High voltage secondary measuring system

Abstract

The present invention refers to an improved digital secondary measuring system for high voltage substations. Furthermore, the present invention refers to a transformer substation containing such high voltage secondary measuring system. Furthermore, the present invention refers to a method of measuring the current and the secondary voltage of a high voltage instrument transformer making use of the instrument transformer system as described herein and providing an easy and reliable measurement for protection, control and metering.

Description

[0001] Description

[0002] High Voltage secondary measuring system

[0003] The present invention refers to an improved high voltage secondary measuring system . Furthermore , the present invention refers to a substation containing such high voltage secondary measuring system . Furthermore , the present invention refers to a method of measuring the secondary current and the secondary voltage of at least two high voltage instrument transformers containing a current trans former and a potential trans former .

[0004] An instrument trans former is a type of electrical device used in power systems to accurately measure current and voltage levels . It is designed to provide a scaled-down replica of the current or voltage in a high-voltage line , which can then be easily measured by low-voltage metering devices . The term instrument trans former contains current trans formers ( CTs ) , potential trans formers ( PTs ) , and voltage trans formers (VTs ) .

[0005] Instrument trans formers are integral to the modern world, particularly in the realm of power generation, transmission, and distribution . They play a critical role in ensuring safety by reducing high voltage levels to safer levels for easy measurement and monitoring, protecting both electrical devices and operators from potential harm . Their ability to provide accurate measurements of current and voltage levels is crucial for the proper functioning and monitoring of power systems . This accuracy is essential in power distribution systems where instrument trans formers are used alongside metering devices to measure power consumption accurately for billing purposes .

[0006] Instrument trans formers also contribute to the protection of power systems . They assist in the operation of protective relays , providing analog measurements based on the current and voltage in the system . These measurements help the protection to detect faults and trigger circuit breakers to disconnect when abnormal conditions are detected, preventing potential damage to the system .

[0007] Moreover, they are essential in power quality monitoring . Instrument trans formers provide accurate voltage and current waveforms at a safe level for monitoring equipment , ensuring the quality of power delivered is maintained . They are also used in control systems to regulate power flow and maintain the stability of the power grid .

[0008] Thus , instrument trans formers contribute signi ficantly to the safety, reliability, and ef ficiency of modern electrical power infrastructure .

[0009] Herein, respective improvements not only relate to the instrument trans formers themselves , but also to the entire measuring loop, including the analog data acquisition unit the control and protections devices ( PACs ) and the connection between them . Including that mid-term to long-term the servicing of respective numbers of substations increases . Thus , there is a need to simpli fy existing measuring systems to enable a faster construction of new substations as well as easier maintenance , servicing and upgrading of existing substations .

[0010] These problems are solved by the products and methods as disclosed hereafter and in the claims . Further beneficial embodiments are disclosed in the dependent claims and the further description . These benefits can be used to adapt the corresponding solution to speci fic needs or to solve further problems .

[0011] According to one aspect the present invention refers to a high voltage secondary measuring system for a high voltage substation, wherein the high voltage secondary measuring system contains at least two high voltage instrument trans formers , a current merging sensor being a stand alone merging unit ( SAMU) and a voltage merging sensor being a stand alone merging unit

