WO2010066303A1

WO2010066303A1 - A system for transient overvoltage protection

Info

Publication number: WO2010066303A1
Application number: PCT/EP2008/067390
Authority: WO
Inventors: Ambra Sannino; Henrik Breder; Lars Liljestrand; Svenn-Erik Rafoss; Wojciech Piasecki
Original assignee: Abb Research Ltd
Priority date: 2008-12-12
Filing date: 2008-12-12
Publication date: 2010-06-17

Abstract

The present invention relates to a power system (1) comprising an electrical apparatus (2), a switchgear (4, 4a) electrically connected by means of a power cable (3) to the apparatus and including a switch (5) arranged to connect and disconnect electrical power to the apparatus via the power cable, wherein transient overvoltages may occur when connecting or disconnecting the power by operating the switch, and the system further comprises a transient overvoltage reducing device arranged to protect the apparatus against the transient overvoltages transmitted through the power cable to the apparatus. The transient overvoltage reducing device (6, 6a, 6b) is arranged inside of or in immediate proximity to the switchgear. The present invention also relates to a switchgear comprising a bushing and a transient overvoltage reducing device for providing transient overvoltage protection for an electrical apparatus, and to a power cable including a cable termination with a transient overvoltage reducing device arranged to protect the apparatus against the transient overvoltages transmitted through the power cable.

Description

A SYSTEM FOR TRANSIENT OVERVOLTAGE PROTECTION

FIELD OF TH E INVENTION

The present invention relates to a power system including an electrical apparatus and a transient overvoltage reducing device.

The present invention also relates to a switchgear comprising a transient overvoltage reducing device for providing transient overvoltage protection for an electrical apparatus electrically connected to the switchgear.

The present invention further relates to a power cable having a first end designed to be connected to an electrical apparatus, and a second end including a cable termination designed to be connected to a switchgear.

The present invention yet further relates to a cable termination to be connected to a switchgear.

Such a power system has many applications, such as an electrical power grid system, a wind farm, or an industrial power system.

PRIOR ART

A power system in electrical power industry comprises a plurality of electrical devices for power generation and transmission. The trend is to use cables instead of overhead lines when building power systems. In a cable power system, long cables are used to connect the electrical apparatus for transmitting generated electrical power. To isolate faults and protect an electrical apparatus, a switchgear is electrically connected by a power cable to the apparatus. A switch is included in the switchgear to connect and disconnect electrical power to the apparatus via the power cable. When the switch is operated to connect or disconnect electrical power via the power cable, the operation causes a short-lived oscillation because of the sudden change of the voltage through the power cable. This phenomenon is called transient overvoltage, in the following denoted TOV. TOVs are dangerous for all types of electrical apparatus, such as power generator, transformers, motors, etc.

The TOVs in cable systems are characterized by rate of rise and repetition rate, which means that not only one voltage step but a high number of transient spikes can be generated. For example, in case vacuum circuit breakers having the capability to interrupt high-frequency currents are used, the TOVs show a repetitive rate. Another type of TOVs is character- ized by amplitude, which may occur in overhead line systems caused by, for example, lightning.

It is known that by installing a TOV reducing device close to the electrical apparatus, the amplitude and the rate of rise of a TOV can be reduced. For example, a surge capacitor can be arranged close to a transformer to reduce the impact of TOVs. A patent application with the publication number WO2008/025242 describes a device that is a series-impedance element. The series-impedance element can be used for protecting an electrical apparatus against TOVs.

