Q 7) explain the basic principle of control in hvdct.1.The current in a dc link depends mainly on the resistance and the difference in voltage at the two ends of the HVDC transmission line, according to Ohm's law. 2.The voltage at the converter station depends on the number of converter groups in series, ac system voltage, converter tap-changer position, circuit impedances and firing angle of the valves. 3.The tap-changer position and firing angle of the valves can be changed for control purposes. 4.For efficient operation, the voltage is maintained high. 5.For minimizing the reactive power consumption of the converters, the firing angle at the rectifier and the commutation margin angle at the inverter side are generally kept low.6. The inverter firing angle is controlled to maintain the commutation margin angle between 15' to 20 and the inverter tap-changers controlled to give the rated dc voltage.7. The rectifier firing angle is then controlled to obtain the required transfer of current or power and the rectifier tap-changer is adjusted to keep the firing angle between 10 to 20'.8. If due to a sudden change in ac system voltage, it is not possible to increase the voltage above the requirement at the rectifier end, the voltage at the inverter end can be reduced by increasing the commutation margin angle at the receiving-end thus obtaining the necessary power transfer.9. Dc links have the ability to switch on or off all or varying amounts of power in a few milliseconds. This facility is used to protect dc equipment against faults.10. In some cases, dc converters are switched off for approximately 0.8 s and full load can be taken again in a few milliseconds. Thus, block-DE block sequence is possible with HVDC transmission.OrThe current in a dc line operating in the steady state is given by Ohm's law as the difference in its terminal voltages divided by its resistance. The current Aid in the line is then given by (1)In this equation cos is used in the numerator and + Rc2 in the denominator if the inverter is operated with constant ignition angle ; cos and Rc2 are used if the extinction angle is constant. For present purposes, the former mode of operation is assumed, because it is the ignition angle that can be directly controlled; the extinction angle y is controlled indirectly through controlling to values computed from the direct current Id, the commutating voltage, and the desired extinction angle. Direct current Id, then, depends on the voltage dropnumerator of Eq. (1)divided by the total resistance (denominator). Since in practice the resistances are fixed, the current is proportional to the difference of the two internal voltages and is controlled by controlling these voltages. The direct voltage at any designated point of the line, as well as the current, can be controlled by controlling the two internal voltages; for example, if the line is uniform and if the two commutating resistances are equal, the voltage at the midpoint of the line is the average of the internal voltages. The direct voltage at any other point of the line is a weighted average of the internal voltages. More generally, any two independent quantities, for example, power and voltage, could be controlled by the two internal voltages. Each internal voltage can be controlled by either of two different methods: grid control or control of the alternating voltage. The internal voltage of the rectifier is written in Eq. (1) as Vd01 cos . Grid control, delaying the ignition angle a (time / ), reduces the internal voltage from the ideal no-load voltage Vdo1 by the factor cos. (I.e the voltage drop due to overlap is represented by the voltage across the com-mutating resistance R01.) The alternating voltage could be controlled by generator excitation, but it is usually controlled by tap changing on the converter transformers. Grid control is rapid (1 to 10 ms), but tap changing is slow (5 to 6 sec per step). Both these means of voltage control are applied cooperatively at each terminal. Grid control is used initially for rapid action and is followed by tap changing for restoring certain quantities (ignition angle in the rectifier or voltage in the inverter) to their normal values.Q2) explain the constant current vs constant voltage control.two alternative ways of operating a dc transmission system while permitting control of transmitted power are as follows:1. Current held constant while power varies as the power does (i.e. constant current control)2. Voltage held nearly constant while current varies as the power does( i.e. constant voltage control)These 2 methods could be used for ac transmission and distribution.parametersconstant current controlconstant voltage control

Definition the rectifier station controls DC current through firing angle, a, which is called constant current (CC) control.The inverter station controls voltage through extinction angle, y, which is called constant voltage control.

Connections Various loads and sources are connected in in seriesVarious loads and sources are connected in parallel

Application Widely used for street lightning circuits or in earlier dc transmission projectsThis scheme is used in ac transmission and distribution as well as in dc distribution systems.

Load limiting Load or source is turned off by bypassing it after bringing its EMF to 0 if it has one.Load or source is taken out of service by opening the respective branch.

Limitation to Short circuit current Sc currents are ideally limited to the value of load current Value of sc c/n is much larger because this c/n are only limited by circuit resistance

I2R lossI2R loss is always same as full load value.I2R loss in the conductors is proportional to the square of the power transmitted

Daily or annual energy lossIf the s/m transmits less than rated power, the daily or annual energy loss is much more than cvc.If the s/m transmits less than rated power, the daily or annual energy loss is much less then ccc.

Voltage dependent losses (corona insulation leakage)lessmore

Conclusion : Thus, consideration of losses favors the constant-voltage system, but limitation of current favors the constant-current system. Q 10) explain the working of bypass valve.Definition : Most of the valve faults that are not self-clearing with the valve in service are cleared by relieving the valve from current for a fraction of a second.. This is the purpose of the bypass valve.To remove an arc-back, current is diverted into a bypass valve. The bypass valve is a separate valve connected across a 6-pulse valve group. This valve has a higher current rating than other valves and is capable of carrying I pu direct current for about 60 seconds.

