Prof. Mohamed Adel Abdallah

High Voltage Engineering

Chapter 1

High Voltage Insulation

* Need for Electrical Insulation

Prevent leakage current from HV parts to nearby grounded

parts

Safety for Workers

* Area of Insulation Application

Phase to earth

Phase to Phase

Inter-Coil(between coils)

Inter-turn (In a coil)

* Function of Electrical Insulation Materials

Mechanical Support

Heat transfer

Arc Quenching

* Basic Electrical Properties of Dielectric Materials

1- DC Resistivity : Ω.m

2- DC Conductivity:

3- Dielectric Permittivity:

=

4- Electric Polarization:

Decrease of the electric field due to the dielectric material with the material tends to neutralize some charges

5- Loss angle and dissipation factor:

Parallel equivalent

V Rp Cp 𝜹( Ic )

()

V

= =

=

Parallel equivalent

VRs

Cs

II

𝜹

𝑰 𝑹𝒔

𝑰 𝑪𝒔

Loss factor (dissipation factor)=

6- Power Loss in a dielectric:

=

𝑪𝑷 ≈𝑪𝒔

7- Breakdown Voltage:

8- Dielectric Strength:

9- Partial discharge and Corona (PD):

* Non Electrical Properties of Dielectrics

Maximum Operating Temp. Hardness Elasticity

Coefficient of expansion.

Thermal conductivity

Specific gravity

Tensile strength

Compressive strength Density

* Classification of Insulating Materials

Gases

Vacuum

Liquids

Solids

Chapter 2

Generation of Testing Voltage

*Direct voltages

1 -A.C. to D.C. conversion

Single-phase half-wave rectifier

full-wave rectifier circuit

Cascade circuits

Cascade circuit

*AC Voltagescascaded transformers.

(1 )Primary windings.(2 )Secondary h.t. windings .(3 )Tertiary exciting windings

Series resonant circuits

𝑽 𝑳𝑽 𝑺

𝑽 𝑳=𝑽 𝑺

𝟏−𝝎𝑳𝑪

𝒇 𝒓=𝟏

𝟐𝝅 √𝑳𝑪

*Impulse Voltage Generation

Single-stage generator

For R2 >> R1 and C1>> C2

=)3 R1

= 0.7( R1 +R2()1+ 2C C )

Multistage generator

Chapter 3

High Voltage Testing

High Voltage tests are applied to determine the ability of the insulation to meet its design requirements

Destructive Nondestructive

For solid insulatorsTo measure the maximum Dielectric strength

For all types of insulatorsTo insure the quality ofInsulation

*HV Test Classifications

1- AC - DC - Impulse ( switching - Lightning)

2- Type tests - Routine Tests - Special Tests

3- Destructive Tests - Nondestructive Tests

4- Acceptance - Design and Research

Test Voltages (I) Direct voltages

Mainly used for:- Pure scientific research workFor testing equipment related to HVDC transmission

systems. For HVAC power cables of long length, as the large

capacitance of those cables would take a high current if tested with a.c. voltages .

AC Impulse

BIL for lightning ( 1.2/50 µs)BSL for switching

Breakdown Tests

For self restoring insulation ( liquid and gas insulations)

Dielectric Loss Tests

tan δ = δ

Ic

VcCpRp

𝐈𝐑𝐩

The Schering bridge

Purposes of tan δ tests

1- to assess the level of degradation suffered by the dielectric2- to assess the quality of the insulation3- to assess the level of contamination (liquid dielectric)4- to assess the presence of cavities ( solid dielectrics)

Partial Discharge Tests

Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation.

The term ‘partial discharge’ includes a wide group of discharge phenomena:

(i) internal discharges occurring in voids or cavities within solid or liquid

dielectrics;

(ii) surface discharges appearing at the boundary of different insulation

materials;

(iii) corona discharges occurring in gaseous dielectrics in the presence of

inhomogeneous fields;

(iv) continuous impact of discharges in solid dielectrics forming discharge

channels (treeing).

The detection and measurement of discharges is based on the

exchange of energy taking place during the discharge. These

exchanges are manifested as:

(i) Electrical pulse currents (with some exceptions, i.e. some types of

glow

discharges);

(ii) Dielectric losses;

(iii) Radiation (light);

(iv) Sound (noise);

(v) Increased gas pressure;

(vi) Chemical reactions.