The electrical power system of a wind park is such a power system, where a number of wind turbines are connected by long cables operated through switches in the wind park substation. An individual wind turbine is connected or disconnected by a switch placed in a switchgear at the bottom of the wind turbine tower. It has been shown that TOVs can occur when operating switches in a wind park. Although such switching operations do not occur frequently during normal operation of a wind park, a large number of maneuvers are carried out during commissioning of the wind park. When the connections to the wind turbines must be energized and tested one by one, electrical apparatus such as the wind turbine transformers may then be stressed by such TOVs during commissioning, resulting in a reduced level of withstand capability of the insulation when they are put in normal operation. Particularly, dry insulated transformers which are used mostly in the wind farm are more sensitive to TOVs characterized by rate of rise and repetition rate than oil-insulated transformers. Furthermore, a wind turbine has a unique construction, meaning that a nacelle is placed at the top of the wind turbine tower. The nacelle houses the electrical generating and/or transforming apparatus such as electrical generator, motor, gearbox, and wind turbine transformer, and it has a Mm- ited space.

A problem in connection with installing a TOV reducing device for reducing the impact of TOVs on the electrical apparatus is the limited space in the nacelle of a wind turbine. It can be true for some other applications beside wind turbines.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a solution to the above- mentioned problem.

According to a first aspect of the invention, this object is achieved by a power system as defined in claim 1 .

Such a power system comprising an electrical apparatus, a switchgear electrically connected by means of a power cable to the apparatus and including a switch arranged to connect and disconnect electrical power to the apparatus via the power cable, wherein TOVs may occur when connecting or disconnecting the power by operating the switch, and the sys- tern further comprises a TOV reducing device arranged to protect the apparatus against the TOVs transmitted through the power cable to the apparatus, and the TOV reducing device is arranged inside of or in immediate proximity to the switchgear.

The invention makes it possible to arrange the TOV reducing device at a distance from the apparatus to be protected. In the case of a wind turbine, the invention makes it possible to arrange the TOV reducing device in the bottom of the wind turbine tower where there is more space, whereas the space in the nacelle of the wind turbine is limited. When the power is connected or disconnected by operating the switch, the amplitude and rate of rise of a TOV passing through the power cable will rise. This causes a short-lived oscillation in the electrical apparatus and as a consequence damages the apparatus. The rate of rise of a TOV, also called the time derivatives of a TOV, will damage the electrical apparatus, for example by stressing the insulation of the apparatus. The higher the rate of rise of a TOV is, the more serious damage it causes to the apparatus.

Because the power system uses the cable to the power connection, and the apparatus connected to the cable is exposed to TOVs characterized by rate of rise and the repetition rate, it is the rate of rise and the repetition rate that stress the apparatus, while the impact of amplitude is relatively low since such a TOV is a low-frequency transient. Although arranging a TOV reducing device at a distance from the apparatus will decrease the reduction of the amplitude, a test and simulation have shown that the reduction of rate of rise and repetition rate for TOVs remains the same and accordingly a suitable TOV protection for the apparatus is achieved.

The invention makes it possible to protect the electrical apparatus against the TOVs characterized by rate of rise and repetition rate. This means that by arranging the TOV reducing device inside of or in immediate proximity to the switchgear in a proper way, the steepness of the rate of rise is reduced and the number of the TOV spikes is reduced. Therefore, the total impact of a TOV on the apparatus is reduced.

It is advantageous to arrange a TOV reducing device inside of or in immediate proximity to the switchgear because it providing the flexibility for the installation location for the TOV reducing device regardless of the position of an electrical apparatus to be protected. For example, a smaller TOV reducing device may be needed to be replaced by a large one in a place where the space for housing the TOV reducing device is limited. In such a case it is advantageous to arrange the TOV reducing device inside or close to the switchgear instead of arranging it close to the apparatus to be protected. Another advantage of the invention is that it provides protection to all electrical apparatus against TOVs regardless of the origin of the TOVs. Such apparatus include the power cable and/or other electrical apparatus between the electrical apparatus and the switchgear, the cable terminations used for connecting the cable to the switch included in the switchgear. For example, if the electrical apparatus is a transformer, and a secondary current transformer is arranged between the transformer and the switchgear for measuring the current and voltage passing through the connection, in this case even the secondary current transformer is protected against TOVs.

