ipts-2011-02-prokop-javora-enu

Presentation 2.1

© OMICRON electronics GmbH 2011 – International Protection Testing Symposium

Low-Power Current and Voltage Sensors for MV Applications

Radek Javora / Vaclav Prokop, ABB PPMV Instrument Transformers and Sensors, Czech Republic

Introduction Current and Voltage Sensors (Electronic Instrument Transformers) offer an alternative way of making the current and voltage measurements needed for the protection and monitoring of medium voltage power systems. Sensors based on alternative principles have been introduced as successors to conventional instrument trans-formers in order to significantly reduce size, increase safety and reliability, and to provide greater rating standardisation and a wider functionality range. The new sensor technologies also require a slight modification of the testing procedures/equipment which is partly different from well-known con-ventional testing methods.

Sensor principles Low-power stand-alone MV sensors produced by ABB are based on well-known principles and proven technologies. The characteristic feature of advanced ABB sensors is the level of output signal, which is fully adapted to fit new microprocessor-based equipment without the need to take up unnecessary power. The behaviour of the sensor is not influenced by the non-linearity and width of the hysteresis curve, which results in a highly accurate and linear response over a wide dynamic range of measured quantities. A linear and highly accurate sensor characteristic in the full operating range enables the combination of metering and protection classes in one winding. In addition, one standard sensor can be used for a broad range of rated currents and is also capable of precisely transferring signals containing frequencies different from rated ones.

Current sensor Current measurement in ABB sensors is based on the Rogowski coil principle. A Rogowski coil is a toroidal coil, without an iron core, placed around the primary conductor in the same way as the secondary winding in a current transformer. However, the output signal from a Rogowski coil is not a current, but a voltage:

dttdiMtu P

S)()( =

Fig. 1 Working principle of a Rogowski coil

Fig. 2 Example of waveforms of primary current and

secondary voltage of a Rogowski coil In all cases, a signal that represents the actual primary current waveform is easily obtained by integrating the transmitted output signal.

Voltage sensor Voltage measurement in ABB MV sensors is based on the two working principles – resistive and capacitive dividers. The output voltage is directly proportional to the input voltage:

IP

US

Presentation 2.2


)()(21

2 tuRR

Rtu PS +=

Fig. 3 Working principle of a resistive divider

)()(21

1 tuCC

Ctu PS +=

Fig. 4 Working principle of a capacitive divider

Fig. 5 Example of waveforms of primary and secondary

voltage of resistive or capacitive voltage dividers

In all cases, the transmitted output signal reproduces the actual waveform of the primary voltage signal.

Sensor applications Nowadays, a wide range of MV applications is covered by sensors, since 11 types of MV sensor product families with 34 different product versions are currently available in the ABB MV sensor products portfolio. One of the main areas where the new sensor technologies are used is MV Air Insulated Switchgears (AIS). The block type sensors designed according to DIN size requirements can be used in primary as well as in secondary AIS. With only 3 voltage levels and 2 current versions, we are able to cover applications from 6kV up to 24kV with rated currents from 80A up to 3200A. Low-voltage ring-type current sensors can be used in primary and secondary AIS. The main advantage is the small size of the sensors, which could offer various possibilities for installing the sensors by using insulated parts of primary circuits already installed in the switchgear (such as bushing insulators, insulated cable, insulated conductors, etc.). If only voltage measurement is required, the voltage sensors can be selected. These sensors fit any AIS application where highly accurate voltage measurement is required. Another area where MV sensors are increasingly used is Gas Insulated Switchgears (GIS) where SF6 gas is used as insulating medium. Bushing-type sensors were designed for GIS applications and combine four functions into one device – voltage and current measurement, voltage indication capability and the bushing function. Due to increased requirements for integrated solutions in MV applications, a new type of circuit breaker was designed where three standard products are combined – a circuit breaker, protection relay and sensors in one effective solution. This new sensor product family was therefore designed to enable current and voltage measurements at the circuit breaker poles. MV sensors are used not only in indoor applications. Bushing-type sensors were designed to enable current and voltage measurements in outdoor switches as well.