The control grid of the bypass valve is normally blocked. Working:When a bridge is to be bypassed, its bypass valve is unblocked and the main valves are simultaneously blocked by discontinuing the transmission of positive pulses to their grids. The direct current shifts from the main valves to the bypass valve not instantly but in a few milliseconds by a process like normal commutation between two main valves.. By simultaneously unblocking the main valve and blocking the bypass valve, the direct current can be transferred back to the main valves.If a bridge is to be removed from service for replacement of a defective valve or for other maintenance work, the direct current is first transferred to the bypass valve, after which the bypass switch (8 in Figure 1) is closed and takes over the direct current. Finally, the ac and de disconnecting switches (9 and 10) are opened, isolating the bridge. For putting the bridge back into service, these switching operations are performed in reverse order.Q 11) explain the current transfer to the bypass valve both for rectifier and inverterTransfer of Current A] Transfer to the Bypass Valve (rectifier) Consider the bridge is operating as a rectifier. Assume that the direct current in the line is kept constant by the action of the dc reactor and the constant-current control. Assume valves 1 and 2 are conducting, the grids of all the main vales are given a blocking signal (that is, the positive grid pulses are turned off) and that valve 7 (the bypass valve) is unblocked; that is, its grid is made positive.At this instant, the anode voltage of valve 7, v7 = vd = vn= - vp, is negative and it cannot ignite.At instant D, valve 3 would normally ignite but cannot ignite due to lack of a grid pulse. Valves 1 and 2 continue to conduct, now dc voltage Vd decreases and at instant E the direct voltage of the bridge becomes zero and starts to reverse. Immediately the bypass valve ignites. the effective circuit is as follows:

Figure consisting L.H.mesh of 3 valves in series & inductance 2Lc.The bypass c/n I7 and the commutating voltage are both 0 at =0 (when the valve 7 is fired)Taking this moment as t=0, the voltage is,

& the bypass c/n lagging 900 is,

The c/n in the main valves are,

After commutation is completed only the R.H.mesh of the circuit is conducting.B] Transfer to the Bypass Valve (inverter)In an inverter, the direct voltage across the bypass valve is normally positive, so that valve ignites as soon as it is unblocked. The direct line voltage immediately becomes zero, and commutation begins from the conducting main valves to the bypass valve. This commutation has some ignition angle which varies somewhat with the instant of unblocking.

Q 8) what are the different malfunctions of mercury arc valve?Following are the commonest malfunctions of valve. Arcback (backfire)it is defined as the Conduction in the reverse direction Arc through (fire-through, shoot-through)it is defined as the Conduction during a scheduled blocking period Quenching (arc quenching, arc chopping)it is defined as the Premature extinction of the arc during a scheduled conducting period Misfire it is defined as the condition where in spite of positive grid and anode voltages valve Failure to ignite

1. Arcback

Causes:Arcbacks are the commonest and best known as well as the severest kind of malfunction of rectifier valves but are less frequent in inverters., arcbacks are random in nature.Since arcback is reverse conduction, it can occur only when there is inverse voltage across a valve.In rectification each valve is exposed to inverse voltage during approximately two-thirds of each cycle but to forward voltage for a much shorter time and with a lower crest value.

the most of the time in the rectifier operation the voltage across, the valve are either 0 or it is a highlynegative & very small portion is positiveWhen the voltage across valve is 0 it is called the conduction period(i.e 3)the longeest period in the rectifier is called as inverse voltage period (i.e.1)the blocking period is the positive value (i.e.2)

An average occurrence of one or two arcbacks per valve per month is considered satisfactory.Among the factors that tend to increase the occurrence of arcbacks are the following: 1. High peak inverse voltage-- if, the voltage across the valve is very high then this may occur2. High voltage jumps( especially of the jump at arc extinction)--if the arc extension at point a is very high jump, because it was conducting thus there is a very huge voltage arising and there is a possibility that voltage is going to very high negative and then it may it may conduct in this period. so this causes the arcback.

3. High rate of change of current at the end of conduction

when the conduction comes to an end c/n may switch depending upon the value of u(i.e. if u period is small, rate of rise is very high which can cause arcback )4. Overcurrent -- If current value is very high even through u is same then again the rate is increasing and the possibility of arcback is there.

