Negative resistance is a phenomenon seen in many non-linear electronic devices.  In this phenomenon, the current increases as the voltage decreases and vice versa.

This is in opposition to Ohm's law, which states that the current should vary linearly with the voltage.  This phenomenon is seen in electronic components, such as the Gunn diode, the tunnel diode and thyristors.

The fluorescent tube light is an example for negative resistance, when conduction starts, the current rises to a high level.  This is accompanied by a drop in voltage, causing the negative resistance.  To prevent the high current from damaging the device, an inductance (ballast) is connected in series.

There are no negative resistance components, such as resistors.  Certain devices exhibit negative resistance in a particular region of the VI curve.

Power system stability is the stability of a system to respond to disturbances in the system.  Power system stability is also known as synchronous stability.  Power system stability is classified into three types.

They are

  • Steady state stability
  • Transient stability
  • Dynamic stability

Steady state stability is the stability of the system to respond during minor disturbances during normal operations.  The variation in the bus voltages and the phase angles are relatively small.  Steady state stability is caused by switching small loads.

Transient stability, on the other hand, is the analysis of the response of the system to sudden and large variations in the system voltage and phase angles.  This kind of variation is caused by a sudden fault or overload, caused by a tripping of a power source.

Power flow studies need to be conducted to study the response of a power system to steady state and transient faults.

Dynamic stability is the stability which is maintained against small variations.  These variations can coalesce into large variations which can cause large fluctuations and tripping.  Dynamic stability is ensured by the use of sophisticated control equipments.