Electrical protection system and method for an offshore wind farm

Abstract

: Electrical Protection System and Method for an Offshore Wind Farm An electrical protection system of an offshore wind farm comprising one or more arrays of wind turbine generators, wherein, for the or each array, a first wind turbine generator provides a connection point connected (a) via a tower cable to a generator of the wind turbine generator, (b) via a collector cable to a subsea substation, and (c) via an inter-array cable to one or more further wind turbine generators of the array This allows power generated by the array to be routed through the connection point of the first wind turbine generator. The system comprises a circuit breaker located on the first wind turbine generator and above the water, and between the connection point and the collector cable.

Description

[0001] Electrical Protection System and Method for an Offshore Wind Farm

[0002] Technical Field

[0003] The present invention relates to electrical protection systems and methods for an offshore wind farm and specifically to an offshore windfarm that comprises a plurality of wind turbine generators connected to a subsea substation and / or a subsea junction box.

[0004] Offshore wind farms are playing an ever increasing role in supplying electrical energy. This is driven by the increasing capacity of wind turbine generators (WTGs) and the increasing numbers of turbines in individual wind farms. Consider for example the Dogger Bank wind farm located off the north east coast of England; this is projected to have a generating capacity of 3.6GW produced by around 600 individual WTGs.

[0005] Figures 1 and 2 illustrate alternative power interconnection systems that may be utilised with an offshore wind farm. In the system of Figure 1 the array comprises two pairs of WTGs 1a, 1b. The WTGs of each pair are connected in parallel to a junction box 2 via respective inter-array power cables 3. In the system of Figure 2 the WTGs 4 are coupled in a daisy-chain fashion by inter array cables 5 that link the WTGs in series. These systems have their own advantages and disadvantages.

[0006] Within a given wind farm, the individual WTGs may be connected to an offshore substation (OSS) - which may be bottom fixed, e.g. monopole or jacket, or floating, or which may be subsea - which delivers power to an onshore grid connection point via a main power supply or “export” cable. Power may be transferred from the OSS to the onshore grid connection point via AC transmission or via High Voltage DC (HVDC) transmission. The WTGs themselves generate AC power and this power is transferred from the WTGs to the OSS, via collector cables, for conversion to DC and / or to step up or down the voltage. The OSS utilises transformers and / or DC to AC converters before transferring the power over the export cable. Whilst the export cable will be a submarine power cable, the OSS is typically located above the water level. Traditionally, an OSS is equipped with control and protection systems including switchgear (SWG), protection relay and telecommunications equipment. Figure 3 illustrates an exemplary configuration with a plurality of daisy-chained WTG arrays 7 (only one of which is shown) connected into the OSS 6 via respective collector cables 8. Each of the collector cables 8 is connected to power lines 9 of the OSS 6 via an associated circuit breaker (CB) 10, where the CBs are provided within switchgear of the OSS. These CBs are high voltage / high current components designed to interrupt the flow of electrical power in the event of a detected failure in the system. In the case of an OSS located above the water level, the CBs are installed within the OSS and the associated costs are considered reasonable in terms of the entire project. It will be appreciated that the CBs installed in the OSS are operable to disconnect WTGs at the array level, i.e. all WTGs in a given daisy-chain array 7 are connected or disconnected together.

[0007] Switchgear (SWG) 11 installed in a WTG traditionally includes three breakers. A commonly used configuration is illustrated in Figures 3 and 4, where the following symbols are used for the different breaker types:

[0008] Circuit breakers - “x” Load switch - “o” Disconnector -

[0009] The skilled person will be aware of the definitions of these terms although the following are provided by way of clarification:

[0010] Circuit breaker: An electrical switch designed to protect an electrical circuit from damage caused by overcurrent / overload or short circuit. Its basic function is to interrupt current flow after protective relays detect a fault. The current flow may also be interrupted manually, which may be by remote operation.

[0011] Load switch: A mechanical switching device configured to make, carry and break currents under normal conditions. The device may be able to carry, for a specified time, currents under defined abnormal circuit conditions such as those associated with a short-circuit. Short-circuit currents may be conducted but not switched off.

[0012] Disconnector: A high-voltage disconnector is used to allow isolation of apparatus for maintenance. The disconnector is usually not intended for normal control of the circuit, but only for safety isolation. Unlike load switches and circuit breakers, disconnectors lack a mechanism for suppression of electric arcs which occur when conductors carrying high currents are mechanically interrupted. Thus, they are off-load devices, with very low breaking capacity, intended to be opened only after the current has been interrupted by some other control device such as the circuit breaker.

[0013] NB. So-called switch disconnectors may combine the properties of load switches and disconnectors. Where “load switches” are referred to in the following discussion, the term is considered to encompass also switch disconnectors.

