In high‑penetration renewable energy grids, the voltage regulation capability of the energy storage system is crucial for maintaining power quality and system stability. However, field failures of step‑up transformers used in energy storage system installations have been frequently reported, with insulation breakdown occurring mainly on the high‑voltage side. Through autopsy and operation data analysis, it has been determined that the root cause is overvoltage at the connection point of the energy storage system. When the grid‑connected voltage exceeds the transformer’s designed continuous working voltage, core saturation and winding overheating accelerate insulation aging, making the transformer vulnerable to transient or temporary overvoltages during disturbances. To ensure reliable operation and effective power support, it is essential to thoroughly investigate the voltage fluctuation characteristics at the energy storage system connection point and propose appropriate insulation design guidelines.
This paper first presents a theoretical analysis of node voltage fluctuations in a distribution network with high photovoltaic (PV) penetration. Taking an energy storage system connected to a typical distribution feeder, the voltage at the point of common coupling (PCC) is derived using DistFlow equations. The influence of PV output variation, energy storage system charging/discharging, system faults, and switching operations on the PCC voltage is studied. Subsequently, detailed time‑domain simulations are conducted on a modified IEEE 14‑bus system to quantify the overvoltage magnitudes and durations under various scenarios. Based on the computed overvoltage levels, an insulation coordination method for the energy storage system step‑up transformer is proposed, ensuring the transformer can withstand all operating conditions.
1 Theoretical Analysis of Node Voltage Fluctuations
1.1 Impact of PV Output on Node Voltage
Consider the simplified equivalent circuit of a grid‑connected PV and storage system shown in Figure 1 (non‑numbered). The voltage at node N can be expressed as:
$$V_N = \frac{V_G}{k} – \frac{R_N P_N + X_N Q_N}{V_N^n}$$
where \(V_G\) is the high‑side voltage of the transformer, \(k\) is the turns ratio, \(R_N+jX_N\) is the equivalent impedance between node N and the system, \(P_N\) and \(Q_N\) are the net active and reactive power flows into node N. With a unity power factor PV system, the net power becomes:
$$P_N = P_{LN} – P_{VN},\quad Q_N = Q_{LN} – Q_{VN}$$
Combining the above, the voltage deviation due to PV output is:
$$V_N = \frac{V_G}{k} – \frac{R_N(P_{LN}-P_{VN}) + X_N(Q_{LN}-Q_{VN})}{V_N^n}$$
When \(P_{VN}\) is large, reverse power flow may cause overvoltage; when PV output is low, undervoltage may occur. In distribution networks with high R/X ratio, reactive power alone is often insufficient for voltage regulation, necessitating the participation of energy storage system.
1.2 Voltage Regulation by Energy Storage System
The energy storage system can rapidly absorb or inject active power to counter voltage deviations. Its state of charge (SOC) dynamics are described by:
$$
\begin{cases}
Q_{ESS}(t) = Q_{ESS}(t-\Delta t) – \frac{P_t^c}{\eta_c}\Delta t + \eta_d P_t^d\Delta t \\[2mm]
Q_{ESS,min} \le Q_{ESS}(t) \le Q_{ESS,max} \\[2mm]
0 \le u_c \le 1,\quad 0 \le u_d \le 1 \\[2mm]
0 \le P_t^c \le P_{c,max},\quad 0 \le P_t^d \le P_{d,max} \\[2mm]
u_c + u_d \le 1
\end{cases}
$$
In addition, system‑level constraints must be satisfied, such as transient voltage security margin, renewable energy disconnection margin, and load shedding capacity. These constraints determine the required active power from the energy storage system to bring node voltages within safe limits.
1.3 Overvoltage Due to System Disturbances
System faults or switching operations can induce temporary or transient overvoltages at the energy storage system connection point. For a DC blocking fault at a converter station, the overvoltage at a nearby renewable energy station with output \(P_w\) and \(Q_w\) can be approximated as:
$$U_w = U_d + \frac{Q_w X_w}{U_d} + \frac{P_w X_w}{U_d}\quad (\text{neglecting longitudinal component})$$
For asymmetric faults, the voltage on healthy phases can rise significantly. For a single‑phase‑to‑ground fault, the healthy‑phase voltage may reach line voltage level. Such overvoltages are directly imposed on the energy storage system transformer.
2 Simulation Case Study
Simulations are performed on the modified IEEE 14‑bus system (Figure 2, not shown) where two synchronous generators at buses 8 and 14 are replaced by equivalent PV plants, and an energy storage system is connected at bus 12. The system base voltage is 35 kV (non‑effectively grounded).
