The present invention relates to a high-voltage (HV) divider used to obtain lower voltage signals in electrical HV networks .

In electrical HV networks, capacitor voltage transformers (CVTs) are electrical equipment employed to step down HV signals so as to provide lower voltage output signals for line measuring and/or protection devices or to be used as coupling capacitors for HF communication signals.

The main elements of a conventional CVT transformer as, for example, one designed by the Trench Group, called TCVT, are schematically illustrated in Figure 1.

In its most basic form, a conventional CVT transformer comprises :

- a capacitor stack comprising two capacitors, a HV

capacitors Ci _H and middle-voltage capacitor C _M across which the transmission line signal is split;

- an inductive element REACT acting as compensating reactor aimed at tuning the device to the line frequency;

- an intermediate inductive intermediate voltage transformer T _IMV to further step down the voltage.

The CVT transformer shown in Figure 1 consists of two main parts, a capacitor voltage divider Al contained inside an insulator body INS and an electromagnetic unit U _EM contained inside a base-box BB _C V _T ·

The capacitor voltage divider Al comprises the capacitor stack and a link connecting a point P _C ci between to the capacitors Ci _H ,C _M to a first intermediate terminal ΤΊ _ΝΤ ι which is one of the input terminals of the base-box BB _C VT · The capacitors Ci _H , C _M are designed to reduce the input primary voltage Vi _N till a value between 5 and 1 0KV useful to

energize the inductive intermediate voltage transformer Ti _M v contained in the electromagnetic unit U _E M -

The insulation bodies INS of the capacitor voltage divider Al of the CVT products developed by the Trench Group have typically an external insulation using porcelain or composite insulators and an internal capacitor insulation comprising oil and polypropylene with or without paper.

The electromagnetic unit U _E M typically comprises a surge arrester SA, the inductive intermediate voltage transformer T vr the inductive element REACT with a neutral end terminal Nl, and a filter circuit FILT against ferro-resonances oscillations. Plate G denotes the ground terminal of the base-box BB _C VT ·

The input high voltage Vi _N is applied at input HV primary terminal Τ _ΪΝ and a set of output voltages V ₀ UT I , ₀ UT3 are measured at a set of output terminals Τ ₀ υτι , Τ ₀ υτ3 located at the base-box level. Output voltage V ₀ UT ₃ is provided at HF output terminal Τ ₀ υτ ₃ (derived from another intermediate terminal Ti _NT3 ) and it is employed for HF communication signals for coupling capacitor purposes. Terminal N _H F is the earthing link for the HF output terminal Τ ₀ υτ ₃ (where the illustrated link is to be disconnected in case a HF

communication device needs to be connected to it) . Subset of output voltages V ₀ UT I, measured at output terminal subset Τ ₀ υτι , is used for measuring the amplitude of the applied input voltage Vi _N at the nominal frequency range (e.g. 50 or 60 Hz), e.g. for performing voltage measurements and/or for supplying voltage to line protection devices.

The output terminal subsets Τ ₀ υτι is denoted with the term "subset" since output voltage VI may conveniently be supplied on more than one terminal according to customer requirements who may for example wish you to have two terminals to be connected to a voltage measurement device and two other terminals to be connected to protection devices.

CVT transformers are normally used on HV systems with an applied input primary voltage Vi _N in the range of 52 to 800 kV. For different primary voltage ranges, only Ci _H needs to be modified so that, advantageously, a standard inductive intermediate voltage transformer Ti _M v can be used for a large range of primary voltage values.

Nowadays, in the majority of HV networks, conventional CVT transformers are used for the already mentioned purposes (e.g. HV measurements, protective relay, HF communication) as a conventional solution.

