High-Voltage Surge Arresters

High-Voltage Surge Arresters

In safe and reliable

protection of electrical equipments, surge arresters are the primary protection against atmospheric and switching over voltages. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrester, in most cases to earth. If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices.
Different relevant standards exists for design and type tests like IEC 60099-4 , ANSI/IEEEC62.11 and other specific standards.

Applications.

1). Protection of AIS and GIS substation equipment
2). HVDC protection
3). Protection of series capacitor banks
4). Protection of cables
5). Protection of transmission lines
6). Polluted areas and areas with high seismic activities.

Types.

1). Silicone-housed surge arrester :-

A flexible type of arrester for line discharge class 2 to 5 and system voltage up to 550 kV.

2). Composite-housed surge arrester :-

A high strength arrester for line discharge class 3 to 5 and system voltage up to 800 kV.

3). Porcelain-housed surge arrester :-

A high strength arrester family for line discharge class 2 to 5 and system voltage up to 800 kV.

Other Types

Horn Gap Arresters

Horn gap arresters are named for their two horn-shaped metal rods. These rods are arranged around a small air gap, and the distance between the two rods increases as they rise 

o two different wires: One attaches to the electrical line. Between it and the horn is a resistance and a choke coil. The resistance regulates the level of current allowed into the arrester at once, and the choke coil increases the reactivity of the arrester when transient frequency occurs. The other wire is connected to a ground, which siphons the excess electricity to the ground.

Multi-Gap Arresters

Multi-gap arresters are made from a series of metal cylinders. These cylinders are all insulated from one another as well as separated by air gaps. The first cylinder gets connected to the electrical line, while all of the other cylinders are attached to the ground through a series resistance, which gradually wears down the power of the current. Some of the gaps between the later cylinders have a shunt resistance that catches a surge when there is an excess of voltage.

Valve-Type Arresters

Valve-type arresters are commonly used in more high-powered electrical systems. They consists of two main parts: a series of spark gaps and a series of non-linear resistor discs. Valve-type arresters work when excessive voltage causes the spark gaps to touch, and the non-linear resisters carry the voltage into the ground. Once the surge of excess power ends, the resisters push the spark gaps apart.

Brief performance data of Typical Lightning Arresters

System voltage	kV	72 - 145	24 - 170	52 - 420	52 - 420	300 - 550
Rated voltage	kV	75 - 120	18 - 144	42 - 360	42 - 360	228 - 444
Nominal discharge current	kApeak	10	10	10	20	20
Line discharge class	Class	2	2	3	4	4
Mechanical strength (SSL)	Nm	1 300	1 600	4 000	4 000	9 000

Building Blocks

The most important component for the surge arresters is the ZnO blocks, the varistors. Stacked in the center of the surge arrester, the varistor is the heart of the surge arrester. The varistor consist of a mix of zink oxide and other metallic powders that are blended and pressed into cylindrical blocks.

Typical type tests performed for different size of ZnO varistor.

Tests on ZnO blocks

Energy withstand test on all blocks - The blocks pass three energy test cycles with cooling in-between. In each cycle, the injected energy is far in excess of the single impulse energy capability.

Classification of all blocks

Visual inspection and classification of the blocks at 1 mA (d.c.) and 10 kA (8/20 μs) and the residual voltages are printed on each block together with a batch identification.

Accelerated life tests on samples

Here power losses after 1 000 hours is calculated from a test with shorter duration (approximately 300 hours) at an elevated temperature of 115 °C at 1.05 times Uc shall not exceed the losses at start of the test.

Impulse current tests on samples

Blocks are subjected to high current impulses (4/10 μs) and long duration current impulses (2 500 μs) of amplitudes verifying catalogue data.

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2). MENTION THE FORMAT OF DRAWING YOU REQUIRED, WHICH WILL BE SENT TO YOU BY RETURN EMAIL. (ONLY SOFT COPY OF DRAWING WILL BE PROVIDED BY US)

3).  AS PER YOUR DRAWING REQUEST, WE WILL ANALYZE, THE PROBLEM, AND EMAIL YOU, ABOUT THE CHARGE/COST OF THE JOB. TYPICAL CHARGES IS 100/- TO 500/-.

4). IF YOU AGREE, SENT YOUR CONFIRMATION, FOR PROCESSING THE REQUEST, BY EMAIL.

