HOW SURGE ARRESTERS WORKS

Along the way of this topic you will be learned how surge arresters works in electrical equipment like an overhead tranmission lines. lets first give the definition of terms and then Classification of arresters and Arrester Selection.

Definition of terms:
Surge arrester
a device used to protect equipment against over voltages caused by incoming surges.

MCOV - maximum continuous operating voltage
Maximum voltage the device can withstand before conduction (clamping) begins. When applying metal oxide SA, the minimum value of this voltage is usually the maximum system line-to-ground voltage.

Duty cycle voltage rating
The designated maximum permissible voltage between terminals at which the arrester is designed to perform its duty cycle.

TOV – temporary over voltage
These are created by faults on the utility power distribution system and can cause extensive damage since their time domain is much longer (ms to seconds to hours)

Duty cycle rating (kV rms)
the designated maximum permissible operating voltage between arrester’s terminal at which it is designed to perform its duty cycle.

Discharge current
the current that flows through an arrester as a result of surge.

Discharge voltage
the voltage that appears across the terminals of an arrester during the passage of discharge current. The discharge voltage resulting from 8/20 us current wave shape reasonably well between the current magnitudes of 5kA and 20kA.

8 x 20 microsecond current wave shape
a current wave shape that rises to crest in 8 microsecond and decays to one-half crest value in 20 microsecond.


Lead length
is the combined length of the line lead and ground lead length in series with the arrester and in parallel with the device or cable being protected.

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Insulation coordination
The process of correlating the insulation withstand levels of the protected equipment and the protective characteristics of surge arresters


Protective Margin
is a measure of surge arrester’s ability to protect a piece of equipment or a system.

**

Nominal Voltage
the voltage by which the system may be designated and is near the voltage level at which the system normally operates.

Maximum System Voltage
the highest phase-to-phase voltage for which equipment is designed for satisfactory continuous operation without derating of any kind.
( maybe 5 to 10 percent higher than the nominal voltage )


Classification of Arrester
Station class
More ruggedly constructed than intermediate and distribution class
Greater surge current discharge ability
Lower IR voltage drop (better protection)
Only class available for use on systems above 150 kV
Recommended for all s/s of large capacity ( 10 MVA and above)

Intermediate type
IR voltage drop higher than station class
Cost saving compared to station class
Available at ratings 3 kV through 120 kV

Distribution Type
protective characteristic are not as good as either station & intermediate
applied at low voltage distribution substation transformers


Selection of Arrester

System Voltage
Line to ground voltage
Voltage regulation
Grounding information
Grounded
Ungrounded/resistance grounded
Arrester Class
Protection Level
Energy Capability
Pressure relief rating


Surge Arrester Specification Sample
Type: Station Arrester
Housing Make: Polymer
Conductive Element: Metal Oxide
Rated Voltage (Duty Cycle): 12KV
MCOV: 10.2KV
Pressure Relief Class: 65KA
Energy Capability: 3.8 kJ/kV


MCOV Rating
Example:
13.8 KV System, Y – grounded, 10% voltage regulation

(13.8/1.732) x 1.05 = 8.37(or 8.4 MCOV)
Duty Cycle Rating – 10 KV (from catalog)


69 KV System, Y - grounded, 10% voltage regulation
(69/1.732) x 1.05 = 41.86 (42 MCOV)
Duty Cycle Rating – 54 KV (from cooper 54-60 KV)
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- Insulation Coordination
- Fault Current
Protective Margins
Discharge Voltage
Full Wave
- Switching Surge
- Lead length
****

Protective Margin
Minimum protective margin is 20% as recommended by ANSI C62.22.1-1996 page19

% margin = (equipment withstand level- 1 / Arrester Protective Level ) x 100%

Note of Caution:
The actual PM offered by an arrester will vary from the calculated PM value. This is because the surge protection industry calculates BIL margin percentages on the basis of the industry-standard 8x20-microsecond wave shape. However, the actual wave shape of a lightning surge can be much faster than that of the 8x20-microsecond wave shape.



Discharge Voltage Margin
(Chopped Wave Withstand – Equivalent front of Wave)

% Margin = (CWW - 1/PL1)x 100%

where:
CWW = Chopped Wave Withstand (1.15 x BIL)
PL1 = Steep Current Residual Voltage at 0.5 sec wave

Full Wave Margin
(Full Wave Withstand Discharge – Discharge Voltage for Impulse Current at Rated kA)

% Margin = (BIL - 1/ PL2)x100%

where:
BIL = Basic Impulse Insulation Level of Protected Equipment
PL2 = Lightning Impulse Residual Voltage at 8/20sec wave & rated kA

Switching Surge Margin
(Switching Surge Withstand – Switching Surge Voltage)

% Margin = (SSWL - 1/PL3) x 100%

where:
SSWL = Switching Surge Withstand Level of Equipment (0.83 x BIL)
PL3 = Switching Impulse Residual Voltage

Lead Length

V= L * di/dt
L= .4 micro Henry per foot
8/20 Wave @ 10 kA = 500 V per ft.
.5 Wave @ 10 kA = 8,000 V per ft.


note: 10 kA is the typically recommended value for the coordinating current.

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