In recent years, in electrical power systems, the assessment of power quality has become a key requirement for the modern management of electrical power systems, being power quality a major concern for electricity providers and their customers. Power quality is evaluated in compliance with industry specifications for determining the harmonic pollution level in the system. Power system harmonics are integer multiples of the fundamental power system frequency and are created by a large variety of non-linear devices connected to the electrical power systems. The risks involved with harmonics pollution are voltage distortions leading to several power quality disturbances, including for example:

- increased heating in the equipment and conductors;

- misfiring in variable speed drives;

- torque pulsations in motors;

- increased losses;

- damage to electrical equipment;

- malfunction of metering and control equipment.

Hence, harmonics pollution, by affecting power equipment performances, is often the cause for customer

dissatisfaction, for load and revenue losses. In order to ensure that power quality criteria are met, energy providers need to determine the origin of the power quality disturbances. To do that, the harmonics level in HV systems need to be identified and measured with a reliable image of the voltage waveform on the HV line.

Unfortunately, the above described conventional CVT

transformers do not provide the industry-required sufficient accuracy for the measurement of voltage harmonics since they provide a poor image of the harmonics on HV networks. In fact, CVT transformers are tuned to resonate at the nominal system frequency (i.e. 50 Hz or 60 Hz) so that the CVT accuracy is obtained on a narrow frequency band. Since, the lowest frequency resonance appears at around a few hundreds Hz, conventional CVTs cannot thus be used for harmonic measurements. In order to be able to at least monitor harmonics through conventional CVTs, in the art have been introduced some equipment which are modifications of conventional CVTs with the addition of extra devices.

For example, a first type of such modified CVT is a

conventional CVT comprising a harmonic monitoring terminal as the one developed by the Trench Group. A second known type of modified CVT is a conventional CVT comprising a PQ-sensor™. With such types of modified CVT transformers it is possible to monitor harmonics in the HV system while simultaneously being still able to carry out the conventional tasks of CVTs (e.g. HV measurements, protective relay, HF communication). The first type of modified CVT equipment contains a harmonics monitoring terminal (HMT) which comprises among other

elements a third capacitor. The Trench Group designed such type of modified CVT equipment under the product name TCVT with HMT terminal. Performances in harmonics monitoring of such equipment product are an accuracy of about ± 5% in a frequency range of about 50Hz to 3kHz.

The second type of modified CVT equipment contains a PQ- sensor™. The technology behind the PQ-sensor™ can be found in the disclosures of document EP 1295133 Bl (2001, Ghassemi Foroozan) which discloses a CVT comprising current sensor means arranged to sense the current flowing in at least one capacitor of the CVT and measurements means arranged to provide from the sensed current values a measurement of the harmonic values of the applied input voltage.

For example, the Trench Group added such PQ-sensor™ to its conventional CVT in order to market such a second type of modified CVT equipment product, e.g. under the product name TCVT with PQ sensor™, such equipment product being suitable for different applications such as harmonics monitoring, revenue metering, protection and PLC. Performances in

harmonics monitoring of such equipment product are an

accuracy of about ± 3% in a frequency range of about 50 Hz to 6 kHz.

In the art, in order measure voltage harmonics with high accuracy, the optimal solution used so far consists of precision HV resistor capacitor dividers (RCD) , another type of HV instrument transformer.

Precision HV RCD dividers are voltage transformers that provide a low-voltage output proportional to the applied HV voltage input and are almost linear from DC up to more than 10 kHz: harmonics can be typically measured with an accuracy better than 1% and a phase deviation better than 1° over this frequency range .

Figure 2 is a simplified electric circuit of an RCD divider. The RCD simplified circuit consists of an assembly A2

comprising a capacitor stack and a resistor stack,

electrically connected in parallel to each other, and a link connecting point P _C c2 to point P _RR then to a second output terminal Τ ₀ υτ2 · The capacitor stack comprises two capacitors C2H , C _L and the resistor stack comprises two resistors R _H ,R _L -

Point P _C c2 and point P _RR are, respectively, a point P _C c between the two capacitors C2H , C _l and a point P _RR between the two resistors R _H ,R _L - The input primary voltage Vi _N is applied at input terminal Τ _ΪΝ and the second output voltage V _0UT 2 is measured at output terminal Τ ₀ υτ2 (where neutral terminal 2 is grounded) . The conditions that enable an accurate measure of the voltage waveform is the frequency independence of the transfer function V _0UT 2 / _IN which requires that: R _H *C2 _H = R _L * C _L . Such RCD devices can be designed to give a frequency

independent transformation ratio and phase angle over a frequency range up to at least 1 MHz and are also suitable for DC measurement.

