Investigation of Ferroresonance Phenomena in Electric Power Systems
Mahmoud Mohamed Hassanin Ibrahim;
Abstract
Both analytical and experimental studies have been performed for the investigation of ferroresonance phenomena. The effect of supply voltage, capacitance, frequency and resistance on ferroresonance phenomena have been studied.Ferroresonance is normally initiated after some types of switching event such as load rejection, fault clearing, transformer energisation, single-phase switching or loss of system grounding.
The experimental and analytical results show that the ferroresonance is not only a function of the capacitance and nonlinear inductance of the network components but also on the supply voltage, this voltage is variable over a wide range in some distribution networks. It has been concluded that the change in supply voltage to a circuit with probable to ferroresonance may directly lead to its occurrence. In distribution networks a slow or fast change in voltage over the feeders may lead to ferroresonance happening in some points where lightly loaded or unloaded transformers are connected. In practical distribution networks; switching transient conditions could cause slight and successive increase in supply voltage till a ferroresonance condition is achieved. It is recommended that thetransformer core configuration must be considered and saturation characteristics above rated voltage must be accurately known.
The power system engineer should be aware that a lightly-loaded transformers operating in the presence of source or shunt capacitance may experience ferroresonance, capacitance exists in the form of cables, series or shunt capacitor banks, or stray capacitances in inadequately-grounded portions of the system. The presence of power factor correction capacitor banks helps the increase of the probability of ferroresonance occurrence.
Testing the transformer at 100 Hz frequency has shown that the knee point occurs at much higher voltage than its occurrence at 50 Hz, this makes the ferroresonance less probable to occur at this frequency. Therefore the tendency of less occurrence of ferroresonance can be correlated to higher frequency occurrence. The 60 Hz system may be better prepared to withstand the ferroresonance phenomena. In the mobile power systems of normal frequency of 400 Hz, ferroresonance phenomena is less probable to occur, So It can be concluded that with higher frequency of networks the occurrence of ferroresonance becomes harder to happen due to the lower level of saturation of transformers under normal voltages. As the studyshowsthat the ferroresonance phenomena depends on the saturation characteristics of transformers. So, it is recommended for transformer’s designer to choose the operating points relatively far from the knee.
The experimental study for the influence of resistance on ferroresonance phenomenon shows that the occurrence of ferroresonance is more likely in the absence of adequate damping. A lightly-loaded or unloaded transformer fed through a capacitive source impedance is a prime candidate for ferroresonance. Damping added to the circuit will weaken the ferroresonant voltage and current. Most damping is due to the load applied to the secondary of the transformer, other damping is present in the form of resistive source impedance, transformer losses, and also corona losses in high voltage systems. So it is recommended to avoid any switching operations that will reconfigure a circuit into a sudden inclusion of capacitance connected in series with transformer with no or light load condition.
The experimental and analytical results show that the ferroresonance is not only a function of the capacitance and nonlinear inductance of the network components but also on the supply voltage, this voltage is variable over a wide range in some distribution networks. It has been concluded that the change in supply voltage to a circuit with probable to ferroresonance may directly lead to its occurrence. In distribution networks a slow or fast change in voltage over the feeders may lead to ferroresonance happening in some points where lightly loaded or unloaded transformers are connected. In practical distribution networks; switching transient conditions could cause slight and successive increase in supply voltage till a ferroresonance condition is achieved. It is recommended that thetransformer core configuration must be considered and saturation characteristics above rated voltage must be accurately known.
The power system engineer should be aware that a lightly-loaded transformers operating in the presence of source or shunt capacitance may experience ferroresonance, capacitance exists in the form of cables, series or shunt capacitor banks, or stray capacitances in inadequately-grounded portions of the system. The presence of power factor correction capacitor banks helps the increase of the probability of ferroresonance occurrence.
Testing the transformer at 100 Hz frequency has shown that the knee point occurs at much higher voltage than its occurrence at 50 Hz, this makes the ferroresonance less probable to occur at this frequency. Therefore the tendency of less occurrence of ferroresonance can be correlated to higher frequency occurrence. The 60 Hz system may be better prepared to withstand the ferroresonance phenomena. In the mobile power systems of normal frequency of 400 Hz, ferroresonance phenomena is less probable to occur, So It can be concluded that with higher frequency of networks the occurrence of ferroresonance becomes harder to happen due to the lower level of saturation of transformers under normal voltages. As the studyshowsthat the ferroresonance phenomena depends on the saturation characteristics of transformers. So, it is recommended for transformer’s designer to choose the operating points relatively far from the knee.
The experimental study for the influence of resistance on ferroresonance phenomenon shows that the occurrence of ferroresonance is more likely in the absence of adequate damping. A lightly-loaded or unloaded transformer fed through a capacitive source impedance is a prime candidate for ferroresonance. Damping added to the circuit will weaken the ferroresonant voltage and current. Most damping is due to the load applied to the secondary of the transformer, other damping is present in the form of resistive source impedance, transformer losses, and also corona losses in high voltage systems. So it is recommended to avoid any switching operations that will reconfigure a circuit into a sudden inclusion of capacitance connected in series with transformer with no or light load condition.
Other data
| Title | Investigation of Ferroresonance Phenomena in Electric Power Systems | Other Titles | بحث ظاهرة الرنين الحديدى فى نظم القوى الكهربية | Authors | Mahmoud Mohamed Hassanin Ibrahim | Issue Date | 2015 |
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