When setting the relay characteristic/torque angle for the three phase directional overcurrent element, a positive angle setting was specified. This was due to the fact that we consider as polarising voltage, the phase voltage value in quadrature of the current under fault conditions. With directional earth fault protection, the residual current under fault conditions lies at an angle lagging the polarising residual earth fault voltage.
Replacing an MWTU11/TWL1111 Reverse Power / Forward Power Relay.
The MWTU11/TWL1111 was not strictly a power measuring relay. The relay operated using
an Icosø characteristic. Provided enough voltage was present to polarise the measuring
element, a 1% setting on a 1A relay would pickup at 10mA independent of the voltage
magnitude applied.
By selecting the 0.01 to 8Ien, model number MiCOM P125B, A setting range of 1 to 100% of
the nominal current in 0.1% steps is available.
2.6.5 Application of the Power Function
The standard configuration of the MiCOM P125 when set to use the wattmetric earth fault
protection can measure the zero sequence power, the connection for this can be found in the
sales publication and service manual. By adopting the connection above the relay can
measure the phase current and phase to phase, or phase to ground voltage, the operate
quantity when using these connections is a single phase power. As power protection is
generally required to operate for balanced three phase conditions basing the protection on a
single phase measured quantity is of no consequence.
When using the Power Function of the relay (Pe) care must be taken when selecting the
relay range. If the MiCOM P125CA relay is considered and is connected phase to ground
with 1A CT secondaries. A 0.2W setting would be equivalent to 0.2/63.5 Watts, 0.3% of the
nominal load.
2.7 Thermal Overload Protection (P126 & P127)
Thermal overload protection can be applied to prevent damages to the electrical plant
equipment when operating at temperatures in excess of the designed maximum withstand. A
prolonged overloading causes excessive heating, which may result in premature
deterioration of the insulation, or in extreme cases, insulation failure.
MiCOM P126 & P127 relays incorporate a current based thermal replica, using load current
to reproduce the heating and cooling of the equipment to be protected. The element thermal
overload protection can be set with both alarm and trip stages.
The heating within any plant equipment, such as cables or transformers, is of resistive type
(I²R x t). Thus, the quantity of heat generated is directly proportional to current squared (I²).
The thermal time characteristic used in the relay is based on current squared, integrated
over time.
The MiCOM P126 & P127 relays automatically use the highest phase current as input
information for the thermal model.
Protection equipment is designed to operate continuously at a temperature corresponding to
its full load rating, where heat generated is balanced with heat dissipated by radiation etc.
Over-temperature conditions therefore occur when currents in excess of rating are allowed to
flow for a certain period of time. It can be shown that temperatures during heating follow
exponential time constants and a similar exponential decrease of temperature occurs during
cooling.
In order to apply this protection element, the thermal time constant (Te) of the plant
equipment to be protected is therefore required.
The following sections will show that different plant equipment possesses different thermal
characteristics, due to the nature of their construction.
