On-Board EV Charger for Disaster Use and Power Demand Adjustment

Introduction

The spread of electric vehicles (EVs) is being promoted in anticipation of their use as emergency power sources during disasters and their ability to help balance power supply and demand as the share of renewable energy increases. In particular, the shift towards higher voltage for batteries and chargers for EV charging to achieve fast charging is making progress.
There are two main types of EV charging: normal charging and fast charging. Normal charging uses a single-phase AC power source, such as that for home use; 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 long time—more than 10 hours in some cases to fully charge a battery. And thanks to the rise in batteries capacities, charging time with this method continues to increase.
On the other hand, fast charging can support large-current output by installing a charging station (off-board charger) connected to the grid, enabling battery charging in a short period of time. Therefore, the on-board chargers are also expected to reduce charging time. Furthermore, in recent years, as represented by V2H (Vehicle-to-Home), there has been increasing expectation of using EVs as emergency power sources during disasters and for power demandresponse functions based on renewable energy. As a result, the development of bidirectional inverters is advancing.
* On-Board Charger (OBC): A system mounted inside an xEV to charge the battery from an AC power source.
* Off-Board Charger: A charging station, also called EVSE (Electric Vehicle Service Equipment).

Challenges

The input for normal chargers is single-phase 100–240 VAC, with a relatively low maximum current and power. Reducing battery charging time requires either increasing the current value or raising the voltage value of the charger along with the battery. Therefore, the charger voltage needs to be boosted internally to support 800V battery systems. In addition, an efficient power supply requires enhancing the power factor. For this reason, chargers are equipped with a power factor correction (PFC) circuit. On the other hand, since the input side is connected to a low-voltage system, the outflow of harmonic currents and voltage fluctuations must be prevented, and this dictates compliance with IEC standards. Additionally, to enable the battery installed in the EV to serve as a source of AC power, the inverter must be designed with a circuit that makes it bidirectional.The demand for these EV chargers is increasing with the spread of EVs, making accurate power and current measurements essential at development sites.

Solutions provided by WT5000

• High-precision power measurement with a basic power accuracy of ±0.03%
• Measurement of high voltage and large current up to 1500 Vdc and up to 2000 Arms using current sensors
• High-precision measurement of power factor and other power parameters
• High-precision measurement of AC/DC and DC/AC conversion efficiencies
• Measurement of integrated power and integrated current
• IEC-compliant harmonics/flicker measurement
• Abnormality check by continuous measurement of power values and waveform data
• Development support for charging equipment using other measuring instruments

Proposals provided by WT5000

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 for the power input elements and can accommodate up to seven inputs. YOKOGAWA’s design technology accumulated over the years has enabled us to house an extremely high-precision measurement circuit inside a compact input element. In addition, you can choose from three types of input elements (rated input 30 A, rated input 5 A, and an element dedicated to current sensor input) to suit your applications and can install, remove, or swap the input elements yourself.

Figure 1. WT5000 Precision Power Analyzer

Measuring high voltages and large currents up to 1500 Vdc and up to 2000 Arms

For normal chargers used in EVs, the input ranges from 100 VAC to 240 VAC, resulting in an output power of approximately 2 kW. However, requirements for their output power are rising every year as the demand for fast charging increases—sometimes as high as 11 kW or 22 kW. As a result, there is a trend towards higher voltage in chargers along with batteries. A single WT5000 can measure voltages up to 1500 Vdc and measure currents up to 2000 Arms (3000 Apeak) using an AC/DC current sensor. Additionally, installing up to seven power input elements on the WT5000 allows simultaneous measurement of seven 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 2. AC/DC current sensors CT1000A (left), CT2000A (center), CT1000S (right)

Figure 3. WT5000 and CT1000A AC/DC current sensor

High-precision measurement of power factor and other power parameters

An on-board charger for EVs converts 50 Hz/60 Hz AC power from a household power outlet or other sources to DC in EVs and supplies it to the batteries. 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 bring the power factor as close to 1 as possible to maximize power. The WT5000 measures the power that passed through a PFC circuit and calculates the power factor. You can simultaneously measure and display parameters other than the power factor, such as voltage, current, active power, apparent power, and reactive power, enabling you to check changes in each parameter at the same time.

Figure 4. Example of displaying voltage, current, power factor, THD, etc.

High-precision 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. This is why they are equipped with PFC circuits.
On-board chargers also contain an internal DC/DC converter to control DC voltage levels. Designing these circuits in a way that minimizes power loss is crucial.
The WT5000 features multi-channel input capability, so it can measure the efficiency of each circuit with high accuracy.

Figure 5. Overview of an on-board charger

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 of time. The integration function integrates the active power (watt-hour), current (ampere-hour), apparent power (volt-ampere-hour), and reactive power (var-hour) values.This function includes two modes: one to measure charging and discharging of, for example batteries, and the other to measure AC power sold or bought.

Figure 6. Example of the integrated power and current measurement screen

Detail of Integration function

Charge/Discharge mode
Measure DC watt hours (for each sampled data item) by polarity.

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

Measurement items related to the integration function

IEC-compliant harmonics/flicker measurement

The WT5000, when combined with the IS8010 Integrated Software Platform, which is PC application software, can perform harmonic testing in accordance with IEC 61000-3-2, as well as voltage fluctuation and flicker testing in accordance with IEC 61000-3-3. Additionally, by using the special model of AC/DC current sensor CT200, you can conduct harmonic and voltage fluctuation/flicker tests exceeding 16 A per phase in accordance with IEC 61000-3- 11 and IEC 61000-3-12. Since normal chargers for EVs use household outlets, they must comply with the aforementioned standards.

Figure 7. Harmonics/flicker test system compliant with the IEC standard

Abnormality checks by continuous measurement of power values and waveform data

When voltage, current, and power data need to be measured continuously for a long period of time, the IS8000 can be used to check and save trends in power parameters in real time. Furthermore, with the WT5000’s /DS (data streaming) option you can simultaneously monitor and record numerical power data and waveform data. For example, by zooming in on the area where there was an abnormality in power measurements, the waveform data at that point can be observed with a single WT5000 unit.

Figure 8. Voltage, current, and power trends displayed on the IS8000

Figure 9. Zooming in on the area of a power drop to check for abnormal waveforms

Development support or charging equipment using other measuring instruments

DL950:
Long-term data recording and high-speed
measurement when abnormal signals occur
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. While acquiring data at a low sampling rate to capture long-term trends, waveforms of sudden transient events can be captured at a high sampling rate. Additionally, the IS8000 enables acquired waveform data and numerical power data from the WT5000 to be synchronized based on IEEE1588 and displayed, allowing more detailed waveform observation during power fluctuations.

Figure 10. Example of measurement with the DL950’s dual capture function

DLM3000/DLM5000:
Observation of PFC circuit waveforms
When you are designing and operationally verifying PFC circuits and bidirectional inverter circuits, we recommend using the DLM3000HD Mixed Signal Oscilloscope, upgraded to 12-bit resolution and offering outstanding operability, or the DLM5000HD with eight channels.