In an embodiment of the invention, the switchgear further comprises a housing including a bushing for insulating a conductor passing through the housing, the power cable including a cable termination adapted for con- nection to the bushing at a connection point, and the TOV reducing device is arranged adjacent to the connection point between the bushing and the cable termination.

In an embodiment of the invention, the switchgear further comprises a housing including a bushing for insulating a conductor passing through the housing, and the TOV reducing device is arranged in the bushing.

In an embodiment of the invention, the power cable includes a cable termination adapted for connection to the switchgear, and the TOV reducing device is arranged in the cable termination.

It is advantageous to integrate the TOV reducing device in the bushing of the switchgear or the cable termination because it is practical for installing, repairing or replacing of the TOV reducing device.

In an embodiment of the invention, the power system further comprises a second TOV reducing device. The switchgear further comprises a housing including a bushing for insulating a conductor passing through the housing, the power cable including a cable termination adapted for connection to the switchgear. The first TOV reducing device is arranged in the cable termination and the second TOV reducing device is arranged in the bush- ing. This means that alternatively two TOV reducing devices can be used in a combined way. An advantage of using two TOV reducing devices is that the impact on the apparatus caused by the TOV will be reduced even more.

In a specific embodiment of the invention, the TOV reducing device is a surge capacitor.

In another specific embodiment of the invention, the TOV reducing device is a choke. A choke is an electrical circuit designed as an inductor or an inductor with a parallel resistor. The choke is characterized in that it has low impedance and resistance for main power frequencies and high impedance for high frequencies. The impedance of an inductor increases with increasing frequency. Adding a resistor in parallel with the inductor makes the characteristic for the choke inductive with low impedance at power frequencies and resistive at higher frequencies for improved damping of high frequency oscillations.

In an embodiment of the invention, the power system is a wind electrical power system and the apparatus is a wind turbine transformer. Such a wind electrical power system includes a wind turbine tower comprising a wind turbine for generating electricity. The wind turbine is electrically connected to the wind turbine transformer, the wind turbine transformer is arranged at the top of the tower, and the switchgear is arranged at the bot- torn of the wind turbine tower.

A wind park comprises a plurality of wind turbines connected by a number of long cables. The properties such as the number of cables, cable lengths and cable connection points have a strong influence on the TOVs caused by switching operations and ground faults, which means that it is important to protect electrical devices against TOVs in the electrical power system of a wind park.

To house electrical components for generating and transferring electricity, for example, motor, gearbox, electrical generator, wind turbine transformer, a nacelle is constructed and installed at the top of the tower. How- ever, the space in the nacelle is limited and the weight of the nacelle should not be too heavy.

It is advantageous to arrange the TOV reducing device in or close to the switchgear that is located at the bottom of the tower, which overcomes the problems caused by a limited space in the nacelle on one hand. On the other hand, this will not add an extra weight to the nacelle.

Another advantage is providing convenience when repairing or replacing the TOV reducing device, or making service for the TOV reducing device.

Yet another advantage is to provide protection to all electrical apparatus after the switch included in the switchgear against TOVs regardless of the origin of the TOVs, for example if a TOV is from connecting and/or discon- necting a feeder cable, or from connecting and/or disconnecting a turbine, or from a ground fault.

This makes the invention cost-effective and compact, while at the same time the TOV reducing device arranged in or adjacent to the switchgear provides the same protection to the wind turbine transformer located in the nacelle as when it is arranged close to the wind turbine transformer. The same concept of the TOV reducing device being arranged inside of or close to the switchgear can be applied when the transformer is placed at the bottom of the wind turbine tower. The invention can be applied to both offshore and land-based wind turbines independently of the voltage level and of the switching equipment used in the switchgear, which means the switching equipment can be any type of load break switch or circuit breaker.

According to a second aspect of the invention, this object is achieved by a switchgear as defined in claim 9.

