Hvdc Transmission Technology Is Fast Advancing And Its Applications Are Rapidly Expanding. This Book Presents The K. R. Padiyar. New Age International. An up-to-date text on HVDC transmission dealing with the state of the art in HVDC transmission technology, and many aspects of interactions of AC/DC systems. Modelling and analysis of DC systems are also discussed in detail. Developed from Padiyar's courses at the Indian institutes. DC transmission at distribution level voltages (using VSC-HVDC) is also being considered for integration of distributed generation in the power grid. Chapter 10 presents power flow analysis in AC-DC systems based on a novel approach. The book should be useful as text/reference to.
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The output of the VCO is a signal proportional to a sawtooth waveform an angle Theta. This waveform is used to generate the Sine-Cosine waveforms which are fed back to the multipliers to generate the error signal. Under steady state, this error is reduced to zero and the output of the Sine-Cosine oscillator will be in synchronism with the commutation voltages.
The small signal block diagram of the DQO grid control unit is shown in Figure , where represents the PI transfer function. The Bode plot of is shown in Figure The solid lines are for the loop transfer function and the dotted lines are for the PI controller and the integrator transfer functions.
One of the limiting factors is the existence of the low order harmonic component in the signal when the three-phase ac source contains harmonics. For example, with a third harmonic injection at the ac bus, the signal will contain a second harmonic ac component. Under such operating conditions, as the gain crossover frequency increases, the error between the synchronizing signal and the fundamental component of the commutation voltage increases as well.
Our studies show that the gain crossover frequency of around 40 Hz provides a good compromise between a fast response and a small synchronizing error. The absence of the low-pass filter allows the DQO circuit to achieve a relatively faster dynamic response than its conventional counterpart.
Experience also shows that the optimization of the low-pass filter requires additional effort when compared to the DQO circuit. A typical fault duration can be for ms giving rise to 5 6 cycle loss of voltage on a 50 60 Hz system.
Under such conditions, the GFU falls back to its free-running mode and continue to provide a synchronizing voltage to the gating unit. On fault recovery, the GFU should rapidly re-synchronize with the commutation voltage. Figure a shows the internal signals from the conventional GFU during a temporary loss of the commutation voltage caused by a fault on the ac commutation bus.
The multiplier output and the low-pass filter output are reduced to zero during the fault period. The Integrator output shows only a small offset voltage during the fault period which is used to modulate the frequency and phase of the Sine-Cosine oscillator stage following it.
The post-fault synchronization dynamics of the conventional GFU show that the output voltage is able to synchronize with the commutation voltage within 1 cycle 20 ms at 50 Hz.
The waveform of also shows that the control loop is slightly under damped and requires a settling time of about 3 cycles. Figure b shows the internal signals from the DQO GFU during a temporary loss of the commutation voltage caused by a three phase fault on the ac commutation bus. During the fault, the three phase commutation voltages and are reduced to zero causing the input to the PI controller to drop to zero.
This results in the output of the sawtooth waveform, Theta, to be at the centre frequency 50 Hz in this case. After the fault, the error is reduced to zero within 1 cycle 20 ms.
In a weak ac system, harmonic distortion is a common occurrence, and the role of the GFUs is to provide a clean output with minimal delay for synchronization purposes. Typical harmonics that are present in the commutation voltage are the characteristic harmonics i. However, even the lowest characteristic harmonic, i. The most onerous condition is, however, posed by the third harmonic which is the closest to the second harmonic and is the most difficult harmonic to filter out.
This harmonic level distorts the outputs of the multiplier and other stages in the GFU. As explained in the theory earlier, a strong second harmonic component is visible in the waveforms of the and Nevertheless, the output voltage of the gird firing unit contains practically no harmonics and is synchronized to the fundamental component of the commutation voltage.
Although, the signal contains a strong second harmonic component, the integrator output Theta smooths the impact of this ac component considerably. The output voltage of the DQO unit contains practically no harmonics and is synchronized to the fundamental component of the commutation voltage.
Similar tests with injections of 5th, 7th harmonic components were carried out and similar results were obtained. A 6-pulse model is used here only to minimize the simulation time. However, the same design principles and the operational characteristics for the GFUs can be extended to a pulse unit. The fixed frequency ac source has an impedance formed by the 2R-L network to provide a short circuit ratio of 2.