Sensor advantages Construction of ABB’s current and voltage sensors is done without the use of ferromagnetic cores. This fact results in several important benefits for the user and the application.

Sensor benefits • Sensor behaviour is not influenced by the

non-linearity and width of the hysteresis curve; this results in an accurate and linear

Us

Up

US

UP

Presentation 2.3


response over a wide dynamic range of measured quantities

• A single device/sensor could be used for both protection and for measurement purposes (no need for a separate design/product)

• There are no hysteresis losses, so sensors also have an excellent frequency response at frequencies different from the rated one, thus providing very precise input to protection functions, allowing more precise fault analysis and efficient fault location

• Sensors do not have dangerous operational states (it is not a problem to keep the output short-circuited or leave it open), resulting in a high degree of safety for surrounding devices and personnel. The output signal remains very low even during faults in the network

• The use of sensors disable the possibility of related ferroresonance phenomena, thus increasing the safety and reliability of the power network even more; furthermore, there is no need for additional protection equipment, special burden or wiring

• Significantly lighter devices, weighing only a fraction of conventional CTs or VTs; therefore, no special machines/tools are needed to carry them and transport costs are less

• Fast connection of sensors to IEDs, without any tools or materials needed, simplifies and reduces the assembly

Due to the linear response and wide dynamic range, sensors are much more standardised devices (compared to a number of different designs of CTs and VTs). Therefore, it is much easier to select the appropriate design (simplifying engineering tasks) and there could be also a reduction in spare parts on the user’s side. ABB sensors are connected to the measurement and protection evaluation devices by means of shielded cables and connectors, providing a high degree of immunity to electromagnetic disturbances. The accuracy of these sensors is verified and tested, including the cabling, so precise information is assured up to the evaluation device.

Sensor accuracy With ABB MV sensors, measuring class 0.5 is reached for continuous current measurement in the extended accuracy range from 5% of the rated primary current Ipr up to not only 120% of Ipr (as is common for conventional current transformers), but even up to the rated continuous thermal current Icth. For dynamic current measurement (protection purposes) the ABB sensors fulfil the requirements of protection class 5P up to an

impressive value, reaching the rated short-time thermal current Ith. This provides the possibility to designate the corresponding accuracy class as 5Pxxx, proving excellent linearity and accuracy measurements.

Example of current measurement range for the KEVCD A sensor with rated current 80 A and accuracy class 0.5/5P630: Metering accuracy class 0.5 is, according to the IEC 60044-8 standard, guaranteed from 5% of Ipr up to Kpcr*Ipr where Kpcr is the rated extended primary current factor and Ipr is the rated primary current. Factor Kpcr is, in the case of conventional CTs, usually just 1.2, but in the case of the KEVCD A sensor the Kpcr factor is several times higher and equals 15.625. Protection accuracy 5P630 is guaranteed for the advanced KEVCD A sensor from the current equal to Kpcr*Ipr up to the current corresponding to Kalf*Ipr, where Kalf is, according to IEC 60044-8, the accuracy limit factor. For this type of sensor, the value of Kpcr*Ipr is equal to the rated continuous thermal current Icth (1250 A) and the value of Kalf*Ipr is equal to the rated short-time thermal current Ith (50 kA). The accuracy limits are described on the graph below.

Fig. 6 Combined current accuracy class 0.5/5P630

Example of voltage measurement range for metering accuracy class 0.5 and protection accuracy class 3P: The accuracy limits are described on the graph below.

Fig. 7 Combined voltage accuracy class 0.5/3P

Presentation 2.4


Compactness Since the sensor elements are particularly small, and the same elements are used for both measurement and protection, the current and voltage sensors can easily be combined in one device – the Combined Sensor, which is still smaller and far lighter than a conventional Instrument Transformer. For example, the weight of the combined KEVCD_A sensor designed for 24 kV is only 15.6 kg and designs for lower voltage levels are even lighter. This enables much easier handling without the need for special lifting devices.