Such a switchgear is electrically connected to an electrical apparatus by means of a power cable, the switchgear comprising a switch arranged to connect and disconnect electrical power to the apparatus via the power cable, wherein TOVs may occur when connecting or disconnecting the power by operating the switch, and the switchgear further comprises a TOV reducing device arranged to protect the apparatus against the TOVs transmitted through the power cable to the apparatus.

According to a third aspect of the invention, this object is achieved by a power cable as defined in claim 12.

Such a power cable has a first end designed to be connected to an electrical apparatus, and a second end including a cable termination designed to be connected to a switchgear comprising a switch arranged to connect and disconnect electrical power to the apparatus via the power cable, wherein transient overvoltages may occur when connecting or disconnecting the power by operating the switch, and the cable termination comprises a TOV reducing device arranged to protect the apparatus against TOVs transmit- ted through the power cable to the apparatus.

According to a forth aspect of the invention, this object is achieved by a power cable as defined in claim 14.

Such a cable termination designed to be connected to a switchgear and comprises a TOV reducing device arranged to reduce TOVs received by the cable termination.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Figure 1 shows a schematic diagram of a first example of a power system, according to the invention, comprising an electrical apparatus, a switchgear and a TOV reducing device arranged inside of the switchgear;

Figure 1 a shows a TOV reducing device in the form of a surge capacitor; Figure 1 b shows a TOV reducing device in the form of a choke;

Figure 2a shows a schematic diagram illustrating a second example of a power system, according to the invention, comprising a TOV reducing device arranged in a bushing of a switchgear;

Figure 2b shows another schematic diagram illustrating a third example of a power system according to the invention, wherein in the system two TOV reducing devices are arranged for protecting against TOVs, one arranged in the cable termination of a power cable and the other arranged in the bushing of a switchgear;

Figure 2c shows a schematic diagram illustrating an example of a power cable according to the invention, and a TOV reducing device arranged in a termination of the cable;

Figure 3 shows a schematic diagram of en electrical power system in the form of a wind turbine, wherein the electrical power system is equipped with a TOV reducing device arranged inside of a switchgear that is located at the bottom of the wind turbine tower;

Figure 4a shows the rate of rise of voltages of a transformer without a

TOV reducing device; and

Figure 4b shows the rate of rise of voltages of a transformer with a TOV reducing device arranged according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows a schematic diagram of a first example of a power system. The power system 1 comprises an electrical apparatus 2, a switchgear 4, a TOV reducing device 6 arranged inside of the switchgear and a power cable 3. The switchgear 4 is electrically connected by the power cable 3 to the apparatus 2 to de-energize the apparatus to allow work, for example service or repair, to be done and to clear faults downstream. The switch- gear includes a switch 5 for connecting and disconnecting electrical power applied to the apparatus 2 via the power cable 3. The switch can be any type of switch, for example a load break switch or a circuit breaker. The power cable 3 electrically connects the apparatus 2 and the switchgear 4. The length of the cable is in the range of a few meters to a few hundred meters. When the power is connected or disconnected by operating the switch 5, TOVs may surge and be transmitted through the power cable 3 to the apparatus 2. Such a TOV is characterized by rate of rise of the TOV and repetitive rate of the TOV. According to the invention, arranging the TOV reducing device 6 inside of the switchgear instead of close to the apparatus 2 has the same protection effect on the apparatus as the TOV reducing device is arranged close to the apparatus. TOVs can be also caused by operating other switches in other switchgear, and they pass through the cable to the apparatus. In this case, the TOV reducing device provides the same protective effect to the apparatus as when the TOV is caused by operating the switch included in the switchgear. The TOV reducing device may even protect the cable, the termination of the cable and the other electrical apparatus between the apparatus and switchgear against the TOVs. Since the TOV reducing device is arranged inside of the switchgear, it is a cost- effective solution in a case where there are several electrical apparatus arranged after switchgear, because all electrical apparatus are protected against TOV by using only one TOV reducing device.

