High Voltage secondary measuring system

Abstract

A measuring system 8 for high voltage substations 9 contains at least two high voltage instrument transformers 3, at least one voltage merging sensor 1 and at least one current merging sensor 2. The voltage sensor measures the secondary voltage of the instrument transformer, and the current sensor measures the secondary current of the transformer. The voltage and current sensors are separate elements and are at most 35 metres from the transformers. The sensors may be directly attached to the transformers with copper cables. The sensors may be connected to different phases. The sensors may be integrated into the transformers. The sensors may transmit data in digital form via fibre optic cables 4.

Description

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 transformer and a potential transformer. An instrument transformer 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 transformer contains current transformers (CTs), potential transformers (PTs), and voltage transformers (VTs). Instrument transformers 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 transformers are used alongside metering devices to measure power consumption accurately for billing purposes . Instrument transformers 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. Moreover, they are essential in power quality monitoring. Instrument transformers 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. Thus, instrument transformers contribute significantly to the safety, reliability, and efficiency of modern electrical power infrastructure. Herein, respective improvements not only relate to the instrument transformers 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 simplify existing measuring systems to enable a faster construction of new substations as well as easier maintenance, servicing and upgrading of existing substations . 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 specific needs or to solve further problems . 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 transformers, a current merging sensor being a stand alone merging unit (SAMU) and a voltage merging sensor being a stand alone merging unit (SAMU), 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 current merging sensor is adapted to measure a secondary current of the current transformer and digitize the measured value, wherein the voltage merging sensor is adapted to measure a secondary voltage of the instrument transformer selected from the group of potential transformers and voltage transformers and digitize 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 transformer, preferably is attached to the current transformer, and the voltage merging sensor is distanced at most 35m, more preferred at most 30m, even more preferred at most 20m, from the instrument transformer selected from the group of potential transformers and voltage transformers, preferably is attached to the instrument transformer selected from the group of potential transformers and voltage transformers, 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 transformer being electrically connected to the functional part of the instrument transformer. Also, it is preferred that a preferred or more preferred upper limit as specified 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 transformer and the current merging sensor being distanced at most 30m from the current transformer. The inventor found that utilizing such separated current merging sensors and voltage merging sensors being stand alone merging units (SAMU) allows to locate such sensors significantly closer to respective instrument transformers to acquire the secondary current and secondary voltage and digitize 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 digitizing 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 affecting the cables or accuracy of respective measurements respectively. Also, capacitive and inductive coupling can occur between adjacent 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 transformer. Enabling to acquire the required data very near to such instrument transformer 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 transformers. 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. As mentioned above the term "instrument transformer" has the common meaning as known to the skilled person and especially includes current transformers (CTs), potential transformers (PTs), and voltage transformers (VTs). The term "high voltage" refers to a voltage as known be the skilled person. Preferably, it refers to a voltage of at least 20kV. Typically, it is preferred that the current merging sensor and / or voltage merging sensor, preferably the current merging sensor and the voltage merging sensor, fulfill the requirements according to IEC61850-9-2 and / or IEC61869-13, more preferred according to IEC 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 utilized in the inventive high voltage secondary measuring system. According to further aspect the present invention refers to a high voltage substation containing at least one inventive high voltage secondary measuring system. According to further aspects the present invention refers to a current transformer, wherein a current merging sensor is integrated into the cur rent transformer, wherein the current merging sensor is adapted to measure a secondary current of the current transformer and digitize the measured value . According to further aspects the present invention refers to an instrument transformer selected from the group of potential transformers and voltage transformers, wherein a voltage merging sensor is integrated into the instrument transformer selected from the group of potential transformers and voltage transformers, wherein the voltage merging sensor is adapted to measure a secondary voltage of the instrument transformer selected from the group of potential transformers and voltage transformers and digitize 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 transformer with a current merging sensor and the secondary voltage of an instrument transformer selected from the group consisting of potential transformers and voltage transformers with a voltage merging sensor, wherein the current merging sensor is connected to a phase of the current transformer and the voltage merging sensor is connected to a different phase of instrument transformer selected from the group consisting of potential transformers and voltage transformers. To simplify 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. Fig. 1 shows a scheme of an inventive high voltage secondary measuring system. According to one aspect, the present invention refers to a high voltage secondary measuring system as specified 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 transformer selected from the group consisting of potential transformers and voltage trans formers by a copper cable. Furthermore, it was noted that it is typically beneficial to connect the current merging sensor and the voltage merging sensor to different phases. According to further embodiments it is preferred that the current merging sensor and the voltage merging sensor are connected to different 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 transformer or be even directly installed in the instrument transformer during manufacturing being a part of the device. Herein, it can be furthermore preferred that no cable is utilized to