High-precision power measurement of EV charging equipment

1. Introduction

The electrification of vehicles (EV) is accelerating for promoting SDGs implementation and preserving the environment, and the number of fast chargers installed for the EV infrastructure is rapidly growing. There are 2 main types of EV chargers: normal charging and fast charging.

Normal charging uses single-phase 100 to 240 Vac domestic supply for battery charging. AC is converted to DC by an AC/DC converter (on-board charger) inside EVs to charge the EV batteries. Generally, normal charging takes a longer charging time, requiring several hours or more to fully charge a battery. On the other hand, fast charging via charging stations (off-board chargers) connected to the grid can charge an EV battery in a short period of time with high current output.

As EV batteries are becoming larger in capacity, charging them with normal charging requires even longer time. For that reason, there is a growing need for installation of fast chargers that can fill batteries in a short time and the number of charging stations installed is rapidly increasing. In addition, high-voltage and high-current products are being developed to shorten the charging time.

2. Challenges

Normal chargers are of single-phase 100 - 240 VAC type with a relatively small maximum current and power. On the other hand, fast chargers of 500  -1000 Vdc and 100 - several hundred Adc class have been developed recently, and there is a demand for accurate measurement of high voltages and high currents to evaluate their capabilities.

Charge capacity (Wh, Ah) is also required to be measured accurately. This may be because the electricity supplied to EV batteries is related to electricity bills.
To supply power efficiently, fast chargers are equipped with a power factor correction (PFC) circuit. Power can be supplied to EVs more efficiently with a better power factor. Since a charger has a conversion circuit inside, it is also important to minimize losses associated with the conversion. Furthermore, as a high current from the grid is drawn into a fast charger, the current harmonic components must be suppressed. This requires to check distortion factor for each harmonic current order and total harmonic distortion (THD).
The demand for EV chargers is increasing along with their widespread use and high-precision power and current measurements are essential in the EV charger development.

3. Solutions by WT5000

4. Proposal by WT5000

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

The WT5000 delivers best-in-class basic power accuracy of ±0.03% (50/60 Hz). It can measure the conversion efficiency of EV chargers with higher accuracy.
The WT5000 employs a modular architecture and can be fitted with up to 7 power input elements. YOKOGAWA’s design technology accumulated over the years has enabled an extremely high-precision measurement circuit to be housed inside a compact input element. Users can choose from three types of input elements (30 A, 5 A, and current sensor element) to suit their applications and can install, remove or swap the input elements themselves.

Figure 1. WT5000 Precision Power Analyzer

Figure 2. WT5000 and CT1000A AC/DC current sensor

4.2 Measurement up to 1500 Vdc and up to 2000  Arms

The output power of EV fast chargers is increasing every year to charge batteries in a shorter period of time. Today, there are fast chargers of several hundred kW class and chargers with higher voltage and higher current are increasing in number.
A single WT5000 can measure voltages up to 1500 Vdc and measure currents up to 2000 Arms (3000  Apeak) with an AC/DC  current sensor. Additionally, installing up to 7 power input elements on the WT5000 allows simultaneous measurement of 7 single-phase systems or several single-phase/three-phase power systems to accurately measure DC voltage, AC voltage, power factor, THD, and Input/Output efficiency.

Figure 3. AC/DC current sensor CT1000A (left) and CT2000A (right)

4.3 Measurement of power factor and other power parameters

EV fast chargers convert 50 Hz/60 Hz AC power from the power grid to DC and deliver it to the battery on EVs. Since the power value of AC power varies depending on the voltage, current, and phase difference, it is important to minimize the phase difference and suppress harmonics to bring the power factor close to 1 in order to increase the power supply.
The WT5000 measures the power that passed through a PFC circuit and calculates the power factor. Other parameters than the power factor, such as voltage, current, active power, apparent power, and reactive power, can be measured and displayed simultaneously, allowing users to check changes in each parameter at the same time.

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

4.4 Measurement of AC/DC and DC/AC conversion efficiencies

Normal chargers for EVs have several conversion circuits inside. As on-board chargers are usually connected to a commercial power source, it is important to suppress harmonics. So, they are equipped with PFC circuits. On-board chargers also contain an internal DC/DC converter to control DC voltage levels.

In the case of off-board fast chargers, DC power is supplied to a battery via several conversion circuits including AC/DC conversion and DC/AC conversion. It is extremely important to design these circuits so as to minimize power losses. The WT5000 with multi-channel input capability can measure the efficiency of each circuit with high accuracy.

Figure 5. Overview of on-board charger

Figure 6. Overview of off-board charger

4.5　Measurement of harmonic data of each order and THD

The WT5000 can measure harmonics of each order up to the 500th order. THD can also be measured, allowing users to easily see whether or not it is below the limits specified by standards or other regulations. The upper limit of the order for distortion factor (13th order, 40th order, 100th order, etc.) can be set freely, which enables tests according to many standards.

Figure 7. Measurement of harmonics of each order and THD

4.6 Measurement of integrated power and integrated current

The W5000 has an integration function to measure power consumption (Wh) and current consumption (Ah) over a long period of time. The integration function integrates the active power (watt hour), current (ampere hour), apparent power (voltampere hour), and reactive power (var hour) values. This function includes 2 modes: one to measure charging and discharging of, for example batteries, and the other to measure AC power sold/bought.

Figure 8. 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 sampled 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	Volt-ampere hours
WQ	Var hours

4.7 Checking abnormalities by continuously measuring power values and waveform data

When voltage, current, and power data need to be measured continuously for a long period of time, the IS8000 Integrated Software Platform, which is PC application software, can be used to check and save trends of power parameters in real time. Furthermore, the WT5000 ’s /DS (data streaming) option allows numerical power data and waveform data to be simultaneously monitored and recorded. For example, by zooming in on the area where there is an abnormality in power measurements, the waveform data at that point can be observed. A single WT5000 is all that is needed to check the situation.

Figure 9. Voltage, current, and power trends displayed on IS8000

Figure 10. Zooming in on the area of power decrease to check abnormal waveforms

4.8 Other measuring instruments useful in development of chargers

DL950: Long-time data recording and high-speed measurement at abnormal signal occurrences
 The DL950 ScopeCorder, which is a multi-channel isolated waveform measuring instrument, has the dual capture function that can capture waveforms at different sampling rates. Waveforms of sudden transient events can be captured at a high sampling rate while acquiring data at a low sampling rate to capture long-term trends. Additionally, the IS8000 enables acquired waveform data and numerical power data from the WT5000 to be synchronized and displayed via IEEE1588, allowing more detailed waveform observation during power fluctuations.

Figure 11. Measurement with the DL950’s dual capture

DLM3000/DLM5000: Observation of PFC circuit waveforms
The DLM3000 Mixed Signal Oscilloscope or DLM5000 with 8 channels, which has excellent waveform observation performance and operability, can be utilized to check the operation of a PFC circuit.