MSCDN – MP2A Applications Guide
1 Introduction
The MSCDN-MP2A relay provides wide bandwidth, true RMS phase-by-phase Resistor Thermal Overload Protection
and Resistor Open Circuit Protection and is suitable for capacitor bank applications. Together with its sister units
MSCDN-MP1 and MP2B, this protection unit offers a complete solution for Main 1 and Main 2 protection of EHV
capacitor banks.
These notes give guidance on the application of the relay and the protection elements integrated in it, reference may be
made to the Commissioning Chapter, which provides detailed set-up instructions.
2 Resistor Thermal Overload Protection
2.1 Fault Setting Principles
The operate time of the thermal elements is given by
(
)
sec
I
I
I
ln
2
2
2
P
2
⎭
⎬
⎫
⎩
⎨
⎧
×
−
−
×
τ
=
B
I
k
t
…(Eq. 1)
Where
I
P
= Previous steady state current level
I
B
= Basic current rating of resistor
k = Multiplier resulting in the overload pickup setting k.I
B
= I
θ
I = The measured resistor current
τ
= Thermal time constant
Thermal
State
Time
100%
Heating
Cooling
Available
Thermal
Capacity
Figure 1 – Exponential heating and cooling curves
For the heating curve:
For the cooling curve:
%
100
)
1
(
2
2
×
−
⋅
=
−
τ
θ
θ
t
e
I
I
….(Eq.2)
θ θ
τ
=
⋅
−
F
t
e
……(Eq.3)
where:
θ
= thermal state at time t
θ
F
= final thermal state before disconnection of device
I = measured thermal current
I
θ
= thermal overload current setting (or k.I
B
)
τ
= thermal time constant
©2010 Siemens Protection Devices Limited
Chapter 5 Page 3 of 7