connect the respective instrument transformer to the respective merging sensor. Utilizing the current merging sensor and voltage merging sensor as specified above it even becomes possible to further reduce the distance between the current merging sensor and the current transformer 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 transformer selected from the group consisting of potential transformers and voltage transformers 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 transformer. Surprisingly, it is possible to directly attach the current merging sensor to the current transformer and the voltage merging sensor to the high voltage instrument trans former selected from the group consisting of voltage trans former and potential transformers. Preferably the sensors are already attached to the respective instrument transformer 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 transformer. Especially, when utilizing 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 transformer. However, just for the sake of security it needs to be pointed out that even without utilizing the further embodiments as described herein allows to reduce the distance between the merging sensors and the high voltage instrument transformer not only working well, but still representing significant improvements over the known systems. 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 transformer selected from the group consisting of potential transformers and voltage transformers. Herein, it can be avoided that any cable connection between current merging sensor and the current transformer and the voltage merging sensor and the instrument transformer selected from the group consisting of potential transformers and voltage transformers is located outside of the respective instrument transformer. Surprisingly, further improving the safety of the maintenance personal under real conditions. Corresponding instrument transformer can also easily be directly provided with respective current merging sensors and voltage merging sensors during their manufacturing. It was further noted that specific 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 transformers . According to further embodiments the current merging sensor and / or the voltage merging sensor fulfills the requirements according to IEC 61869-13, preferably the current merging sensor and the voltage merging sensor fulfill 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 transformer by a cable having a length of at most 35m, more preferred at most 30m, even more preferred at most 20m. An example of a current merging sensor that can be utilized for the inventive Instrument transformer systems is the current merging sensor CMS as available from the company Grid to great being designed according to IEC 61869-13. An example of a voltage merging sensor that can be utilized for the inventive Instrument transformer systems is the voltage merging sensor VMS as available from the company Grid to great being designed according to IEC 61869-13. While it is possible to directly attach the current merging sensor and the voltage merging sensor to the high voltage instrument transformer, 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. Im, more preferred at least 0.2m, even more preferred at least 0.5m, from the high voltage instrument transformer. 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 difficult for the field personnel to detach the respective merging sensor during maintenance it was noted that in extreme cases the possibility to utilize tool allowing to leverage some force in between is surprisingly beneficial to simplify the work of respective colleagues . To further reduce the requirements of cables utilized 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 utilizing 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 realized even in case the respective current merging sensor and the voltage merging sensor are directly attached to the high voltage instrument transformer. According to further aspects the present invention refers to a current transformer, wherein a current merging sensor is integrated into the current transformer, wherein the current merging sensor is adapted to measure a secondary current of the current transformer and digitize the measured value. According to further embodiments it is preferred that the current transformer contains at least two integrated current merging sensors . According to further aspects the present invention refers to an instrument transformer selected from the group of potential transformers and voltage transformers, wherein a voltage merging sensor is integrated into the instrument transformer selected from the group of potential transformers and voltage transformers, wherein the voltage merging sensor is adapted to measure a secondary voltage of the instrument transformer selected from the group of potential transformers and voltage transformers and digitize the measured value. According to further embodiments it is preferred that the instrument transformer selected from the group of potential transformers and voltage transformers contains at least two integrated voltage merging sensor. 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 transformer and an instrument transformer selected from the group consisting of potential transformers and voltage transformers, 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 transformer and the voltage merging sensor is distanced at most 35m, preferably at most 30m, even more preferred at most 20m, from the instrument transformer selected from the group consisting of potential transformers and voltage transformers, wherein the current merging sensor and the voltage merging sensor are separate elements. According to further aspects the present invention refers to a method containing the step of measuring the secondary current of a current transformer with a current merging sensor and the secondary voltage of an instrument transformer selected from the group consisting of potential transformers and voltage transformers with a voltage merging sensor, wherein the current merging sensor is connected to a phase of the current transformer and the voltage merging sensor is connected to a different phase of instrument transformer selected from the group consisting of potential transformers and voltage transformers. Experimental data: Experimental instrument transformer 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 trans former . No . Prim. Equip Voltage (kV) Current (A) Core / Win-ding Types Type Sec. Current (A) 1 CT 145 600 3P / 2M Traditional 1A 2 CT 145 600 3P / 2M Merging Sensor 1A 3 CT 245 600 3P / 2M Traditional 1A 4 CT 245 600 3P / 2M Merging Sensor 1A 5 CT 550 600 3P / 2M Traditional 1A 6 CT 550 600 3P / 2M Merging Sensor 1A 7 VT 145 600 3P / 2M Traditional 8 VT 145 600 3P / 2M Merging Sensor 9 CT 145 600 3P / 2M Traditional 5A 10 CT 145 600 3P / 2M Merging Sensor 5A 11 CT 245 600 3P / 2M Traditional 5A 12 CT 2 4 5 600 3P / 2M Msrgin Sensor 5 A 13 CT 550 600 3P / 2M Traditional 5A 14 CT 550 600 3P / 2M Merging Sensor 5A 15 VT 145 600 3P / 2M Traditional - 16 VT 145 600 3P / 2M Merging Sensor No . Burden Required (VA) Burden Selected (VA) Prim. Equip. Price (€) Price Reduction Compared with Traditional Cu Cable L (m) Cable Reduction Compared with Traditional 1 5 30 5250 2095 - 2 2.5 2.5 4600 12 35 98 3 5 30 7650 2095 - 4 2.5 2.5 6550 14 35 98 5 5 30 14750 2095 - 6 2.5 2.5 