Low inductive power resistors for public supply network impedance simulation

Author: Martin Kufner, Head of Development, Spitzenberger-Spies

**Industry/Application Area:**** **Test and measurement system, EMC immunity and emission tests

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**Product Used: **Powertron metal foil power resistors FHR 2-80xxx

**The ****Challenge**

For flicker measurements (= measurements of voltage fluctuations) an impedance is necessary according to the standard IEC 61000-3-3 and IEC 61000-3-11. Figure 1 shows the typical test setup for flicker measurements.

The equipment under test (EUT) is connected to a voltage source (G) with an impedance

Z = (R_{A}+R_{N}) + j(X_{A}+X_{N}). Measuring equipment (M) measures the voltage fluctuation caused by the current of the EUT. For currents up to 16A the reference impedance Z_{ref} = 0.40W + j0.25W is defined which represents the typical supply network impedance according to IEC/TR 60725. For currents more than 16A a test impedance Z_{test} lower than Z_{ref} is needed to adjust the steady-state voltage drop caused by the equipment within the range of 2% to 9% of the supply voltage. The ratio of inductive to resistive components of Z_{test }(= X_{test}/R_{test}) has to be within the range of 0.5 to 0.75.

*Figure 1: Schematic diagram for flicker measurements with resistors R _{A} and inductors X_{A} in line conductor and R_{N} and inductor X_{N} in neutral conductor *

Therefore, an adjustable arrangement of low-ohmic resistors and inductors for currents up to 1,500A is necessary. Despite the low resistor values, the maximum continuous power loss per individual resistor value can reach 30kW due to the high currents.

To reach the required measurement accuracy for flicker values the tolerance of the resistor and inductor values has to be lower than 4%. This means, when switching on and off the individual resistor values the inductance mustn`t change more than 4% even with the lowest individual inductor value.

Furthermore, the resistor values should be as stable as possible over the temperature range of about 60°C.

Despite a low thermal resistance, the dielectric strength of the resistors should be at least 2.1kV_{DC}.

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**The Solution**

The necessary impedance for flicker measurements is a part of the analyser impedance system (AIS) of Spitzenberger & Spies. It includes a harmonic analyser and a flickermeter as well as the adjustable impedance network for public supply network impedance simulation. The analyser impedance system is available with different nominal currents (16A, 32A, 75A, 125A, 250A, 375A, 500A or 750A). Figure 2 shows the arrangement of resistors and inductors in the analyser impedance system with 125A nominal current.

*Figure 2: Arrangement of resistors and inductors in analyser impedance system with 125A (AIS 125/3/P)*

With the implemented binary stepped resistor and inductor bank the impedance can be adjusted from 0.0125W + j0.0078W up to 0.40W + j0.25W in steps of 0.0125W + j0.0078W. The individual resistor values are bypassed by low-ohmic pneumatic switches.

Market research showed, that the metal foil power resistors FHR 2-80xxx from Powertron are the most suitable resistors which fulfil the requirements especially due to the possibility of customized optimization.

The FHR 2-80xxx resistors come with a high power rating (up to 2,500W per individual resistor) and the needed low-ohmic values (resistors in the 10mW and 100mW range) are available. Due to the high individual power rating, only a few resistors have to be connected in parallel which significantly reduces the wiring effort. At the same time the high precision of the resistors (2% tolerance) guarantees a uniform power distribution.

Another advantage of the metal foil power resistors FHR 2-80xxx is the lower temperature coefficient (±25ppm/K) compared to wirewound power resistors (in the range of ±100ppm/K) which leads to better stability and accuracy from ambient temperature to shutdown temperature of 80°C.

In contrast to wirewound resistors, the FHR 2-80xxx resistors also have a very low inductance (<50nH).This was one of the most important reasons to use these resistors, because the inductive part of the total impedance shouldn`t change when switching the individual resistors on and off.

**The User Explains**

As described above the impedance is available for different nominal currents. Depending on the nominal current the increment of the resistor values varies. The higher the nominal current the smaller the steps. The resistor and inductor in figure 2 - which can`t be short-circuited - symbolizes the basic resistance and inductance that arise from the construction and wiring of the impedance network itself.

In Figure 3 you can see the metal foil power resistors FHR 2-80xxx mounted on a heat sink for air cooling for 125A.

*Figure 3: FHR 2-80xxx resistors mounted on heat sink*

With higher nominal currents water cooling instead of air cooling is used due to the higher power losses. The resistors are protected from overcurrent by a circuit breaker and temperature measurement is done on the heat sink. If the shutdown temperature is reached the impedance is bridged. If the impedance network is used combined with a voltage source of Spitzenberger & Spies the very fast current limitation functionality of the voltage source can provide additional protective function against short-term overload of the resistors.

To reach the required accuracy of the impedance network each resistor and inductor has to be adjusted after wiring and assembly of the impedance network. Figure 4 shows the assembled impedance network for one phase.

*Figure 4: Resistor and inductors for one phase*

As it is easier to adjust the resistor value with a parallel balancing resistor than with a balancing resistor in series the needed tolerance range for the FHR 2-80xxx resistors is 0% to +2%. If the standard tolerance range of ±1% would be used, the resistor value could be lower than the nominal value and an adjustment with a parallel balancing resistor wouldn’t work. This modified tolerance range is a custom option.

Another customized optimization is the higher insulation strength. According to the datasheet the dielectric strength is 1.5kV_{DC}. Because of the use for nominal voltages up to 230V_{AC} the dielectric strength has to be 2.1kV_{DC}. Powertron modified the FHR 2-80xxx resistors to reach the higher requirements with hardly any effect on the thermal resistance.

Due to these modifications, the Powertron metal foil power resistors FHR 2-80xxx are ideal for this application.

**“ The low inductivity and temperature coefficient of **

metal foil power resistors FHR 2-80xxx

helps us to fulfil the requirements for

supply network impedance simulation.”

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**Acknowledgement:**

Spitzenberger & Spies GmbH & Co. KG is a manufacturer of linear power amplifiers which are used in different test systems (EMC, automotive, avionic, photovoltaic etc.).

Spitzenberger & Spies is a privately held German company, specialized in developing, manufacturing and selling test and measurement systems in the low frequency range with linear power amplifiers as the main component. The portfolio covers solar simulation up to 1,500V, automotive and avionics test systems up to 300kHz, AC/DC 4-Quadrant voltage and current sources up to 400kVA, EMC test equipment with rise times less than 5µs and many customized solutions as well as fully automatic and software-controlled test equipment. Spitzenberger & Spies was founded in 1963 and has currently about 70 employees, among them engineers, technicians, selling representatives and more.

Contact Information

- Martin Kufner
- Head of Development
- Spitzenberger-Spies
- Viechtach,Germany
- Email: info@spitzenberger.de
- Phone: 49 9942 956-0

Customer Statement

"The low inductivity and temperature coefficient of metal foil power resistors FHR 2-80xxx helps us to fulfil the requirements for supply network impedance simulation."

- Martin Kufner

Spitzenberger-Spies

Case Study

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