# Effect of Harmonics on Ferroresonance in Low Voltage Power Factor Correction System--A Case Study
## 4 Experimental Results and Discussion
### Experimental Results for C1  
Case C1 represents the normal operation, where current and voltage waveforms, electrical quantities and harmonic distortions of the studied system are analyzed under normal operating conditions. The experimental results for C1 are given as follows.  
The steady-state data for the low-voltage side of the transformer station when the capacitor stage is switched on and off are given in Table 1. The voltage and current waveforms of transformer T1 are distorted as shown in Figure 8. In C1, transformers T1, T2, and T3 are on, and since they have the same ratings and operate in parallel, it can be assumed that the same conclusions apply to T2 and T3. Therefore, the secondary voltage of the transformers is distorted and the current waveform is typical of a system with three-phase diode bridge rectifiers. The value of \(THD_{U}\) is acceptable by industrial standards. After switching on the capacitor stage, the harmonic distortion of voltage and current has decreased because the capacitor stage partially filters the current harmonics.  
The voltage and current waveforms of the transformer T1, the reactor and the capacitor of the switched-on stage during switch-on are shown in Figure 9. When the capacitor stage is switched on, the reactor goes into saturation and the maximum values of the stage current reach values up to 15 times the nominal value. The transient state lasts less than 50 ms, after which the reactor reaches the normal operating state.  
The reactor voltage, capacitor voltage, and stage current in the steady state after the stage is switched on are shown in Figure 10. The stage current is \(I_{S}=74.2\) A, and \(THD_{I_{S}}=38.7\) %. The stage current is distorted because the series LC circuit filters some of the current harmonics. The highest harmonic in the stage current is the 5 th harmonic.  
The spectrum of reactor voltage, capacitor voltage and stage current is shown in Figure 11. The harmonic magnitudes are given as a percentage of the fundamental and the scale is logarithmic. It can be seen that the 5 th and 7 th harmonics of the stage current have significant values. The current of the 5 th harmonic is 26.2 A.  
The measurement study was also performed on the medium voltage side of the transformers. The waveforms of the voltage and current of the medium voltage feeder are also distorted, as shown in Figure 12. The total harmonic distortion (THD) of the current is \(THD_{l,T1}=3.6\)% and the THD of the voltage is \(THD_{U,T1}=1.9\)%. The harmonic distortion on the medium voltage side is lower than that on the low voltage side, which is  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**State** & \(P_{T1}\) [kW] & \(Q_{T1}\) [kvar] & \(cos\varphi_{T1}\) & \(I_{T1}\) [A] & \(THD_{l,T1}\) [\%] & \(THD_{u}\) [\%] \\ \hline Cap. stage off & 544.6 & 111.5 & 0.98 & 797.9 & 7.8 & 3.5 \\ Cap. stage on & 542.7 & 93.4 & 0.99 & 789.0 & 7.1 & 3.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: C1: Electrical quantities on the low-voltage side of the transformer station.  
Figure 8: C1—Secondary voltage and current of transformer T1 before switching on the capacitor stage.  
expected since the current harmonic sources are connected to the low voltage side of the transformer station.  
Figure 10: C1—Capacitor voltage, reactor voltage and stage current after the stage is switched on.  
Figure 9: C1—Transformer secondary voltage, capacitor voltage, reactor voltage and stage current when the stage is switched on.