13400 9 35 98 7 - - - - 2095 - 8 - - - - 35 98 9 100 100 7100 2095 - 10 2.5 2.5 4600 35 35 98 11 100 100 8300 2095 - 12 2.5 2.5 6550 21 35 98 13 100 100 15400 2095 - 14 2.5 2.5 13400 13 35 98 15 - - - - 2095 - 16 - - - - 35 98 Table 1 5 The traditional systems realize the measurements by directly connecting the protection control and metering devices to the instrument transformers via copper cables. Respective systems typically provide a distance between the high voltage instrument transformers and the current sensor and voltage sensor 10 of around 2000m. Test showed that the distance can also be reduced to around 1000m without suffering significantly with regard to the reliable provision of measured data of the current and secondary voltage of the high voltage instrument transformer. Shortening the distance further resulted in significant impairing the quality of the measurements rendering the utilization continuously problematic without spending significant effort 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. Utilizing 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, affecting the accuracy of the current and voltage measurements. Problems like capacitive and inductive coupling. Such capacitive and inductive coupling can occur between adjacent cables, leading to crosstalk and interference in the measurements. Also, ground loops based on different parts of the measurement system being grounded at different points. Herein, such ground potential differences can cause unwanted currents to flow in the copper cables. Leading to erroneous measurements. However, such problems surprisingly can be avoided by utilizing the inventive high voltage secondary measuring system, wherein the close placement of the separated current merging sensor and voltage merging sensor do not suffer from problems allowing such significantly simplified 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 transformer while saving 98% of the cable required when utilizing 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 transformer. While it is not shown in Table 1 the current merging sensor and voltage merging sensor can also be directly attached to the instrument transformer. The inventor noticed that placing the current merging sensor in immediate vicinity to the current transformers reduces significantly the consumed energy by the measuring loop (current merging sensor + protection or control or metering devices), which allows to utilize even current transformers with a reduced size of the core (secondary winding). Resulting in a reduction of the overall size of the entire current transformer. The burden of the current transformer core is based on the length of the cable that is between the current transformer 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. With that given the current merging sensor mitigate the unnecessary high secondary burden in the analogue measurement loops between the current transformers 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 size of the core of a current transformer. A larger core size 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 transformer for fast and accurate measurements. A larger core will result in a physically larger and heavier current transformer, which may not always be desirable, especially in space-constrained applications. Bigger cores require more magnetic material, which increases the cost of such current transformer. The integration of single-phase current merging sensor and voltage merging sensor in current transformers and voltage transformers can even provide a 100% copper wire reduction in the measuring loop. 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 specified in IEC 61850-9-2. Allowing to significantly reduce the amount of cables satisfying the strict requirements to enable a measurement of the current and secondary voltage of the high voltage instrument transformer allowing to simplify building new substations and upgrading existing substations by cutting off available cables not required anymore, for example, to let them be refurbished. While the mering sensors are placed near the high voltage instrument transformer. Also benefitting from the insight that said part of the existing cables near the high voltage instrument transformer 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 simplifying future maintenance actions. 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 specific problems or fulfilling specific needs. The scope of the protection should be understood to be only limited by the claims attached. 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 transformers 3 being a current transformer and a voltage transformer. Herein, two voltage merging sensors 1 are directly attached to a voltage transformer and two current merging sensors 2 are directly attached to the current transformers 3. Alternatively, differing 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 transformers 3 as specified above at most 35m from the high voltage instrument transformers providing the benefits as specified. In such inventive embodiments the voltage merging sensors 1 and the current merging sensors 2 are preferably connected to the high voltage instrument transformer 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 transformer 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. Not well visible in figure 1 is that the voltage merging sensors 1 and the current merging sensors 2 could be connected to different phases of the respective high voltage instrument transformers 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 transformer. 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 transformer. Herein, the current merging sensors and the voltage merging sensors are fulfilling the requirements according to IEC 61869-13. 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 transformers 3 by means of fiber optic cables 4. Allowing to essentially negate the specified 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 transferred to such cloud, but is directly utilized by the protection, control and metering system 6 tasked with protecting, for example, the high voltage equipment such as power transformers 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. 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. 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 specific problems or fulfilling specific needs. The scope of the protection should be understood to be only limited by the claims attached.

Claims

1. 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 the voltage merging sensor (1) is a single-phase merging sensor and / or the current merging sensor (2) is a single-phase merging sensor.

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

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 distanced at least 0.1m from the high voltage instrument transformer (3).

10. 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.

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

12. 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 a current 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.

13. Method according to claim 12, 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 (3) selected from the group consisting of potential transformers and voltage transformers with a voltage merging sensor (1),5 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.10

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