The Trench Group developed a product family of precision HV RCD dividers under the product name RCVT, which are available for voltage levels from 24kV up to 765kV. Performances of such equipment product are an accuracy up to about 0.1 ~6 , in a frequency of about 0 to 50 kHz. Such RCVT product range cover all IEC and ANSI metering requirements: class 0.2 or lower and can also be used for protection purposes, according IEC or ANSI industry specifications. Such equipment product is suitable for different applications such as harmonics metering, revenue metering and protection (low power) .

RCVTs have typically an external insulation using porcelain or composite insulators and internal insulation using oil with paper-polypropylene or SF6 with polypropylene.

RCVTs are not affected by the trapped charges phenomena. Table 1 below summarizes at a glance the features of the three above described HV equipment types. TCVT + CVT + RCD

HMT PQ sensor™

AC Voltage Output Yes Yes Yes

(nominal (but low frequency) power only)

HF Communication Yes Yes No

DC Voltage Output No No Yes

Harmonics Yes Yes Yes

Measuring (accuracy (accuracy of (accuracy of about ± about ± 3% in of about ± 5% in a a frequency 0.1% in a frequency range of about frequency range of 50Hz to 6kHz) range of about 50Hz about 0 to to 3kHz) 50kHz)

Table 1 : summary of equipment features

As summarized in Table 1, even if the above illustrated two types of modified CVT equipment enable harmonics metering functionalities, it is evident that, where high accuracy harmonics measurement and low-power is required, RCD-type equipment represents the optimal solution. On the other hand, where voltage measuring, voltage protection and HF communication functionalities are required, it is instead the CVT-type equipment which represents the desirable solution.

Hence, in HV networks where best performances are required, it is often the case that both equipment types of instrument transformers need to be installed, i.e. CVTs and RCDs .

Moreover, due to the strict power quality requirements of modern HV network management, energy providers are more and more required to install RCD transformers in electrical substations, in addition to the conventional or modified CVT transformers already present. Unfortunately, CVT and RCD transformers are HV equipment of large sizes due to their physical characteristics. This can prove to be a great problem in electrical substations which have a rigid layout and where space is a serious constraint.

It is therefore aim of the present invention to overcome the above mentioned drawbacks, by providing a HV divider which provides at least some functionalities of the CVT whilst enabling a high accuracy harmonics measurement.

The aforementioned aim is achieved by a HV divider for stepping down an input high voltage in HV systems, wherein at the divider input terminal is applied the input high voltage and wherein, at the divider output terminal set, a set of output voltages are provided;

wherein said divider comprises a primary part and a secondary part electrically connected to each other so that the output of the primary part is the input of the secondary part;

wherein the input terminal of the primary part is the divider input terminal and the output the secondary part is the divider output set; wherein the primary part comprises a first assembly and a second assembly included within insulating means and having for input the same input terminal which is the divider input terminal ;

the first assembly being a first capacitor stack comprising a first high voltage capacitor and a middle voltage capacitor; the second assembly comprising a second capacitor stack and a resistor stack electrically connected in parallel to each other; the second capacitor stack comprising a second high voltage capacitor and a low voltage capacitor; the resistor stack comprising a high voltage resistor and a low voltage resistor;

wherein the output of the primary part is a set of

intermediary terminals comprising at least:

- a first intermediate terminal connected to a point between the two capacitors of the first capacitor stack;

- a second intermediate terminal connected to a link

connecting a point between the two capacitors of the second capacitor stack and a point between the two

resistors of the resistor stack;

wherein the secondary part comprises an electromagnetic unit and wherein the secondary part output set comprises at least:

