RGPV EX7102 UNITIII

1

READING MATERIAL FOR B.E. STUDENTS

OF RGPV AFFILIATED ENGINEERING COLLEGES

BRANCH VII SEM ELECTRICAL AND ELECTRONICS

SUBJECT EHV AC AND DC TRANSMISSION

Professor MD Dutt Addl General Manager (Retd)

BHARAT HEAVY ELECTRICALS LIMITED

Professor(Ex) in EX Department

Bansal Institute of Science and Technology

Kokta Anand Nagar BHOPAL

Presently Head of The Department ( EX)

Shri Ram College Of Technology

Thuakheda BHOPAL

Sub Code EX 7102 Subject EHV AC AND DC TRANSMISSION

UNIT III

Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

2

EX 7102

RG PV Syllabus

UNIT III EHV AC AND DC TRANSMISSION

Components of EHV DC system, convertor circuits rectifier and inverter valves. Reactive power requirement, harmonic generation. Adverse effect , classification, remedial measures to suppress , filters and ground return. Convertor faults and protection, harmonics mis operation. Commutation failure, Multi terminal DC lines.

INDEX

S No Topic UNIT III Page1 Components of EHV DC system, convertor circuits rectifier

and inverter valves3- 6

2 Reactive power requirement, harmonic generation. 7-93 Adverse effect , classification, remedial measures to suppress ,

filters and ground return9-12

4 Convertor faults and protection, harmonics mis operation 12-205 Commutation failure, Multi terminal DC lines.). 20-23


3

COMPONENTS OF EHV DC SYSTEM, CONVERTOR CIRCUITS

RECTIFIER AND INVERTOR VALVES

The conversion from AC to DC and vice versa is done in HVDC convertor stations bu using three phase bridge converters. A converter has two types of circuits.

1) Main circuit through which high power flows, the main circuit comprises of converter transformers, thyristor valves, bus bars, series reactors etc.

2) Control and protection circuit for firing or blocking the valves in desired sequence, monitoring etc.

The main circuit of converters in HVDC terminal stations are made up of series connected thyristor valves. A thyristor can be turned ‘ON’ when the anode is positive with respect to the cathode and a gate signal is provided. Once the thyristor is turned ON, it can be turned OFF only when the current through it goes to zero and there is minimum commutation margin when the voltage across the thyristor is reverse biased i.e. the anode is negative with respect to cathode. Now the current is blocked until a control pulse is applied to the gate. When not conducting, the thyristor should be capable of withstanding the forward or reverse bias voltage appearing at its cathode and anode. The convertor valves are connected between convertor transformer and DC switchyard. Each thyristor valve is made up several thyristor connected in series. Individual thyristor rating is about 1.5Kv, 600 to 4000 A. The configuration of rectifier and inverter is identical.. The triggering pulse to thyristor gates are delayed by angle ‘α’ called the delay angle, w.r.t the instant of natural commutation i.e the instant when the phase voltage across subsequently conducting valve exceeds that across the proceeding valve. The delay angle is varied by means of control circuit which gives the triggering pulses to the gates of thyristor in each arm of the bridge in a definite sequence. With delay angle less than 90˚, the convertor acts as rectifier mode and the delay angle ‘α’ between 90˚ and 180˚ the convertor works as inverter .

CONVERTER CIRCUITSThe basic HVDC converter is the three phase, full wave bridge circuit as shown in figure below. This circuit is also known As “ GRAETZ BRIDGE”. This is a six pulse converter and the 12 pulse converter is composed of two such bridges in series supplied from two different three phase transformers with voltage differing in phase by 30˚. The pulse number of a converter is defined as the number of pulsation ( cycle of ripple) of direct voltage per cycle of alternating voltage. The configuration for a given pulse number is selected in such a way that both the valves and converter transformer


4

utilization is maximum. The GRAETZ BRIDG has been universally used for HVDC converters as it provides better utilization of converter transformer and a lower voltage across the valve when not conducting. The voltage across the valve when it is not conducting is referred to as the peak inverse voltage and is an important factor that determines the rating of valves.

THREE PHASE , FULL WAVE BRIDGE CIRCUIT (GRAETZ CIRCUITS) The converter transformer has on load taps on the AC side for voltage control. The AC side windings of the transformer are usually star connected with grounded neutral; the valve side winding are delta connected or star connected with ungrounded neutral. The purpose of analysis, the following assumptions are made:-

1) The AC system including the converter transformer, may be represented by an ideal source of constant voltage and frequency in series with lossless inductance representing mainly the transformer leakage reactance.

2) The direct current Id is constant and ripple free;( because of the large smoothing reactor)

3) The valves have zero resistance when conducting and infinite resistance when not conducting.

The equivalent circuit shown below is representation of the bridge circuit.

