Voltage Protection; Frequency Protection; Underfrequency Protection - GE MiCOM P40 Agile Technical Manual

Application Notes
MiCOM P40 Agile P442, P444
4.12

Voltage protection

Voltage protection contains undervoltage and overvoltage protection.
Undervoltage conditions may occur on a power system for a variety of reasons, some of
which are outlined below:
• Increased system loading. Generally, some corrective action would be taken by
voltage regulating equipment such as AVRs or On Load Tap Changers, to bring the
system voltage back to its nominal value. If the regulating equipment is unsuccessful
in restoring healthy system voltage, then tripping using an undervoltage relay is
required following a suitable time delay.
• Faults occurring on the power system result in a reduction in voltage of the phases
involved in the fault. The proportion by which the voltage decreases is directly
dependent on the type of fault, method of system earthing and its location with respect
to the relaying point. Consequently, co-ordination with other voltage and current-based
protection devices is essential to achieve correct discrimination.
This function will be blocked with Voltage Transformer Supervision (VTS) logic or could be
disabled if CB open.
Both the under and overvoltage protection functions can be found in the relay menu "Volt
Protection". The following table shows the undervoltage section of this menu along with the
available setting ranges and factory defaults. Different time settings may be required
depending upon the severity of the voltage dip.
In the majority of applications, undervoltage protection is not required to operate during
system earth fault conditions. If this is the case, the element should be selected in the menu
to operate from a phase to phase voltage measurement, as this quantity is less affected by
single-phase voltage depressions due to earth faults.
The voltage threshold setting for the undervoltage protection should be set at some value
below the voltage excursions which may be expected under normal system operating
conditions. This threshold is dependent upon the system in question but typical healthy
system voltage excursions may be in the order of -10% of nominal value.
Similar comments apply with regard to a time setting for this element, i.e. the required time
delay is dependent upon the time for which the system is able to withstand a depressed
voltage.
The inclusion of the stages in the settings and their respective operating characteristics
allows for a number of possible applications:
• Use of the IDMT characteristic gives the option of a longer time delay if the
overvoltage condition is only slight but results in a fast trip for a severe overvoltage. As
the voltage settings for both of the stages are independent, the second stage could
then be set lower than the first to provide a time delayed alarm stage if required.
• Alternatively, if preferred, both stages could be set to definite time and configured to
provide the required alarm and trip stages.
• If only one stage of overvoltage protection is required, or if the element is required to
provide an alarm only, the remaining stage may be disabled within the relay menu.
This type of protection must be co-ordinated with any other overvoltage relays at other
locations on the system. This should be carried out in a similar manner to that used for
grading current operated devices.
4.13

Frequency protection

The frequency protection menu contains underfrequency and overfrequency protections,
individually activated when the corresponding status is activated.
4.13.1

Underfrequency protection

Frequency variations on a power system are an indication that the power balance between
generation and load has been lost. In particular, underfrequency implies that the net load is
in excess of the available generation. Such a condition can arise, when an interconnected
system splits, and the load left connected to one of the subsystems is in excess of the
P44x/EN AP/Hb6
(AP) 5-115