[0012] ( SAMU) , wherein the at least two high voltage instrument trans formers contain a current trans former and an instrument trans former selected from the group consisting of potential trans formers and voltage trans formers , wherein the current merging sensor is adapted to measure a secondary current of the current trans former and digiti ze the measured value , wherein the voltage merging sensor is adapted to measure a secondary voltage of the instrument trans former selected from the group of potential trans formers and voltage trans formers and digiti ze the measured value , wherein the current merging sensor is distanced at most 35 m, more preferred at most 30m, even more preferred at most 20m, from the current trans former, preferably is attached to the current trans former, and the voltage merging sensor is distanced at most 35m, more preferred at most 30m, even more preferred at most 20m, from the instrument trans former selected from the group of potential trans formers and voltage trans formers , preferably is attached to the instrument transformer selected from the group of potential trans formers and voltage trans formers , wherein the current merging sensor and the voltage merging sensor are separate elements . Naturally, such distance is measured starting from the nearest connection of the instrument trans former being electrically connected to the functional part of the instrument trans former . Also , it is preferred that a preferred or more preferred upper limit as speci fied herein for the distance of the current merging sensor and the voltage merging sensor applies for both sensors in the same way . So that it is , for example , preferred that an embodiment refers to the voltage merging sensor being distanced at most 30m for the potential trans former and the current merging sensor being distanced at most 30m from the current trans former . The inventor found that utili zing such separated current merging sensors and voltage merging sensors being stand alone merging units ( SAMU) allows to locate such sensors signi ficantly closer to respective instrument transformers to acquire the secondary current and secondary voltage and digiti ze the measured values to be used for control protection and metering . Herein, traditional completely analogue measuring systems require to locate the data acquisition devices ( Intelligent Electronic devices or Electromechanical Relays ) at a distance of 1000m and more . Furthermore , the existing digiti zing devices known as merging units (MUs ) combine current and voltage measurements . Due to this fact placing in immediate vicinity is not possible and they are located in a distance of more than 50m . For example , in traditional and digital substations with merging units the high electromagnetic fields are af fecting the cables or accuracy of respective measurements respectively . Also , capacitive and inductive coupling can occur between adj acent cables leading to crosstalk and interference in the measurements . Based on that having a fiber optic cable instead of copper cable brings many technological advantages . However, it was noted that it is possible to place current merging sensors and voltage merging sensors in the vicinity of the instrument trans former . Enabling to acquire the required data very near to such instrument trans former and only transmit the data to the rest of the respective devices like protections devices . Resulting, for example , in thousands of meter cable saved for each monitoring device or even completely eliminating the need of a copper cable out of respective systems measuring the voltage and secondary current of instrument trans formers . Reducing not only the requirements for building new substation . But also , reducing the requirements for servicing and upgrading existing substations . Overall providing not only such benefit , but also providing a surprisingly more reliable and safer substation in total .

[0013] As mentioned above the term " instrument trans former" has the common meaning as known to the skilled person and especially includes current trans formers ( CTs ) , potential trans formers ( PTs ) , and voltage trans formers (VTs ) . The term "high voltage" refers to a voltage as known be the skilled person . Preferably, it refers to a voltage of at least 20 kV .

[0014] Typically, it is preferred that the current merging sensor and / or voltage merging sensor, preferably the current merging sensor and the voltage merging sensor, ful fill the requirements according to IEC61850- 9-2 and / or IEC61869- 13 , more preferred according to TEC 61850- 9-2 and IEC61869- 13 . It was noted by the inventor that such current merging sensors and voltage merging sensor are simultaneously very beneficial to be utili zed in the inventive high voltage secondary measuring system .

[0015] According to further aspect the present invention refers to a high voltage substation containing at least one inventive high voltage secondary measuring system .

[0016] According to further aspects the present invention refers to a current trans former, wherein a current merging sensor is integrated into the current trans former, wherein the current merging sensor is adapted to measure a secondary current of the current trans former and digiti ze the measured value .

[0017] According to further aspects the present invention refers to an instrument trans former selected from the group of potential trans formers and voltage trans formers , wherein a voltage merging sensor is integrated into the instrument trans former selected from the group of potential trans formers and voltage trans formers , wherein the voltage merging sensor is adapted to measure a secondary voltage of the instrument trans former selected from the group of potential trans formers and voltage trans formers and digiti ze the measured value . According to further aspects the present invention refers to a method containing the step of measuring the secondary current of a current trans former with a current merging sensor and the secondary voltage of an instrument trans former selected from the group consisting of potential trans formers and voltage trans formers with a voltage merging sensor, wherein the current merging sensor is connected to a phase of the current trans former and the voltage merging sensor is connected to a di f ferent phase of instrument trans former selected from the group consisting of potential trans formers and voltage trans formers .

[0018] To simpli fy understanding of the present invention it is referred to the detailed description hereafter and the figures attached as well as their description . Herein, the figures are to be understood being not limiting the scope of the present invention, but disclosing preferred embodiments explaining the invention further .

[0019] Fig . 1 shows a scheme of an inventive high voltage secondary measuring system .

[0020] According to one aspect , the present invention refers to a high voltage secondary measuring system as speci fied above . According to further embodiments it is preferred that the current merging sensor is connected to the current transformer by a copper cable and the voltage merging sensor is connected to the instrument trans former selected from the group consisting of potential trans formers and voltage transformers by a copper cable .

