The internal causes that give rise to over-voltages will be discussed in detail below:
Internal Cause # 1. Switching Operations on Unloaded Line:
A switching operation produces a sudden change in the circuit conditions. When an open-ended line is connected to a source of voltage, travelling waves are set up which rapidly charge the line. On reaching the open end of the line, these waves are totally reflected without change of sign, thereby producing voltage-doubling at that end. These reflected waves travel back to the supply end, giving rise to further reflections.
Actually, owing to losses, the waves gradually attenuate and the line ultimately attains the normal voltage. Hence the maximum potential attained by the line at the instant of switching-in cannot exceed twice the maximum supply voltage Vmax or 2√2 Vrms where Vrms is rms value of phase voltage. In practice, owing to the dissipation of energy, the potential attained by the line would be somewhat less than this theoretical maximum. If the surge impedance of the line is Z0 then current will fluctuate between
and 
sinusoidally.
Analogous phenomena are produced on switching out an open-ended line. Travelling waves are set up and the line momentarily attains the voltage of the magnitude not exceeding twice the supply voltage at the instant of disconnection.
Internal Cause # 2. Sudden Opening of Loaded Line:
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If a line carrying load is suddenly opened, a transient voltage of value given by e = i Z0 is set up, where i is the instantaneous value of the current at the instant of opening of line, and Z0 is the natural or surge impedance of the line.
If i = 250 A and Z0 = 500 ohms, the transient rise of voltage, however, rapidly the current is interrupted cannot possibly exceed 250 x 500, i.e., 125,000 volts, because this is the maximum value of the voltage wave necessary to store in the electrostatic field the whole of the energy stored in the magnetic field at the instant of break. If Vmax is the peak value of the operating voltage, the maximum voltage to which the line may be subjected is Vmax + 125,000 volts.
From the above discussion it is obvious that transient voltage rise by the sudden interruption of load is not a function of the line voltage and, therefore, low voltage transmission systems are liable to overvoltage of the same magnitude as high voltage systems. The higher safety factor of the insulation on low voltage systems is thus justified.
The worst case that could happen would be the interruption of the short circuit current of the system at its peak value, when voltage rises of several hundred kilovolts might be caused. Fortunately, the use of oil circuit breakers has practically eliminated all danger on account of short circuits, as these breakers open the circuit when the current wave has its zero value.
Internal Cause # 3. Insulation Failure:
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The insulation failure in a power system may take place in various ways such as between the conductors of an overhead line or the cores of an insulated cable, or between one conductor, or core, and earth.
The failure of insulation between the line and earth is very frequent. When there is a breakdown of insulation to earth, the potential at fault suddenly falls from maximum to zero and, therefore, a negative voltage wave of very steep front in the form of surge travels from the fault in both directions.
Let the line AB be earthed at point F. Let the instantaneous voltage of the line with respect to earth at the instant of failure of insulation be 2v. The potential at the fault F suddenly falls to zero and therefore a negative voltage, of amplitude v, and of very steep front, travels from the fault in both directions.
The reduction of voltage to one-half is due to the equal dimension of the energy into electromagnetic and electrostatic energy. Each voltage wave is accompanied by a current wave of amplitude v/Z0 and consequently the current to earth, if there is an extremely low tower footing resistance -in the case of an overhead line is – (2v/z0).
As these waves travel to the ends of the line they reduce the voltage to zero; and when they reach the open ends, reflected waves are set up which reduce the voltage to v – v – v, i.e. – v and the current is neutralized.
When the reflected waves reach F, the portions of the line along which they have travelled will be charged to – v. The current at F can be reversed by a flashover in the opposite direction, and the result is a periodic flashover with reversals of potential on the line and currents at F until the stored energy is dissipated by damping.
Internal Cause # 4. Arcing Grounds:
It is experienced in insulated neutral system. Consider an alternator, whose one phase has been connected to a long line which has got distributed inductance and capacitance to earth, as shown in Fig. 9.2. The alternator winding can be imagined to be connected to earth through its capacitance.
Let a spark gap be connected between the line and earth. The alternating emf impressed on the circuit by the alternator will charge the line at each half cycle and if the line pd becomes high enough the gap will breakdown and the circuit will then resonate at its own natural frequency.
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Similar conditions arise when an earth fault develops on a transmission line connected to an all- insulated system. On occurrence of flashover, as by the puncture of an insulator on an overhead line, accidental contact with trees or by the breakdown of a weak spot in the insulation of an underground cable, the conductor is discharged through the agency of a transient high-frequency oscillation.
The faulty conductor is thus brought down to earth potential, and the other two conductors of the three phase system correspondingly rise in voltage from the phase to the line voltage. As soon as the conductor is discharged to ground, the spark gap to earth opens, and the conductor then charges again from the power supply of the system, and the cycle is repeated.
In themselves, oscillations such as the above are not particularly dangerous, but, if continued, the resulting voltage rise may result in a breakdown of the insulation at some other point, and possibly on another conductor. In such a case double fault to earth will be formed, and it will be equivalent to a short circuit.
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Surges and other transient phenomena due to switching operations are taking place in such a way that the initiating voltage is the system voltage, are rarely dangerous in themselves, partly because attenuation may limit serious voltage rise, and partly because the associated energy is small. The danger is when a flashover may open a path for a power arc, much as the stepped leader of a lightning flash paves the way for the main discharge.
An example is the double fault which may result from an arcing ground, as explained above. Because of the troubles due to arcing grounds power systems are rarely operated with insulated neutral. Instead the system neutral is grounded solidly (i.e., without any impedance between the neutral and the ground) or through a resistance or reactance.
Internal Cause # 5. Resonance:
Resonance in an electrical circuit implies that the impedance of the circuit is purely resistive and the power factor is unity. Thus at resonance the inductive reactances and capacitive reactances cancel out.
In usual transmission lines the capacitance is usually so small that resonance cannot occur at the fundamental supply frequency, but if the generator emf wave is distorted, trouble may be experienced due to resonance at one at the higher harmonics.