EQUIVALENT CIRCUIT FOR THREE PHASE FULL WAVE BRIDGE CONVERTERLet the instantaneous phase voltages be ea = Em sin (ωt +150˚) eb = Em sin (ωt +30˚)Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

5

ea = Em sin (ωt - 90˚)Where Em is the peak value of line neutral voltage

Then the line voltage eac = ea-ec =√3 Em sin (ωt +120˚)Similarly eba = √3 Em sin (ωt) ecb = √3 Em sin (ωt -120˚)

To ease the understanding of the operation of bridge converter, let us assume negligible source inductance Lc=0 and no ignition delay

NEGLECTING SOURCE INDUCTANCE As shown the cathodes of valves 1,3 and 5 and anodes of valves 2,4 and 6 are connected together. The valves are numbered in the order of firing. When the phase voltage of phase a is more positive than the voltages of the other two phase, valve 1 conducts. The potential of the cathode of the three valves are now equal to that of the anode of valve 1. As the cathodes of valve 3 and 5 are at a higher potential than their anodes, these valves do not conduct. Similarly valve 2 conducts when phase c voltage is more than the other two phases.

WAVE FORMS OF VOLTAGES AND CURRENTS OF GRAETZ BRIDGE CIRCUIT(a) SOURCE LINE TO NEUTRAL AND LINE-LINE VOLTAGES(b) VALVE CURRENTS AND PERIODS OF CONDUCTION(c) Phase current Ia


6

From the wave form we see that valve 1 conducts when ωt is between -120˚ and 0˚, since ea is greater than eb and ec . Valve 2 conducts when ωt is between -60˚ and 60˚. As ec is more negative than ea and eb during this period, this shows the period of conduction of each valve, and the magnitude and duration of current in it. Since the direct current Id is assumed constant, the current in each valve is Id when conducting and zero when not conducting.

Valve switching sequence without ignition delay and overlap

Just before the period ωt= o, valves 1 and 2a re conducting, just after ωt =0, eb becomes more positive than ea and valve 3 ignites, valve 1 is extinguished because its cathode is now at higher potential than its anode. For the next 60˚, valves 2 and 3 conduct. At ωt=60˚, ea is more negative than ec, causing valve 4 to ignite and valve 2 to extinguish and the sequence continues. The period and the conducting valves is given in the table below:-

SNo Period Conducting Valves1 -60˚ to 0˚ 1 and 22 0˚ to 60˚ 2 and 33 60˚ to 120˚ 3 and 44 120˚ to 180˚ 4 and 55 180˚ to 240˚ 5 and 66 240˚ to 300˚ 6 and 1

REACTIVE POWER REQUIRMENT, HARMONIC GENERATIONProf MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

7

Both the active power P and reactive power Q are equally important for satisfactory operation of HVDC converters. The rectifier as well as the inverter absorb the reactive power. The reactive power is required fora) Mainly due to the delayed current conduction in the converter.b) Partly due to the reactance of the converter transformer.Supplying leading reactive power is called reactive power compensation. AC system supplies active power P as well as some reactive power Q to the convertor; but this not enough. Hence additional compensation is provide on AC side of the converter by means of AC filter capacitors, AC shunt capacitors, synchronous condensers or static VAr source. Reactive power compensation has a significant influence on the cost of HVDC substation.As we know CosΦ = cosα

Where cos Φ is the displacement factor or vector power factor and angle ‘Φ’ is the angle by which the fundamental line current lags the line to neutral source voltage. When the delay angle ‘α’ is zero, the fundamental component of current is in phase with the line to neutral source voltage, As the delay angle is increased the displacement angle between the fundamental component of the current and the line to neutral voltage increase and current lags behind the voltage. Thus, the converter which may be a rectifier or an inverter draws reactive power from the AC system.

The reactive power compensation/ consumption of converters vary mainly with :-1) The active power2) The delay angle ‘α’ of rectifier and extinction angle ‘γ’ of the inverter.3) AC bus voltage and DC pole voltage.4) The commutating reactance of converters5) Mode of operation of HVDC system (monopolar , bi polar)

Variation of reactive power with active power

The reactive power demand of converter varies between 20% to 60% of the active power flow. At full load, the reactive power absorbed by the converter is about 60% of the active power flow for the normally used values of transformer reactance,


8

delay angle and extinction angle . In practice reactive power is controlled in narrow margin.

Generally AC filter capacitors are arranged in suitable switchable banks such that the requirement of AC harmonics filters and reactive power compensation on AC side are fulfilled by the AC filter banks under various conditions of power and voltage of Ac and DC network condition.The variation of reactive power Qd versus active power Pd is shown above in the diagram.In case higher compensation is required additional shunt capacitors are installed. Synchronous condensers are use in special cases where AC bus needs compensation of reactive power as well as additional short circuit level for satisfactory converter operation and rotating inertia for improvement in dynamic stability.At the inverter terminal, the inverter end feeds power to AC load. So in addition to reactive power required by inverter, the reactive power required by the AC load should also be supplied. At the rectifier terminal the reactive power required by the converter is supplied partly by filter capacitors and partly by AC network. Thus the MVAr of filter capacitors at the rectifier end may be less than the MVAr of filter capacitors at the inverter end. The reactive power requirements of a converter can be reduced to zero or even reversed if forced commutation is used. This also helps in avoiding commutation failure sin inverters. Forced commutation is achieved by addition of a voltage component to the normal commutation voltage to shift the zero crossing. This cam be implemented easily by providing series capacitors. Forced commutation is feasible if gate turn off ( GTO) or MOS controlled thyristors ( MCT) are usd in converters. Or else , the auxiliary circuits required for forced commutation along with increased rating of thyristor valves may be expensive as compared to the cost of reactive power compensation devices required without forced commutation.