[0021] Furthermore , it was noted that it is typically beneficial to connect the current merging sensor and the voltage merging sensor to di f ferent phases . According to further embodiments it is preferred that the current merging sensor and the voltage merging sensor are connected to di f ferent phases . It was noted that such arrangements provide even further reduction of the copper cables and even allow the complete elimination of the need of a cable as the current merging sensor and the voltage merging sensor can be attached to the instrument trans former or be even directly installed in the instrument trans former during manufacturing being a part of the device . Herein, it can be furthermore preferred that no cable is utili zed to connect the respective instrument trans former to the respective merging sensor .

[0022] Utili zing the current merging sensor and voltage merging sensor as speci fied above it even becomes possible to further reduce the distance between the current merging sensor and the current trans former is most 15m, more preferred at most 10m, even more preferred at most 0m, and the distance between the voltage merging sensor and the instrument trans former selected from the group consisting of potential trans formers and voltage trans formers is to at most 15m, more preferred at most 10m, even more preferred at most 0m . Preferably, no cable is needed to connect the current merging sensor and / or the voltage merging sensor to the respective instrument trans former . Surprisingly, it is possible to directly attach the current merging sensor to the current trans former and the voltage merging sensor to the high voltage instrument transformer selected from the group consisting of voltage transformer and potential trans formers . Preferably the sensors are already attached to the respective instrument trans former during production . According to further embodiments it is preferred that the current merging sensor and the voltage merging sensor are directly connected to the high voltage instrument trans former . Especially, when utili zing further embodiments as described herein minor drawbacks can be easily compensated providing an overall reliable and highly sensitive measurement of current and voltage of the high voltage instrument trans former . However, j ust for the sake of security it needs to be pointed out that even without utili zing the further embodiments as described herein allows to reduce the distance between the merging sensors and the high voltage instrument trans former not only working well , but still representing signi ficant improvements over the known systems .

[0023] According to further embodiments it is preferred that the current merging sensor is integrated into the current transformer and the voltage merging sensor is integrated into the instrument trans former selected from the group consisting of potential trans formers and voltage trans formers . Herein, it can be avoided that any cable connection between current merging sensor and the current trans former and the voltage merging sensor and the instrument trans former selected from the group consisting of potential trans formers and voltage trans formers is located outside of the respective instrument trans former . Surprisingly, further improving the safety of the maintenance personal under real conditions . Corresponding instrument trans former can also easily be directly provided with respective current merging sensors and voltage merging sensors during their manufacturing .

[0024] It was further noted that speci fic types of current merging sensors and voltage merging sensors are especially useful for the inventive application . According to further embodiments it is preferred that the current merging sensor is a singlephase merging sensor and / or the voltage merging sensor is a single-phase merging sensor . Preferably the current merging sensor and the voltage merging sensor are single-phase merging sensors . Such merging sensors are especially useful and provide very reliable and precise measurements when being used in close distance to high voltage instrument trans formers .

[0025] According to further embodiments the current merging sensor and / or the voltage merging sensor ful fills the requirements according to IEC 61869- 13 , preferably the current merging sensor and the voltage merging sensor ful fill the requirements according to IEC 61869- 13 . According to further embodiments it is preferred that the current merging sensor and the voltage merging sensor are connected to the high voltage instrument trans former by a cable having a length of at most 35m, more preferred at most 30m, even more preferred at most 20m .

[0026] According to further embodiments it is preferred that the current merging sensor and the voltage merging sensor each contain a digital signal processing element , wherein the digital signal processing element is adapted to enhance the signal quality and extract speci fied measurement parameters . It was noted that it is possible to directly include such digital signal processing element in the current merging sensor and voltage merging sensor allowing to directly process the respective data .

[0027] According to further embodiments it is preferred that the current merging sensor and the voltage merging sensor each contains a sensing element , an analog-to-digital conversion element , a digital signal processing element , a data formatting element , a time element , and a network interface , wherein the digital signal processing element is adapted to enhance the signal quality and extract speci fied measurement parameters , wherein the data formatting element is adapted to format the digital data according to a predefined format , wherein the time element is adapted to assign a time data to the measured data . Such speci fied measurement parameters can be , for example , the magnitude and phase of the measured current and the measured voltage . Such predefined format is preferably a commonly available standard for digital data . Enabling a reliable and easy possibility to process the data subsequently independent from a highly speci fic software solution provided by a respective device manufacturer . The time data preferably represents a time stamp of the measurement of the measured data, for example , enabling a synchroni zation of all devices within a respective substation . Such sensing element can be , for example , a conventional type sensing element like a wound type trans former or a non-conventional type sensing element like a optical sensing element or a Rogowski coil . Such analog-to-digital conversion element is preferably a high resolution analog to digital conversion element . Such network interface is preferably an ethernet based interface .