Fig. 8 Weight comparison – KEVCD 24A_ sensor Another example could be the combined bushing sensor KEVCY _R, which in addition fulfils the bushing function. The KEVCY_R sensor is designed for 24 kV and for continuous currents up to 630 A. Its weight is only 1.75 kg.

Fig. 9 Weight comparison – KEVCY 24RE1 sensor

Rated parameters Because the sensors are highly linear devices within a very wide range of currents and voltages, the same single sensor can be used for the various rated currents and voltages associated with each specific application up to the specified maximum voltage for equipment. There is no need to specify other parameters such as burden, safety factor, etc. since they are standard over the defined range. To achieve the correct function of the protection and control IED, the selected rated current and voltage, as well as the rated transformation ratio, must be properly set on the IED.

Energy savings concept As there is no iron core and no necessity for high burden values, there is the possibility for very low current losses. Apart from that, only one secondary winding is needed, therefore ABB sensors exhibit extremely low energy consumption that is just a fraction of that transferred to heat in conventional CTs/VTs. This fact contributes to huge energy savings during its entire operating life, supporting the world-wide effort to reduce energy consumption. A comparison of total costs of ownership (TCO) for a current transformer (600/5/5 A, 25 VA, cl. 0.5, 5P20) and a current sensor using a Rogowski coil is summarised in Fig. 10.

Fig. 10 Comparison of TCO for the use of a

conventional inductive CT and an LPCS using a Rogowski coil

To be able to compare the application and user advantages, the purchasing costs of the CT as well as of the sensor were kept on the same level, regardless of type. It is assumed that the inductive CT is working at its rated current and rated burden for the whole expected life time (30 years). As one can see, the energy consumed and dissipated in the CT during its whole life time is significantly higher than that of the low-power stand-alone current sensor. Therefore, low-power sensors contribute to higher efficiency and environmentally friendly measurement in MV networks. Furthermore, the temperature rise caused by internal heating up due to the current flowing through the sensor is very low and creates the further possibility of upgrading the switchgear current ratings, or the other applications, and/or reduces the need for artificial ventilation.

Correction factors The amplitude and phase error of a current and voltage sensor is, in practice, constant and independent of the primary current and primary voltage. Due to this fact it is an inherent and constant property of each sensor and it is not considered as unpredictable and influenced error. Hence, it can be easily corrected in the IED by using appropriate correction factors, stated separately for every sensor.

Presentation 2.5


Values of the correction factors for the amplitude and phase error of a current and voltage sensor are mentioned on the sensor label and should be uploaded without any modification into the IED before the sensors are put into operation. To achieve the required accuracy classes it is recommended to use all correction factors (Cfs): the amplitude correction factor (aU) and phase error correction factor (pU) of a voltage sensor; and the amplitude correction factor (aI) and phase error correction factor (pI) of a current sensor. Values of the correction factors for the amplitude and phase error of a current and voltage sensor are determined for rated values (100% of nominal primary voltage Upn and 100% of rated primary current Ipr). The amplitude correction factor is a number by which the output of sensor must be multiplied in order to have minimum amplitude error. Below are two examples how the amplitude correction factor of a current sensor is determined. The amplitude correction factor is calculated directly from ratio of rated secondary voltage and measured voltage:

9947,07992,150

150UU

sm

sr ===mV

mVaI

where aI is the amplitude correction factor of a current sensor Usr is the rated secondary voltage (reference voltage) – 150 mV at 50 Hz or 180 mV at 60 Hz Usm is the measured secondary voltage

Or, at first, amplitude error is obtained as:

%53,0100150

1507992,150100 =⋅−=⋅−

=mV

mVmVUUU

sr

srsmamε

where �am is the amplitude error of a current sensor Usr is the rated secondary voltage (reference voltage) – 150 mV at 50 Hz or 180 mV at 60 Hz Usm is the measured secondary voltage then the amplitude correction factor is calculated as:

( ) 9947,0

1100/53,01

1)100/(1 =

+=

+=

am

aIε

The phase error correction factor is a number by which the output of the sensor must be increased or decreased (depending on the sign) in order to have minimum phase error. Below is an example how the phase error correction factor of a current sensor is determined. The phase error correction factor is calculated directly from phase error using the opposite sign:

°−=−=−= 293,0´)60.17()( epI ϕ Where

pI is the phase error correction factor in degrees �e is the phase error in minutes

Safety New sensor technologies used in ABB sensors introduce many other advantages as well as those described above. Since the output signal is very low (in mV for current measurement and in V for voltage measurement), there is no significant transfer of power from primary to secondary. These low levels of output signals, which can be designated as communication signals instead of power signals, are very safe for the user. The secondary circuit of the current sensor can be left open with no risk of high voltage on the secondary terminals. Conversely, the secondary circuit of the voltage sensor can be short-circuited with no risk of explosion or danger to personnel.

Ferroresonance in power networks The ferroresonance phenomenon is a typical, unpredictable aspect of network with VTs inserted usually in cable networks with an isolated neutral or not efficiently grounded. VTs are usually efficiently designed to operate close to the knee of magnetisation curve and having burden close to the no-load state. In case the capacitance of a power line together with the VT inductance in saturated state creates an oscillating circuit (RLC), the circuit will not reach these conditions unless some transients occur. Although the cause of the saturation ceases (for example, a ground fault), a transient oscillation (i.e. a multiple or sub-frequency of that of the network) is maintained by the resonant state, exchanging reactive energy within the oscillating circuit. Owing to the frequency of this oscillation, a permanent and high circulation of current is produced in the primary winding. Since this current is only magnetising (not consumed by the burden), the secondary winding is not very involved, so there is a lot of heating at the primary and negligible heating on the secondary side. Abnormal heating of the windings always produces a strong internal pressure, consequently breaking the external housing, thus creating a risk to personnel. In order to avoid such conditions, various additional devices are being put in the open-delta winding of conventional VTs, in order to properly damp resulting oscillations. Ferroresonance phenomena are very unpredictable, so it is very difficult to have one single device that will safely damp such oscillations for all VT designs and in all different types of networks that exist around the world. The only safe solution to avoid ferroresonance caused by interaction of any kind of power network and VTs is to have a power network without capacitance or completely avoid using non-linear saturable VTs. The first condition is not possible in our networks nowadays; nevertheless the second condition could be met by using ABB MV sensors instead. All ABB MV sensors are constructed

Presentation 2.6


without the use of any ferromagnetic components. Therefore, it is a solution that is absolutely ferroresonance-free, very safe, highly reliable and not dangerous to personnel. Network operators can fully rely on that.

IED (Intelligent Electronic Device) product portfolio ABB IEDs offer a wide range of products for the protection, control, measurement and supervision of power systems. With the seven types of ABB IEDs that support sensor technologies for analogue measurement we are able to cover various applications from low-end up to high-end solutions.

Fig. 11 Example of ABB IEDs supporting sensors

Protection and control IEDs incorporate the functions of a traditional relay, as well as allowing new additional functions. The information transmitted from the sensors to the IED is very accurate, providing the possibility of versatile relay functionality. However, the IED must be able to operate with sufficient accuracy at the sensor’s low input signal level, and the signal from the Rogowski coil must be integrated. Modern IEDs are designed for such sensor use, and they are also equipped with built-in integrators for Rogowski coil sensor inputs. Modern digital apparatuses (microprocessor-based relays) allow protection and measurement functions to be combined. They fully support current and voltage sensing realised by a single sensor with a double accuracy class designation (e.g.: current sensing with combined accuracy class 0.5/5P630 as well as voltage sensing with combined accuracy class 0.5/3P for KEVCD A sensors).

