High-Precision Power Measurement for High-Voltage On-Board EV Chargers

Introduction

The shift towards electric vehicles (EVs) is accelerating due to environmental goals and sustainability. To enable faster charging, EV batteries and chargers are moving to higher voltage levels.

There are two main types of EV chargers: normal and fast.
Normal chargers use a single-phase domestic AC supply, converting AC to DC with an on-board charger inside the EV.
This process typically requires over 10 hours for a full charge.
As battery capacities grow, normal charging times increase.

In contrast, fast chargers at charging stations can deliver high-current DC, significantly reducing charge time. The rising demand for quicker on-board charging is also influenced by the need for bidirectional AC power, summarized as V2X, for applications like emergency power and outdoor use.

Challenges

Normal chargers operate on a 100 – 240 VAC single-phase supply with relatively low maximum current. To reduce charge times, it is necessary to increase either the current or voltage of the charger and battery. This means that modern chargers must be able to boost voltage and be compatible with 800 V battery systems. Fast chargers incorporate power factor correction (PFC) circuits to maximize power efficiency.
However, they must also comply with IEC standards to prevent harmonic currents and voltage fluctuations.

As the demand for EV chargers grows, high-precision power and current measurements are essential for charger development.

Solutions by WT5000

• High-Precision Power Measurement with ±0.03% Accuracy
• Measurement Range up to 1500 Vdc and 2000 Arms
• Power Factor and Parameter Measurement
• AC/DC and DC/AC Efficiency Measurement
• Harmonic Data and THD Analysis
• IEC-Compliant Harmonic and Flicker Measurements
• Abnormality Detection through Continuous Monitoring
• Additional Instrumentation for Charger Development

WT5000 Proposal

High-Precision Power Measurement with ±0.03% Accuracy

The WT5000 provides class-leading accuracy of ±0.03% (at 50/60 Hz), enabling precise EV charger efficiency measurements.
With a modular design, the WT5000 accommodates up to seven power input elements, offering flexibility with 30 A, 5 A, and current sensor options.

Figure 1. WT5000 Precision Power Analyzer

Measurement Range up to 1500 Vdc and 2000 Arms

EV fast chargers increasingly operate at high voltage and high current levels. The WT5000 supports measurements up to 1500 Vdc and 2000 Arms (3000 Apeak) with AC/DC current sensors. Its seven power input elements allow simultaneous measurement across single- or three-phase systems, ensuring precise monitoring of DC voltage, AC voltage, power factor, THD, and input/output efficiency.

Figure 2. AC/DC current sensor CT1000A (left) and CT2000A (right)

Figure 3. WT5000 and CT1000A AC/DC current sensors

Power Factor and Parameter Measurement

Typical EV chargers convert 50 Hz/60 Hz AC power from the grid into DC for battery charging. Since the power value of AC depends on voltage, current, and phase difference, it is crucial to minimize the phase difference and suppress harmonics. Achieving a power factor close to 1 is essential for enhancing overall power supply efficiency. The WT5000 measures power through a PFC circuit while simultaneously displaying key parameters, including voltage, current, active power, apparent power, and reactive power. This allows for real-time monitoring of changes across all metrics.

Figure 4. Display of voltage, current, power factor, THD, etc.

AC/DC and DC/AC Efficiency Measurement

EV chargers can contain multiple conversion circuits. Since on-board chargers are typically connected to a commercial power source, suppressing harmonics is important; thus, they are equipped with PFC circuits. Additionally, on-board chargers include an internal DC/DC converter to regulate DC voltage levels.

Off-board fast chargers supply DC power to batteries through various conversion stages, including AC/DC and DC/AC conversions. Designing these circuits to minimize power losses is vital. The WT5000, with its multi-channel input capability, accurately measures the efficiency of each conversion circuit.

Figure 5. Overview of on-board charger

Harmonic Data and THD Analysis

The WT5000 can measure harmonics up to the 500th order, with the capability to assess Total Harmonic Distortion (THD) as well. This allows users to easily determine whether THD falls within the limits set by various standards and regulations.
Additionally, the upper limit for the distortion factor can be freely adjusted (e.g., 13th, 40th, or 100th order), enabling compliance testing according to multiple standards.

Figure 6. Measurement of harmonics of each order and THD

[Detail of Integration function]

Charge/Discharge mode (Charge/Discharge)
Measure DC watt hours (for each sampled data item) by polarity.
Sold/Bought mode (Sold/Bought)
Measure AC watt hours (for each update interval data item) by polarity.

Measurement items related to the integration function

ITime Integration time
WP	Sum of positive and negative watt hours
WP+	Sum of positive P values (consumed watt-hours)
WP-	Sum of negative P values (watt-hours returned to the power supply)
q	Sum of positive and negative ampere hours
q+	Sum of positive I values (ampere-hours)
q-	Sum of negative I values (ampere-hours)
WS	VA hours
WQ	Var hours

IEC-Compliant Harmonic and Flicker Measurements

The WT5000 integrates seamlessly with the IS8010 measurement software platform, enabling harmonic tests in compliance with IEC61000-3-2, as well as voltage fluctuation and flicker tests according to IEC61000-3-3. Additionally, by utilizing the specialized AC/DC current sensor model CT200, the WT5000 can conduct harmonics, voltage fluctuation, and flicker tests for currents exceeding 16 A per phase, in accordance with IEC61000-3-11 and IEC61000-3-12. Since standard EV chargers typically connect to household outlets, adhering to these regulations is important to ensure safety and reliability.

Figure 7. Harmonic/flicker test system that meets IEC regulations

Abnormality Detection through Continuous Monitoring

When continuous measurement of voltage, current, and power data is required over an extended period, the IS8000 Integrated Software Platform, a PC application, allows for real-time monitoring and saving of power parameter trends.
Additionally, the WT5000’s /DS (data streaming) option enables simultaneous monitoring and recording of both numerical power data and waveform data. This feature enables users to zoom in on areas with abnormalities in power measurements and analyze the corresponding waveform data, positioning the WT5000 as a comprehensive solution.

Figure 8. Voltage, current, and power trends displayed with IS8000

Figure 9. Zooming into areas of reduced power to inspect waveform
abnormalities

Additional Instrumentation for Charger Development

DL950 ScopeCorder:

Captures long-term data trends and high-speed measurements during transient events. Integrated with the IS8000, it synchronizes waveform data from the WT5000 via IEEE1588 for detailed analysis during power fluctuations.

Figure 10. Measurement with the DL950’s dual capture

DLM3000/DLM5000 Oscilloscopes:

Ideal for observing PFC circuit waveforms, with multi-channel capabilities to monitor PFC and bidirectional circuits.