[0028] An example of a current merging sensor that can be utili zed for the inventive Instrument trans former systems is the current merging sensor CMS as available from the company Grid to great being designed according to IEC 61869- 13 .

[0029] An example of a voltage merging sensor that can be utili zed for the inventive Instrument trans former systems is the voltage merging sensor VMS as available from the company Grid to great being designed according to IEC 61869- 13 .

[0030] While it is possible to directly attach the current merging sensor and the voltage merging sensor to the high voltage instrument trans former, it can be beneficial to include a minimum distance for practical reasons . According to further embodiments it is preferred that current merging sensor and the voltage merging sensor are distanced at least 0 . 1m, more preferred at least 0 . 2m, even more preferred at least 0 . 5m, from the high voltage instrument trans former . While it seems trivial it was noted that for practical applications such minimum distance can be surprisingly beneficial . For example , in case the connection is somehow impaired and it is di f ficult for the field personnel to detach the respective merging sensor during maintenance it was noted that in extreme cases the possibility to utili ze tool allowing to leverage some force in between is surprisingly beneficial to simpli fy the work of respective colleagues .

[0031] To further reduce the requirements of cables utili zed to forward the measured data over longer distances it is preferred for typical applications that fiber optic cables are used . According to further embodiments it is preferred that the current merging sensor and / or the voltage merging sensor are adapted to transmit measured data by means of fiber optic cables , preferably wherein the current merging sensor and the voltage merging sensor are adapted to transmit measured data by means of fiber optic cables . Allowing to avoid a complex shielding of such cables compared to normal network cables utili zing electrical signals . While such electricity based network cables can also be beneficial for certain applications it was noted that surprisingly the conversion of measured data into optical data can be easily reali zed even in case the respective current merging sensor and the voltage merging sensor are directly attached to the high voltage instrument trans former .

[0032] According to further aspects the present invention refers to a current trans former, wherein a current merging sensor is integrated into the current trans former, wherein the current merging sensor is adapted to measure a secondary current of the current trans former and digiti ze the measured value .

[0033] According to further embodiments it is preferred that the current trans former contains at least two integrated current merging sensors .

[0034] According to further embodiments it is preferred that the current merging sensor is a single-phase merging sensor being connected to a single phase of the current trans former .

[0035] According to further aspects the present invention refers to an instrument trans former selected from the group of potential trans formers and voltage trans formers , wherein a voltage merging sensor is integrated into the instrument trans former selected from the group of potential trans formers and voltage trans formers , wherein the voltage merging sensor is adapted to measure a secondary voltage of the instrument trans former selected from the group of potential trans formers and voltage trans formers and digiti ze the measured value . According to further embodiments it is preferred that the instrument trans former selected from the group of potential trans formers and voltage trans formers contains at least two integrated voltage merging sensor .

[0036] According to further embodiments it is preferred that the voltage merging sensor is a single-phase merging sensor being connected to a single phase of the instrument trans former .

[0037] According to a further aspect the present invention refers to a method of measuring a secondary current and a secondary voltage of a substation containing a current trans former and an instrument trans former selected from the group consisting of potential trans formers and voltage trans formers , containing the step of measuring the secondary current with a current merging sensor and the secondary voltage with a voltage merging sensor, wherein the current merging sensor is distanced at most 35m, preferably at most 30m, even more preferred at most 20m, from the current trans former and the voltage merging sensor is distanced at most 35m, preferably at most 30m, even more preferred at most 20m, from the instrument trans former selected from the group consisting of potential trans formers and voltage trans formers , wherein the current merging sensor and the voltage merging sensor are separate elements .

[0038] According to further aspects the present invention refers to a method containing the step of measuring the secondary current of a current trans former with a current merging sensor and the secondary voltage of an instrument trans former selected from the group consisting of potential trans formers and voltage trans formers with a voltage merging sensor, wherein the current merging sensor is connected to a phase of the current trans former and the voltage merging sensor is connected to a di f ferent phase of instrument trans former selected from the group consisting of potential trans formers and voltage trans formers .

