Purpose
With the increasing demands of the energy storage intake to facilitate Net-Zero in the power network, inoperative tertiary windings of existing supergrid transformers provide opportunities to connect Battery Energy Storage System (BESS) to achieve local system services, i.e., frequency and voltage control. One major concern is the impact of BESS connection on the local network power quality, due to the appearance of power electronic devices. BESS system is connected through a transformer to the Point of Connection (POC), which is further connected to the tertiary windings through cables and transformers. To accurately assess the impact of BESS connection on local system power quality, it is necessary to conduct transient analyses with accurate modelling of the transformer. This paper aims to perform the harmonic assessment for such BESS connection application in the UK 400 kV substation and to improve the transformer transient modelling.
Modelling methods
A representative local 400 kV substation network model was modelled in ATP-EMTP. BCTRAN and Hybrid transformer models and distributed pi cable models were configured to model the 400/132/13 kV and 33/13 kV transformers and 13 kV and 33 kV cables in the substation. In order to assess the harmonic penetration in the network, a single-frequency three-phase harmonic scan from 2nd order to 50th order by harmonic current source with 5A amplitude was carried out. In addition, a reverse engineering procedure was taken to obtain an accurate high-frequency model of the 33/0.4 kV transformer, which is directly connected with the BESS, based on limited structural specification and one FRA measurement reference.
Results
The simulation results showed the harmonic penetration in the local network is dependent on the frequency of the harmonic injection, which is related to the switching frequency of the BESS. From the harmonic scan, one harmonic resonance was observed at 1.75 kHz, which showed the heaviest harmonic penetration and was possibly caused by coincidental resonance between cable capacitances and transformer leakage impedances. Nevertheless, the voltage harmonic amplitudes attenuated greatly across the transformers. The modelled 33/0.4 kV transformer successfully reproduced the FRA spectrum as to the measurement reference.
Conclusions
The transformers showed apparent suppression effects on the voltage harmonic propagation in the local network. The Total Harmonic Distortion (THD) values at the system source and load ends were below 5%. A harmonic resonance at 1.75 kHz was observed in the studied configuration, which indicates a need of careful tuning of the BESS control system to avoid this resonance frequency. The successfully established high-frequency 33/0.4 kV transformer model enables the local network model to be integrated with the BESS control system and enables more practical harmonic scenarios to be studied.

Harmonic,Battery Storage System,Transformer High Frequency Modelling,ATP-EMTP,Tertiary Winding