- a first output terminal subset, deriving from the first intermediate terminal processed through electromagnetic unit, arranged to provide a first output voltage subset to be used for measuring the amplitude of the input voltage at the nominal frequency range;

- a second output terminal, deriving from the second

intermediate terminal, arranged to provide a second output voltage to be used for measuring the waveform of the input high voltage. In invention embodiments, the HV divider may advantageously comprise :

- a third intermediate terminal connected to the output

terminal of the first assembly;

- a third output terminal, deriving from the third

intermediate terminal, arranged to provide a third output voltage to be used for high frequency communications.

In invention embodiments, the first capacitor stack and the second capacitor stack may conveniently physically be

realized through two different column arrangements, the first column arrangement including a set of first-stack capacitor elements physically stacked together one above the other and electrically connected and the second column arrangement including a set of second-stack capacitor elements physically stacked together one above the other and electrically

connected .

In invention embodiments, the first capacitor stack and the second capacitor stack may preferably be physically realized through one single column arrangement, the single column arrangement including a number of capacitor elements

belonging to both capacitor stacks; wherein the capacitor elements of both capacitor stacks are physically stacked together one above the other together and wherein only the capacitor elements belonging to the same capacitor stack are electrically connected.

In invention embodiments, the electromagnetic unit may preferably comprise at least an inductive middle voltage transformer, a compensation reactor and a filter circuit. Embodiments of the invention enable to use only one HV instrument transformer equipment instead of two, whilst having the required functionalities. This leads to space and cost savings for energy providers at the HV electrical sub ^¬ stations. Savings are also achieved in terms of the equipment installation costs and civil works required at electrical sub-stations. For example, such benefits are particularly relevant for customers operating under tight space

constraints or in regions with adverse environmental

conditions where mechanical robustness requirements play a key role.

Embodiments of the invention enable greater flexibility by allowing employing the pre-existing sub-station layout.

Embodiments of the invention enable to select one or more functionalities to be used, at best technical performances without compromises.

Embodiments of the invention enable a simple and cost- effective manufacturing by employing components of the conventional instrument transformer product portfolio.

The invention will now be described in preferred but not exclusive embodiments with reference to the accompanying drawings, wherein:

Figure 1 is a schematic circuit diagram of a

conventional CVT transformer (prior art, previously described) ;

Figure 2 is a schematic circuit diagram of a RCD

divider (prior art, previously described) ; Figure 3 is a schematic circuit diagram of a HV divider according to an example embodiment of the present invention; Figure 4 is a schematic illustration of two different example embodiments of the physical

realization of the two capacitor stacks. In the drawings, like reference signs refer to same or similar elements.

Figure 3 is a schematic circuit diagram of a HV divider according to an example embodiment of the present invention. At least some embodiments of the present invention addresses the above described issue in which a HV divider for measuring an input high voltage Vi _N in HV systems has an input terminal I _N to which it is applied the input high voltage Vi _N and which has an output terminal set Τ ₀ υτι,Τ ₀ υτ2 where a set of output voltages V ₀ UTI/ ₀ UT2 are provided.

The divider comprises a primary part PI and a secondary part P2 electrically connected to each other so that the output of the primary part PI is the input of the secondary part P2; wherein the input terminal of the primary part PI is the divider input terminal Τ _ΪΝ and the output the secondary part P2 is the divider output set Τ ₀ υτι,Τ ₀ υτ2·

The primary part PI comprises a first assembly Al and a second assembly A2 included within insulating means INS and having for input the same input terminal which is the divider input terminal ΤΊ _Ν . The first assembly Al is a first

capacitor stack comprising a first high voltage capacitor Ci _H and a middle voltage capacitor C _M . The second assembly A2 comprises a second capacitor stack and a resistor stack electrically connected in parallel to each other. The second capacitor stack comprises a second high voltage capacitor C2H and a low voltage capacitor C _L . The resistor stack comprises a high voltage resistor R _H and a low voltage resistor R _L , both preferably being high accuracy/low inductance resistors. The output of the primary part PI is a set of intermediary terminals T _INTI ,T _INT2 comprising at least:

- a first intermediate terminal ΤΊ _ΝΤ ι connected to a point

between the two capacitors Ci _H ,C _M of the first capacitor stack;

- a second intermediate terminal Ti _NT2 connected to a link

connecting a point P _C c between the two capacitors C _2H ,C _l of the second capacitor stack and a point P _RR between the two resistors R _H , R _L of the resistor stack.

The insulating means INS around the two assemblies A1,A2 may be realized through three alternative solutions:

1) An insulating body containing the two assemblies A1,A2; where the insulating body is in the ambient air (Air

Insulated Substation AIS) .

2) An insulating body containing the two assemblies A1,A2; where the insulating body is inside a gaseous insulating area (e.g. with SF6 gas) contained in a metallic envelope

(Gas Insulated Substation GIS) .

3) The two assemblies A1,A2 are inside a gaseous insulating area (e.g. with SF6 gas) contained in a metallic envelope (Gas Insulated Substations GIS) .

The insulation body may be a cylindrically-shaped porcelain body (with or without sheds) or a fiberglass tube (with or without silicone sheds) . The sheds are typically used fur insulating bodies located in AIS substations. The two

assemblies Al, A2 - when inside the insulator body - are immersed in an insulating material which can be liquid (e.g. oil), gaseous (e.g. SF6) or solid (e.g. resin). It is noted that in latter case, i.e. when the two assemblies A1,A2 are immersed in a solid insulating material like resin, they may not require an insulator body around them since the solid insulating material can act as insulator body. Moreover, it is noted that the metallic envelope (also called enclosure) of the GIS usually contains other types of

substation equipment as well.

Each capacitor may be preferably made by alternating foils of aluminium, paper and/or polypropylene.

One side of the primary part PI is connected to high voltage, the other side is connected through a bushing to a base-box BB containing the electromagnetic unit U _E M and to a terminal block, which may be inside or outside the base-box BB , where the second output terminal Τ ₀ υτ2 is provided.

The secondary part P2 comprises an electromagnetic unit U _E M and an output terminal set Τ ₀ υτι,Τ ₀ υτ2 comprising at least:

- a first output terminal subset Τ ₀ υτ _ΐ/ deriving from the

first intermediate terminal ΤΊ _ΝΤ ι processed through

electromagnetic unit U _E M, arranged to provide a first output voltage subset V ₀ UT I to be used for measuring the amplitude of the input voltage Vi _N at the nominal frequency range;

- the second output terminal Τ ₀ υτ2, deriving from the second intermediate terminal Ti _NT 2, arranged to provide a second output voltage V ₀ UT2 to be used for measuring the waveform of the input high voltage Vi _N .

In one invention embodiment, the electromagnetic unit U _E M may comprise at least an inductive middle voltage transformer T vr a compensation reactor REACT and a filter circuit FILT. In one invention embodiment, the HV divider may

advantageously comprise:

- a third intermediate terminal Ti _NT 3 connected to the output terminal of the first assembly Al ;

- a third output terminal Τ ₀ υτ3 , deriving from the third intermediate terminal Ti _NT 3 , arranged to provide a first out voltage V ₀ UT3 to be used for HF communication.

The first and third output terminal Τ ₀ υτΐ _/ Τ ₀ υτ3 are the

terminals providing the functionalities typical of

conventional CVTs, the first terminal subset Τ ₀ υτι is used for connection of carrier frequency equipment (e.g. for measuring and protecting purposes) and the third terminal Τ ₀ υτ3 is used for HF communication.

The second output terminal Τ ₀ υτ2 is providing the

functionality typical of RCDs, e.g. harmonics measurement. It may also be advantageously used by the customers for

connection of low-voltage-power equipment.