[0039] Experimental data : Experimental instrument trans former systems have been examined . Herein, a standard arrangement with a location of the respective sensors far remote has been compared to inventive high voltage secondary measuring systems as described herein . One example of a traditional system has been listed in Table 1 in contrast to such system as described herein, wherein a current merging sensor and a voltage merging sensor have been placed in the vicinity of the high voltage instrument transformer .

[0040]

[0041] Table 1

[0042] The traditional systems reali ze the measurements by directly connecting the protection control and metering devices to the instrument trans formers via copper cables . Respective systems typically provide a distance between the high voltage instrument trans formers and the current sensor and voltage sensor of around 2000m . Test showed that the distance can also be reduced to around 1000m without suf fering signi ficantly with regard to the reliable provision of measured data of the current and secondary voltage of the high voltage instrument trans former . Shortening the distance further resulted in signi ficant impairing the quality of the measurements rendering the utili zation continuously problematic without spending signi ficant ef fort into shielding measures for cables . While the possibility to simply replace the cable to the high voltage instrument to remove such potential failure origin now becomes feasible and was noted to be surprisingly well received and interesting solution .

[0043] Utili zing merging sensors in short distance , however, provides possibilities to mitigated existing problems like electromagnetic interference (EMI ) . Resulting in HV substations often having high electromagnetic fields , which can induce noise in the copper cables . Thus , af fecting the accuracy of the current and voltage measurements . Problems like capacitive and inductive coupling . Such capacitive and inductive coupling can occur between adj acent cables , leading to crosstalk and interference in the measurements . Also , ground loops based on di f ferent parts of the measurement system being grounded at di f ferent points . Herein, such ground potential di f ferences can cause unwanted currents to flow in the copper cables . Leading to erroneous measurements . However, such problems surprisingly can be avoided by utili zing the inventive high voltage secondary measuring system, wherein the close placement of the separated current merging sensor and voltage merging sensor do not suf fer from problems allowing such signi ficantly simpli fied arrangements . Herein, as exemplarily shown in Table 1 the respective sensors have been placed at the construction on which the CT / VT are installed with a copper cable connection of 35m providing the measurements of current and secondary voltage of the high voltage instrument trans former while saving 98 % of the cable required when utili zing traditional setups . Such current merging sensors and voltage merging sensors were provided as sensor unit kits already attached to a copper cable to be connected to the high voltage instrument trans former . While it is not shown in Table 1 the current merging sensor and voltage merging sensor can also be directly attached to the instrument trans former . The inventor noticed that placing the current merging sensor in immediate vicinity to the current transformers reduces signi ficantly the consumed energy by the measuring loop ( current merging sensor + protection or control or metering devices ) , which allows to utili ze even current trans formers with a reduced si ze of the core ( secondary winding) . Resulting in a reduction of the overall si ze of the entire current trans former .

[0044] The burden of the current trans former core is based on the length of the cable that is between the current trans former and the relay and also the consumption of the relay . In the past the relays need higher VA (volt ampere ) to work properly but nowadays with the relays in the market this burden is normally lower than 0 . 5VA, making the cable the big part of the burden . This burden is calculated based on the current we have and the resistance of the cable with the formula P=IA2xR .

[0045] With that given the current merging sensor mitigate the unnecessary high secondary burden in the analogue measurement loops between the current trans formers and the protection, control and measurements system in of a high voltage substation . Due to the high resistance of the copper conductors , which leading to a bigger si ze of the core of a current trans former . A larger core si ze of a CT leads to a slower response time due to its increased inductance . This can affect the performance of protection devices that rely on the current trans former for fast and accurate measurements . A larger core will result in a physically larger and heavier current trans former, which may not always be desirable , especially in space-constrained applications .

[0046] Bigger cores require more magnetic material , which increases the cost of such current trans former . The integration of single-phase current merging sensor and voltage merging sensor in current trans formers and voltage trans formers can even provide a 100% copper wire reduction in the measuring loop .

