High-Precision Power Measurement for EV Fast Charging Equipment

Introduction

The rapid electrification of vehicles (EVs) is crucial for advancing Sustainable Development Goals (SDGs) and environmental preservation. The proliferation of fast chargers, or off-board chargers, connected to the grid allows for quick EV battery charging with high current output. As EV batteries increase in capacity, there is a growing demand for fast chargers with high voltage and high current that can quickly replenish these batteries, leading to a rise in the number of installed charging stations.
* Off Board Charger: A charging station, also called EVSE (Electric Vehicle Service Equipment).

Challenges

Advanced fast chargers operating between 500 to 1000 Volts (DC) and capable of handling hundreds of Amps (DC) are currently under development, necessitating precise measurement of high voltages and currents. Minimizing conversion losses within chargers and suppressing harmonic components from the grid’s high current are critical. This involves checking the distortion factor for each current harmonic order and total harmonic distortion (THD).
Additionally, the metering function of each charging station must be thoroughly validated for both repeatability and accuracy in measuring charge capacity (in Wh and Ah), which directly impacts billing.

The Solution: WT5000

• High-precision power measurement with a basic accuracy of ±0.03%
• Measurement up to 1500 Vdc and up to 2000 Arms
• Power parameter calculations including Power factor
• Measurement of AC/DC and DC/AC conversion efficiencies
• Measurement of integrated power and integrated current
• Harmonic data of each order and THD
• Abnormality inspection with measurement trending and waveform streaming

Figure 1. WT5000 Precision Power Analyzer

Relevant WT5000 Features

High-precision power measurement with a basic power accuracy of ±0.03%

The WT5000 offers a best-in-class basic power accuracy of ±0.03% (50/60 Hz). It measures the conversion efficiency of EV chargers with the highest accuracy. Its modular architecture supports up to 7 power input elements. Yokogawa’s extensive design technology integrates an ultra-precise measurement circuit into a compact input element. Users can choose from three types of swappable input elements (30 A, 5 A, and current sensor element) to suit their needs.

Measurement up to 1500 Vdc and up to 2000 Arms

As the need for faster battery charging increases, the output power of EV fast chargers continues to rise, with several hundred kW class chargers becoming more common. These chargers operating at increasingly higher voltages and currents.
The WT5000 is equipped to handle these demands, capable of measuring voltages up to 1500 Vdc and currents up to 2000 Arms (3000 Apeak) with an AC/DC current sensor. With up to 7 power input elements, simultaneous measurement across 7 single-phase systems or configurations of singlephase/ three-phase power systems are possible with a single WT5000. This enables accurate measurements 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

In terms of sensors, Yokogawa offers precision high current split-core type sensors. A split core mechanism facilitates easy installation and removal and is ideal for retrofit applications and temporary setups. This feature allows for measurements not only during development and validation stages but also in the field when precision high current measurements are required.

Figure 4. Split core current sensor CT1000S

Power parameter calculations including Power factor

EV fast chargers convert 50 Hz/60 Hz AC power from the grid to DC and deliver it to the EV battery. Since the real power delivered depends on the AC voltage, AC current, and phase difference, it is crucial to minimize the phase difference and suppress harmonics in the charger design. Chargers typically contain a Power Factor Correction (PFC) circuit with the aim to achieve a power factor close to 1, which maximizes active power and enhances power supply efficiency.
The WT5000 can verify the effectiveness of a PFC circuit and precisely measure and calculate the associated power factors. In addition, other parameters such as voltage, current, active power, apparent power, and reactive power can be measured and displayed simultaneously. This allows users to monitor changes in each parameter in real time.

Figure 5. Display of voltage, current, power factor, THD, etc.

Measurement of AC/DC and DC/AC conversion efficiencies

For off-board EV fast chargers, DC power is supplied to a battery through several conversion circuits, including AC/DC and DC/AC conversion. It is extremely important to design these circuits to minimize power losses. The WT5000, with its multi-channel input capability, can accurately measure the efficiency of each circuit.

Figure 6. Typical Charger Conversion Circuits

Measurement of integrated power and integrated current

The WT5000 has an integration function to measure power consumption (Wh) and current consumption (Ah) over a long period. This function integrates values for active power (watthour), current (amp-hour), apparent power (volt-amp-hour), and reactive power (var-hour). It includes two modes: one for measuring charging and discharging, such as in batteries, and the other for measuring AC power sold or bought.

Figure 7. Integrated power and current measurement screen

[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

Abnormality inspection with measurement trending and waveform streaming

When voltage, current, and power data need to be measured continuously for a long period, the IS8000 Integrated Software Platform, a PC application, can be used to monitor and save trends of power parameters in real-time.
Additionally, the WT5000’s /DS (data streaming) option allows numeric power values and waveform data to be simultaneously monitored and recorded. For instance, by zooming in on the area where there is an abnormality in power measurements, the waveform data at that point can be observed.

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

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

Additionally, with a split core sensor, the test point can be easily changed, making it an effective means to isolate the problem area.