The basic PD test circuit

Ca = C`a +C``a

Va

Let us now assume that the sample was charged to the voltage Va but the terminals A, B are no longer connected to a voltage source. If the switch S is closed and Cc becomes completely discharged, the current ic(t) releases a charge δqc = Cc δVc from Cc, a charge which is lost in the whole system as assumed for simulation. By comparing the charges within the system before and after this discharge, we receive the voltage drop across the terminal δVa as

𝜹𝑽 𝒂 (𝑪𝒂+𝑪𝒃 )=𝜹𝑽𝒔 (𝑪𝒂+𝑪𝒃+𝑪𝒌)

=

= =

SURGE OVER VOLTAGE DUE TO TRAVELING WAVE ON TRANSMISSION LINES

Chapter 4

Wave Propagation on Transmission Line

SURGE OVER VOLTAGE PROTECTION

Chapter 5

DIRECT LIGHTNING STROKES

A direct lightning stroke consists

of the injection of a current wave

into the line. Depending on the

typical impedance of the line, the

current wave generates a voltage

surge wave. A flashover device,

at the isolator on the overhead

line, shunts part of the current,

but does not always limit the

peak value.

INDUCED VOLTAGE SURGES

A voltage surge can also be produced by a lighting stroke that hitsclose to the line, an “induced lighting stroke”.

The voltage surge is induced in the loop formed by the line and the earth through the magnetic field created by lightning that strikesnear the line.

Each part of the network is characterised by its typical impedance:

Zo, given in ohms.It characterizes the conditions of electromagnet wave propagation along the conductors.

Zo = √L/C with:

L: inductance per unit lengthC: capacitance per unit length (not taking resistances and conductance into account).

.

Surge Suppressors ( Arresters)

Spark Gap

With a slow impulse, the protective gap’s flashover voltage is constant and indicated with the letters Uo. This voltage must always be 20% lower than the dielectric withstand value of the equipment to be

protected (BIL)

Switchgear can be destroyed through internal flashover, if the wave from the lightning impulse has a rapid current rise front and sharp decrease, even if the protective gap is in good condition.

Surge Arresters (Non-

Liner Resistors)

Resistor Property

Its resistance diminishes sharply as the voltage across it increases.

kVI

α (measure of nonlinearity) : Depends on the resistor material.

k (constant): is likewise dependent on the material, but in addition it is controlled by the geometry of the element ( directly proportional to cross sectional are and inversely to its length.

There are Two Types of nonlinear resistors (Arresters) used in the applications:

Silicon Carbide ( SiC) : 2< α<6 Traditional material

Metal Oxide (ZnO) : 20< α<100

Principle of Operation

21

21.ZZZZ

Z s

221

.)( ZZZageSourceVolttV

Metal Oxide Arrester ( ZnO)Consists of 90% Zinc Oxide and10% other metal OxidesThe compound is mixed and pressed

Discharge current.

The surge current which flows through the arrester after the spark over.

Impulse spark-over. Highest value of voltage attained during an impulse of given polarity, of specified wave shape applied between the line terminal and earth terminal of an arrester before the flow of discharge current.

Chapter 6

System Earthing&

Protective Earthing

Earthing :An electrical connection to the general mass of earth ( Volume of soil / rock etc.) whose dimensions are very large compared to the electrical system.

The most often quoted reasons for having an earthed system are:

To provide a sufficiently low impedance to facilitate satisfactory protection operation under fault conditions.

To ensure that living beings in the vicinity of substations are not exposed to unsafe potentials under steady state or fault conditions.

To retain system voltages within reasonable limits under fault conditions (such as lightning, switching surges or inadvertent contact with higher voltage systems), and ensure that insulation breakdown voltages are not exceeded.

Custom and practice.Graded insulation can be used in power transformers.•To limit the voltage to earth on conductive materials which enclose

electrical conductors or equipment. Less often quoted reasons include:

To stabilize the phase to earth voltages on electricity lines under steady state conditions, e.g. by dissipating electrostatic charges which have built up due to clouds, dust, sleet, etc.

A means of monitoring the insulation of the power delivery system.

To eliminate persistent arcing ground faults.

To ensure that a fault which develops between the high and low voltage windings of a transformer can be dealt with by primary protection.

To provide an alternative path for induced current and thereby minimize the electrical "noise" in cables.

Provide an equipotential platform on which electronic equipment can operate.