HARMONIC GENERATIONAny periodic non sinusoidal waveform can be resolved into a fundamental sine wave of the same frequency and several other sinusoidal wave form of higher frequency order known as HarmonicsHarmonic voltage and currents are integral multiple of fundamental frequency. Therefore, any periodic non sinusoidal 50Hz current or voltage waveform is considered to be the sum of basic 50Hz current or voltage and some combination of harmonics.Operation of thyristor converters in HVDC terminal substation generates several current and voltage harmonic in AC and Dc system. These harmonics travel into AC system and DC lines creating problems. Effect of each harmonic is analyzed mathematically by applying the rules of circuit analysis of sinusoidal periodic functions. The effect of original non sinusoidal waveform on the circuit is predicted


9

by summing up the effect of component sinusoidal waveform by using superposition theorem. Harmonics are eliminated by using AC shunt filters and DC shunt filters. AC harmonic filters are provided on the bus bar of converter transformer. These AC filters are R,L,C circuits connected between phase and earth. The value of L,C parameters are so selected that the filter offers low impedance to harmonic frequencies. Thereby harmonic currents are passed to earth and the harmonic content in the AC network is minimized to acceptable level. The AC harmonic filters provide reactive power required for satisfactory converter operation. The requirement of AC filters and shunt compensation depends on the condition of AC network. The AC harmonic filters constitutes a significant capital cost of the HVDC terminal substations. A large area in converter substation is covered by AC filters.DC filters are R,L,C shunt circuits connected between DC pole and neutral bus. Smoothening reactor also helps in reducing DC current harmonics.

ADVERSE EFFECT S, CLASSIFICATION, REMIDIAL MEASURES TO SUPPRESS , FILTERS, GROUND RETURN

Harmonic caused by operation of HVDC converter travels into AC network and along DC lines and have the following harmful effects:-

a) Harmonics cause heating of filter capacitors.b) Harmonics cause additional heating of rotating machines such as synchronous

generators, synchronous motors, induction motors, power transformers etc, and machines are to be derated .

c) Harmonics influence torque, speed characteristics of induction motors. The speed as well as torque is reduced.

d) Harmonics of particular order may cause resonance in AC circuit and thereby over voltage, flash over’s and insulation failures.

e) Harmonic causes noise in communication system.f) Harmonic cause disturbance, in accuracy and instability in constant current

control system of HVDC converters.g) Shunt capacitors and shunt reactors are heated due to harmonics. h) DC harmonics cause telephone interference in adjacent telephone linesi) DC harmonics cause disturbance in control system of HVDC converter.

The telephone interference TI is the most significant trouble caused by harmonics and most difficult to overcome.

CLASSIFICATIONOperation of HVDC converter gives predominant current harmonics on AC side and predominant voltage harmonics on DC side. AC current harmonics pass through AC network impedance and produce voltage harmonics corresponding to the voltage drop


10

in AC network impedance due to harmonic current. DC voltage harmonics travel along HVDC line and generate corresponding DC current harmonics due to voltage harmonics and DC impedance.The harmonics are of various order from 2 to 60 , harmonics are classified as:-

1) Characteristics harmonics2) Non Characteristics harmonics.

Characteristics Harmonics :- Harmonics which can be predicted by mathematical analysis and are generally predominant are called Characteristics Harmonics. Characteristics harmonics are always present even under ideal operation i.e balance Ac voltages, symmetric three phase network and equidistant pulses.By mathematical analysis of the original AC and DC waveforms of HVDC converters the following characteristics harmonics are obtained.

nac = pq ± 1 i ndc = pq iiq= integers 0,1,2,3p= pulse number of convertor ( 6 or 12) Harmonics given by equation i are called characteristics harmonics on AC sideHarmonics given by equation ii are called characteristics harmonics on DC side

These are harmonics of definite order. Necessary filters are provided for diminishing these harmonics.

The order of characteristics harmonics depends on the pulse number of the converter. From nac = pq ± 1 Pulse no p=6 pulse no p=12

q 0 1 2 3 0 1 2 31 5 11 17 nac 1 11 23 35

7 13 19 13 25 37

The amplitude of harmonic waveform goes on reducing as the order of the harmonic increases. Hence harmonics of 60th order and above are usually too small and are neglected . for calculation for designing of filters, they are considered only for designing filters with reference to permissible limits of telephone interference.

