# Effect of Harmonics on Ferroresonance in Low Voltage Power Factor Correction System--A Case Study
## 3 System Description
### Operating Conditions  
The transformer station operates at different operating points with different load requirements. Depending on the load requirement, the low voltage side is supplied by two or three transformers connected in parallel. At different operating points, the ratio of nonlinear and linear loads to the total power demand is different.  
When the nonlinear loads constitute the majority of the loads, the harmonic distortion on the low voltage side is high, but still below the limits. However, the experimental results have confirmed that under such conditions, switching on the capacitor stage triggers the ferroresonance of the detuned reactor, which is supported by the power system. In contrast, the capacitor stages switched on before the transition from the low-distortion to the high-distortion operating point do not enter the ferroresonance, but exhibit high current harmonics in the steady state. Although the stage current is lower than the rated value, the fifth harmonic current is higher than the rated values of detuned reactors given in Tables 10 and 11, which is not acceptable. This is due to the high value of the fifth voltage harmonic at the low-voltage busbar at these operating points, which is caused by the injection of the fifth harmonic current from variable-speed drives, and the resonant frequency of the detuned reactor and power correction capacitor (210 Hz), which is very close to the fifth harmonic. Therefore, the phenomenon of ferroresonance in the transformer station occurs under certain levels of harmonic distortion, and different ratios of linear and nonlinear loads can form the conditions under which ferroresonance can be triggered. To analyze this phenomenon in more detail, the measurement results under two different operating points are presented and analyzed: Case 1 (C1), where the conditions for ferroresonance are not met, and Case 2 (C2), where the conditions for ferroresonance are met and ferroresonance occurs when the stage is switched on.  
Case C1 is analyzed as a reference point for normal operating conditions, where the harmonic distortion of the bus voltage is lower. In C1, the low voltage side of the transformer station is supplied by three transformers connected in parallel. The total nonlinear load demand is about 500 kW, which is less than 30% of the total power demand. In C1, the capacitor stages can be switched on and off and no ferroresonance occurs.  
Ferroresonance occurs at operating conditions in C2 where the harmonic distortion of the bus voltage is high. In C2, the low voltage side of the transformer station is supplied by two transformers connected in parallel, and the third transformer is disconnected. The total demand of the nonlinear loads is about 700 kW, which is more than 50% of the total power demand. In C2, switching on the capacitor stages triggers the undamped ferroresonance supported by the grid. In this case, the ferroresonance can only be stopped by switching off the capacitor stage under ferroresonance. Capacitor stages which are switched on before operating state C2 is reached do not go into ferroresonance but exhibit fifth harmonic current overload.