Curtailment signals are control commands or data inputs that instruct an energy system to reduce, limit, or temporarily stop power consumption or generation. In EV charging, curtailment signals are used to throttle charging power during grid constraints, site demand peaks, or market events—helping chargers stay within available capacity and support grid stability.
What Are Curtailment Signals?
A curtailment signal is a “reduce power now” instruction that can apply to:
– A single charger connector
– A group of chargers at one site
– A whole charging network or fleet depot
Curtailment can be temporary (minutes/hours) or scheduled (peak tariff windows), depending on the grid or site requirements.
Why Curtailment Signals Matter in EV Charging
Curtailment signals matter because EV charging is a flexible electrical load that can be adjusted without immediate safety risk. They help:
– Prevent site overload and main breaker trips, improving uptime
– Comply with connection tariffs and contracted capacity limits
– Reduce peak demand costs in commercial sites and depots
– Support grid congestion relief and local transformer protection
– Enable participation in demand response and flexibility programs
For operators, curtailment is often the difference between scalable deployments and costly grid upgrades.
Common Sources of Curtailment Signals
Curtailment signals can originate from several systems:
Utility or Grid Operator Requests
– Instructions triggered by grid congestion, local feeder constraints, or emergency conditions
– Often tied to formal demand response or flexibility programs
These signals may be time-critical and require fast response.
Site Energy Management System (EMS/BMS)
– Signals based on building load, HVAC peaks, production equipment use, or facility limits
– Often driven by current transformer (CT) measurements at the main incomer
This is common in workplaces, commercial real estate, and depots.
CPMS or Charger Control Policies
– Operator-defined rules (time-of-day caps, user group priority, peak pricing windows)
– Network-wide controls during incidents or capacity planning events
This supports consistent policy enforcement across multi-site rollouts.
On-Site Generation and Storage Coordination
– Curtailment to align charging with solar output, battery limits, or microgrid constraints
– Helps reduce import peaks and improve self-consumption strategies (clean energy matching)
How Curtailment Signals Affect Charging Sessions
When a curtailment signal is applied, the system typically:
– Reduces the allowed charging current or power setpoint
– Maintains safe charging behavior and communication with the vehicle
– May queue or prioritize vehicles based on rules (fleet readiness, departure time)
In AC charging, limits are commonly communicated to the vehicle via control pilot (CP) (PWM current limit). In DC charging, limits are applied as power/current setpoints governed by the BMS charging requests.
Curtailment vs Load Balancing
These concepts are related but not identical:
– Load balancing: sharing available capacity across multiple chargers to stay under a site limit
– Curtailment: an external or higher-level instruction to reduce available capacity (which load balancing then distributes)
Curtailment often triggers load balancing behavior, but curtailment can also be applied to a single charger or connector directly.
Typical Use Cases
Curtailment signals are used in:
– Fleet depots to avoid demand spikes during simultaneous plug-in events
– Corporate campuses to keep charging within building capacity during business hours
– Public sites with constrained grid connections or dynamic grid limitations
– Flexibility programs where the operator is paid to reduce load during peak grid stress
– Sites with limited connection capacity waiting for grid reinforcement (connection lead time constraints)
Data and Control Requirements
Reliable curtailment requires:
– Fast and stable communications between EMS/CPMS/chargers
– Clear priority and fairness rules (who gets power first)
– Monitoring and logging to verify curtailment response and duration
– Analytics to measure impact on session outcomes (charging session analytics)
– User communication strategy (expected reduced speed during certain periods)
Common Pitfalls
– Curtailment applied without clear rules, causing unpredictable user experience
– Over-curtailment leading to vehicles not reaching required SoC for fleets
– Slow signal propagation causing short overload events and breaker trips
– Missing measurement validation (CT mapping errors lead to wrong curtailment triggers)
– Not tracking curtailment events in analytics, hiding true performance and utilization patterns
– Confusing curtailment-induced slow charging with charger faults or connectivity issues
Related Glossary Terms
Load Balancing
Active Power Throttling
Control Pilot (CP)
Current Transformer (CT)
Grid Congestion Management
Connection Tariffs
Charging Session Analytics
Battery Management System (BMS)
Uptime