2.1 Overvoltage Caused by PV Output Variation
Irradiance increases from 200 W/m² to 1000 W/m² between 5 s and 10 s, causing PV output to rise from 300 kW to 900 kW. The PV bus (bus 8) voltage reaches 1.2 p.u., and the adjacent energy storage system bus (bus 12) voltage reaches 1.1 p.u. The overvoltage persists about 1.5 s after the irradiance begins to decrease.
2.2 Overvoltage Due to Energy Storage System Charging/Discharging
When the energy storage system switches from discharging to charging at 1.0 s, the PCC voltage decreases by 0.03 p.u. When switching from charging to discharging at 1.5 s, the voltage increases by 0.05 p.u. The impact on adjacent PV bus is negligible.
2.3 Overvoltage During System Disturbances
Three types of faults are applied on the line between buses 12 and 13 (fault duration 0.1 s):
- Symmetric three‑phase fault: After fault clearance, the voltage at bus 12 recovers with a maximum overshoot of 1.09 p.u. lasting about 0.07 s.
- Two‑phase‑to‑ground fault: The healthy phase (C) voltage rises to 1.5 p.u. during fault. After clearance, all phases return to nominal.
- Single‑phase‑to‑ground fault (phase A): During fault, the healthy phases voltage rises to 1.7 p.u. At clearance, a transient surge of 2.0 p.u. appears on the faulted phase for about one cycle.
Additionally, sudden load removal at bus 6 causes a temporary voltage rise of 1.1 p.u. at the energy storage system bus.
3 Summary of Overvoltage Levels
The computed overvoltage values are compared with the limits specified in IEC 60071 and GB/T 50064 for a 35 kV non‑effectively grounded system:
| Condition | Type | Standard Limit (p.u.) | Simulated Max (p.u.) |
|---|---|---|---|
| Normal operation | Maximum continuous voltage | 1.1 | 1.1 |
| PV output variation | Temporary overvoltage | 3.0 (1.73√3) | 1.7 |
| Single‑phase fault | Transient overvoltage | 4.0 | 2.0 |
| Two‑phase fault | Temporary overvoltage | 3.0 | 1.5 |
| Load switching | Temporary overvoltage | 3.0 | 1.1 |
The results show that while the temporary and transient overvoltages remain within the standard limits, the continuous maximum voltage (1.1 p.u.) must be considered for the transformer’s highest working voltage selection.
4 Insulation Coordination for Energy Storage System Transformer
Based on the analysis, the following insulation requirements are proposed for the energy storage system step‑up transformer:
- Highest working voltage: The transformer should be rated for at least 1.1 p.u. of the nominal system voltage, i.e., the maximum continuous voltage expected at the PCC.
- Power‑frequency withstand voltage: Since the temporary overvoltage reaches 1.7 p.u. (within the 3 p.u. limit), the standard power‑frequency test voltage for 35 kV class can be selected according to the relevant standard.
- Switching impulse withstand voltage: The maximum switching overvoltage computed is 2.0 p.u., well below the 4.0 p.u. limit. Hence, standard switching impulse levels are adequate.
- Lightning impulse protection: A surge arrester of proper voltage rating should be installed at the transformer high‑voltage side to limit lightning‑induced overvoltages.
It is important to note that the energy storage system transformer should not be selected solely based on nominal voltage. The actual voltage profile at the connection point, including temporary overvoltages caused by PV fluctuations and faults, must be taken into account. Proper insulation coordination ensures the energy storage system can reliably provide voltage support throughout its lifetime.
5 Conclusion
This paper analyzes the overvoltage characteristics at the connection point of an energy storage system in a high‑PV‑penetration distribution network. Theoretical derivations and time‑domain simulations on a modified IEEE 14‑bus system reveal that:
- The energy storage system’s own charging/discharging causes minimal voltage variation (≤0.05 p.u.), whereas PV power fluctuations can induce a sustained overvoltage of up to 1.1 p.u. at the adjacent energy storage system bus.
- System disturbances, particularly single‑line‑to‑ground faults, generate the most severe overvoltages: during the fault, healthy‑phase voltage can reach 1.7 p.u., and transient surges at fault clearance can reach 2.0 p.u.
- For a 35 kV non‑effectively grounded system, all computed overvoltages stay within the permissible limits of relevant international and national standards. Nevertheless, the transformer’s highest working voltage must be set at 1.1 p.u., and standard power‑frequency, switching, and lightning impulse withstand levels are sufficient.
The proposed insulation coordination method ensures that the energy storage system transformer can endure all anticipated operating conditions, thereby enhancing the reliability and effectiveness of voltage regulation in modern power grids. Future work will include experimental validation on a test platform to confirm the simulation findings.