5 RESIDUAL OVERVOLTAGE PROTECTION
On a healthy three-phase power system, the sum of the three-phase to earth voltages is nominally zero, as it is the
v
ector sum of three balanced vectors displaced from each other by 120°. However, when an earth fault occurs on
the primary system, this balance is upset and a residual voltage is produced. This condition causes a rise in the
neutral voltage with respect to earth. Consequently this type of protection is also commonly referred to as 'Neutral
Voltage Displacement' or NVD for short.
This residual voltage may be derived (from the phase voltages) or measured (from a measurement class open
delta VT). Derived values will normally only be used where the model does not support measured functionality (a
dedicated measurement class VT). If a measurement class VT is used to produce a measured Residual Voltage, it
cannot be used for other features such as Check Synchronisation.
This offers an alternative means of earth fault detection, which does not require any measurement of current. This
may be particularly advantageous in high impedance earthed or insulated systems, where the provision of core
balanced current transformers on each feeder may be either impractical, or uneconomic, or for providing earth
fault protection for devices with no current transformers.
5.1 RESIDUAL OVERVOLTAGE PROTECTION IMPLEMENTATION
Residual Overvoltage Protection is implemented in the RESIDU
AL O/V NVD column of the relevant settings group.
Some applications require more than one stage. For example an insulated system may require an alarm stage and
a trip stage. It is common in such a case for the system to be designed to withstand the associated healthy phase
overvoltages for a number of hours following an earth fault. In such applications, an alarm is generated soon after
the condition is detected, which serves to indicate the presence of an earth fault on the system. This gives time for
system operators to locate and isolate the fault. The second stage of the protection can issue a trip signal if the
fault condition persists.
The product provides two stages of Residual Overvoltage protection with independent time delay characteristics.
Stage 1 provides a choice of operate characteristics, where you can select between:
● An IDMT characteristic
● DT (Definite Time)
The IDMT characteristic is defined by the following formula:
t = K/( M - 1)
where:
● K= Time multiplier setting
● t = Operating time in seconds
● M = Derived residual voltage setting voltage (VN> Voltage Set)
You set this using the VN>1 Function setting.
Stage 1 also provides a Timer Hold facility.
Stage 2 can have definite time characteristics only. This is set in the VN>2 status cell
The device derives the residual voltage internally from the three-phase voltage inputs supplied from either a 5-limb
VT or three single-phase VTs. These types of VT design provide a path for the residual flux and consequently permit
the device to derive the required residual voltage. In addition, the primary star point of the VT must be earthed.
Three-limb VTs have no path for residual flux and are therefore unsuitable for this type of protection.
P54A/B/C/E Chapter 10 - Voltage Protection Functions
P54xMED-TM-EN-1 231
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