A Solution for Accurate High-Voltage Power Measurement

This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.

With the ongoing electrification of society, there is a rapidly growing need for more electrical power. One way to meet this need is to increase system voltage. Obvious applications are ultra-high-speed chargers for EVs, all kinds of power converters, and solid-state transformers to increase flexibility and reduce losses of the power grid. Power measurement instruments like the HIOKI PW8001 Power Analyzer can measure power up to 1500 Vdc. The VT1005 high-voltage divider is the solution for accurate voltage and power measurements from 1500V up to 5kVrms.

 

Image used courtesy of Bodo’s Power Systems [PDF]

 

High Noise Resistivity

The VT1005 is highly resistant to both common-mode and high-frequency noise, allowing it to measure voltage accurately even in noisy environments. Since conversion devices like inverters are sources of noise, noise resistivity is very important in power efficiency evaluation. The design of the VT1005 with the floating differential input eliminates common mode noise because the same noise signal will be picked up in both input connection cables and canceled out in the differential amplifier, as it only amplifies the differences at the input.

 

Figure 1. Common-mode noise is canceled due to the differential input. Image used courtesy of Bodo’s Power Systems [PDF]

 

SiC and even more GaN power devices are characterized by fast voltage rising and falling response, and their output waveforms contain numerous high-frequency components. When we compare the output voltage of the VT1005 with another voltage divider on the market, we find that the latter shows a ringing effect, whereas the VT1005 does not. (see fig. 2) Such a ringing effect happens because the HV input pulses cause parasitic capacitances and inductances in the circuit to resonate at their characteristic frequency. Parasitic capacitances and inductances are not part of the design but just by-products of the components used to construct the circuit and the circuit layout. This ringing effect has a negative impact on the accuracy of the voltage and power measurement and must be avoided.

 

Figure 2. Output voltage waveform from an inverter using SiC semiconductors at 50 kHz carrier frequency. Image used courtesy of Bodo’s Power Systems [PDF]

 

Frequency Flatness is essential

To further increase the efficiency of power systems, SiC and GaN semiconductors are becoming popular switching devices. The switching frequency of SiC semiconductors is 50kHz or higher, and GaN can even go up to 1MHz. This makes it hard to accurately measure power efficiency because accuracy must be guaranteed over a broad frequency spectrum from DC up to 1 MHz or even more. To realize such accurate power measurement, HIOKI power measurement systems have excellent frequency flatness, i.e. measurement stability, which is essential to realize constant measurement accuracy over a wide frequency range,

For example, to measure an efficiency improvement of 0.01%, it is necessary to have a gain accuracy of 0.1%, and the phase error must be within 0.1o.

 

Figure 3. Gain and phase characteristics of the VT1005 HVD (solid lines) in comparison to a competitor (dotted lines). Image used courtesy of Bodo’s Power Systems [PDF]

 

The above two graphs show the gain and phase error for three individual VT1005s and another three-phase high voltage divider available on the market. What is striking is that you can hardly differentiate the three solid lines of the VT1005 as they are almost identical and therefore overlapping, whereas the three dotted lines of the other voltage divider are much more divergent, indicating that the accuracy is lower. Furthermore, the gain and phase error at higher frequencies are greater with this voltage divider, which makes it unsuitable for applications in which SiC or GaN semiconductors are used.

Because both the VT1005 high-voltage divider and the current sensors are designed and produced by HIOKI, they are optimized for use in combination with the HIOKI power analyzers, for performance both in gain and phase accuracy over the entire frequency range when the phase shift correction function of the power analyzer is activated.

 

Figure 4. Gain and Phase characteristics of the VT1005, before (red) and after (yellow) phase correction with a Hioki Power Analyzer. Image used courtesy of Bodo’s Power Systems [PDF]

 

Wireless Power Transfer System (WPT)

WPT efficiency measurement is an application that requires accurate high-voltage measurement. The voltage output tends to be higher, even up to 3 kV, because transmitting power at a higher voltage can reduce power losses in transmission. Therefore, the power efficiency measurement requires a high voltage measurement. Since WPT transfers power through coils, the transmit/receive part has a very low power factor. When the power factor is low, the phase error greatly affects the measured value, so power measurement with a low phase error is essential.

 

Figure 5. WPT coils are used for power transmission resulting in low power factor. Image used courtesy of Bodo’s Power Systems [PDF]

 

The VT1005’s measurement band ranging from DC to 4 MHz allows for the measurement of voltage from DC to high frequencies. In addition, the flatness of the amplitude and phase characteristics in the measurement band enable highly accurate power measurement and evaluation of power efficiency.

 

Loss Measurement of HF Reactors and Transformers

The noise resistivity, gain, and phase accuracy at higher frequencies make the VT1005 High Voltage Divider, in combination with the PW8001 power analyzer and CT6904A current sensor the optimum solution to measure loss of high voltage, high-frequency reactors, and transformers used in inverter drives and power converters like for instance onboard chargers for EVs. With this setup accurate loss measurements can be guaranteed even at a frequency of 300 kHz.

 

Figure 6. High frequency and voltage transformer in a SiC-based power converter. Image used courtesy of Bodo’s Power Systems [PDF]

 

In light of the increased use of SiC and GaN semiconductors, high-frequency and high-voltage applications are on the rise. These require highly accurate measurement solutions like the VT1005 with HIOKI power analyzers to prove even small efficiency gains.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.