AC Voltage Variation Studies

As the UK power system transitions to net zero, Battery Energy Storage Systems (BESS); for example, are stepping up to provide flexibility, stability, and support for renewables. But before they can connect to the grid, every project must demonstrate that it can ride through disturbances safely. One of the key checks is the AC Voltage Variation Study.

Why AC Voltage Variation Studies Matter

The grid is exposed to short-term voltage excursions every day caused by faults, breaker trips, or line switching. These Temporary Over-Voltage (TOV) events, if uncontrolled, can damage equipment and destabilise the system.

AC Voltage Variation Studies gives network operators confidence that:

• The plant can withstand extreme but credible voltage swings.
• It won’t worsen over-voltages during faults or breaker trips.

How AC Voltage Variation Studies Work

Using advanced EMT (Electromagnetic Transient) simulations, engineers model a range of credible scenarios, such as:

• Faults at different network busbars.
• Breaker trips at substations and Points of Connection.
• Plant operating in various control modes and configurations (if applicable)

The simulations check whether voltages remain inside Grid Code limits (typically below 1.4 p.u. RMS during a TOV event) and whether the system recovers smoothly. Temporary over-voltages (TOVs) are defined as voltages above the maximum network voltage that last for two cycles or more. Causes of TOVs include single, double and three phase fault inception and clearance, load/generation rejection, line/cable re-energisation by means of auto-reclose and parallel line resonance conditions. The limits for TOVs in England and Wales are defined in TGN(E) 288. The limits for RMS phase-to-ground voltages are specified by the voltage profile in Fig. 1. The maximum RMS value of a TOV must not exceed 140% of the maximum continuous operating voltage (e.g., 145 kV for 132 kV nominal). The instantaneous limit is 200% of peak voltage under maximum continuous voltage conditions. The limit for RMS voltage applies only to unplanned events while the limit for instantaneous voltage applies to both planned and unplanned events.

Comparing Method 1 and Method 3 for TOV Compliance

When demonstrating compliance with TGN(E) 288, different levels of modelling detail can be applied. Among these, Method 1 (IEC 60071-4 / TGN 261 based) and Method 3 (Thevenin worst-case assumption) represent the two extremes.

Method 1 – IEC 60071-4 / TGN 261 (Preferred by NGET)

• Builds a detailed network model, including at least two substations away from the Point of Common Coupling (PCC).
• Includes major generators, capacitor banks, reactors, and other nearby assets.
• First, the worst TOV profile is determined at the PCC without the connectee equipment. Then, the connectee’s plant is added to evaluate its impact.
• Produces a realistic, credible TOV profile that reflects the true behaviour of the system.
• Advantage: This is the most accurate and accepted approach, giving NGET confidence that studies reflect real system conditions.
• Disadvantage: Requires significant data availability and modelling effort.

Method 3 – Simplified Thevenin Worst-Case Assumption

• Assumes that the PCC voltage is already at the maximum TOV limit (1.4 p.u.) before the connectee is added.
• Represents the grid as a simple Thevenin source with fixed 1.4 p.u. voltage magnitude and corresponding fault-level impedance.
• The connectee equipment is then tested under these extreme conditions.
• Produces conservative results, which ensure compliance but may overstate the risk by ignoring damping, protection action, or realistic network dynamics.
• Advantage: Straightforward and requires minimal network modelling.
• Disadvantage: Often onerous for the connectee, as it assumes the harshest possible case without considering realistic conditions.

In summary:

• Method 1 is the engineering-based, preferred method, providing realistic and credible results.
• Method 3 is a simplified, worst-case assumption used when detailed modelling is not feasible, but it can lead to more conservative (and sometimes unnecessarily strict) outcomes.

Managing Plant Tripping During TOV Studies

During testing, analysis may show that the connectee plant trips in certain overvoltage conditions, particularly when using the conservative Method 3 approach. In such cases:

1. Check HVRT (High Voltage Ride-Through) settings
2. Refer to the OEM to confirm or adjust the HVRT protection settings.
3. Sometimes slight adjustments to the ride-through curve may remove false tripping without reducing compliance with the Grid Code.
4. Revert to Method 1 for more detailed analysis
5. If no viable solution is feasible based on Method 3 assumptions, a more detailed Method 1 study should be carried out.
6. This reduces the conservatism by quantifying the actual network behavior, and can demonstrate the plant’s continuing compliance without tripping under realistic conditions.

Case Study – BESS Plant

National Grid has provided guidance on the approved methodologies for assessing compliance with TGN(E) 288. The guidance lists three possible methodologies, for this study methodology 3 has been selected. In this methodology the external network is modelled using a Thevenin equivalent. The external network is assumed to be operating at the TOV limit prior to connection of the plant thus representing a worst-case scenario. This is achieved by using dynamic voltage source to reproduce the voltage profile in Fig. 1. The plant model is then connected and the TOV assessed for several transient conditions, such as internal and external faults, and plant tripping

At the BESS project (50 MW at 132 kV in Scotland), our AC Voltage Variation Study demonstrated that:

• Voltages stayed within the 1.4 p.u. limit under all tested conditions.
• The system showed controlled recovery following three-phase and phase-to-phase faults.
• Breaker trips confirmed that the plant could disconnect safely without creating additional over-voltages.

For the following results, 1 p.u. is defined as the maximum continuous operating voltage of 145 kV RMS line-to-line. For all cases, the TOV is evaluated at the plant 132 kV POC busbar. Voltage response under three-phase-to-Ground fault at the POC and the 33kV Customer Substation are shown in figures 2 and 3 below: