Accelerating EV adoption poses risk to electric grid reliability in US – report

The US Environmental Protection Agency moved on April 12 to tighten emission standards for conventional gas-fueled cars and trucks in a bid to accelerate the transition to electric vehicles that the Biden Administration backs.

A day earlier, however, a white paper laid out potential issues that could impact EV charging in the event of disturbances on the bulk power system.

The paper called for “early engagement and information exchange” between electric utilities and manufacturers to “facilitate anticipation and timely resolution of potential grid reliability issues.”

It warned in particular that the effects of grid dynamics, controls, and system stability due to the power electronic behaviour of EV charging loads “may create new risks of widespread, cascading blackouts.”

The report, Electric Vehicle Dynamic Charging Performance Characteristics during Bulk Power System Disturbances, was issued by the North American Electric Reliability Corp. (NERC), the California Mobility Center and the Western Electricity Coordinating Council and was released April 11. It was prepared as part of a joint EV grid reliability working group.

Faster EV adoption

Citing a number of sources, the report said that electricity consumption by the transportation sector alone could increase by more than a factor of 12 between 2021 and 2050 (from 12 billion kWh in 2021 to more than 145 billion kWh in 2050). And electricity consumption in 2050 could represent a 66% increase over total 2022 electricity consumption.

In announcing its new emission standards, the EPA said that since early 2021, EV sales have tripled while the number of available models has doubled. At present, there are more than 130,000 public EV chargers across the country, a 40% increase over 2020. The private sector has also committed more than $120 billion in domestic EV and battery investments since Congress passed the Inflation Reduction Act last autumn.

Grid challenges

But the NERC-backed white paper warned that the rapid growth in demand from EV charging is “unprecedented” and is taking place at the same time electricity system operators and planners are also focused on integrating “rapidly growing levels of inverter-based generation resources, extreme weather impacts, and increasingly malicious security threats.”

These impacts must be managed in order to maximize the speed of adoption. Examples included:

• Demands on distribution providers to process EV charging load interconnection requests may increase faster than can be managed and lead to delays in new interconnections.
• Significant increase in distribution system hosting capacity and loading effects may cause operational problems in distribution systems requiring expensive, last-minute upgrades in order to accommodate EV charging demands.
• Large-scale changes to demand profiles due to unmanaged EV charging behaviour, time-of-use rates, and distributed renewable energy resources may lead to resource adequacy shortfalls and create needs for short-term, emergency rationing, such as planned, rolling blackouts.
• The need for flexible ramping resources and reserves carried by balancing authorities and transmission operators may grow faster than what has been anticipated in current long-term planning and operational planning studies.
• Effects of grid dynamics, controls, and system stability due to the power electronic behaviour of the EV charging loads may create new risks of widespread, cascading blackouts.

Control strategy

The report said that typical electric end-use loads such as resistive heating, lighting, and cooking were considered grid friendly because their electrical characteristic is constant impedance. That means that as the voltage dropped slightly, the device power draw would also reduce and hence support the grid’s stable steady-state operation.

By contrast, many, if not most, of today’s electronically coupled loads do not exhibit this constant impedance characteristic. Instead, their power electronic controls seek to maintain either a constant current level or a constant power level regardless of system voltage or frequency. When they seek to maintain a constant power level, these loads are not considered grid friendly from a grid dynamics and stability perspective.

The report said that a constant power load exacerbates system instability because, during events on the system when voltage reduces, the load draws more current in order to maintain constant power. In contrast, constant current loads maintain a fixed level of current consumption, independent of voltage, which is grid friendly. That is, the behaviour allows power consumption to drop slightly when system voltages decline.

Based on this, the report recommended that EV chargers and EV supply equipment should use a steady-state control strategy that relies on constant current control rather than constant power level control during normal operations.

The report said that following this recommendation would enable variations in power consumption in direct proportion to changes in voltage. It said that when grid voltages are depressed during unexpected emergencies, EVs and related equipment “support the grid by reducing their power consumption, improving overall grid voltage stability.”

Supporting grid reliability

The report also said that EVs and equipment can support grid reliability by actively contributing to system frequency response. It said the ability of end-use loads (and generators) to support grid frequency is a “core tenet of overall grid reliability,” and said that all newly interconnecting generating units on the bulk power system are required to have active power-frequency controls.

It recommended that EV chargers and equipment should have a programmable current consumption droop characteristic with a programmable range and a default value of 5%. This 5% frequency droop (equal to, say, 3Hz on a 60Hz system) would mean that current consumption would go from 100% to 0% for a frequency drop from 60Hz to 57Hz.

Event ride-through

In addition, the report said it is also critical to have load response coordinated with under-frequency load shedding (UFLS) actions. The first step of UFLS is typically set around 59.3Hz (equal to a 0.7Hz change from nominal). It recommended programming EV chargers and equipment with the capability to rapidly reduce current consumption for severe frequency excursions before UFLS levels are reached.

Turning to issues related to event ride-through operations, the report said it would be important for EV chargers and equipment to dynamically respond to measured terminal conditions with an appropriate charging performance characteristic.

It said doing so requires rapid measurement through the power electronics and sufficiently fast response times to manage current consumption while ensuring equipment integrity.

It said, for example, that EV chargers and equipment should dynamically control current consumption with any ramping down and ramping up of current done in a way that supports grid reliability. According to the report, a grid-friendly EV charging load nearly instantaneously reduces current consumption for a short amount of time. A grid-unfriendly EV charging load, by contrast, does not respond to the event until after it is already cleared and voltage has recovered.

In essence, the report said, “the response of the EV charging load in the latter example does not occur until after the event is over, which does not support grid reliability, and can potentially be detrimental to the overall grid performance, reducing reliability.”

This article was first published on power-grid.com