In collaboration with SSEN & UK Power Networks
According to the Electricity Networks Association (ENA), Electric Vehicle Supply Equipment (EVSE) is considered “potentially disturbing equipment” to the distribution network – i.e. equipment that has the potential to cause voltage and current distortion. Power quality and harmonic assessment of EVSE and heat-pumps is an area that needs more research. This problem equally applies to the growth of heat pumps and other non-linear loads.
This project has researched the state-of-the-art relating to the power quality grid impacts of plug-in electric vehicles. The assessment goes beyond the information available on equipment to date – typically single devices in isolation, charging at rated power, and operation under ‘normal’ conditions. The tests performed under this work package collected harmonics emission data of different AC On Board Chargers (OnBCs) and a DC rapid charger in light of current and anticipated acceleration transport electrification. The project provided an accurate understanding of different EV chargers’ harmonic emissions and the potential impact of smart charging on these emissions. The data collected was used to assess the potential impact of high penetration of EV charging facilities on the quality of power supply in future distribution networks.
In a future power system, where most devices are connected via power electronics converters, power quality could become a limiting factor for new connections even before thermal or voltage constraints occur (as defined by G54-1/IEEE 519/IEC 61000).
The project objective was to provide inputs for DNOs to update their EV strategies and associated engineering recommendations and best practices. This project was seen as a timely opportunity to investigate and develop solutions and approaches to mitigate unacceptable network power quality. Investigating this issue now and developing an appropriate set of tools and processes will prevent network level harmonics and phase unbalance from becoming a future barrier to the net-zero transition and required grid integration of large volumes of LCTs.
Key project tasks
To assess their impacts, tests have been conducted on EV Supply Equipment (EVSE) points under rated power and ‘smart’ (de-rated) charging conditions. The network-level impacts of EVSEs under these charging conditions have been assessed by integrating recorded harmonic data for five different EVs (Electric Vehicle) into network simulations under a variety of scenarios.
The tasks carried out were as follows:
Quantify the variation of EVSE harmonic signatures across different device types and models via field data collection: Capture charging data for a minimum of 3 EVSE devices, including the existing DC rapid charger at PNDC.
For AC charging, evaluate the correlation between device-level harmonic current magnitude
and the operating setpoint (e.g. due to smart charging or high state of charge) – via testing and data collection.
Evaluate the impact of varying levels of background voltage harmonics on both AC and DC EVSE device level current harmonics – via testing and data collection.
Explore how device-level interactions have a positive or negative impact on network-level power quality and how this phenomenon can be used in Engineering Recommendations – via lab testing, modelling and simulation
Model device level variables at various levels of EVSE adoption to simulate and quantify future expected network power quality levels under a range of proposed operational scenarios and EV uptake forecasts.
Which PNDC assets were utilised?
Two AC EV charge points in PNDC LV lab to operate and capture charging data for minimum of 5 vehicles.
Installed three-phase charging point with one of the tested EVs:
One DC rapid EV charge point outside the PNDC facility to operate and capture the charging data of an EV using a DC charge point.
ABB Terra 53 charger with Nissan Leaf charged (through DC CHAdeMO port) at 50kW.:
The following instruments were used to measure and record EVSE harmonics:
1. Fluke 435-II Power Quality and Energy Analyzer
2. National Instrument Compact-Rio
3. Voltage Divider
4. Amplifier circuit and EVSE PWM control circuit
5. PWM generator (PicoScope 4824A)
6. DC power supply
8. Distribution switchboard
Data logging and charging control setup:
Key findings of the project are that for the EVs considered, derated charging did not adversely affect the network hosting capacity of EVs compared to charging at fully rated power. Due to higher cancellations present with larger numbers of EVs operating at different setpoints, smart charging can improve the hosting capacity for EVs within G5/5 voltage harmonic limits.
Different EV makes and models have different harmonic emission signatures despite having almost the same ratings and number of phases. Specifically, the tested EVs OnBCs (On Board Chargers) showed significant variation in harmonics emission magnitude and phase angle. Therefore, diversifying the EV chargers can contribute to harmonics emission cancellation. This is likely to be the case in public settings as the choice of different EVs increases to consumers. However, DNOs need to be conscious of large depot-based fleet deployments of the same vehicle type that may not experience the same levels of beneficial harmonic cancellation.
PNDC have provided recommendations for the modification of IEC standards and have shown that voltage harmonic limits in the ENA G5/5 standard can limit DNO’s network capacity for EVs.
It has been recommended that the IEC 61000-3-12 standard should be revised to include limits for higher order (h>13) current harmonics for single-phase equipment.
It has also been shown that G5/5 limits for higher order triplen voltage harmonic reduces EV hosting capacity, particularly for weaker grids.
Based on the harmonic emissions measurements conducted by PNDC on 5 different EVs, it has been recommended that DNOs consider the impacts of higher order triple harmonics when specifying the minimum grid strength for EVSE connections.
Evidence about inaccuracy of IEC 61000-3-6 harmonics summation factor for EV emission has been found and reported.
While a larger scale test might be required to confirm this conclusion, this evidence, if confirmed, is critical for the accuracy of DNOs’ new connection assessment of large charging hubs and overall LV networks studies with high EV charging share.
Evidence about a mild unbalance and low neutral current in a three-phase on-board charger was found and reported.
Furthermore, evidence about capacitive power factor of one on-board charger is found.
Similarly, three phase chargers were found to be emitting low triplen harmonics.
Such findings highlight the discrepancy in power consumption and power quality metrics among EVs of different make.
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PNDC is one of the University of Strathclyde’s industry-facing innovation centres and focuses on accelerating the development and deployment of novel energy and transport technologies through multiple collaboration models and open access facility provision.