NON CHARACTERISTICS HARMONICSSeveral other non characteristics harmonics may be present in small portion. Non characteristics harmonics are harmonics of other order than characteristics harmonics and are less predominant than the characteristics harmonics. The actual waveform contain various characteristics and non characteristics harmonics. Non characteristics harmonics are generated due to :

a) Imbalance in the operation of the bridges.


11

b) Firing angle errorsc) Unbalance and distortion in AC voltagesd) Unequal transformer leakage impedances.

The harmonics produced due to imbalance in the operation of bridge is termed as residual harmonics. These are produced mainly because of the difference in the firing angles in the two bridges of a 12 pulse converter. The unequal leakage reactance’s of the two converter transformer feeding the two bridges also lead to residual harmonics.

The last three cause can lead to the generation of triplen and even harmonics.

However such non characteristics harmonics are usually of smaller magnitude and therefore separate tuned filters are generally not provide to filter them.

REMIDIAL MEASURES TO SUPPRESS

The operation of thyristor converters in HVDC system produces several characteristics and non characteristics harmonics in AC and Dc wave form. Harmonics have harmful effects on AC generators, Induction motors , capacitors, transformers , supply circuit etc. They cause in the AC and DC waveform within specified limits.Harmonics are minimized by

1) The use of converter bridges of higher pulse number.2) The use of DC smoothening reactor.3) The use of Ac and DC harmonic filters.

There are basically two type of filters that are used in HVDC system. They are:-a) Passive filtersb) Active filters

The passive filters have been used in HVDC system right from the early days and are still used. Passive filters are basically LC circuits whose inductance and capacitance are so selected that the complete LC circuit resonates at a particular frequency. At this resonate frequency, the filter offers low impedance to the harmonics of that frequency and high impedance to other frequency harmonics. Thus the filters used to pass and stop certain frequencies.In HVDC systems, AC shunt filters and DC shunt filters are used for suppressing characteristics and non characteristics harmonics. Series filters are used only for power line carrier frequency range (80to 500Khz). Normally series filters are not used as main harmonic filters.

FILTERSAC FILTERS


12

In HVDC converter station AC filters are installed at AC yard side. These filters comprising of R,L,C banks connected in shunt i.e between line and ground at the AC bus bar. Harmonics have to be filtered out sufficiently at the terminal so that the harmonics entering the AC system are small and distortion caused by harmonic currents is within limits. The penetration of harmonic into the AC system and resonance conditions depends on the harmonic impedance of AC network. The harmonic impedance is constantly varying as the circuit are being added or switched out and also the system operating conditions are varying. In filter design elaborate studies are to be undertaken considering various factors including the possible system resonance.

An LC circuit resonates at a frequency given by

f 0 = 1 2π√LCAt this frequency, the filter offers low impedance to harmonic of f 0 frequency and high impedance to other frequencies. A typical AC filter has several parallel branches tuned for specific frequencies such as 7th 9th 11th 13th etc. The respective tuned branches offer low impedance to respective harmonic frequency currents order and conducts them to earth. Hence, the harmonic contents in the network are reduced. The impedance of AC network changes with load. The Ac filter requirements and the AC shunt compensation requirement is affected by the equivalent impedance of the AC network. Hence the Ac filter requirement changes with the power transfer through the DC link.

AC filters are grouped in separately switchable groups, each having certain tuned branches and a high pass branch.To adjust the L and C parameters of a filter to some specific resonant frequency to achieve desired harmonic suppression is know as tuning of a filter. Tuning signifies the sharpness of a filter. The sharpness of tuning is normally expressed in terms of Q factor of an inductor or a capacitor or a circuit.

The Q factor = 2π[Maximum energy stored ] Energy dissipated per cycleIn a complete cycle of sinusoidal AC wave, the maximum energy stored in an inductor is ½ LIm².Similarly average power of an inductor with resistance R is I²R. thus energy dissipated per cycle,

= { Im }² R √2 fWhere ‘f’ is the frequency, Thus the Q factor of an inductor


13

= 2π ½ LIm² = 2πfL ( Im/√2 )²R R fThe Q factor for an inductor = ωL/RFor a capacitor with capacitance ‘C’ and resistance ‘R’The maximum energy stored= ½ CVm² = 1 Im² 2 ω²C And the energy dissipated per cycle { Im }² R √2 F

The Q factor of a capacitor = 1 ω0 CRFor a parallel RLC circuit resonance, the factor = ω0 CR

At the resonant frequency ω1 the current is maximum ω2 are the corresponding frequencies when the value of current is 0.707Im. The power delivered to the circuit at ω1and ω2 frequencies is( 0.707Im)² times i.e half of the maximum power Im²R.