[0047] The measured data can be easily transmitted from the current merging sensor and the voltage merging sensor to remotely located protection control and metering devices via fiber optic cables connected to a network, for example , according to a communication standard as speci fied in IEC 61850- 9-2 . Allowing to signi ficantly reduce the amount of cables satis fying the strict requirements to enable a measurement of the current and secondary voltage of the high voltage instrument trans former allowing to simpli fy building new substations and upgrading existing substations by cutting of f available cables not required anymore , for example , to let them be refurbished . While the mering sensors are placed near the high voltage instrument trans former . Also benefitting from the insight that said part of the existing cables near the high voltage instrument trans former provide the highest likelihood of being undamaged . Allowing to also repair respective damages of existing substations in many cases by simply replacing the existing monitoring systems by the inventive one also simpli fying future maintenance actions .

[0048] The present invention was only described in further detail for explanatory purposes . However, the invention is not to be understood being limited to these embodiments as they represent embodiments providing benefits to solve speci fic problems or ful filling speci fic needs . The scope of the protection should be understood to be only limited by the claims attached .

[0049] Figure 1 shows a scheme of an inventive high voltage secondary measuring system 8 as included in a substation 9 . Herein, the high voltage secondary measuring system 8 exemplarily shows two high voltage instrument trans formers 3 being a current trans former and a voltage trans former . Herein, two voltage merging sensors 1 are directly attached to a voltage trans former and two current merging sensors 2 are directly attached to the current trans formers 3 . Alternatively, di ffering from the example shown in figure 1 the voltage merging sensor 1 and the current merging sensor 2 according to the invention as described herein could also be distanced from the high voltage instrument trans formers 3 as speci fied above at most 35m from the high voltage instrument trans formers providing the benefits as speci fied . In such inventive embodiments the voltage merging sensors 1 and the current merging sensors 2 are preferably connected to the high voltage instrument trans former 3 by means of a copper cable . However, placing the voltage merging sensor 1 and the current merging sensor 2 directly at the respective high voltage instrument trans former 3 allows to provide a very simple arrangement providing reliable results . As visible in figure 1 the voltage merging sensors 1 and the current merging sensors 2 are separate elements .

[0050] Not well visible in figure 1 is that the voltage merging sensors 1 and the current merging sensors 2 could be connected to di f ferent phases of the respective high voltage instrument trans formers 3 the voltage merging sensors 1 and the current merging sensors 2 are attached to . Also , it is not visible that the example as shown in figure 1 contains that the voltage merging sensors 1 is a not a single-phase merging sensors and the current merging sensors 2 is not a single-phase merging sensors being directly attached to the respective instrument trans former . In alternative embodiments not shown in figure 1 the current mering sensor 2 is a single-phase merging sensor and the voltage merging sensor 1 is a single phase merging sensor allowing to eliminate the needs of a copper cable for connecting the respective merging sensor and the respective instrument trans former . Herein, the current merging sensors and the voltage merging sensors are fulfilling the requirements according to IEC 61869- 13 .

[0051] The current merging sensors and the voltage merging sensors are connected to a control , protection and metering system 6 located distanced from the high voltage instrument trans formers 3 by means of fiber optic cables 4 . Allowing to essentially negate the speci fied above problems . The protection, control and metering system 6 processes the data acquired and the corresponding protection, control and metering functions could function normally . The example as shown in figure 1 send to a cloud 7 . Through said cloud 7 the data is further provided to a user interface 10 and a database 11 . Yet , in alternative embodiments preferred for typical application cases the data does not need to be trans ferred to such cloud, but is directly utili zed by the protection, control and metering system 6 tasked with protecting, for example , the high voltage equipment such as power trans formers and controlling, for example , high voltage equipment like disconnectors and circuit breakers . Thus , such alternative embodiments do not necessarily require such cloud 7 like the example as shown in figure 1 .

[0052] While the power for running the respective sensors can be acquired from many points of such substation 9 the example as shown in figure 1 receives the requires electricity from the location of the protection, control and metering systems 6 by means of a power cable 5 .

[0053] The present invention was only described in further detail for explanatory purposes . However, the invention is not to be understood being limited to these embodiments as they represent embodiments providing benefits to solve speci fic problems or ful filling speci fic needs . The scope of the protection should be understood to be only limited by the claims attached .

