to the onshore power station (2) through the submarine cable (4) when a significant power load drop or a significant overvoltage is detected or anticipated.
2 The AC transmission system according to claim 1, characterized in that the electrical heating system (6) is a direct electrical heating (DEH) system.
3 The AC transmission system according to claim 1, characterized in that the electrical heating system (6) is an inductive power factor electrical heating system.
4 The AC transmission system according to any of claims 1 to 3, characterized in that the electrical heating system (6) comprises a 3-to-2 phase transformer (7).
5. The AC transmission system according to claim 4, characterized in that the 3-to-2 phase transformer is positioned under water, adjacent the pipeline (5).
6 The AC transmission system according to any of the preceding claims, characterized in that the detected overvoltage is at least 10% higher than the nominal voltage value.
7 The AC transmission system according to any of the preceding claims, characterized in that the detected overvoltage is at least 20% higher than the nominal voltage value.
8 The AC transmission system according to any of the preceding claims, characterized in that the offshore plant (3) is an offshore platform for oil/gas extraction.
9 A method for preventing/reducing overvoltage in an AC transmission system adapted to transmit electric power from an onshore power station (2) to an offshore plant (3) through a submarine cable (4), comprising the following steps: transmitting a high power to the offshore plant, monitoring the used power load at the offshore plant, connecting an electrical heating system arranged in a submarine pipeline to the submarine cable to control the power factor of the transmission system when a significant power load drop or a significant voltage increase is detected or anticipated.
10. The method according to claim 9, characterized in that the electrical heating system (6) is a direct electrical heating system.
11.The method according to claim 9 or 10, characterized in that the significant power load drop is at least 20%.
12. The method according to any of claims 9 to 11, characterized in that the significant power load drop is at least 30%.
13. The method according to any of claims 9 to 12, characterized in that the significant power load drop is at least 50%.
14. The method according to any of claims 9 to 13, characterized in that the significant voltage increase is at least 10%.
15. The method according to any of claims 9 to 14, characterized in that the significant voltage increase is at least 20%.