The distance between ω1 and ω2 measured in Hz is called the band width of the filter.The Q factor or quality factor can be expressed in terms of the bandwidth as

Qr = ω0 = f0 ω2 - ω1 f2 - f 1

The AC shunt filters thus1) Divert harmonics to ground and reduce the harmonic contents in the main AC

network.2) Provide reactive power compensation required on the AC side. Part of the

reactive power required by converter is supplied by AC shunt filters and the remaining requirements is provided by shunt capacitor banks.

CHARACTERISTICSOF SERIES RLC FILTERThe design criteria for AC harmonic filters include


14

1) To achieve the required reduction in the harmonic contents in the AC network according to the specifications and statutory rules.

2) To reduce telephone interference.3) To supply the required reactive power or a major part of it.4) Minimum cost.

TYPES OF AC FILTERSThe following are the various type of AC filters that can be used:-

a) Single tuned filterb) Double tuned filterc) High pass filter

a) Single tuned filter, b) Double tuned filter c)Second order high pass d) High pass ’C’ type

IMPEDANCE CHARACTERISTICS AS A FUNCTION OF FREQUENCYa) Single tuned b) Double tuned c) High pass filter

DC FILTERSThe harmonics in the DC voltage contain both characteristics and non characteristics harmonics. These harmonics result in the current harmonic in DC lines and cause telephone interference. The harmonic current varies with the distance from the converter station.


15

DC filters are used to reduce DC harmonics to minimize the telephone interference. The choice of DC filters also affect the overvoltage due to DC line resonance and line fault. Therefore, in addition to the telephone interference criteria, the DC filters are designed to avoid the resonance between DC filters and the DC line at lower order harmonics.

The design aspect of the DC filter is similar to the AC filter except that the reactive power of DC filter is not significant. The rating of DC filter capacitors is determined by maximum DC voltage, required capacitance and thermal rating due to harmonic current.The size or MVAr rating of DC filter is much smaller than that of AC filter. This is because the smoothening reactor smoothens the ripples in DC current or reduces the DC harmonic current substantially. The various components in the configuration of Dc filter branch include:-

1) Oil and air filled smoothening reactors connected in the pole bus.2) Shunt tuned filter branches consisting of capacitors ,reactors and resistors.3) High pass filter comprising of a capacitor and an inductor resistor combination4) Surge capacitors connected in parallel between the pole bus and the neutral bus

and the earth.

GROUND RETURNHVDC transmission lines use ground or sea water as the return conductor either continuously in the monopolar mode or for short time in case of emergency in bipolar operation. These return paths are called ground return even when the sea water is used as a return path ground electrodes are used at both ends of the HVDC link to transfer current from the convener to the ground. There are three types of electrodes. Namely

1) Land electrodes2) Shore electrodes located on a sea beach at a short distance from water line.3) Sea electrodes located in the water at some distance from the coastline.

A monopolar line with ground return is more economical than a bipolar line due to the elimination of the return conductor. For the same length of transmission, the resistance offered by the ground in case of DC is much less as compared to CA transmission as the DC spreads over a very large cross sectional area in both depth and width. In fact the earth resistance in case of DC in independent of line lengths ( for long lines) and is equal to the sum of the electrode resistance. As resistance in case DC is low, the power losses are also reduced. Besides this, there are two more significant advantages of using ground as the return, They are:-

a) A DC line can be built in two stages if the initial load requirement is low. Initially the line will operate as a monopolar line with ground return and later on it can be built as a bipolar line. Thus considerable part of the investment can be deferred until the second stage.

b) The use of ground return improves the reliability / availability of the HVDC link. In the event of an outage of one of the conductors of a bipolar line, it can be temporarily operated at almost half of its rated power by the use of the healthy


16

line and the ground. Thus, the reliability of a bipolar line is equal to that of double circuits 3 phase line.

CONVERTOR FAULTS & PROTECTION, HARMONIC MISOPERATION

The Faults in a DC system are caused by1) The malfunctioning of the equipment and controllers2) The failure of insulation caused by external sources such as lightning pollution

In HVDC transmission system, the faults and abnormal conditions can occur in the transmission line, converter, DC yard, AC yard, auxiliaries etc. The faults have to be detected and the system has to be protected so that the disruption in the power transmission is minimized.For satisfactory operation of the converters, the valves of the converter bridge should fire in a definite sequence and conduct in a definite direction.There are three basic types of faults that can occur in converters and given below:-

a) Faults due to malfunction of valves and controllers1) Arc backs or back fire2) Arc through or fire through3) Misfire4) Quenching or current extinction

