Tag Archive for: BESS Inverters

LVRT and HVRT are the two grid rules that keep BESS and solar inverters online during a short voltage sag or spike, instead of letting them shut off and add to a larger grid failure.

This guide explains what the terms mean, why they exist, and how BESS developers can meet them.

What LVRT and HVRT Mean

Voltage ride-through means an inverter stays online through a short grid event. LVRT covers sags, when voltage drops below normal. HVRT covers swells, when voltage rises above normal. Faults, lightning, switching, and sudden load shifts can all cause these events.

Older rules worked the other way. Under IEEE 1547-2003, inverters tripped off the moment voltage moved outside a narrow band. A National Renewable Energy Laboratory review calls this “sensitive voltage tripping.” That rule was fine when solar made up a tiny share of power. However, it became a problem once solar and storage grew large. At that scale, one fault could knock out a big share of regional power in seconds.

Why LVRT and HVRT Matter More for BESS

A battery system feels both sides of this problem more than solar alone. During a sag, a BESS can discharge to help voltage recover. During a swell, it can charge to soak up the extra energy. Because of this two-way skill, LVRT and HVRT rules shape how much a storage asset can help, not just how well it survives.

Why LVRT and HVRT Became Mandatory

Grid operators did not add these rules for fun. Instead, they added them after real failures. On August 16, 2016, the Blue Cut Fire in Southern California triggered a transmission fault that knocked out about 1,178 MW of solar PV output, per a NERC/WECC disturbance report. On October 9, 2017, the Canyon 2 Fire caused a separate set of faults that cut roughly 900 MW of solar PV output, per a second NERC disturbance report. In both cases, inverters shut off during brief voltage dips instead of riding through them.

Those failures changed the rules. Now, most DER must stay connected through defined voltage swings. It must also help the grid during that time. As a result, ride-through moved from a nice-to-have feature to a hard certification requirement.

Balancing Worker Safety With Grid Stability

Utilities still need inverters to trip for real faults on their own lines. This is because a downed line stays dangerous to line workers if power keeps flowing. Grid codes solve this with clear voltage-and-time limits. Inside the limit, the plant must ride through. Outside it, tripping is allowed. That line is the whole point of an LVRT and HVRT curve.

How Ride-Through Curves Work

Every LVRT and HVRT rule is drawn as a curve. The curve plots voltage against time. A voltage of 1.0 p.u. is normal. A voltage of 0.0 p.u. is a dead short at the terminals. For each voltage level, the curve sets the shortest time an inverter must stay connected.

Mandatory Operation, Momentary Cessation, and Trip

IEEE 1547-2018 names three responses inside this curve. First, mandatory operation. The inverter must keep sending active and reactive current as set by the rule. Second, momentary cessation. The inverter can pause briefly, usually below 0.5 p.u., then restart fast once voltage returns. Third, trip. This is only allowed once the event falls outside both zones. Meanwhile, IEEE 2800-2022, the newer rule for large plants, limits momentary cessation even further. That pause behavior helped cause the California events.

Reactive Current Injection Under LVRT and HVRT

Modern codes ask for more than staying online. During a sag, the inverter must push extra reactive current to help raise local voltage. During a swell, it pulls reactive current to help bring voltage back down. This response is set by a gain value, called a k-factor. Most codes set k between 2 and 6. As a result, a bigger sag gets a bigger response, up to the inverter’s current limit.

IEEE 1547-2018 and IEEE 2800-2022: The US Framework

In the US, smaller grid-connected systems follow IEEE 1547-2018. Meanwhile, large, transmission-connected plants follow IEEE 2800-2022. Both set clear LVRT and HVRT rules. NERC PRC-024 sets outer voltage and frequency limits. Therefore, no bulk-system plant may trip inside those limits. It acts as a backstop for both standards.

DER Categories and LVRT and HVRT Coverage

IEEE 1547-2018 splits inverters into three groups. Each group has its own ride-through table.

A utility or public commission picks the category for each project. Today, most utility-scale BESS projects use Category III. That is because it gives the longest ride-through time and the most grid support.

