Convert voltage converters to the Hz cycle

Voltage converters and sensors for power quality measurements in medium voltage

ORELich welcome dear friends of protection, control and electrical engineering to our new guest contribution by Roland Bürger (MBS AG) and Jens Schübel (PQ Plus), this time on the subject of voltage converters and sensors for PQ measurement. Here we go!

introduction

A reliable energy supply has meanwhile become an important location factor for many companies. While power failures and voltage fluctuations were among the most important parameters of supply quality in the past, voltage transients and voltage harmonics are becoming more and more important. This is mainly due to the increasing number of non-linear consumers and many decentrally connected renewable energy sources.

In order to guarantee uniform standards for electrical energy supply in Europe, the minimum requirements for voltage quality are defined in a European standard. This is EN 50160, which is the heading

"Characteristics of the voltage in public electricity supply networks"

wearing. This standard is to be understood as a product standard for electrical energy and is therefore also used as a valid product standard in electricity supply contracts. In February 2014 the Federal Court of Justice made it unequivocally clear that electricity is also subject to the Product Liability Act. The distribution network operator is thus liable for damage to electrical consumers that can be traced back to poor voltage quality on the part of the distribution network operator. For this reason, many measuring device manufacturers now offer measuring devices that prepare automated quality reports in accordance with EN 50160. Digital meters are also increasingly offering power quality functions in accordance with EN 50160. While the measuring devices can process the voltage directly in the low voltage, we are dependent on voltage converters or voltage sensors in the medium voltage. The voltage quality is usually measured on older systems. However, the built-in voltage converters usually do not give any indication of the transmission behavior at higher frequencies on the rating plate. The devices are only specified for the 50 Hz fundamental frequency of the network. However, measurements according to EN 50160 require a frequency range of up to 2 kHz. We want to investigate the question of whether the existing devices are suitable for measurements up to 2 kHz.

Almost without exception, the built-in voltage converters are inductive converters that work according to the transformer principle.

In detail, the primary coil not only consists of inductively acting copper windings, but capacities also result from the individual layers that are isolated from one another. The capacities between the individual turns also contribute to the total capacitance of the primary coil. This results in an oscillating circuit made up of inductance, capacitance and ohmic resistance, which must also have a corresponding resonance frequency.

In order to find this resonance frequency, a commercially available 10 kV voltage converter is now run through in the "frequency sweep process" with 6,400 measuring points up to 10 kHz.

We have found what we are looking for!

A resonance point can be seen at approx. 6 kHz. While the transducer transmits the primary signal in an acceptable manner up to approx. 5 kHz, an amplitude error of approx. 100% and a phase error of 87 ° results at approx. 6 kHz. A reliable PQ analysis up to z. B. 9 kHz can therefore not be carried out with this voltage converter.

Despite the normatively regulated voltage levels, every converter manufacturer has a large number of different converters with different primary coils in order to be able to meet the most diverse secondary configurations on the part of the customer. These converters have already been delivered and installed in measuring fields. The manufacturer can only carry out a rough calculation of the first resonance point in connection with the archived production documents. The resonance point measured in practice can often deviate from the calculation result by a few kHz. It is therefore very difficult for converter manufacturers to make reliable statements for devices that have already been delivered.

A contribution from the technical-scientific organization CIGRE / CIRED offers the measuring point operators good support. A guideline for power quality measurements was published here, which provides a meaningful table with regard to the frequency transmission behavior of voltage converters.

It can be seen that 10 kV voltage transformers up to the 50th harmonic (2.5 kHz) can generally be used for PQ measurements. This statement is consistent with our measurement result in Figure 3.

In the 20 kV range, however, devices have already been found according to the table that do not provide any reliable measured values ​​on the secondary side from the 21st harmonic. In the 30 kV range, there is even a general release up to the 7th harmonic. We state that only 10 kV voltage converters can be used in existing systems for reliable EN 50160 measurements. In the voltage levels 20 and 30 kV, information must be provided by the converter manufacturer.

Furthermore, the question arises whether measurements up to 2 kHz are still sufficient. As became clear in the specialist article "Power Quality Measurements in the Low Voltage Network - Best Practice", current measurements up to 9 kHz have already been declared the standard. For voltage measurements, however, EN 50160 continues to apply to the EVU, so that a voltage measurement of up to 2 kHz would be sufficient in terms of standards. However, if the measuring device already has high sampling rates and enables harmonics evaluation up to 9 kHz, a reliable voltage measurement up to 9 kHz would also be desirable.

MBS AG therefore offers frequency-optimized voltage converters up to 24 kV for the measuring range up to 9 kHz. The accuracy requirement for these converters is defined in IEC 61869-6. The amplitude or phase error is as follows.

The converter holds class 0.5 up to approx. 8 kHz. From 8 kHz, class 1 is clearly maintained. These voltage converters thus enable reliable PQ measurement up to 9 kHz and, like all other medium-voltage converters from MBS AG, are also SF6-free.

There are already tendencies that in the future a PQ measurement of up to 150 kHz could also be carried out in medium voltage. Applicable standards for the low-voltage network already define limit levels of up to 150 kHz. The current mobile PQ analyzers also measure up to at least 150 kHz, which can be absolutely necessary for an extensive fault analysis. This frequency range is normatively named in the current DIN EN 61000-2-2: 2020-05 Electromagnetic Compatibility (EMC) - Part 2-2. While this standard only refers to public low-voltage networks, the EN 50160 standards committee is also discussing measured values ​​up to 150 kHz. The current draft already contains references to standard values ​​up to 150 kHz. From the current point of view, it could therefore make sense to at least plan for PQ measurements up to 150 kHz in order to counteract expensive retrofit measures. The PQ officers of the EVUs would certainly be grateful.

