Electrical circuits - lecture

The effects of three-phase harmonics

The effects of three-phase harmonics on circuits are similar to the effects of stress

and high blood pressure on the human body. High levels of stress or harmonic distortion can lead to problems for the utility's distribution system, plant distribution

system and any other equipment serviced by that distribution system. Effects can

range from spurious operation of equipment to a shutdown of important plant

equipment, such as machines or assembly lines.

Harmonics can lead to power system inefficiency. Some of the negative ways that

harmonics may affect plant equipment are listed below:

❏ Conductor Overheating: a function of the square rms current per unit volume of the conductor. Harmonic currents on undersized conductors or cables

can cause a "skin effect", which increases with frequency and is similar to a

centrifugal force.

❏ Capacitors: can be affected by heat rise increases due to power loss and reduced life on the capacitors. If a capacitor is tuned to one of the characteristic

harmonics such as the 5th or 7th, overvoltage and resonance can cause dielectric failure or rupture the capacitor.

❏ Fuses and Circuit Breakers: harmonics can cause false or spurious operations and trips, damaging or blowing components for no apparent reason.

❏ Transformers: have increased iron and copper losses or eddy currents due

to stray flux losses. This causes excessive overheating in the transformer

windings. Typically, the use of appropriate "K factor" rated units are recommended for non-linear loads.

❏ Generators: have similar problems to transformers. Sizing and coordination is critical to the operation of the voltage regulator and controls. Excessive harmonic voltage distortion will cause multiple zero crossings of the

current waveform. Multiple zero crossings affect the timing of the voltage

regulator, causing interference and operation instability.

❏ Utility Meters: may record measurements incorrectly, resulting in higher

billings to consumers.

❏ Drives/Power Supplies: can be affected by misoperation due to multiple

zero crossings. Harmonics can cause failure of the commutation circuits,

found in DC drives and AC drives with silicon controlled rectifiers (SCRs).

❏ Computers/Telephones: may experience interference or failures.

HIGH-VOLTAGE (HV) TRANSMISSION LINE PROTECTION SCHEMES

Most transmission lines are protected by directional distance relays. These may serve as backup protection to other schemes in service, or these may be the sensing components in various forms of differential protection.

Figure below shows the basic elements of a PLC (power line carrier) system extensively used for protection of HV transmission lines.

 

An HV transmission line is capable of simultaneous functions of communications and electrical energy transmission. PLC equipment consists of three distinct parts: terminal assemblies consisting of transmitters, receivers, and protective relays; the coupling and tuning equipment, which connects the terminals to selected points on the transmission line; and the transmission line itself, which provides a suitable channel for the transmission of carrier energy in PLC bands of frequencies between terminals.

 

Coupling to lines is accomplished by means of high-voltage capacitors, which provide a low-loss path to carrier signals and block 60 Hz power frequency energy from the carrier equipment. Line traps minimize the loss of carrier power into adjacent lines and prevent external ground fault currents from short-circuiting the carrier signal of the unfaulted line.

 

Carrier Frequencies.

Frequencies in the range of 30 kHz to 500 kHz have been employed for PLC relaying and other communication purposes. The range is high enough to be isolated from the transmission line and the noise it creates and yet not so high as to give rise to excessive attenuation.

 

There are two basic types of signals used for teleprotection channels. Keyed carriers are sometimes referred to as AM, amplitude modulation: It is normally off and intelligence is transmitted by turning the carrier on and off.

 

This type of signal is normally used in blocking-type relaying systems. The frequency may be in range from 29 kHz to 31 kHz, and the signal could be applied to a single sideband (SSB) PLC channel.

 

Frequency Shift Keyed Carrier.

This signal is always on, which provides a means of continuously monitoring the channel. The frequency shift keyed (FSK) carrier is less susceptible to noise and has a greater operating range. FSK channels have been available with two-frequency operations, high and low shift frequencies for additional security.

 

Blocking Schemes.

Transmission line faults are detected using either high-speed phase comparison, in which the phase of the currents at the two terminals are compared, or direction comparison relaying. The scheme operates in a trip permissive mode.

 

A received signal is used to block tripping of the protected line for external faults. The blocking scheme is biased toward dependability because channel or remote relay failure will result in operation of the local blocking relay.

 

Tripping Schemes.

A phase comparison tripping scheme channel is keyed to the trip condition every half cycle during the fault. The scheme is biased toward security so that a failure of the channels or relays would result in non operation of the local relay for external and internal faults. Directional distance relays can be used both for phase as well as ground fault conditions.

-Power system protection blog

String Effeciency and methods to improve String Effeciency

The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency i.e.,

 

String efficiency is an important consideration since it decides the potential distribution along the string. The greater the string efficiency, the more uniform is the voltage distribution. Thus 100% string efficiency is an ideal case for which the voltage across each disc will be exactly the same. Although it is impossible to achieve 100% string efficiency, yet efforts should be made to improve it

as close to this value as possible.

 

Methods of Improving String Efficiency

The maximum voltage appears across the insulator nearest to the line conductor and decreases progressively as the cross arm is approached. If the insulation of the highest stressed insulator (i.e. nearest to conductor) breaks down or flash over takes place, the breakdown of other units will take place in succession. This necessitates to equalize the potential across the various units of the string i.e. to improve the string efficiency.

The various methods for this purpose are :

 

By using longer cross-arms. The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K

can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.

 

By grading the insulators. In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required. However, good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.

By using a guard ring. The potential across each unit in a string can be equalized by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.

Voltage ranges

Synchronization process

There are five conditions that must be met before the synchronization process takes place. The source

(generator or sub-network) must have equal line voltage, frequency, phase sequence, phase angle, and

waveform to that of the system to which it is being synchronized.

 

Waveform and phase sequence are fixed by the construction of the generator and its connections to the system.

During installation of a generator, careful checks are made to ensure the generator terminals and all control

wiring are correct so that the order of phases (phase sequence) matches the system. Connecting a generator

with the wrong phase sequence will result in a short circuit as the system voltages are opposite to those of the

generator terminal voltages.

 

The voltage, frequency and phase angle must be controlled each time a generator is to be connected to a grid.

Generating units for connection to a power grid have an inherent droop speed control that allows them to share

load proportional to their rating. Some generator units, especially in isolated systems, operate with isochronous

frequency control, maintaining constant system frequency independent of load.

Sizing of protective earthing conductor

Below is based on IEC 60364-5-54. This table provides two methods of determining the appropriate c.s.a. for both PE or PEN conductors.

 

 

	c.s.a. of phase conductors Sph (mm2)	Minimum c.s.a. of PE conductor (mm2)	Minimum c.s.a. of PEN conductor (mm2)
			Cu                       AI
Simplified method (1)	Sph≤ 16	Sph(2)	Sph(3)	Sph(3)
	16 < Sph ≤ 25	16	16
	25 < Sph ≤ 35			25
	35 < Sph ≤ 50	Sph/2	Sph/2
	Sph > 50			Sph/2
Adiabatic method	Any size	(3)  (4)

(1) Data valid if the prospective conductor is of the same material as the line conductor. Otherwise, a correction factor must be applied.
(2) When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected:

§  2.5 mm² if the PE is mechanically protected

§  4 mm² if the PE is not mechanically protected

(3) For mechanical reasons, a PEN conductor, shall have a cross-sectional area not less than 10 mm² in copper or 16 mm² in aluminium.
(4) Refer to table G53 for the application of this formula.

Wikipedia