Aug 08, 2015 - 07:05 PM
Short Circuit Ratio
In order to represent the AC systems a short circuit ratio (SCR) has been applied. The SCR is an index for describing the strength of the AC system. It is determined by the Thevenin impedance of the AC system. If the Thevenin impedance is low, the system stability is better. Very strong grids can often be idealized by a stiff grid. A stiff grid is represented by only an AC power source. However if the Thevenin impedance is higher, the system is weaker. Should an AC fault occur, the AC voltage would drop substantially. The SCR is defined as the short circuit level (SCL) at the AC busbar divided by the rated DC power Pd given by Eqn. 3.49,
Using the definition od SCR, a system can be categorized as,
• Strong system, SCR > 3.
• Weak system, 2 < SCR > 3.
• Very weak system, SCR < 2.
The rest of the section will develop equivalent impedance circuits to represent the strength of the system, i.e. the SCR. The AC impedance absolute value |Zac| and its angle φ is calculated with ,