b) Commutation failure in inverters.c) Short Circuits

ARC BACKThe arc back is the failure of the valve to block in reverse direction and results in temporary destruction of the rectifying property of the valve due to conduction in the reverse direction. This is a major fault in mercury arc valves and is of random nature. This is a non self clearing fault and results in severe stresses on transformer windings. Arc back is more common during rectification than during inversion. Thyristor have the advantage that they do not suffer from arc back.ARC THROUGHA malfunction in the gate pulse generator or the arrival of a spurious pulse can fire a valve which is not supposed to conduct, but is forward biased. For example, in a bridge, when valve 1 is successfully commuted its current to valve 3, the initial voltage across it is negative for the duration of the extinction angle and then become positive. If valve 1 is fired at this time, the current will transfer back to valve 1 from valve 3. This is known as ARC THROUGH. The arc through fault is likely to occur mainly at the inverter end, as the valve voltages at the inverter is positive most of the time when they are not in conducting mode. The effects of arc through are similar to that of commutation failure- the voltage across the bridge falls as valve 4 is fired ( with valve 1 conducting) and the AC current goes to zero when valve 2 current goes to zero. The firing of valve 5 is unsuccessful and the bridge recovers to normal operation after valve 6 is fired and subsequent firings are according to the normal sequence. Thus a single arc through in thyristor valves can occur due to malfunction in the control system, but


17

probability is very small. The protection against persistent arc through is also through the converter differential protection scheme.MISFIREWhile an “arc through” is caused by the presence of unwanted gate pulse, misfire occurs when the required gate pulse is missing and the incoming valve is unable to fire.

A misfire can occur in rectifier or inverter stations, but the effects are more severe at the inverter station. This is due to the fact that in inverters, persistent misfire leads to the average bridge voltage going to zero, while an AC voltage is injected into the link. This results in large current and voltage oscillations in the DC link as it presents a lightly damped oscillatory circuit viewed from the converter. The DC current may even extinguish and result in large over voltage across the valves. The chances of misfire are very small in modern converter stations because of duplicated converter controls, monitoring and protective firing.The effects of a single misfire are similar to those of commutation failure and arc through. However, at the end of the cycle, the normal sequence of valve firings is restored. Thus a single misfire is also self clearing.

CURRENT EXTINCTIONThe extinction of current in a valve can occur if the current through it falls below the holding current. This can arise at low values of the bridge currents when any transient can lead to current extinction. The current extinction can result in over voltages across the valve due to current chopping in the oscillatory circuit formed by the smoothing reactor and the DC line capacitance.The problem of current extinction is more severe in the case of short pulse firing method. However, in modern converter stations, the return pulses coming from thyristor levels to the valve group control, indicate the build up of voltage across the thyristor and initiate fresh firing pulses. It may happen that a number of firing pulses may be generated during a cycle when the link current is low.

PROTECTION AGAINST OVERCURRENTThe over current protection in convertors is based on principle similar to those used in AC systems. The factors considered in designing a protection system are:-

1) Selectivity2) Sensitivity3) Reliability4) Back up

The main feature of converter protection is that it is possible to clear faults by fast controller action (in less than 20ms) by blocking gate pulses or current regulation and control. The selectivity is enhanced by high impedances of the smoothing reactor and the converter transformer. The converters are divided into independent valve groups such that the protection system must be able to switch off only the affected valve group.


18

Consider a converter station of a 12 pulse converter ( 2 valve group per pole). The protection system used per pole is shown in the figure below

OVERCURRENT PROTECTION IN A POLE

The protection against converter faults discussed earlier is provided by valve group differential protection, which compares the current on the valve side of converter transformer to the DC current measured on the line side of the smoothing reactor. The differential protection is employed because of its selectivity and fast detection. The over current protection used as back up. The level of over current required to trip must be set higher than that of the valve group differential protection to avoid tripping with faults outside the stations.The pole differential protection is used to detect ground faults which may not be otherwise detected.The fault clearing action of these protection circuits is to block the valves and at the same time trip the AC circuit breaker of the affected group or pole. The fast tripping sequence is used for internal faults where there is danger of valve damage. This involves increasing of the delay angle of the rectifier to about 150˚ combined with the signal to trip the Ac circuit breaker. The pulses are blocked after 29ms. This allow the inverter action at the rectifier station to try to reduce the current before the converter is blocked. The faults are classified as:-

a) Internal faults which cause high over currents but are very infrequent. The thyrister surge current ratings must be chosen to withstand these over currents.

b) Line faults which cause over currents in the range of 2 to 3 p.u These are limited by current control.

c) Commutation failures at the inverters may be quite frequent, However, the over currents are small and limited by current control.

OVER VOLTAGE IN A CONVERTER SYSTEMThe over voltage in a converter station are caused by :-Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

19

a) Disturbances on the AC sideb) Disturbances on the DC sidec) Internal faults in the converter

The type of over voltage can be classified into three categories.1) The switching over voltage ( with wave front of more than 100ms)2) Temporary over voltage ( lasting few seconds)3) Steep front over voltages ( with wave front times in the range of 0.3 -3 msec)

DISTURBANCE OF THE AC SIDEThe lightning strokes in the AC network cause steep-fronted high over voltages. These are, however, reduced in magnitude and steepness by AC filters. After they pass through the converter transformer, they appear only as highly damped switching surges across the converter.The initiation and clearing of the faults in the AC system result in switching surges and temporary over voltages. The energization of a converter transformer can cause high over voltage ( upto 1.6p.u) due to the inrush magnetizing currents that last up to 100cycles. The voltage is also distorted due to even harmonics. This type of temporary over voltage can cause severe stresses on the surge arrestors due to excessive energy dissipation. Pre insertion resistors in the circuit breakers are very effective in controlling these over voltages. The temporary over voltages due to load rejection can be quite serous for converter stations connected to weak AC systems. The load rejection may be caused by blocking of the converters in response to the action of the protection system.

