TRANSIENTS IN SUBSTATION BASIC INFORMATION AND TUTORIALS

What are the origin of transients in substation?

a) High-voltage switching.

Opening or closing a switching device to de-energize or energize a section of substation bus is generally accompanied by arcing and will initiate a high-frequency transient. The frequency will be determined by the self-inductance and shunt capacitance of the high-voltage conductors involved. The resulting overvoltages can exceed two per unit. Both electric and magnetic coupling between high-voltage and low-voltage conductors can result in high-level transients in the low-voltage system.

b) Capacitor switching.

Switching a capacitor bank causes a current transient which is a function of the bank size and the circuit constants back to the source. If other capacitors are already connected nearby to the same line or bus, they lower the impedance seen by the switched capacitor, increasing the magnitude and frequency of the transient. Energy stored in the nearby bank may contribute further to the severity.

The circuit between banks is likely to ring at a high frequency because of the low inductance in the short line connecting the banks and the reduced effective capacitance considering the banks in series. This phenomenon further enhances the tendency of the transient to interfere with nearby circuits.

c) Transmission line switching.

This phenomenon is similar to capacitor bank switching, with the difference being the distributed nature of the inductance and capacitance of the line. The magnitude of the line charging current tends to be substantially less than that for capacitor bank switching. The frequency of the transient current or voltage is inversely proportional to the line length.

d) Coupling capacitor voltage transformers (CCVT).

The capacitors in these devices, in conjunction with inductances of the power system conductors, constitute a resonant circuit whose frequency can be in the megahertz range. Unless the base of the CCVT has a low-surge impedance to the substation ground grid, a high voltage can appear between the CCVT secondary terminals and the grid. The high voltage will be generated primarily during air-break switching operations.

e) Ground voltage rise (GVR).

GVR is the voltage rise proportional to the magnitude of the ground current and to the ground resistance. Under normal conditions, the grounded electrical equipment operates at essentially zero ground voltage within the substation yard. During a fault, the portion of fault current which is conducted by a ground electrode in the earth causes a rise of the electrode voltage with respect to remote earth (see IEEE Std 80-1986 and [B26]).

f) Ground voltage rise differences.

Both electromagnetic coupling and conduction can contribute to substantial ground voltage rise differences, particularly at the higher frequencies typical of many transients occurring on a high-voltage power system. Even well designed grounding grids that extend over the large areas needed for high-voltage switchyards have sufficient inductance to cause high voltage differences.

Electromagnetic coupling to the ground grid is directly proportional to the rate of change of flux and the length and orientation of the current-carrying conductor and inversely proportional to the height of the conductor above the ground grid.

Conduction of power system transients to the ground grid is typically provided through metallic grounding of transformer neutrals and capacitive paths, such as bushings, coupling capacitors, and CCVTs. These are low-impedance high-energy paths that can induce common-mode voltages on control circuits (see IEEE Std 367-1987 ).

g) Other transient sources.

Other phenomena that generate transients occur in power systems. Some examples are undesirable time spans between the closing of the poles of a circuit breaker, fault occurrence, fault clearing, load tap changing, line reactor de-energizing, series capacitor gap flashing, arcing ground faults, failing equipment, lightning, GIS surges, and capacitor reinsertion. Normally, the magnitudes of such transients are less than those of other phenomena described herein.