LV EQUIPMENT PROTECTION
*Recommended to use ETAP (Electrical Simulator and Analysis) ver. 16.1
PROTECTIVE DEVICES (Fuses, ACB, MCCB, MCB, MCP)
*Requirement for PEE:
Phase Fault Protection
Protection Coordination
Ground Fault Protection *MCP – Motor Circuit Protector
Short Circuit Protection and Voltage Drop
Overload Protection
Arc Flash
LV EQUIPMENT
*IEC and ANSI Standard (Differs in Symbols, Methods and Terms)
Switchgears
Ex: ANSI – Ground
Switchboards
IEC – Earth
LV MCC
Short Circuit Calculation Methods
LV Panels
Transformers
Motors SYSTEM VOLTAGE LEVELS (IEC 60038)
Low Voltage - 1000V
LV CIRCUIT BREAKER CHARACTERISTICS Medium Voltage - 1000V to 35kV
High Voltage - 35kV to 230kV
Thermal (Small MCCBs, MCB)
Extra High Voltage -245kV to 1200kV
Magnetic (MCP)
Thermal-Magnetic (MCCB, MCP)
*Log,Log Graph (Necessary for Coordination)
LV FUSES
Inverse Characteristic *Inverse – The higher the current Thermal – Overload *Thermal in most MCCB is fixed
the faster it act. Magnetic – Short Circuit
LV-ACB (PCB)
Short Time Pickup (STPU) *Fuse is reliable because its fast acting PROTECTION CURVES
Long Time Pickup (LTPU)
Long Time Delay (LTD) Thermal – Magnetic Magnetic Thermal
Short Time Delay
Instantaneous
Sensor / Plug
AT – Ampere Trip *Only in MCB, MCCB
AF – Ampere Frame *Physical size of the breaker *MCP – L Graph (uses magnetic for short circuit protection only)
*Interchangeability purposes Ex: Fire Pump Motor needs to run even on overload or grounded
conditions (will not run on short circuit condition)
*No Ampere Trip in ACB only Ampere Frame because it can be setup) *Magnetic-Only Circuit Breaker is Rare
Locked Rotor Current: 5x FLA
SAMPLE SWITCHGEAR (ACB)
Ex: 20A Motor FLA
LRA = 5(IFL)
= 5 (20)
= 100 A
*Protection system must be fast enough to protect but not fast enough to
lose coordination
Consider Starting Characteristic of Motor
Notes:
*ACB – Consider location and capability to maintain
*For Safety – selection of rating
*Cross and Latch and Interrupting or Making Rating
*Two Rating of Breaker
-Making
-Breaking
*Adjust design based on actual availability of equipment
SWITCH GEAR PROTECTION
234 for copper (zero resistance @ -234.5°C)
Example Problem: 228 for aluminum
Note:
√ *For XLPE: T2=250°C; T1=90°C
CABLE DAMAGE CURVE
Assume Demand Load: MDL = 90A or 100A
MATRIX:
MATRIX:
T (sec) I (A)
T (sec) I (A) (Min) (Max)
0.25 20 kA
1000 500 1 10 kA
0.015 1800
70 1000
PLOT:
Note:
* Directly acting element has tolerance (BAND)
* Band (min and max) PLOT:
* Tolerance is available at product catalog
For conductor protection side:
Use: 70mm2 XLPE (based on catalog)
PLOTTING OF CONDUCTOR (CONSIDERED IN PROTECTION)
ANSI FORMULA: WHERE:
I = ISC (Short circuit, Amps)
( ) ( ) A = Conductor area (Circular mils)
t = Time of short circuit (secs)
IEC FORMULA: T1 = Max operating temp (ex. 75°C)
T2 = Max short circuit temp (ex.150°C)
( ) * ( )+
TRANSFORMER DAMAGE CURVE
Through Fault Protection Curve Example Problem:
Reference:
- Through Fault: IEEE C57
- Short Circuit: IEEE C37
- ANSI Transformer Handbook Assume: ACB, Square D Micrologic 3.0
Check suppliers or catalog (Data can be found at catalogs)
MATRIX:
Graph Point t (sec) (x) multiples @Secondary @Primary
Note: (Not Amps but Multiples)
1 300 3 1506 720
2 100 4 2008 960
Question:
3 20 8 4016 1920
IS 20MVA, 220V Secondary possible?
