Areva MiCom P120, Техническое руководство

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Application Guide P12x/EN AP/A96
MiCOM P120/P121/P122/P123
Page 29/80
11. THERMAL OVERLOAD PROTECTION (P122 & P123 ONLY)
Thermal overload protection can be applied to prevent damages to the equipment of the
electrical plant when operating at temperatures that are above the values designed for
maximum withstand. A prolonged overloading causes excessive heating, which may result in
premature deterioration of the insulation, or in extreme cases, insulation failure.
MiCOM P122 & P123 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 the current squared
(I²). The thermal time characteristic used in the relay is based on current squared, integrated
over time.
MiCOM P122 & P123 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.
11.1 Time Constant Characteristic
This characteristic is used to protect cables, dry type transformers (e.g. type AN), and
capacitor banks.
The thermal time characteristic is given by:
( )
( )
( )
2
p 2
2
FLC
2
t
I I
I k I
e
−
× −
=
⎟
⎠
⎞
⎜
⎝
⎛
τ
−
Where:
t = Time to trip, following application of the overload current, I
τ
= Heating and cooling time constant of the protected plant equipment
I = Largest phase current
I
FLC
= Full load current rating (relay setting ‘Thermal Trip’)
k = 1.05 constant, allows continuous operation up to < 1.05 I
FLC
I
P
= Steady state pre-loading current before application of the overload
The time to trip varies depending on the load current carried before application of the
overload, i.e. whether the overload was applied from “hot” or “cold”.
Curves of the thermal overload time characteristic are presented in the chapter
P12x/EN TD/C55 of the Technical Guide.