Global LVRT and HVRT Codes Compared

Exact limits shift by country. The core idea stays the same everywhere. In the US row below, remember that IEEE 2800-2022 applies specifically to transmission-connected plants, not smaller distribution-tied systems.

Germany and the EU tend to demand a faster fault-current response than the US baseline. Their grids already carry more inverter-based power, so the margin for delay is smaller. ERCOT asks for two test profiles from both BESS and solar: a legacy dip and a step-by-step voltage-dip curve. Because of this, a plant controller must line up the reactive response from every inverter at one shared point.

Why Project-Specific Studies Still Matter

A generic grid-code curve sets the floor. However, it is not the final word. The interconnection study for one project can tighten that curve. Specifically, it looks at local grid strength, fault current, and protection settings. For that reason, developers should treat the study, not the general code, as the rule that governs a live project.

LVRT vs. HVRT: Key Differences

LVRT and HVRT share one framework. They differ in cause and response.

• Trigger: LVRT reacts to sags from faults or heavy switching. HVRT reacts to swells, often from sudden load loss or capacitor switching.
• Reactive response: LVRT asks for pushed current to raise voltage. HVRT asks for pulled current to lower it.
• Typical severity: LVRT events tend to run deeper and happen more often. Short circuits are simply more common than large load losses.
• BESS behavior: A BESS can discharge to help LVRT and charge to help HVRT. A solar-only plant cannot do both.

How BESS Inverters Achieve LVRT and HVRT Compliance

Meeting a curve on paper is easy. Meeting it in the field, under a real fault, depends on how the inverter is built.

Grid-Following vs. Grid-Forming Response

Most inverters today are grid-following. They read grid voltage and frequency through a phase-locked loop, then respond with current. Grid-following units can meet LVRT and HVRT rules. However, their speed is capped by how fast that loop can track a distorted wave during a fault. Grid-forming inverters work differently. Instead, they set their own voltage reference, which gives a faster LVRT and HVRT response. They act more like a spinning generator. Increasingly, more grid codes now favor this design in high-growth areas.

Reactive Current Priority and Current Limits

During a deep sag, an inverter’s total current is capped by its hardware. So, the control system must split that limited current between active power and reactive support. Most codes put reactive current first, since it does the most to fix voltage. Any leftover current then goes to active power. Getting this order wrong is a common reason inverters fail a compliance test, even when the timing is correct.

Testing and Certification

LVRT and HVRT compliance is tested, not assumed. In the US, UL 1741 certification checks baseline inverter behavior. Meanwhile, large projects also need project-specific Model Quality Testing. ERCOT now requires this test for both solar and BESS plants.

What Model Quality Testing Covers

This test runs the full LVRT and HVRT curve under controlled conditions. It starts with a flat-start check and a small voltage test. Next comes LVRT testing under both a legacy curve and a voltage-dip curve. HVRT testing follows the same pattern. Other tests check small frequency shifts during charge and discharge, and grid strength across several fault levels. Finally, a phase-angle-jump test, run in software like PSCAD, closes out the sequence. Importantly, the plant controller is tested with every inverter together, not alone. Otherwise, the combined response at the shared connection point can differ from any single unit’s result.

Design Considerations for LVRT and HVRT Compliance

Treat LVRT and HVRT compliance as a design choice, not a final checklist item.

• Confirm the DER category early. The utility’s choice of Category I, II, or III sets both the inverter type and the ride-through curve. This choice is hard to change later.
• Size reactive headroom on purpose. Saving current for reactive support cuts the active power on hand during a fault. This shapes how you manage state of charge.
• Coordinate the plant controller model. For hybrid solar-plus-storage sites, test the plant controller with every inverter together. Do not test each unit alone.
• Track changing standards. IEEE 2800 updates and ERCOT’s guide keep shifting. A BESS built to an old curve may fail today’s interconnection study.

Key Takeaways

Frequently Asked Questions

Further Reading

• BESS PCS Functions
• Understanding BESS Specifications
• Fast Frequency Response (FFR)
• C&I vs. Utility-Scale BESS
• AI Data Center Energy Storage