PLOT:
Consider:
*Band = Range
*ST Band
Short Circuit Current:
*Margin = 1.25%
√ LTPU: 115% ~ 125% of IFL
= 502 (1.2)
~ 600 A
√
*Primary not more than 125% of
rated primary current.
LSIG SETTING, Sensor = 800A
No, Because Short Circuit current
is too high.
⁄
COMBINED LOAD CHARACTERISTIC SHORT CIRCUIT PROTECTION COORDINATION
Phase Fault Protection
Ex: Motor + Static Loads Ground Fault Protection
VARIAC DIAGRAM: Copper Loss
TRANSFORMER SHORT CIRCUIT TEST
*Open Circuit Test – Core Loss
*Do not reach Vn because it will short circuit
VRDG = VAPP = IL ZØ
%Z or Percent Impedance
Z or Per-unit Impedance (w/o 100)
ZPER UNIT
Note:
*Considered in Generator:
*Winding = Copperloss or LineLoss = IL2 R
-Max Step Load (Bulk Increase in load)
-Max Rejection of Load (Bulk Decrease in load)
Z=
-Transient Stability
*Inrush current is only from source (ex. Primary energization)
(x8 of current)
*Linear means not direct acted but simulated electronically
DERIVIATION OF √ POWER TRIANGLE
S3Ø = 3 S1Ø
S1Ø = VØIØ
If Wye: If Delta:
VECTOR DIAGRAM ( ) ( )
√ √
√ √
̅̅̅ ̅̅̅̅ ̅̅̅̅̅ ̅̅̅̅ *Wye or Delta do not affect power formula
POWER BASE: (PB)
1. √
2.
3.
√
√
4.
√ ( √ )
NOTE:
√ *Voltage should not change ZPER UNIT
*Per-unit is unitless
√
THREE PHASE FAULT SHORT CIRCUIT FORMULA Example Problem:
√ √ √
( )
√
( )
√
LINE TO LINE FAULT
√
( )
LINE TO GROUND FAULT
NOTE:
*In Solidly Grounded System:
ZOT = ZP-SYS
*In Resistance Grounded System:
ZOT = ZP-SYS + 3Zg
MATRIX For POWERBASE (PB), Use the highest MVA (Utility/Source) @1500MVA
1ST Cycle (Max) 30 Cycles (Min)
Bus VL (kV) Z1 IF3Ø (kA) Z1=Z2 ZOT IFL-L IFL-G IF3Ø * +* +
B1 13.80 9.86 6.36 11.50 10.50 4.76 5.62 5.50
B2 4.16 34.73 5.99 41.50 3152.67 4.35 0.192 5.02 @1500MVAb (POWERBASE)
B3 0.40 147.50 14.70 161.50 120.00 11.61 14.66 13.41 *Convert first to base unit
Sizing of kA and Bus Protection Setting
IMPEDANCE DIAGRAM
[ ]
Z2(-) *just remove power
source
[ ]
*Hila hila method, Z1
[ ]
[ ][ ]
[ ][ ]
[ ][ ]
Z1 - Determine total impedance at Buses
*Use Buses (B1, B2, B3) as reference (hila-hila) then use Ohm’s Law.