DISTURBANCE OF THE DC SIDEThe steep wave front surges in DC overhead lines are produced by lightning strokes. However, when they reach the converter through the smooting reactor they appear as switching surges.The switching surges at the converter are also caused by ground faults on a pole of bipolar DC link. Because of the capacitive and inductive coupling between conductors, the surges also occur at the healthy pole. The magnitude and the wave shape of these surges arriving at the terminal are dependent on the type of termination; inductive capacitive or resistive, The rate of decrease of voltage at the terminal is a variable and is utilized in line fault detection. The over voltage can also arise from the oscillation of current and voltage in the line caused by sudden jumps in the converter voltage due to commutation failure and other converter faults or injection of AC voltages of fundamental frequency and second harmonic.The switching of DC filter, parallel connection of poles can cause transient currents and over voltages which will stress the neutral bus and filter reactors.

OVER VOLTAGE CAUSED BY CONVERTER DISTURBANCES


20

The series connection on thyristors and the spread in delay times of thyristor turn on result over voltages across the device during turn on. These over voltages are repetitive and have to be taken into account in the valve design and the choice of snubber circuit parameters. The commutation over shoot also result in repetitive over voltages.Transient over voltages of very steep front may result from internal converter faults, such as a ground fault at the valve side of the smoothing reactor. The firing of bypass pairs or closing of bypass switch across one converter generates over voltages across the remaining converters.The energization of the DC line from the rectifier side with the remote terminal blocked can cause high over voltages at the inverter which is open ended. Such things should be avoided by deblocking the inverter and limiting the rate of decrease of delay angle.

PROTECTION AGAINST OVER VOLTAGES

The basic principles of over voltages protection are given below:-1) The over voltage stresses in equipment must be limited by providing surge

arresters. The protection level of the surge arresters must be lower than the breakdown voltages of the insulation.

2) Self restoring insulation such as air may be allowed to breakdown where there is no danger to the safety of the personnel.

3) The operation of surge arresters must not be frequent. Hence the protective level of arresters must be higher than the maximum operating voltages in the system.

4) There must be proper coordination of the insulation and over voltage protection in different parts of the system.

The over voltages generated on the CA side should be limited by the arresters on the Ac side. The over voltages generated on the DC side must be limited by Dc line. DC bus and neutral bus arresters. The critical components such as valves are directly protected by arresters connected close to the components.

HARMONIC MIS OPERATION Due to harmonic present in the system following operational problems are encounteredLOSSES AND HEATING DUE TO HARMONICSHarmonic caused by HVDC converters travel in connected AC network and cause additional losses and temperature rise in the synchronous generators, synchronous motors, induction motors, power transformers etc. Harmonics have higher frequency order and therefore cause additional hysteresis losses, eddy current losses.

Hysteresis loss is directly proportional to the frequency. Eddy current loss is proportional to the square of frequency. The stray fluxes ( leakage fluxes) are also increased with increased harmonics. The skin effect increases with increased frequency.

The total iron loss with harmonics is much higher than total iron loss for the fundamental frequency. Higher iron losses cause more loss and more heating. With


21

higher harmonic content the motor , generator, transformer will have higher losses, lesser efficiency and temperature rise with the same ampere load.

TORQUE AND SPEED OF INDUCTION MOTOR

Harmonic cause harmonic fluxes in air gap of induction machines. These rotate either positive or negative sequence given by

n=6q± 1 ( as per expression of a 6 pulse converter)where n is the order of harmonic and q is integers 1,2 etc

With +ve sign of 1, the flux is of +ve sequence i.e in the direction of rotating fundamental magnetic field. With-ve sign of 1 the flux is of –ve sequence i.e opposite to the rotating magnetic field of fundamental frequency.Hence for harmonics 5,11,17 etc negative sequence magnetic flux is produced in air gap resulting into reduced torque and speed. COMMUTATION FAILUREFailure to complete commutation before the commutating voltage reverses is known as commutation failure.The extinction angle is given by γ = 180 – α - µBecause of the turn off time requirements of thyristor there is a need to maintain a minimum value of extinction angle γmin. The overlap angle µ is a function of the commutation voltage and the DC current. The reduction in the voltage or increase in the current or both can result in an increase in the overlap angle which can result in γ<γmin.This gives rise to commutation failure. The current in the incoming valve ( say valve 3) will diminish to zero and the outgoing valve ( valve 1) will be left carrying the full link current.The firing of the next valve in sequence ( valve4) will result in the short circuit of the bridge as both valves in the same arm will conduct. If commutation from valve 2 to valve 4 is successful, only valves 1 and 4 are left conducting and this state continues until the valve 6 is fired. The firing of valve 5 prior to the firing of valve 6 is unsuccessful as the valve 5 is reverse biased at the time of firing.If the commutation from valve 4 to valve 6 is successful, the conduction pattern returns to normal except that the bridge voltage is negative at the instant where valve 4 stops conduction. If the causes which led to commutation failure in valve 1 in the first instance have disappeared, the bridge operation returns to the normal state. Thus, single commutation failure is self clearing.The failure of two successive commutations in the same cycle, is called ‘ double commutation failure’ . If the commutation failure occurs when valve 4 is fired also, the valves 1 and 2 are left in conducting state until the instant in the next cycle when valve 3 will be fired. The double commutation failure is more severe than the single commutation failure. Double commutation failure is very rare but like the single commutation failure, it is self clearing.The following are the effects of a single commutation