[0014] The configuration shown in Figures 3 and 4 and incorporated onto the WTG comprises (1) a circuit breaker CB connected between a central inter-connection CP and a Tower Cable connected to the generator of the WTG, (2) a load switch (LS) connected between the central inter-connection point (CP) and the inter-array cable connected to a preceding WTG of the same daisy chain array (assuming that the WTG is not an end WTG), and (3) a Disconnector (D) connected between the central inter-connection point CP and a subsequent WTG or the OSS (if the WTG is the first WTG). It will be appreciated that the central inter-connection point (CP) referred to here may be a node or other component including, for example, a bus bar.

[0015] This arrangement represents a cost-effective solution that does not jeopardize selectivity and protection. The CBs 10 on the OSS 6 have the main protection function for the collector and inter-array cable (IAC), while the WTG CBs protect the tower cable and the WTGs. The LSs on the WTGs are useful for disconnection of upstream WTGs without the need to depower the complete inter-array string. Finally, the Disconnector on a WTG serves the purpose of isolating the WTG from downstream electrical infrastructure during maintenance.

[0016] The protective function of the WTG CB is typically limited to over-current protection. It is not common practice to install advanced protection relays that use both current and voltage measurements. A fault pass indicator (FPI) is typically used to identify the location of the fault. The FPI registers if the fault current has passed through the branch / cable and returns a binary output; it is not used to control any breakers.

[0017] It may be beneficial in some circumstances to locate parts of the electrical system infrastructure, in addition to cables, subsea. For example one might envisage subsea OSSs and / or subsea junction boxes.

[0018] Figure 5 illustrates an alternative architecture involving a subsea substation (SSS) 115 and subsea junction box 116 to which a plurality of WTGs are connected in a star or branched-off configuration and where each WTG is provided with a tower CB between the inter-array cable and the tower cable. With such a configuration, provision of a subsea switchgear within the SSS 115, in the form of one CB for each collector cable, may be beneficial to avoid clearing of a single fault in any of the inter-array and collector cables necessitating clearing by the onshore export cable circuit breaker and a complete shutdown of the wind farm. In an alternative architecture, subsea switchgear may be installed at the first subsea junction box in a string. In this way only a string is affected in case of a fault on a cable.

[0019] Subsea electrical architectures require a revisit of the traditional control and protection philosophies to realise lean and cost-effective subsea electric architectures whilst ensuring selectivity and protection. New systems and methods may be required to achieve such architectures given the availability and cost of certain components for use subsea including for example subsea CBs. Considering for example an architecture similar to that of Figure 3 but with the OSS located subsea, it may be commercially impractical to implement this with subsea CBs.

[0020] According to a first aspect of the present invention there is provided an electrical protection system of an offshore wind farm comprising one or more arrays of wind turbine generators, wherein, for the or each array, a first wind turbine generator provides a connection point connected

[0021] (a) via a tower cable to a generator of the wind turbine generator,

[0022] (b) via a collector cable to a subsea substation, and

[0023] (c) via an inter-array cable to one or more further wind turbine generators of the array. This allows power generated by the array to be routed through the connection point of the first wind turbine generator. The system comprises a circuit breaker located on the first wind turbine generator and above the water, and between the connection point and the collector cable.

[0024] The remaining wind turbine generators of the array may be connected, via further interarray cables, in a daisy chain configuration behind the first wind turbine generator.

[0025] The inter-array cable may be connected to a junction box and the remaining wind turbine generators of the array may be connected, via further inter-array cables, in a star or branched configuration, to the subsea junction box. The circuit breaker may be a circuit breaker of a switchgear of the first wind turbine generator, the switchgear further comprising a further circuit breaker between said connection point and said tower cable and / or a disconnector or load switch or circuit breaker between the connection point and the inter-array cable.

[0026] According to a second aspect of the present invention there is provided an offshore wind farm comprising one or more arrays of wind turbine generators and an electrical protection system according to the above first aspect of the invention.

[0027] According to a further aspect of the present invention there is provided a method of activating the wind farm of the second aspect and comprising energising an export cable, connecting the subsea substation to an onshore location, and collector cables, whilst energising the inter-array cables at a later time.

[0028] Brief Description of the Drawings

[0029] Figure 1 illustrates two pairs of wind turbine generators coupled in series to a subsea junction boxes;

[0030] Figure 2 illustrates a set of wind turbine generators connected in a daisy chain configuration;

[0031] Figure 3 illustrates a plurality of daisy chain connected wind turbine generator arrays connected in parallel to a surface offshore substation;

[0032] Figure 4 illustrates a switchgear arrangement of a wind turbine generator;

[0033] Figure 5 illustrates one of a plurality of wind turbine generator arrays, the wind turbine generators of each array connected in a star configuration to a subsea junction box, and the plurality of arrays connected via respective collector cables to a subsea substation;

[0034] Figure 6 illustrates a plurality of daisy chain connected wind turbine generator arrays connected in parallel to a subsea offshore substation;

[0035] Figure 7 illustrates a switchgear arrangement of a wind turbine generator;

[0036] Figure 8 illustrates an array of wind turbine generators with one of the generators connected between a subsea substation and a subsea junction box in a daisy chain configuration and with the remaining wind turbine generators being connected in a star configuration to the junction box.