Z2 – Total impedance from reference Bus to utility (No Motor load)
Contribution to utility and motor in Short Circuit (•) @ 1 cycle
Ex. @BUS1
√ √
*No motor contribution at 30 cycles @BUS 3
ZOT (Impedance to ground) Z0B3 = 120
@BUS 1
Z0B1 = 10.5
Fault Currents at 30 Cycles
@BUS 1
@ 30 cycles
√ √
@BUS 2
√ √
( ) ( )
Z0B2 = XT2 + 3ZG
√
⁄
√
( ) @BUS 2 @BUS 3
√
Z0B2 = 30 + 3(1040.89)
Z0B2 = 30 + 3122.67
Z0B2 = 3152.67
COMMON PROTECTION CURRENT TRANSFORMER CONCEPT
WHERE: CT POLARITY
50 = Instantaneous Overcurrent • Polarity Mark
51 = Time Delay Overcurrent
= Current Transformer
*S = 5 A (ANSI)
= Primary/Secondary *S = 1 A (IEC)
Current Ratio
= Device 52, Circuit Breaker Threshold (relay reading)
N = Ground Protection * +
( )
(Setting)
SAMPLE DIAGRAM (3 PHASE)
@8 kA Fault (3 Phase)
* +
*Partial Saturation
@7kA Fault (Line to Line)
* +
L-L Fault
*Additional Relay for L-G Fault for “Back-up Protection”
(To delay action)
*By Zero Sequence Current:
= 200/3 A *Inductive kick
* +* +
For Transformer:
(Set to half of reading)
*Multifunction Relay
*87T is recommended for ≥5MVA TF
*Overlapping of Zone
*Bus Differential
ZCT – Usually used for motor feeder
*87 - Differential Protection (Sensitive)
- No Coordination needed
*Differential Relay must be same manufacture date, model, brand
*Insulation Percent
-100%
For Motor: “PROTECTION IS
-133% SCIENCE AND ART”
-177%
-Jimenez, PEE
*References
-IEEE Colored Books
-Stevenson’s
-Schaumm’s Outline
*Restricted Earth Fault
*Full Protection – All Lines
*GIS Switchgear
*Auto Transformer (Big Capacity)
*If has shielding:
SHIELDING > GROUND WIRE > ENTER THE CT
Ungrounded System
> GROUND BUS
*MGB – Main Ground Bus
SAMPLE PROTECTION RELAY SYSTEM TRANSFORMER DIFFERENTIAL
RC – Restraining Coil OC – Operating Coil
*Always consider Polarity: Input
“Away – Away
“Away – Towards”
*Need to maximize protection, not just transformer
*Up to switchgear to protect the cables
Dyn11
*Phase-Shifting Transformer is utilized to
D – Higher Voltage prevent harmonics.
y – Lower Voltage *Delta Configuration – Theoretically has
n – Neutral neutral.
“Agree or Disagree with
11 – O’clock (Phase-Shift) a right reason”
-Jimenez, PEE
*Just re-arrange the wirings to match the phase
Total Current:
@MOTOR:
Total Load: √
√
Ground Protection: 10 to 30% if IFL of Transformer
Instantaneous/STPU: set according to total load requirements
Ground Setting: 10 to 30% if IFL
@PRIMARY
*use L-L if delta-wye
*Characteristic of motor
starting: √
-Cold Motor If 3Ø fault is one unit: ( )
-Hot Motor
RA51 Setting:
*CLASS 10 – Its trips 10 Pickup 600Max
seconds of starting current.
Set:
*Self-protection up to 1.25
of FL current. * +* + 100 A @4160V, 1040 A @400V
300 A
*Service Factor (SF) – can
overload up to certain EI-IEC (Extremely Inverse)
percentage @ rated
ambient temperature. Where; K = 80, M = Multiple
*Up to 13.41 kA
(Short Circuit)
[ ] Characteristic K
Normal Inverse 0.02 0.14
@TRANSFORMER: Very Inverse 1.0 13.5
Ext. Inverse 2.0 80
* + Long Time 1.0 120
*Allow up to 1.25x *TCC – Time Current Curve
PROTECTION COORDINATION GRAPH