Figure 1 a shows a TOV reducing device in the form of a surge capacitor. To provide protection against TOVs, the surge capacitor 7 is connected to the cable 3. A surge capacitor generally is designed to reduce the steep fronted waves of overvoltages or overcurrents and prevent damage to the turn-to-turn insulation of rotating machines and transformers. It may be used together with a surge arrester to form comprehensive protection to the electrical apparatus. With the surge arrester, the amplitude of TOVs can be effectively reduced.

Figure 1 b shows a TOV reducing device in the form of a choke. To provide protection against TOVs, the choke 10 is connected to the cable 3. The choke 10 is, in this example, an electrical circuit with an inductor 8 and a resistor 9 arranged parallelly. It acts as a resistor at high frequencies while giving small voltages drop and losses at main power frequency. Alternatively, the choke may also be an inductor.

Figure 2a shows a schematic diagram illustrating a second example of a power system, a TOV reducing device arranged in a bushing of a switch- gear. In this example, the switchgear 4a comprises a switch 5 and a housing 24 including a bushing 20 for insulating a conductor passing through the housing 24. The cable 3 includes a cable termination 22. The bushing is cone-shaped and hollow, allowing a conductor 21 to pass along its centre and connect at both ends 23, 23' to other devices; in this example, the first end 23 is connected to the cable termination 22 and the second end is connected to the switch 5. The cable termination 22 is formed so that the bushing 20 can be plugged into it. In this example, the cable termination is an elbow connector, but it can also be a T-connector or other types of connectors. The TOV reducing device 6 arranged in the bushing can be, for example a surge capacitor shown in Figure 1 a or a choke shown in Figure 1 b. As shown in the figure, the TOV reducing device is arranged in the bushing of the switchgear 4a that is adjacent to the connection point 26.

Figure 2b shows another schematic diagram illustrating a third example of a power system. This embodiment may be considered as an extension of the second example shown in the Figure 2a. In this example, a second cable termination 22' is arranged in between the first cable termination 22 and the bushing 20 of the switchgear 4a, and a second TOV reducing device 6' is arranged in the second cable termination to provide an enhanced protective effect against TOCs for the apparatus 2. As shown in the figure, both the first TOV reducing device and the second TOV reducing device are arranged adjacent to the connection point 26. The first TOV reducing device can be an electrical choke, and the second one can be a surge capacitor, or vice versa.

Figure 2c shows a power cable, a TOV reducing device arranged in a termination of the cable. The power cable 3' comprises a first end 30 and a second end 30'. The first end is designed to be connected to an electrical apparatus to be protected against TOVs. The second end 30' includes a cable termination 22". The cable termination may, for example, be connected to a switchgear. The cable termination 22" comprises a TOV reduc- ing device 6 for reducing the impact of TOVs on the apparatus. Such TOVs may be caused by, for examples, operating a switch included in the switchgear.

Figure 3 shows a schematic diagram of en electrical power system in the form of a wind turbine, wherein the electrical power system is equipped with a TOV reducing device arranged inside of a switchgear that is located at the bottom of the wind turbine tower. The wind turbine 50 comprises a nacelle 52 for housing electrical apparatus for generating and transferring electricity, hub 55 and rotor blades 54, 54' for converting wind energy to low-speed rotational energy, a wind turbine tower 64, a switchgear 60 for connecting and disconnecting electrical power to connected electrical devices located in the nacelle and three power cable connections via power cables 40, 40', 40".

The nacelle 52 houses a generator component including an electrical generator 57, the control electronics not shown in the figure, and a gearbox 56 component for converting the low speed incoming rotation to highspeed rotation suitable for generating electricity, and a wind turbine transformer 58 for transforming the generated electricity.

A first power cable 40 is for the connection to the wind turbine transformer 58 arranged in the nacelle, a second power cable 40' is an outgoing feeder for connection to the next wind turbine, and a third power cable 40" is an incoming feeder for connecting an incoming electrical power to the wind turbine.