The range up to 150 kHz is technically not feasible with inductive voltage converters. With 24 kV devices, the first resonance point can only be shifted into the range of 10 to 20 kHz. Voltage sensors based on the principle of the voltage divider offer an alternative. As a reminder, the basic principle is presented here again.

Already today, voltage sensors are mainly installed in existing local network stations that require additional voltage measurement on the medium-voltage side. Measuring fields with conventional inductive voltage converters can only be retrofitted in rare cases for reasons of space. A tried and tested method is to mount sensors in so-called T-connectors. This solution is space-saving and the assembly is carried out by trained personnel in a reasonable time window. While the cone of the symmetrical plug is standardized according to IEC 50181, the cone of the compact T-plug has slightly different dimensions depending on the manufacturer. The VAPxx-S voltage sensor intended for the compact T-connector can, thanks to its patented design, be used in the slightly different T-connectors from all well-known manufacturers without having to fear partial discharges. For the new compact T-connector from Nexans (480 TB) there is already a suitable sensor with the VCPxx-S.

MBS AG also has a sensor for air-insulated switchgear and measuring panels that is already used in new systems and retrofits.

While the accuracy class at 50 Hz is noted on each rating plate of the sensors and is therefore the responsibility of the manufacturer, the manufacturer often does not provide any protocols or reliable statements for measurements beyond 50 Hz. In the market, end users often hear the prejudice that ohmic dividers can generally transmit harmonics very well. This will be examined in the following.

An ohmic divider basically consists of two almost ohmic resistors, but these resistors always have parasitic inductive and capacitive components. A capacitance also forms around the high-voltage resistor, so that the technical literature does not speak of ohmic dividers, but of ohmic-capacitive dividers.

As can be seen in the block diagrams, the primary capacitance must also be balanced on the secondary side. In order to be able to manufacture high-precision sensors, a calibration network is also required in the sensor, through which fine calibration can be carried out after potting. However, this consists exclusively of passive electronic components.

The question now arises as to whether ohmic-capacitive sensors in medium voltage can easily be used for PQ measurements. In the following figure, a competitor and an MBS own 50 Hz sensor from 50 to 150 kHz have been measured with regard to amplitude and phase errors.

Both sensors violate the minimum requirements of class 1 for PQ measurements according to IEC 61869-6. For an optimal transmission behavior, the adjustment network must also be optimized for higher frequencies. MBS AG can currently provide voltage sensors for T-plugs and for air-insulated systems with an optimized transmission behavior of up to 150kHz.

Frequency-optimized sensors are absolutely necessary for PQ measurements in order to be able to comply with the minimum requirement (class 1 according to IEC 61869-6). Transfer curves from current customer projects are shown in the following figures.

The voltage sensors are usually installed with corresponding current sensors. Unlike conventional current transformers, a voltage signal is output on the secondary side.

MBS AG offers special current sensors, the dimensions of which are perfect for mounting on the usual T-plugs

When selecting the measuring device, it should be noted that the voltage sensors mentioned here can provide a maximum of 10 volts on the secondary side. The 3.25 / √3 V standard has already established itself in Germany. The current sensors usually use 225 or 333 mV. In contrast to the traditional inductive devices (100 / √3 V or 1/5 A outputs), so-called small signal inputs are absolutely necessary on the measuring device. While the input impedance of the current sensor inputs only needs to have a value greater than 20 kOhm, the voltage sensors are precisely matched to the input impedance of the measuring device and the length of the connecting cable. A subsequent exchange of the measuring device or the parallel connection of a second measuring device is often not possible because the voltage sensor is only adjusted to a specific burden.

While local network stations with measurements on the medium-voltage side are already being upgraded on a large scale in other countries, Germany is still lagging a little behind this development. Since there is no uniform configuration of the local network stations in Germany, only voltage or current sensors were retrofitted during the first retrofit measures. The measuring devices should be designed in accordance with the CAT IV insulation regulations. A universal measuring device that is currently being used more and more is the UMD 710MVU from PQ Plus.

In addition to the required sensor measurement inputs for voltage sensors, the device also has an optional FFT analysis up to 9 kHz. The voltage sensors see an input impedance of 200 kOhm on the measuring device. This value must be communicated to the sensor manufacturer in addition to the required cable length. The sensors are precisely adjusted to this configuration in the factory. A flexible load range as with conventional voltage converters is not possible. This would require active electronics on the sensor side, which can significantly shorten the service life of sensors. MBS AG has therefore decided to offer only passive sensors that are designed for a service life of approx. 30 years.

While the 24 kV range is well covered by the three T-connector sensors, a 36 kV variant is still in development.

Here, too, a frequency-optimized version is planned in order to be able to carry out PQ measurements up to 150 kHz. In addition to good transmission up to 9 or 150 kHz, the following points should be taken into account with voltage sensors:

🌐 Resistance to temperature and humidity fluctuations over the entire service life

🌐 The sensors should have the largest possible working and storage temperature range

🌐 The hexagon nut should be made of metal, since the pure cast resin version can cause wear or damage at higher tightening torques

🌐 The overall height should be as small as possible so that the switchgear door can still be closed properly.

🌐 The manufacturer should be able to guarantee the resistance of the sensors to fluctuations in temperature and humidity over the entire service life

🌐 It would also be helpful if the voltage sensors remained undamaged in the T-connectors during the VLF cable test.

🌐 The sensors should be purely passive in order to keep life cycle costs low.

Authors: Roland Bürger(MBS AG), Jens Schübel (PQ Plus)

HEARTgreetings

Your SCHUTZTECHNIK team

[1] https://www.juracademy.de/rechtsprechung/article/produkthaftung-fuer- Elektrizitaet-des-netzbetreibers-bei-ueberspannungsschaeden

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