[0037] Figure 9 illustrates a method of energising components of a wind farm. Detailed

[0038] A wind farm electrical power interconnection architecture with subsea substation (SSS) 12 and daisy-chained wind turbine generators (WTGs) 13 is illustrated in Figure 6. [NB. For simplicity, whilst the architecture will generally be a three-phase architecture, the multiple phases are shown in combined form.] In order to achieve a lean and cost- effective configuration, subsea Circuit Breakers (CBs) are not provided at the SSS 12. Rather, the SWG configuration 14 of the first WTG is changed such that the disconnector feeding the collector cable is replaced with a CB that replaces the functionality of the CB typically installed topside on an OSS (see Figure 3) but now no longer present in the SSS 12. This is shown in more detail in Figure 7. It is noted that the switchgear referred to here may be switchgear in a lower part of the WTG, for example within the transition piece of the WTG, i.e. between the base, e.g. jacket, and the tower. Other switchgear may be present in other parts of the WTG, e.g. in the nacelle.

[0039] The collector CB may therefore include a protection relay and installation of both current and voltage measurement sensors, e.g. utilising current transformers and voltage transformers. With the proposed configuration, in the event of a fault on the inter-array cable (IAC) system, the collector cable CB on the first WTG will ensure selective disconnection of only one WTG string and avoid the trip of the entire export cable system for the wind farm. It will be appreciated that the SWG of the second, third etc WTGs of a daisy chain array may be unchanged, i.e. using for example the configuration of Figure 4.

[0040] It is worth noting that a fault in a collector cable must be cleared by the onshore export cable CB. To be able to identify the location of a fault downstream of the collector CB, current monitoring can be included on the feeders of the subsea substation (SSS). By monitoring the current of each collector cable feeder it is possible to determine whether the fault is internal to the SSS or within one specific collector cable.

[0041] Figure 8 illustrates a still further architecture in which a plurality of WTGs 20 of a given array are connected in a star configuration to a junction box 21 via respective interarray cables. However, a first WTG 22 of the array is connected in a daisy-chain configuration to the junction box 21. Whilst the switchgear 23 of each of the star connected WTGs comprises a single Circuit Breaker (CB), the switchgear 24 of the daisy chained WTG 22 comprises a load switch coupled between the WTG interconnection point (CP) and the junction box 21 , a first CB connected between the CP and the tower cable, and a second CB connected between the CP and the collector cable leading to the SSS. As discussed previously, at least the second CB may comprise a protection relay and voltage and current measurement sensors. This configuration is relevant for junction boxes in both star and branched off configurations.

[0042] The architecture of Figure 8 allows standard switchgear to be utilised on the majority of WGTs of a WTG array. The only modification required is to the switchgear of the single daisy chain connected WTG. In particular, the second CB connected between the CP and the collector cable leading to the SSS avoids the need for subsea switchgear at the SSS. Nonetheless, in the event of a fault on the inter-array cable (IAC) system, the collector cable CB on the first WTG will ensure selective disconnection of only one WTG string and avoid the trip of the entire export cable system for the wind farm.

[0043] With the architecture of Figure 8, system energisation may be simplified to some extent, as the export and collector cables can be energised, with the inter-array cables being simultaneously energised at a later time. This is illustrated in Figure 9.

[0044] It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.

Claims

CLAIMS:

1. An electrical protection system of an offshore wind farm comprising one or more arrays of wind turbine generators, wherein, for the or each array, a first wind turbine generator provides a connection point connected(a) via a tower cable to a generator of the wind turbine generator,(b) via a collector cable to a subsea substation, and(c) via an inter-array cable to one or more further wind turbine generators of the array, such that power generated by the array is routed through the connection point of the first wind turbine generator, the system comprising a circuit breaker located on the first wind turbine generator and above the water, and between the connection point and the collector cable.

2. An electrical protection system according to claim 1 , wherein the remaining wind turbine generators of the array are connected, via further inter-array cables, in a daisy chain configuration behind the first wind turbine generator.

3. An electrical protection system according to claim 1 , wherein the inter-array cable is connected to a junction box and the remaining wind turbine generators of the array are connected, via further inter-array cables, in a star or branched configuration, to the subsea junction box.

4. An electrical protection system according to any one of the preceding claims, wherein the circuit breaker is a circuit breaker of a switchgear of the first wind turbine generator, the switchgear further comprising a further circuit breaker between said connection point and said tower cable and / or a disconnector or load switch or circuit breaker between the connection point and the inter-array cable.

5. An offshore wind farm comprising one or more arrays of wind turbine generators and an electrical protection system according to any one of the preceding claims.

6. A method of activating the wind farm of claim 5 when appended to claim 3 and comprising energising an export cable, connecting the subsea substation to an onshore location, and collector cables, whilst energising the inter-array cables at a later time.

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