Medium Voltage Application Guide

Time (s)
SWITCHGEAR
Protection Devices
Protection devices are used in medium voltage distribution systems to protect line and cable feeders, busbar
systems, transformers, motors, generators and power factor correction banks. Abnormal conditions can be
detected based on secondary current and voltage measurement, or temperature monitoring using thermal devices.
When an abnormal condition is detected, correct coordination of the protection devices will rapidly isolate the fault
to a specific zone in the system.
Older protection devices relied on electromechanical relays to measure system parameters. Modern protection
devices are exclusively low consumption, digital, and microprocessor based, with many communication options
available. Modern high-end protection devices incorporate many parameter settings along with programmable
logic control which provides not only protection functions, but switchgear control and interlocking.
Protection devices are primarily covered by IEC 60255-1.
Protection functions
Overcurrent
This is the most widely used form of protection. In a 3-phase power system, all three line currents are measured
using current transformers. The secondary output of a 1 A or 5 A current transformer is connected to the current
input of a protection device. Within the protection device, analog signals are filtered and sampled before being
converted to digital signals for processing. A trip condition occurs if a preset current level is exceeded for a specific
time.
There are two basic methods of overcurrent protection:
Time-overcurrent protection - provides overload protection similar to a bimetallic thermal overload
device, except thermal modelling adjusts the trip curve shape to allow for dynamic heating and cooling
conditions. If the measured current reaches a point on the overload curve, a trip will occur.
Time-overcurrent is also referred to as "inverse" (I) protection
Instantaneous overcurrent protection - provides medium level and high level short circuit protection. If a
set current level is exceeded for set time period, a trip will occur. This protection is also referred to as
definite minimum time (DMT) protection.
Devices may combine time-overcurrent and instantaneous overcurrent protection. Time-overcurrent is normally
only applied to electrical machines such as transformers and motors, whereas instantaneous overcurrent is applied
to cables, busbar systems etc. Overcurrent protection can be directional, which is sometimes used for more
advanced selective isolation of faults.
Overcurrent protection
1
Time overcurrent (I)
Instantaneous overcurrent (DMT)
2
Current (A)
13753.A
710-12280-00A Medium Voltage Application Guide Page 121