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INDUCTANCE OF A CONDUCTOR DUE TO INTERNAL FLUX

INDUCTANCE OF A CONDUCTOR DUE TO INTERNAL FLUX

The inductance of a transmission line is calculated as flux linkages per ampere. If permeability µ is constant, sinusoidal current produces sinusoidally varying flux in phase with the current. The resulting flux linkages can then be expressed as a phasor λ, and

 If i, the instantaneous value of current, is substituted for the phasor I in above eq., then λ should be the value of the instantaneous flux linkages produced by i. Flux linkages are measured in weber-turns, Wbt.

To obtain an accurate value for the inductance of a transmission line, it is necessary to consider the flux inside each conductor as well as the external flux. Let us consider a long cylindrical conductor whose cross section is shown in Fig. We assume that the return path for the current in this conductor is so far away that it does not appreciably affect the magnetic field of the conductor shown. Then, the lines of flux are concentric with the conductor.

  

By Ampere's law the magnetomotive force (mmf) in ampere-turns around any closed path is equal to the net current in amperes enclosed by the path. The mmf equals the line integral around the closed path of the component of the magnetic field intensity tangent to the path and is given by,

Where, H = magnetic field intensity, At/m

s = distance along path, m

I = current enclosed, A

Note that H and I are shown as phasors to represent sinusoidally alternating quantities.

Let the field intensity at a distance x meters from the center of the conductor be designated H_x. Since the field is symmetrical, H_x is constant at all points equidistant from the center of the conductor. If the integration indicated in Eq. above is performed around a circular path concentric with the conductor at x meters from the center, H_x is constant over the path and tangent to it. We get,

Where, I_x is the current enclosed. Then, assuming uniform current density, 

  Where, I is the total current in the conductor. Then, we obtain,

 The flux density x meters from the center of the conductor is,

Where, µ is the permeability of the conductor.

In the tubular element of thickness dx the flux dφ is B_x times the cross-sectional area of the element normal to the flux lines, the area being dx times the axial length. The flux per meter of length is,

 The flux linkages dλ per meter of length, which are caused by the flux in the tubular element, are the product of the flux per meter of length and the fraction of the current linked. Thus,

Integrating from the center of the conductor to its outside edge to find λ_int, the total flux linkages inside the conductor, we obtain, 

  For a relative permeability of 1, µ = 4π X 10^-7 H/m, and,

Hence, we have computed the inductance per unit length (henrys per meter) of a round conductor attributed only to the flux inside the conductor.

 

TYPES OF CONDUCTORS IN POWER TRANSMISSION SYSTEM

TYPES OF CONDUCTORS IN POWER TRANSMISSION SYSTEM

Symbols identifying different types of aluminum conductors are as follows:

AAC- All Aluminum Conductors.
AAAC- All Aluminum Alloy Conductors.
ACSR- Aluminum Conductor Steel Reinforced.

HTLS- High Temperature Low Sag.

In the early days of the transmission of electric power conductors were usually copper, but aluminum conductors have completely replaced copper for overhead lines because of the much lower cost and lighter weight of an aluminum conductor compared with a copper conductor of the same resistance. The fact that an aluminum conductor has a larger diameter than a copper conductor of the same resistance is also an advantage. With a larger diameter, the lines of electric flux originating on the conductor will be farther apart at the conductor surface for the same voltage. This means there is a lower voltage gradient at the conductor surface and less tendency to ionize the air around the conductor.
Ionization produces the undesirable effect called corona.

Aluminum-alloy conductors have higher tensile strength than the ordinary electrical-conductor grade of aluminum. ACSR consists of a central core of steel strands surrounded by layers of aluminum strands. ACAR has a central core of higher-strength aluminum surrounded by layers of electrical conductor grad e aluminum.
Alternate layers of wire of a stranded conductor are spiraled in opposite directions to prevent unwinding and to make the outer radius of one layer coincide with the inner radius of the next layer. Stranding provides flexibility for a large cross-sectional area. The number of strands depends on the number of layers and on whether all the strands are of the same diameter. The total number of strands in concentrically stranded cables, where the total annular space is filled with strands of uniform diameter is 7, 1 9, 37, 61, 91, or more.
Figure shows the cross section of a typical steel-reinforced aluminum cable (ACSR). The conductor shown has 7 steel strands forming a central core, around which there are two layers of aluminum strands. There are 24 aluminum strands in the two outer layers. The conductor stranding is specified as 24 Al/7 St, or simply 24/7. Various tensile strengths, current capacities, and conductor sizes are obtained by using different combinations of steel and aluminum.