Fig. 12 Example of ABB IED REF615 and ABB sensor

KEVCD 24 AE3

Type tests and Routine tests according to the IEC standard Low-power stand-alone sensors are covered by the existing standards for electronic instrument transformers, IEC 60044-7 and IEC 60044-8. There are “just” two IEC standards available nowadays; nevertheless, the significant popularity and interest in sensors resulted in the redesign of these standards. A new set of standards for “non-conventional” instrument transformers is being prepared by IEC TC38/WG37 and is planned to be covered in many more parts starting from IEC 61869-6. In the present IEC 60044-7 & -8 standards, there are different requirements for sensors that contain some active electronic components and for sensors without active electronic components. In the future IEC 61869 series, these devices will be split into different parts so it will be more transparent what belongs to the “electronic” part, and easier to read. Low-power stand-alone sensors with analogue output have to fulfil some type tests and routine tests. Dielectric tests on primary terminals are the same as in the case of conventional ITs. Differences can be found in dielectric tests on the secondary side (low-voltage components) as the low-power sensors do not power the IEDs and they just provide a kind of communication signal to the IED. Therefore, different voltage test levels apply than in the case of conventional ITs. Apart from these tests described by IEC, there are some other development tests, which are performed on sensors in order to understand the actual range of measurement limits, performance under different conditions and use in several different applications. As there are new and new potential applications for existing sensors, the experience and number of producers is growing. Also the laboratories are being more active.

Recommended site tests on sensors installed in switchgears To check the correct assembly and performance of the equipment at the site before it is powered up is one of the key factors to ensure the perfect operation of MV primary devices. To enable long-term performance of the sensors, the following site tests are recommended on sensors installed in switchgears:

• Conductivity test The conductivity test is applicable only on sensors with the primary conductor connected to the primary circuit. The purpose of the test is to confirm that the

Low-end solutions

High-end solutions

Presentation 2.7


joints on primary terminals are correctly tight and maintaining good contact. The recommended values for current supply are 100-200 A DC.

• Insulation test on the primary circuit The purpose of the test is to confirm there is sufficient line-to-earth insulation. The recommended testing voltages are 2.5 kV or 5 kV DC. The test is not applicable for low-voltage sensors.

• High voltage test The purpose of the test is to confirm that the line-to-earth insulation is in good condition and can withstand overvoltage stress. The recommended testing voltage is 80% of power frequency test voltage for 1 minute. The test is not applicable for low-voltage sensors.

• Insulation test on the secondary circuit The purpose of the test is to confirm that the insulation of secondary circuits to earth is sufficient. The recommended testing voltage is 500 V DC. The test is applicable for current sensors only.

• Primary injection The purpose is to confirm that the IEDs are getting correct current and voltage values from the sensors.

IED secondary testing Checking the correct setting and performance of the protection relay before is put into operation is one of the key steps to ensure correct and reliable operation of MV systems. Since a lot of modern IEDs use low power sensors instead of conventional ITs for current and voltage sensing, this necessitates some small differences in secondary testing of protection relays. Generally from the IED point of view, the protection functions work in the same way, the only difference is the way the signal is transferred from the primary side (MV network) to the secondary side (protection relay analogue inputs) and how the secondary signal is processed by the IED (analogue input module).

a)

b)

Fig. 13 Relay input card for ITs (a) and sensors (b) As it can be seen in Fig. 13, the application of sensor inputs resulted in greater simplification and relay design reduction. In addition, available space was increased, and the weight and cost of the relay were reduced. It is also possible to have IED inputs that fit either conventional ITs or non-conventional EITs, or have mixed modules combined to support both technologies. Since the output of the current sensor is a voltage in mV which is then integrated by the IED, the testing equipment should be able to supply the voltage signal within the same range. One of the most important things is to ensure the correct phase displacement of the secondary signal generated by the IED. Since the secondary output from the Rogowski coil is shifted by 90°, the testing equipment should be able to automatically generate the signal with the correct phase displacement without any manual correction being necessary. Unfortunately, this is not followed by all manufacturers of testing equipment. Another important difference is using correction factors. Once the correction factors are set in the IED, they should also be respected during testing. The signal generated from the testing device is multiplied by the correction factor for amplitude correction or increased/decreased by the correction factor for phase error correction in the IED.