The switchgear 60 is placed at the bottom of the wind turbine tower. It comprises a TOV reducing device 6 for providing protection against TOV, two switches 66, 66' for connecting and disconnecting electrical devices in the wind turbine, a fuse 68 for providing protection against overcurrents. The first switch 66 is for the power connection or disconnection to the wind turbine transformer 58 via the third power cable 40, and the second switch 66' is for the connection to the next wind turbine on the same feeder cable via the power cable 40'. Typically, the switch 66' is a load break switch that can be operated with a normal load current. The switch 66 can be either a load break switch or a circuit breaker that is operated automatically in case of faults to prevent from overloads or short circuit currents. In case a circuit breaker is arranged as the switch 66, the fuse is not needed. The TOV reducing device 6 is arranged inside of the switch- gear, or in the bushing of the switchgear as shown in the figure 2a. Alter- natively, it can be arranged in the cable termination included in the cable 40 as shown in Figure 2c.

TOVs may occur when connecting and disconnecting the wind turbine by operating the switch 66 or 66', when connecting or disconnecting the whole feeder cable including several wind turbines, or when ground faults occur in the internal park grid. This means the TOVs can come from the incoming feeder though the third cable 40", or through the first cable 40 by operating the switch 66 or 66', or through the second cable 40' by operating another switch including in another switchgear.

For example by closing the switch 66 when energizing the wind turbine transformer 58, the wind turbine transformer 58 is stressed by a step voltage with a higher amplitude of the phase to ground voltage. However, in this case the concern is not the amplitude but the rate of rise since the amplitude is relatively low.

The surge impedance of a cable connected to a transformer is crucial for the rates of rise of TOVs stressing the transformer insulation. There is a significant difference in surge impedance for overhead lines power sys- terns and cable power systems. The surge impedance for an overhead line is, for example, in the range of 300-400 Ω, while the surge impedance for a cable is at least 10 times less or below 40 Ω. The difference in surge impedances has a direct impact on the time derivatives, or the rates of rise of the TOVs in the two systems: the lower the surge impedance, the higher the time derivative of the transient overvoltage. The consequence is that a transformer installed in a cable system will be stressed by surges with 10 times shorter rise time compared to a similar transformer installed in an overhead line system.

With the TOV reducing device arranged in the bushing, protection to all electrical apparatus after the switchgear against TOVs is achieved regardless of the origin of the TOVs. Alternatively, the TOV reducing device can also be arranged in the cable termination of the first cable 40, or a second TOV reducing device can be arranged in the cable termination of the first cable 40.

It should be understood that the wind turbine transformer can also be placed at the bottom of the wind turbine tower, but the arrangement of the TOV reducing device is the same and it provides the same protection against TOVs.

It can be understood that, as it is shown in Figure 3, the nacelle houses a number of electrical apparatus, which leaves very little space for the TOV reducing device. By arranging the TOV reducing device close to the switchgear the problem with a limited space in the nacelle is overcome.

Figure 4a shows the rate of rise of voltages of a transformer without a TOV reducing device. The figure shows that without the TOV reducing device, the front 70 of the TOV is very steep, which means a high rate of rise, in this example, about 0.048 μs when the surge impedance of the ca- ble is 22 Ω. The transformer is a three-phase transformer. The capacitance of the transformer between each phase and earth is about 1 nF.

Figure 4b shows the rate of rise of voltages of a transformer with a TOV reducing device. The figure shows that with the TOV reducing device, in this example a surge capacitor with a capacitance 0.1 μF or 100 nF, the steepness of the front 70' or the rate of rise of the TOV has been reduced to about 4.8 μs when the surge impedance of the cable is 22 Ω. This means that the reduction of the rate of rise of the TOV is in proportional to the capacitance of a surge capacitor. The range of a surge capacitor could be 0.1 μF to 1 μF.