22

1) The bridge voltage remains zero for a period exceeding 1/3rd of c cycle, during which the Dc current tends to increase.

2) There is no Ac current for the period in which the two valves in an arm are left conducting.

The recovery from a commutation failure depends ona) The response of the gamma controller at the inverter.b) The current control in the linkc) The magnitude of the Ac voltage

On detection of a commutation failure, if the angle of advantage β is increased, subsequent commutation failures may be averted. However, it depends upon the control of DC current and the magnitude of AC voltage. The initial rate of rise of current in the inverter is limited by the smoothing reactor and the current controller at the rectifier helps to limit the current in the case of persistent commutation failure. It may be necessary to reduce the current reference to limit the overlap angle in the case of low voltages caused by faults in the Ac system.Commutation failures are self clearing although in the case of persistent commutation failures, the converter differential protection removes the converter out of service. During commutation failures when the two valves in an arm of bridge are left conducting, the CA current goes to zero while the DC current continues to flow.

The commutation failure in a bridge can lead to consequential commutation failures in the series connected bridge.

MULTITERMINAL DC LINESA multiterminal Dc (MTDC) transmission system consists of three or more HVDC terminal substations and interconnecting HVDC transmission line. MTDC systems are more attractive and may fully exploit the economic and technical advantages of HVDC technology. The MTDC system interconnects three or more AC systems through HVDC lines. Some terminal stations of the MTDC act in rectifier mode and transfer power from AC system to the DC system where as after terminal operate in inverter mode and transfer power from DC system to Ac system. The total power taken from rectifier substation should be equal to the total power supplied by the inverter substation if the losses are neglected.MTDC systems with parallel connected converters are now preferred over series connected converters. MTDC system provide solution to the control problems of interconnection of three or more large AC systems. By MTDC system few HVDC interconnection of high capacity can provide asynchronous tie ups between three or more large AC systems.

MTDC NETWORK CONFIGURATION

There are two possible schemes for a MTDC system:-1) Series scheme with constant current


23

2) Parallel scheme with constant voltageThe series connected MTDC scheme is normally run with constant current setting set by the master converter station. The master station determines its voltage and other converter station have to vary the Dc voltage. Flexibility of the power transfer requires a wide range of transformer tapping at the series station. Thus , a large tap changing ratio, large reactive power requirement, varying DC voltage are the main disadvantages of a series connected MTDC system. Parallel MTDC schemes are therefore widely used and they provide practical solution to the operational problems. In the parallel connected MTDC scheme the converters are connected in parallel and operate at a common voltage. The connections can be either radial or mesh. The currents of each converter are adjusted to share the power allocation.

PARALLEL CONNECTED MTDC SCHEME WITH RADIAL NETWORK

In this system some substation poles acts in rectifier mode and others in the inverter mode. For a parallel connected MTDC system

Sum of rectifier outputs = sum of inverter out put + sum of lossesAs seen from the direction of power in the diagram, station 1 and 4 acts as rectifier whereas station 2 and 3 acts as inverter. The currents Id1, I d2,I d3 and I d4 are adjusted to get the required P1,P2,P3 and P4.Parallel connected MTDC system offer minimum operational and control problems. As compared to series connected scheme, it results in fewer line losses, easier control and offer more flexibility for future extension. The majority of proposed applications of MTDC have considered parallel configuration with radial type connection as the mesh connections requires greater length of DC lines.Practical applications of parallel connected MTDC system include.

1) Parallel Tap off2) Three terminal Dc system3) MTDC system for bulk power transfer4) MTDC system for interconnection

The MTDC system interconnect three or more network by asynchronous DC tie. MTDC system have technical and economical advantage over equivalent two terminal HVDC systems. They provide greater flexibility in dispatching the power between three or more AC systems. In large interconnected systems, MTDC systems can provide Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

24

precise and faster control over dispatching of power. The frequency oscillations in the interconnected systems can be quickly damped out by MTDC system control. The inherent overload capability of MTDC systems can increase the transient stability limits.


Engineering

RGPV EX7102 UNITIII