Claims

Patent claims1. High voltage secondary measuring system (8) for a high voltage substation, wherein the high voltage secondary measuring system (8) contains at least two high voltage instrument transformers (3) , a voltage merging sensor (1) and a current merging sensor (2) , wherein the at least two high voltage instrument transformers contain a current transformer and an instrument transformer selected from the group consisting of potential transformers and voltage transformers wherein the voltage merging sensor (1) is adapted to measure a secondary voltage of the instrument transformer selected from the group consisting of potential transformers and voltage transformers and digitize the measured value, wherein the current merging sensor (2) is adapted to measure a secondary current of the current transformer and digitize the measured value, wherein the voltage merging sensor (1) is distanced at most 35m from the instrument transformer selected from the group consisting of potential transformers and voltage transformers and the current merging sensor (2) is distanced at most 35m from the current transformer, wherein the voltage merging sensor (1) and the current merging sensor (2) are separate elements.

2. High voltage secondary measuring system (8) according to claim 1, wherein the voltage merging sensor (1) is connected to the instrument transformer selected from the group consisting of potential transformers and voltage transformers by a copper cable and the current merging sensor (2) is connected to the current transformer by a copper cable.

3. High voltage secondary measuring system (8) according to any of the preceding claims,wherein the voltage merging sensor (1) and the current merging sensor (2) are connected to different phases.

4. High voltage secondary measuring system (8) according to any of the preceding claims, wherein the distance between the voltage merging sensor (1) and the instrument transformer selected from the group consisting of potential transformers and voltage transformers is most 15m and the distance between the current merging sensor (2) and the instrument transformer selected from the group consisting of potential transformers and voltage transformers are distanced at most 15m.

5. High voltage secondary measuring system (8) according to any of the preceding claims, wherein the voltage merging sensor (1) is directly connected to the instrument transformer selected from the group consisting of potential transformers and voltage transformers and the current merging sensor (2) is directly connected to the current transformer.

6. High voltage secondary measuring system (8) according to any of the preceding claims, wherein the current merging sensor (2) is integrated into the current transformer and the voltage merging sensor (1) is integrated into the instrument transformer (3) selected from the group consisting of potential transformers and voltage transformers .

7. High voltage secondary measuring system (8) according to any of the preceding claims, wherein current merging sensor (2) and the voltage merging sensor (1) each contains a sensing element, an analog-to-dig- ital conversion element, a digital signal processing element, a data formatting element, a time element, and a network interface, wherein the digital signal processing element is adapted to enhance the signal quality and extract specified measurementparameters , wherein the data formatting element is adapted to format the digital data according to a predefined format, wherein the time element is adapted to assign a time data to the measured data.

8. High voltage secondary measuring system (8) according to any of the preceding claims, wherein the voltage merging sensor (1) is a single-phase merging sensor and / or the current merging sensor (2) is a single-phase merging sensor.

9. High voltage secondary measuring system (8) according to any of the preceding claims, wherein the voltage merging sensor (1) and the current merging sensor (2) are connected to the high voltage instrument transformer (3) by a cable having a length of at most 35m.

10. High voltage secondary measuring system (8) according to any of the preceding claims, wherein the voltage merging sensor (1) and the current merging sensor (2) are distanced at least 0.1m from the high voltage instrument transformer (3) .

11. High Voltage secondary measuring system (8) according to any of the preceding claims, wherein the voltage merging sensor (1) and / or the current merging sensor (2) are adapted to transmit measured data by means of fiber optic cables (4) in a digitized form.

12. Substation (9) containing at least one high voltage secondary measuring system (8) according to any of the preceding claims .

13. Method of measuring a current and a secondary voltage of a high voltage instrument transformer (3) , containing the step of measuring the secondary current of acurrent transformer with a current merging sensor (2) and the secondary voltage of an instrument transformer selected from the group consisting of potential transformers and voltage transformers with a voltage merging sensor (1) , wherein the current merging sensor (2) is distanced at most 35m from the current transformer at most 35m and the voltage merging sensor (1) is distanced from the instrument transformer selected from the group consisting of potential transformers and voltage transformers at most 35m, wherein the current merging sensor (1) and the voltage merging sensor (2) are separate elements.

14. Method according to claim 12, containing the step of measuring the secondary current of a current transformer with a current merging sensor (2) and the secondary voltage of an instrument transformer (3) selected from the group consisting of potential transformers and voltage transformers with a voltage merging sensor (1) , wherein the current merging sensor (2) is connected to a phase of the current transformer and the voltage merging sensor (1) is connected to a different phase of instrument transformer selected from the group consisting of potential transformers and voltage transformers.

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