Claims

1 . A power system (1 ) comprising an electrical apparatus (2), a switchgear (4, 4a) electrically connected by means of a power cable (3) to the appara- tus and including a switch (5) arranged to connect and disconnect electrical power to the apparatus via the power cable, wherein transient over- voltages may occur when connecting or disconnecting the power by operating the switch, and the system further comprises a transient overvoltage reducing device arranged to protect the apparatus against the transient overvoltages transmitted through the power cable to the apparatus, characterized in that the transient overvoltage reducing device (6, 6a, 6b) is arranged inside of or in immediate proximity to the switchgear.

2. The system according to claim 1 , wherein the switchgear further com- prises a housing including a bushing for insulating a conductor passing through the housing, the power cable including a cable termination adapted for connection to the bushing at a connection point, and the transient overvoltage reducing device is arranged adjacent to the connection point between the bushing and the cable termination.

3. The system according to claim 1 , wherein the switchgear further comprises a housing including a bushing for insulating a conductor passing through the housing, and the transient overvoltage reducing device is arranged in the bushing.

4. The system according to claim 1 , wherein the power cable including a cable termination adapted for connection to the switchgear, and the transient overvoltage reducing device is arranged in the cable termination.

5. The system according to claim 1 , wherein the power system further comprises a second transient overvoltage reducing device, the switchgear further comprises a housing including a bushing for insulating a conductor passing through the housing, the power cable including a cable termination adapted for connection to the switchgear, the first transient overvolt- age reducing devices is arranged in the cable termination and the second transient overvoltage reducing devices is arranged in the bushing.

6. The system according to any of claims 1 , wherein the transient over- voltage reducing device is any of a surge capacitor or a choke.

7. The system according to any of claims 1 -6, wherein the system is a wind electrical power system and the apparatus is a wind turbine transformer.

8. The system according to claim 7, wherein the system includes a wind turbine tower comprising a wind turbine for generating electricity, which is electrically connected to the wind turbine transformer, the wind turbine transformer is arranged at the top of the tower, and the switchgear is arranged at the bottom of the tower.

9. A switchgear (4, 4a) electrically connected to an electrical apparatus (2) by means of a power cable (3, 3'), the switchgear comprising a switch (5) arranged to connect and disconnect electrical power to the apparatus via the power cable, wherein transient overvoltages may occur when connecting or disconnecting the power by operating the switch, characterized in that the switchgear comprises a transient overvoltage reducing device (6, 6a, 6b) arranged to protect the apparatus against the transient over- voltages transmitted through the power cable to the apparatus.

10. The switchgear according to claim 9, wherein the switchgear further comprises a housing including a bushing (20) for insulating a conductor passing through the housing, and the transient overvoltage reducing device is arranged in the bushing.

1 1 . The switchgear according to any of claim 9 or 10, wherein the tran- sient overvoltage reducing device for protecting transient overvoltage is any of a surge capacitor or a choke.

12. A power cable (3, 3') having a first end (30) designed to be connected to an electrical apparatus, and a second end (30') including a cable termi- nation (22', 22") designed to be connected to a switchgear comprising a switch arranged to connect and disconnect electrical power to the appara- tus via the power cable, wherein transient overvoltages may occur when connecting or disconnecting the power by operating the switch, characterized in that the cable termination comprises a transient overvoltage reducing device (6, 6a, 6b) arranged to protect the apparatus against the tran- sient overvoltages transmitted through the power cable to the apparatus.

13. The power cable according to claim 12, wherein the transient over- voltage reducing device is any of a surge capacitor or a choke.

14. A cable termination (22', 22") designed to be connected to a switch- gear, characterized in that the cable termination comprises a transient overvoltage reducing device (6, 6a, 6b) arranged to reduce transient overvoltages received by the cable termination.

15. The cable termination according to claim 14, wherein the transient overvoltage reducing device is any of a surge capacitor or a choke.