System Earthing:

A) Un-Earthed Systems:-

B) Earthed Systems

1- Impedance Earthed System

Resistor ( Limit Fault Current – Damp Over Voltage )

Reactors (Limit Fault Current – Avoid excessive Over Voltage)

Arc-Suppression Coil ( Ground Fault Neutralizers) Neutralize the capacitive current of the healthy phases,

so that minimize the fault current)

2- Solidly Earthed System ( Low Voltage )

Low Voltage System Earthing:

Identification of the system types is thus defined by means of 2 letters:the first one for transformer neutral connection (2 possibilities): T : for « connected » to the earth, I : for « isolated » from the earth;the second one for the type of application frame connection (2 possibilities): T : for «directly connected» to the earth, N : for «connected to the neutral» at the origin of the installation, which is connected to the earth

transformer frame Neutral T T

I N

i.e. TT, TN and IT.

The TN system includes several sub-systems:TN-C; if the N and PE neutral conductors are one and the same (PEN)TN-S: if the N and PE neutral conductors are separateTN-C-S: use of a TN-S downstream from a TN-C (the opposite is forbidden).

Typical T N-S system

Supply Single Point Earthed. SNE Mains and Service Cables. Customer provided with earth terminal from the sheath of the service cable.

Typical TN-C-S (Protective Multiple Earth) Supply

Neutral Earthed by Supplier at a number of locations. CNE Mains and Service cables. Customer provided with earth terminal connected to the service neutral.

Typical TT System

This is a system where the supply is earthed at one point only, but the cable sheaths and exposed metalwork of the customer's installation are connected to earth via a separate electrode which is independent of the supply electrode.

Typical IT System

Source isolated from earth or connected to earth through a high impedance. All exposed conductive parts of the installation are connected to an independent earth. Not permissible for public supply.

Protective Earthing

• Safety for persons.

• Proper operation and long life

time for equipment

• Earthing systems allow unwanted electrical

currents to flow harmlessly to earth.

• Their main function is to provide low impedance

(not only resistance) paths for high-energy

discharges and high frequency, particularly

lightning strikes, other transients and fault

currents.

The main markets for installing earthing systems:- utility power generation, transmission and distribution. lightning protection for buildings and high structuresPrivate electricity distribution networks in industrial and

commercial premises.Protection of electronic equipment e.g. computer

installations, telecommunications.Domestic housing and small commercial premisesSituations where a build-up of electrostatic potential

couldbe dangerous, including oil refineries, petroleum filling

stations, grain storage, hospitals.

Components of Grounding Systems:-

1- Earth 2- Earthing Electrode 3- Down conductors 4- Bonding

RESISTANCE OF GROUNDING SYSTEMS

The value of resistance to ground of an electrode system is the resistance between the electrode system and another "infinitely large" electrode in the ground at infinite spacing.

The soil resistivity is a deterministic factor in evaluating the ground resistance. The soil resistivity depends on:Type of soil , moisture content, dissolved salts, grain size ,temperature and pressure.

Typical Values of Resistivity of Some Soils

Resistivity( Ω • m)

Type of soil

5-50 Loam, garden soil8-50 Clay

60-100 Sand and gravel10-500 Sandstone

200-10,000

Rocks

Resistance of a Grounding Point Electrode

The simplest possible electrode is the hemisphere

Resistance of Driven Rods

The driven rod is one of the simplest and most economical form of electrodes. Its ground resistance :

Driven RodThe Standard rod is Copper – Clad rod consists of a length of steel with 5 mil coating of copper

Resistance and surface potential distribution

The most frequently used electrode materials are:

Steel Galvanized steel Steel covered by copper High-alloy steel Copper and copper alloys.

Earthing, Backfilling Materials

SOIL ENHANCEMENT OPTIONS

1. Conductive Concrete30 to 90 ohm-meters

2. Bentonite2.5 ohm-meters

3. Carbon-Based Backfill Materials0.1 to 0.5 ohm-meters

4. Clay-Based Backfill Materials (GAF)0.2 to 0.8 ohm-meters depending on moisture content

5- Marconite 0.1 ohm.meters

Grounding Grids

A common method for obtaining a low ground

resistance at high voltage substations is to use

interconnected ground grids. A typical grid system for a

substation would comprise solid copper conductors

buried at a depth of from 30 to 60 cm, spaced in a grid

pattern of about 3 to 10 m. At each junction, the

conductor: are securely bonded together.

Earthing Conductors

Stranding Size

7/1.04 67/ 1.70 16

7/2.14 2519/1.33 3519/1.78 5019/2.14 7019/2.52 95

31 /2.03 12037/2.25 15037/2.52 185

61 /2.25 24061 /2.52 30061 /2.85 400

Step & Touch Potential

Step Potential

Rf = 3 ρs

Maximum Permissible Step voltage 76 volt

Touch Potential

Earthing resistivity Measurement

Earthing resistivity Measurement

Earthing resistance Measurement