TECHNICAL PARTICULARS OF ACSR 'PANTHER', 'ZEBRA' & 'MOOSE' CONDUCTOR.

1.Specification to which the finished conductor conforms: IS-398Part-II-1976
2.Purity of Aluminium Rods: 99.5% Minimum
3.Percentages of Carbon in steel wire/rods: 0.50 to 0.85 (Preferably 0.65%)
4.Purity of Zinc: 99.95%

		Unit	ACSR 'Panther'	ACSR 'Zebra'	ACSR 'Moose'
5.	Particulars of Aluminium strands
i)	Diameter
	a) Standard	mm	3.00	3.18	3.53
	b) Maximum	mm	3.03	3.16	3.55
	c) Minimum	mm	2.97	2.19	3.51
ii)	Cross sectional area of standard diameter wire	mm2	7.069	7.942	9.787
iii)	Weight per Km
	a) Standard	Kg	19.11	21.47	26.45
	b) Maximum	Kg
	c) Minimum	Kg
iv)	Minimum breaking load
	a) Before stranding	KN	1.17	1.29	1.57
	b) After stranding	KN	1.11	1.23	1.49
v)	Maximum D.C. resistance at 200C	Ohm/Km	4.107	3.651	2.921
vi)	Joints in strands of 12 wire Aluminium layer if any		No joint	No joint	No joint
6.	Particulars of Steel strands
i)	Diameter
	a) Standard	mm	3.00	3.18	3.53
	b) Maximum	mm	3.06	3.21	3.60
	c) Minimum	mm	2.94	3.14	3.46
ii)	Cross sectional area of standard diameter wire	mm2	7.069	7.942	9.787
iii)	Weight per Km
	a) Standard	mm	55.13	61.95	76.34
	b) Maximum	mm	57.36	63.12	79.39
	c) Minimum	mm	52.95	60.40	73.33
iv)	Minimum breaking load
	a) Before stranding	KN	9.29	10.43	12.85
	b) After stranding	KN	8.83	9.91	12.22
v)	Elongation of 200 mm length on breaking	%	4	4	4
vi)	Coating of steel core
	a) Quality of Zinc		99.95% purity	99.95% purity	As per IS: 209-1979
	b) Process of galvanising		Hot Dip	Hot Dip	Hot Dip
	c) Minimum weight of coating	gm/m2	240	250	260
	d) Minimum no of dips of one minute duration which the strand can withstand (under preace test)	Nos.	3	3	3
7.	Particulars of Complete Conductor
i)	Code words, if any		ACSR 'Panther'	ACSR 'Zebra'	ACSR 'Moose'
ii)	Copper equivalent Area	mm2	310	260	325
iii)	Nominal aluminium area	mm2	200	420	520
iv)	Sectional area of aluminium	mm2	212.10	428.91	528.50
v)	Total sectional area	mm2	261.50	484.50	597.00
vi)	Overall diameter	mm	21.00	28.62	31.77
vii)	Stranding, lay and wire diameter
	a) Aluminium	mm	30/3.00 Right Hand lay	54/3.18	54/3.53
	b) Steel	mm	7/3.00	7/3.18	7/3.53
viii)	Lay ratio	Min. mMaxm
	a) Steel core
	i) 6 wire Layer	Max.	28	28	28
		Min.	13	13	16
	b) Aluminium
	i) 12 Wire Ist Layer	Max.	16	15	14
		Min.	10	10	12
	ii) 18 Wire IInd layer	Max.	14	14	13
		Min.	10	10	11
	iii) 24 Wire IIIrd Layer	Max.	-	12	12
		Min.	-	10	10
x)	Approximate calculated breaking load	KN	89.67	130.32	159.60
xi)	Final modules of Elasticity (Practical)	GN/m2	80	69	69
xii)	Coefficient of linear expansion	Per0C	17.8x10-6	193x10-6	19.35x10-6
xiii)	Approximate total weight per Km
	a) Steel Section	Kg	388.00	437.00	537.00
	b) Aluminium Section	Kg	586.00	1184.00	1461.00
	c) ACSR Composite	Kg	974.00	1621.00	1998.00
xiv)	Calculated D.C. resistance at 200C (Maximum)	Ohm/Km	0.1390	0.06885	0.25595
xv)	Standard length of conductor (with tolerance, if any)	Metres	1500+5%	1500+5%	1500+5%
xvi)	Number of standard length in one reel (drum)	Nos	One	One	One
xvii)	Random lengths (Maximum percentage of the lengths ordered)	%	10	10	10