A transformer tap changer is a mechanism that adjusts a transformer’s turns ratio to regulate output voltage. In EV charging infrastructure, tap changers help keep voltage within acceptable limits as site load changes (for example, when many chargers start simultaneously) or when the upstream grid voltage varies. Stable voltage supports charger reliability, prevents derating, and reduces nuisance faults across multi-charger sites.
What Is a Transformer Tap Changer?
A tap changer changes the effective transformer ratio by switching between predefined connection points (“taps”) on the transformer winding. This allows the transformer to:
– Raise voltage when supply voltage is low or load is heavy
– Lower voltage when supply voltage is high or load is light
– Keep downstream voltage closer to nominal under varying conditions
Tap changers are commonly found on distribution and substation transformers and may be fixed-setting or adjustable under controlled conditions.
Types of Tap Changers
Two main tap changer types are used in power systems:
– Off-circuit tap changer (OCTC): adjusted only when the transformer is de-energized (used for seasonal or commissioning adjustments)
– On-load tap changer (OLTC): can change taps while energized and supplying load (used for continuous voltage regulation)
In many LV distribution transformers, voltage adjustment may be limited to off-circuit taps, while OLTCs are more common at MV/HV substations or large customer supplies.
Why Tap Changers Matter for EV Charging Sites
EV charging can introduce large, fast-changing loads that affect voltage:
– Multiple chargers starting can cause voltage dip at the site
– Long cable runs increase voltage drop under high current
– Weak grid impedance at the Point of Common Coupling (PCC) amplifies voltage variation
– Peak charging periods can push voltage outside operational limits
Tap changers support:
– Reduced charger power derating due to low voltage
– Fewer charging interruptions and fault events
– Better performance of load management strategies (predictable voltage margins)
– Improved compatibility with site equipment sharing the same supply
How Tap Changer Settings Are Selected
Tap settings are typically chosen based on:
– Utility supply voltage range and local transformer standards
– Expected maximum load and voltage drop across cables/switchgear
– Power quality targets at the PCC (including voltage limits)
– Seasonal variation and feeder loading conditions
– Future expansion plans (adding more chargers later)
For OCTC transformers, the setting is often defined during commissioning and revisited if site loading changes materially.
Tap Changers and Charger Performance
If voltage is consistently low at the charger terminals, EVSE may:
– Reduce available charging power to stay within current and thermal limits
– Trigger undervoltage faults or unstable operation
– Experience higher current draw for the same power, increasing losses and heating
– Contribute to poorer user experience (slower charging, session interruptions)
Proper tap selection can create a healthier voltage profile, especially on long feeder runs or heavily loaded sites.
Risks and Practical Considerations
– Wrong tap setting can cause overvoltage at light load or undervoltage at peak load
– OLTC operation can introduce switching transients and maintenance needs
– Voltage regulation must remain within equipment limits for the whole site, not only chargers
– Tap changes should be coordinated with utility requirements and protection settings
– If voltage issues are severe, tap changes alone may not fix them—cable sizing, transformer rating, or grid reinforcement may be required
Related Glossary Terms
Voltage drop
Line-to-line voltage
Line-to-neutral voltage
Grid capacity assessment
Point of Common Coupling (PCC)
Power derating
Load management
Grid reinforcement
Power quality
On-site transformer