Presentation 2.8


Fig. 14 Example of IED secondary testing settings Manual recalculation of the generated signal in the testing equipment makes secondary testing of the IED uncomfortable. Therefore, taking the correction factors into account when considering the settings of the testing equipment enables the manufacturers to further improve the testing equipment and make it more user-friendly.

Connecting the testing equipment and the IED The sensors are designed with two types of cable connectors. The new type of sensor connector is RJ-45 Cat.6 but the older type of sensors, still in production, use Twin-BNC connectors. The same two types of connectors are also used in ABB IEDs. To provide connectivity between sensors and IEDs with different types of connectors, a group of connector adapters was designed. The purpose of the connector adapters is to adapt the sensor connector type to the IED connector type (if they do not match directly) without influencing the measured signal.

Fig. 15 Example of direct connection between a sensor

and an IED Consequently, depending on the testing equipment and connected IED, there could either be a direct

connection from the testing equipment to the IED or via a connector adapter. Below is an example of testing equipment with Twin-BNC connectors, a Twin-BNC/RJ-45 connector adapter and an IED with an RJ-45 connector.

Fig. 16 Example of connection between testing equipment and an IED via a connector adapter

Some testing equipment also has available accessories which enable connection of IEDs with different connectors.

Fig. 17 Example of testing equipment accessories for

connection to IEDs with an RJ-45 connector (CMLIB REF6xx Interface Adapter)

Conclusion Low Power Current and Voltage Sensors represent a new way of making the current and voltage measurements needed for the protection and monitoring of medium voltage power systems with many benefits for the users and the applications. The new sensor technologies bring also different testing procedures which place new requirements on testing equipment and related operators.

Generated signal Amplitude: 150mV/0.9947 Phase: -90°-0.293° = - 90.293° Frequency: 50Hz

Settings Ratio: 80A/150mV Frequency: 50Hz Correction factors: aI=0.9947 pI=0.293°

Current measurement Amplitude: 80A Phase: 0°

Tester

IED

Presentation 2.9


Literature

[1] Javora, R., Vano, P.: Design of Transducers Matching Requirements of Microprocessor-Based Equipment. Electric Power Engineering 2010, pp. 431-436, Brno, Czech Republic

[2] Javora, R., Stefanka, M., Mahonen, P., Niemi, T., Rintamaki, O.: Protection in MV networks using electronic instrument transformers, CIRED2009, 2009, p. 4, Prague, Czech Republic

[3] KEVCD A Indoor combined sensor, Indoor current sensor Catalogue 1VLC000588. ABB

[4] ABB MV Sensors Catalogue 1VLC000712. ABB

[5] CMLIB REF6xx Interface Adapter, OMICRON web page (http://www.omicron.at/en/products/pro/secondary-testing-calibration/accessories/cmlib-ref6xx/)

About the Authors Vaclav Prokop (30 October 1981) Graduated from Brno Uni-versity of Technology, De-partment of Electrical Power Engineering in 2005. Since 2005 he has been working for ABB as a Protection Relay Engineer, Commissioning En-

gineer and Service Coordinator. In 2008-2009, he worked as a Service Coordinator and Service Engineer in UAE. Presently he is working as Product Manager for MV Sensors. He is responsible for sensor products and their development in MV applications.

Radek Javora (19 March 1975) Graduated from Brno Uni-versity of Technology, Depart-ment of Electrical Power Engineering in 1998. He con-tinued his research activities in Kanazawa University in Japan, focusing of ferro-

resonance phenomena. After his return, he started working for ABB and finished his Ph.D., related to Ferroresonance phenomena in electric power networks, at Brno University of Technology in 2004. Presently, he is working as the head of the Sensor and Simulation Development Department in the ABB s.r.o. Technology Centre in Brno, Czech Republic. He is responsible for development of new current and voltage sensors for medium voltage applications. He has also been active in IEEE and IEC working groups dealing with non-conventional instrument transformers from 2006.

ipts-2011-02-prokop-javora-enu

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