5. Condensation of mercury vapor on anodes 6. Impurity of materials in anodes and grids

in mercury arc valve there is one anode one cathode and grid (viz filled with gas) as shown in figso if there is any impurity present in the grid then that may cause the conduction because the mercury valve works on the ionization of the gas viz inside the valve.7. High rate of increase of inverse voltage

here whenever there is a change in the conduction sequence (say A,B,C) the rate of change of db/dt is very high thus other valve can conduct during the u period

precautions: Most of these factors can be controlled. Factors 1 and 2 can be reduced by decrease of rated valve voltage; 3 and 4, by decrease of rated current. These measures, however, reduce the power handled per valve and, hence, raise the cost of the converter per unit of power. Factors 2 and 3 can be improved by the use of small converter angles (,, , ); but these angles must be increased to large values temporarily in such control operations as starting up, maintaining constant current during dips of alternating voltage, or in causing the transfer of line current from the bypass valve to the main valves of an inverter. These operations do increase the incidence of arcbacks. Factor 5 is minimized by maintaining the anodes at a higher temperature than the cathodes. Factor 7 is minimized by the use of RC damper circuits in parallel with each Factor 3 can be made to occur less frequently by not allowing operation with very small overlap

Results:this malfunction of valves results into L-L short ckt and may be 3 phase s ckt.(e.g. if valve 6 and 1 are conducting and if suddenely valve 3 starts to conduct causing dc sc)It also generates some harmonics.

ARCTHROUGHCauses This is also known as fire through or short through.

Arcthrough occurs during blocking period of valve that is when the voltage across the valves is positive. Since the positive voltage across the valve is more during the inverter operation; the chance of this malfunction is also more in inverters than the rectifiers. It is similar to commutation failure.

It can be caused by failure of the negative grid bias, by a defect in the grid circuit, by the too early occurrence of a positive grid pulse, or by a sufficiently great positive transient overvoltage on the grid or anode.

This malfunction is mainly due to (causes) failure of negative grid pulse--normally the voltage is very positive across the valve and a negative gate pulse is always maintained across the valve, because in practical there are lots of jumps and dents present in voltage across the valves, so there is possibility that it can conduct due to any furious signal. thus to avoid this problem negative pulse is always provided & absence of it can cause malfunction.

early occurrence of positive grid pulse--positive grid pulse is usually given by function generator in a sequence i.e. after 60 degree in order to valve 1, 2, 3 up to 6 and again 1. So due to any fault in controller can cause the false operation of pulse generator i.e. it will give firing pulse earlier to 60 which may lead to malfunction of valve.

sufficient high-positive transient over voltage on grid or anode.presence of high transient may lead to malfunction.

The main problems with arc through are that(after effect) It reduces delay angle ().

It introduces dc component into transformer current. It changes harmonic components. Short circuit occurs once/cycles until arc-through is removed or the bridge is bypassed.

Misfire :As its name, it is a failure of valve to ignite during a scheduled conducting period whereas arc through is the failure to block a valve during a scheduled non-conducting period. This can occur either in rectifier or in inverter but it is more severe when occurs in inverters. It may be either due to negative gate pulse or positive anode to cathode voltage or fault in valves. The effect of misfire in inverter is similar to commutation failure and arc through. Let valves 6 and I are conducting and valve 2 fails to ignite. Valves 6 and I continue to conduct and thereafter valve 3 will conduct and dc short circuit occurs for smaller durations. There is a small jump of voltage at the beginning of short-circuit and large jump at the end of short-circuit.

Q 1) explain the power reversal characteristics in hvdc s/m & give the significance of c/n margin.id and the difference of internal voltages are always positive because of the unilateral conduction of the valves If it is desired to reverse the direction of power transmission, the polarity of the direct voltages at both ends of the line must be reversed while maintaining the sign of their algebraic difference. Station 2 then becomes the rectifier and station 1 the inverter. The terminal voltage of the rectifier is always greater in absolute value than that of the inverter, although it is lesser algebraically in the event of negative voltage. In many dc transmission links each converter must function sometimes as a rectifier and at other times as an inverter. At times both converters are called on to work as inverters in order to DE energize the line rapidly. Therefore each converter is given a combined characteristic, as shown in Figure 8,

consisting of three linear portions : C.I.A., CC., and C.E.A. With the characteristics shown by solid lines, power is transmitted from converter l to converter 2. If the characteristics are changed to those shown by the broken lines, the direction of transmission is reversed by the reversal of direct voltage with no change in direct current. Both stations are given the same current command, but, at the station designated as inverter, a signal representing the current margin is subtracted from that current command, giving a smaller net current command.The difference between the current command of the rectifier and that of the inverter is called the current margin and is denoted by Id. It is generally 15% of the rated current, although it could be made smaller, It must be great enough so that the two steep constant-current lines do not cross each other in spite of errors of current measurement. When it is desired to reverse the direction of power, the margin signal must be transferred to the station that becomes the inverter station. During the reversal of power and voltage the shunt capacitance of the line must be first discharged and then recharged with the opposite polarity. This process implies a greater current at the end of the line initially the inverter than at the end initially the rectifier. The difference of terminal currents can-not exceed the current margin. Hence the shortest time of voltage reversal is

where C is the line capacitance, A Vd the algebraic change of direct voltage, and Aid the current margin. The current margin signal corresponds to the horizontal separation Aid of the constant-current characteristics of the two converters along the horizontal axis or between the corners P1 and P2 in Figure 8. Because of the slope of these C.C. characteristics, the actual separation between them varies with Vd, being least at the normal working point. The margin signal must be great enough to maintain a positive margin there in spite of errors in the current measurement and regulation. Operation at the intersection of the two steep C.C. characteristics, with both current regulators operating, would be erratic.