Overview of Cable Electrical Parameters

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Date: 2021-10-19Read: 7

Overview of electrical parameters of cables, mainly introducing various physical quantities involved in cable engineering, helps us understand how cables operate from a physical perspective, and also assists us in conducting physical analysis and calculations of cables.

In the process of learning these basic concepts, some of the problems that trouble us will suddenly become clear to us; There are also some things that allow us to re understand from different perspectives, and I hope you can learn and gain something from them.

4.19 Overview of Electrical Parameters

The term is used within the scope of all electrical engineering. The practical applications in cable engineering can be found in section 4.2 of this chapter, and important formulas include:

E =IR （4-21）

O= It （4-22）

E = LdI/dt (4-23)

Q= CE （4-24）

（4-25）

W=EIt （4-26）

In the formula, E is the electromotive force (V); I is the current (A); R is the resistance (Q); Q is the total electricity (C); L is inductance (H); C is a capacitor (F); P is power (W); W is electrical energy (J); T is time (s).

Note: In 4-22, O is the amount of electricity (coulomb), the amount of electricity transported by a current of 1 ampere in 1 second, and 1 coulomb=1 ampere · 1 second

4.19.1 Resistance

Resistance R is a scalar（Scalar, also known as "no vector". Some physical quantities only have numerical values without direction, and some have positive or negative distinctions. In physics, scalar (or scalar) refers to a physical quantity that remains invariant under coordinate transformation. In layman's terms, a scalar is a quantity with only magnitude and no direction.），Resistance is the rate at which electrical energy is converted into thermal energy at a given currentThe thermal energy value is obtained by multiplying the current squared by the resistance value.

In DC circuits:

In the formula, R is the resistance (Ω); E is electromotive force (V); I is the current (A); P is power (W).

4.19.2 Conductivity

Conductivity G isUnder the condition of a given potential difference at both ends of a part of the circuit in the loop，The rate at which electrical energy is converted into thermal energyThe value of thermal energy is equal to the product of the square of the potential difference and the electrical conductivity.

In the formula, P is power (W); E is the relative ground voltage (kV); G is conductivity (S). The unit of conductivity is Siemens (S), and conductivity is the reciprocal of resistance.

In the formula, G is the conductivity (S); R is the resistance (Ω).

4.19.3 Conductivity

The conductivity of a certain material refers to the direct current conductivity between the relative cross-sectional area of the material per unit lengthIt is the reciprocal of resistivity.

In the formula, G is conductivity;It is the area; L is the length.

4.19.4 Volume resistivity

The volume resistivity of a material is the reciprocal of its conductivityThe unit of volume resistivity isIt means the resistance per unit length and cross-sectional area of a material.

In the formula, R is the resistance;It is volume resistivity;It is the area; L is the length.

4.19.5 Inductance

Inductance L is a scalar,It is a change in potential difference caused by changes in current within one circuit or between two circuits.

4.19.5.1 Self perception

Self perception refers to being inWithin a circuit, the current changes at a specified rate, resulting in a potential difference within the circuit.

The unit of inductance is Henry (H), 1HIt refers to the 1V potential difference generated when the current in a closed circuit changes at a speed of 1A/s.

In the formula,andAre parameters of the same circuit; L is the self sensing coefficient.

4.19.5.2 Mutual inductance

yesThe performance of two adjacent circuits affected by changes in the current of one circuit on the other circuit.

4.19.6 Inductance of Multi conductor Cable

The arrangement of multiple conductors is consistent with that of other conductors and follows the following formula:

In the formula; GMD is the geometric distance (in) between conductors; GMR is the geometric radius (in) of a conductor.

4.19.7 Cable mutual inductance

Yes, that's rightSingle core metal shielded cablefor,When current flows through a conductor, an electromotive force is generated on the metal screen.If shielding forms a closed loop at certain points, it will generate currentThe mutual inductance conforms to the following formula:

In the formula,The neutral line mutual inductance (H) per 1000ft;It is the shielding radius (in).

4.19.8 Coaxial Cable Inductance

There are three types of inductors to consider in coaxial cables, namelyAir gap inductance, inner conductor inductance, and outer conductor inductance.When the frequency is greater than 50kHz, only the air gap inductance needs to be consideredAnd the error will be within 0.5%. The inductance of coaxial cables with outer conductors arranged in a tubular shape conforms to the following formula:

In the formula,It is the inductance per centimeter of cable length (H);It is the inner diameter of the outer conductor (in);It is the radius of the inner conductor (in).

If the outer conductor is twisted or braided, the inductance will be slightly higher.

4.20 capacitor

The capacitance of a cable refers toThe variation of voltage and current over time in electrical systems containing conductors and insulation materials results in different electromotive forces between conductors.

The unit of capacitance is F, which meansWhen the charge in the circuit increases by 1C, the potential difference in the circuit increases by 1V.

4.20.1 Cable capacitance

The classic capacitance formula for a 1cm long insulated cable is as follows:

In the formula, C is a capacitor;It is the dielectric constant of insulating materials; D is the outer diameter of the insulation; D is the inner diameter of the insulation.

Usually, the following formula can also be used:

4.21 reactance

The reactance isThe ratio of voltage to current effective valueMultiply by the sine of the phase angle difference between current and voltage, because the circuit part does not consider the power supply.The total reactance of a circuit is the sum of inductive reactance and capacitive reactance.

4.21.1 Sensory Resistance

Sensory resistance conforms to the following relationship:

In the formula, X， To measure the impedance (Ω/ft) of the neutral wire; F is the frequency (Hz); L is the inductance (H/ft).

The reactance can be calculated according to the following formula:

4.21.2 Capacitive Resistance

In the formula,For capacitive impedance (Ω/ft); C is the capacitance (pF/ft).

4.21.3 Total reactance

The total reactance of the circuit isThe sum of sensory and capacitive resistance：

4.22 Impedance

Impedance is the ratio of the effective value of the potential difference at both ends to the effective value of the current.

4.23 admittance

Admittance (Y) is the reciprocal of impedance.

4.24 Power factor of power engineering

Power factor in power engineering and "power factor" in cable engineeringDifferent meaningsInPower factor in power engineering is the ratio of active power to apparent powerThe apparent power (S) consists of two parts, active power. and reactive powerTheir sum is the apparent power. The power factor formula is as follows:

In power engineering, power factor is used to calculate the amount of useful workFor example, in an electric motor, there is impedance and a considerable amount of inductive reactance, causing the current to lag behind the voltage. If the power factor is 1.00 and there is no reactance, every 10kVA of power flowing into the motor (ignoring losses) can be converted into 10kW of useful work. When there is reactance and assuming a power factor of 0.8, 8/10 of the current flows into each phase, and only 8kW will reach the motor.The current of different phase components will increase the total current, leading to an increase in heat loss

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