Effect of load rejection

Load rejection is the sudden loss of a large load, for example when a breaker at the receiving end trips while the generator stays connected to the line. It causes a temporary overvoltage that can last from several cycles up to seconds.

Effects

1. Loss of voltage drop. Before rejection, the load current caused a drop across the generator and transformer reactances. When it vanishes, the terminal voltage jumps towards the internal emf $E'$, which may be 1.1–1.2 pu.
2. Ferranti effect. The line is suddenly at no load. Its charging current raises the receiving-end voltage by the factor $1/\cos\beta l$, which is significant on long EHV lines.
3. Generator overspeed. The prime-mover input cannot be cut instantly, so the machine accelerates. Voltage rises in proportion to speed (and frequency), until the governor acts.
4. Self-excitation. A generator feeding a long open line sees a capacitive load. Its armature current then aids the field flux, and the voltage can build up uncontrolled if the line charging exceeds the machine's capability.
5. The overvoltage is at power frequency and lasts long. Surge arresters cannot absorb it for long, so it sets the arrester rating and the insulation design for temporary overvoltage.

  Gen --[X']--+------- long line -------x CB trips
              |                         (load lost)
          V rises: E' + Ferranti + overspeed

Remedies

• Shunt reactors at the line ends.
• Fast-acting AVR and governor; SVC or STATCOM.
• Generator transformer and line protection that trips the line or generator if the voltage stays high.