Figure 4 is a schematic illustration of two different example embodiments of the physical realization of the two capacitor stacks .

In a first embodiment, as illustrated in Figure 4a, the first capacitor stack and the second capacitor stack are physically realized through two different column arrangements COLl,

COL2. The first column arrangement COLl comprises a set of first-stack capacitor elements CE1 physically stacked

together one above the other and electrically connected. The second column arrangement COL2 includes a set of second-stack capacitor elements CE2 physically stacked together one above the other and electrically connected. The electrical

connections, as shown by the dashed line connecting the capacitor elements tags TG, may preferably be connections in series. In other embodiments, the electrical connections may be parallel connections or a combination of series and parallel . In a second embodiment, as illustrated in Figure 4b, the first capacitor stack and the second capacitor stack are physically realized in one single column arrangement COL12. Such single column arrangement includes a number of capacitor elements CE1,CE2 belonging to both capacitor stacks. In the single column arrangement COL12, the capacitor elements of both capacitor stacks are physically stacked together one above the other together and only the capacitor elements CE1,CE2 belonging to the same capacitor stack are

electrically connected, as shown by the dashed line

connecting capacitor elements. The electrical connections, as shown by the dashed line connecting the capacitor elements tags TG, may preferably be connections in series. In other embodiments, the electrical connections may be parallel connections or a combination of series and parallel. The electrical connection of capacitor elements of the same capacitor stack is implemented via a given mixing rule. In one example embodiment, as the one illustrated in Figure 4b, the mixing rule may be connecting the capacitor elements in an alternating sequence. As the skilled in the art easily appreciates one may implement other types of mixing rules to electrically connect the capacitor elements CE1, CE2 : e.g. mixing rules with alternating tuples (single, pairs, triples and so on or any combination thereof) of capacitor elements CE1, CE2 electrically connected together each to form one of the two corresponding capacitor stacks. The advantage of the first embodiment is that the simple design of electrical connections among capacitor elements is easy to implement so as to obtain manufacturing cost savings. The advantage of the second embodiment is that the dimensions and weight of the primary part PI can be reduced.

In fact, having one single column arrangement COL12 instead of two column arrangements COL1, COL2 leads to volume savings when an insulator body is required also because only one hardware fixing system for the capacitor elements is required instead of two and also because of the typical cylindrical shape of the insulator bodies. As a result, when an insulator body is required, the interstitial space to be filled by the insulating material around the two assemblies Al, A2 is also reduced. Having then less insulating material may

advantageously lead to reduced weight of the primary part PI (e.g. in case of liquid or solid filling) and/or less

environmental impact (e.g. in case of oil or of SF6 filling) .

It is noted that the reduced environmental impact of

equipment is becoming an important customer requirement in countries and regions where regulations impose limitations on the used quantity of oil (ground pollution) and/or of SF6 gas (emission restricted by Kyoto protocol) in equipment.

Moreover, the reduced weight of the primary part PI leads also to advantages in term of the civil works and

installation costs required, especially in AIS substations. In fact a less heavy HV divider requires a simpler and more economic metallic pedestal solution for ground fixing in order to fulfill the same mechanical robustness requirements. Mechanical robustness requirements in AIS substations are a sensitive issue for customers operating in regions with adverse climatic and geological conditions (e.g. strong winds, hurricanes, earthquakes etc.). Although the invention has been described with particular reference to some preferred embodiments, it will be evident to those skilled in the art, that the present invention is not limited thereto, but further variations and modifications may be applied without departing from the scope of the invention as defined by the annexed claims.

Acronyms of used terms

AIS Air Insulated Substation

ANSI American National Standard Institute

CVT Capacitor Voltage Transformer

DC Direct Current

GIS Gas Insulated Substation

HF High Frequency

HV High Voltage

IEC International Electro-technical Commission

LV Low Voltage

MV Middle Voltage

PLC Programmable Logic Controller

RCD Resistor Capacitor Divider