Rockwell Automation 825 Smart Motor Manager User Manual - Manual (Page 63)

Functions 3-30
Publication 825-UM001B-EN-P January 2001
A rule of thumb for industrial medium voltage networks is to allow about 1 A of capacitive
earth current for every 1 000…1 500 kVA of system power. Usually, the displacement voltage
is measured at a single neutral point and is assumed to be representative of the entire network.
The earth (ground) fault is localized by using an earth (ground) fault current detector, such as
the Smart Motor Manager with earth (ground) fault protection, in the motor feeders. Often,
operation can continue since the earth currents are comparatively insignificant and the
insulation of the non-faulty phases can be operated at a higher voltage for a short period of
time.
Isolated or High-Impedance Earth Networks
In the case of earth capacitances, the neutral point of the network assumes earth potential and
the sum of the currents flowing through the earth capacitances is zero. Also, no current flows
in normal operation in the high-value earth resistor (Figure 3.16, Figure 3.18, and Figure 3.20)
in the case of transformer with neutral point. It avoids extreme overvoltages in the event of
intermittent earth faults, such as can occur in isolated networks.
If, for example, phase conductor 3 (Figure 3.15 and Figure 3.17) becomes connected to earth
due to an earth fault, the two other phase conductors carry a line-to-line voltage with respect
to earth. Through their earth capacitances, C
N
(on the power supply side as seen from the
Smart Motor Manager) and C
M
(on motor side), a capacitive current flows toward earth and
back to phase conductor 3 through the fault location. In the case of high-impedance earthing
(Figure 3.16, Figure 3.18, and Figure 3.20), the neutral point voltage, now at a high value,
causes an additional current that is limited by the earthing resistor through the fault location.
In the event of an earth fault on the power supply side of the measuring location (current
transformer installation location), the basic unit measures the component of the earth current
flowing via C
M
. The response sensitivity must be selected such that in this case, the Smart
Motor Manager does not trip. On the other hand, earth fault detection by the Smart Motor
Manager should be as sensitive as possible since, in the case of earth faults in the motor
windings, the displacement voltage becomes smaller the closer the fault location is to the
neutral point. The fault current decreases proportionally. Normally, a response threshold is
selected that is greater than 5…10% of the current that flows in the event of a dead earth
fault at the motor terminals.
Neutralized Networks
Systems with earth fault neutralizers, resonant-earth system, Petersen coil. Although
compensated industrial networks are rare, their main features are shown in Figure 3.16,
Figure 3.18, and Figure 3.20. Under fully compensated conditions, the compensation reactor
supplies a current of the same magnitude as that of the capacitive fault current but phase
shifted by 180
° so that only a small ohmic residual current flows via the fault location.