Formula Sheets: Switchgear & Protection | Power Systems - Electrical Engineering (EE) PDF Download

 Page 1


P o w er Systems: Switc hgear & Protection F orm ula
Sheet for Electrical GA TE
Protectiv e Rela ys
• Plug Setting Multiplier (PSM) : Ratio of fault curren t t o rela y pic kup curren t.
PSM =
I
fault
I
pic kup
where I
fault
is the fault curren t, I
pic kup
= CT ratio× Plug se tting.
• Time Multiplier Setting (TMS) : A djusts op erating tim e of in v erse time rela ys.
t = TMS× Time from c haracteristic curv e
• Op erating Time for In v erse Definite Minim um Time (IDMT) Rela y :
t =
(
k
PSM
a
-1
)
× TMS
where k and a are constan ts (e.g., for standard in v erse: k = 0.14 , a = 0.02 ).
Circuit Break ers
• Breaking Capacit y : Maxim um fault curren t a break er can in terrupt.
Breaking Capacit y (MV A) =
v
3×V
LL
×I
fault
where V
LL
is line-to-line v oltage (k V), I
fault
is fault curren t (kA).
• Making Capacit y : P eak curren t a break er can close in to.
Making Capacit y = 2.55× Symmetrical Breaking Curren t
(F or 50 Hz system, p eak factor = 2.55).
• Short-Time Rating : Curren t a break er can withstand for a short duration.
I
short-time
= Rated short-circuit curren t
Protection Sc hemes
• Differen tial Protection : Curren t difference b et w een t w o p oin ts.
I
diff
=|I
1
-I
2
|
Rela y op erates if I
diff
> I
bias
(bias curren t setting).
1
Page 2


P o w er Systems: Switc hgear & Protection F orm ula
Sheet for Electrical GA TE
Protectiv e Rela ys
• Plug Setting Multiplier (PSM) : Ratio of fault curren t t o rela y pic kup curren t.
PSM =
I
fault
I
pic kup
where I
fault
is the fault curren t, I
pic kup
= CT ratio× Plug se tting.
• Time Multiplier Setting (TMS) : A djusts op erating tim e of in v erse time rela ys.
t = TMS× Time from c haracteristic curv e
• Op erating Time for In v erse Definite Minim um Time (IDMT) Rela y :
t =
(
k
PSM
a
-1
)
× TMS
where k and a are constan ts (e.g., for standard in v erse: k = 0.14 , a = 0.02 ).
Circuit Break ers
• Breaking Capacit y : Maxim um fault curren t a break er can in terrupt.
Breaking Capacit y (MV A) =
v
3×V
LL
×I
fault
where V
LL
is line-to-line v oltage (k V), I
fault
is fault curren t (kA).
• Making Capacit y : P eak curren t a break er can close in to.
Making Capacit y = 2.55× Symmetrical Breaking Curren t
(F or 50 Hz system, p eak factor = 2.55).
• Short-Time Rating : Curren t a break er can withstand for a short duration.
I
short-time
= Rated short-circuit curren t
Protection Sc hemes
• Differen tial Protection : Curren t difference b et w een t w o p oin ts.
I
diff
=|I
1
-I
2
|
Rela y op erates if I
diff
> I
bias
(bias curren t setting).
1
• Distance Protection (Imp edance Rela y) :
Z =
V
I
Rela y op erates if measured imp edance Z falls within the protection zone (e.g., Z <
Z
set
).
• Ov ercurren t Protection Pic kup Curren t :
I
pic kup
= CT ratio× Plug setting
• Zone of Protection for Distance Rela y :
Z
zone
= 0.8 to 0.9×Z
line
( Zone 1)
Z
zone
= 1.2 to 1.5×Z
line
( Zone 2)
F ault Calculations for Protection
• F ault Curren t for Three-Phase F ault :
I
f
=
V
pre-fault
Z
1
where Z
1
is p ositiv e sequence imp edance.
• Single Line-to-Ground F ault Curren t :
I
f
=
3V
pre-fault
Z
1
+Z
2
+Z
0
+3Z
f
where Z
0
, Z
1
, Z
2
are zero, p ositiv e, and negativ e sequence imp edances, Z
f
is fault
imp edance.
Key Notes
• CT Ratio : Curren t transformer ratio, e.g., 400/5 A.
• Standard IDMT Curv es : Normal In v erse (k = 0.14 , a = 0.02 ), V ery In v erse
(k = 13.5 , a = 1 ), Extremely In v erse (k = 80 , a = 2 ).
• Co ordination Time In terv al (CTI) : T ypically 0.3–0.4 s f or rela y co ordination.
• Base V alues : Use system base MV A and k V for p er-unit calculations.
• Zones of Protection : Zone 1 (80–90% of line), Zone 2 (120–150% of line), Zone 3
(bac kup).
2