This constitutes a weak ac system and provides a difficult synchronization task for the GFUs. To achieve this the value of the predicted gamma is stored in a holding circuit. The actual gamma is measured one cycle later and subtracted from the previously held value of gamma. The prediction error is defined as: After filtering, is used as a correction signal for the reference value one period cycle later. Thus the complete expression for the firing condition is defined as: The prediction process has an inherent individual phase character.
If no further action were taken, each valve would fire on the minimum area margin condition. To counteract this a special firing symmetrizer is used. When one valve has fired on the minimum margin condition, the following five valves are then fired equidistantly by means of the voltage controlled oscillator. Once a measurement of the gamma angles from the six valves of the convertor are obtained, the minimum value is selected.
This value is then compared to the desired value of gamma and an error signal is generated and fed to a PI regulator. This gamma error signal is used in a similar manner to the current controller at the rectifier to generate the firing pulses for the converter. The terminal can be divided into sub-sections i. Each pole can be further subdivided into the star valve group and the delta valve group depending on the transformer configuration used.
Each valve group comprises a 6-pulse converter. A supplementary power modulation signal can also be inputted at this stage, if required. Maximum power and minimum power limits to the excursions of the power controller are imposed. Finally, the power order is divided by the DC voltage measured value to derive a current order which is sent to the two Pole Controllers. For this circuit, in case of start up routine when the dc voltage may be zero or low value, a bias circuit is required to counteract any problem due to a divide-by-zero function.
The supplementary current input can be added to this to achieve any modulation of the order if desired. The current input is subjected to upper and lower limits for protective purposes. After limitation, the current order is compared to the measured value of dc current to generate an error signal Another signal which modifies the current order is the current margin which is required only at the inverter end to bias off the current controller so that the gamma controller can take over.
The current controller uses the PI regulator to provide dynamic properties to the control loop, and provides the alpha order at its output. The VG controller has two separate secondary loops associated with it: Tap Changer TC Controller This is a relatively slow-acting loop time constant of the order of several hundreds of milli-seconds which maintains the tap position of the converter transformer.
Its function is to maintain the firing angle alpha within a nominal range of about 15 degrees whenever it hits any limits by either raising or lowering the tap position. This will then minimize the reactive power consumption of the converter, and provide sufficient margin for dynamic operation of the converter. The Commutation Failure CF Controller This loop detects the possibility of a CF from measurements of the ac current, commutation voltage and the dc current.
Rapid pre-programmed changes to the alpha order can be made as a function of the CF detector for assisting the recovery of the dc system from a CF. Machida and Y. PAS, March , pp. IEE, Vol. PAS, No. Hingorani and P. Ekstrom and G. IEEE Trans. Ekstrom, P. Jackson and G. Nishimura, A. Watanabe, N. Fujii and F.
Part 1: Bhattacharya and H. Sato, K. Yamaji, M. Funaki, and K. As power systems grow and become more integrated, interconnections to neighboring ac systems are becoming increasingly necessary to enhance stability, security of supply, flexibility and for other economic benefits. Primarily for stability reasons, the trend is for such interconnections to be asynchronous HVDC ties. And usually these interconnections feed into locations where the ac power systems are weak.
Utility system planners realized that the critical element in the HVDC inter-tie was the thyristor converter, which has a fundamental limitation that it requires a reliable and adequately stiff voltage source for valve commutation purposes. The traditional yardstick for assessment of the quality of this commutation voltage has been the Effective Short Circuit Ratio ESCR at the converter ac bus. For instance, a typical ac system with an approximate value of is considered adequate for a Line Commutated Converter LCC with some enhanced control techniques; systems with values of ESCR Chapter 5 96 Although these advantages have been known for a long time, the disadvantages consistently blocked any serious applications of the technique until the early s.
It is important to realize that the commutation process is a function of both circuit-dependent and switch-dependent parameters: Circuit-dependent parameters depend on circuit topology, and include components such as transformer leakage inductor , commutation capacitor, auxiliary switching device, etc.
For the 6-pulse bridge configuration, the most important circuit-dependent parameter for commutation is the finite transformer leakage inductance ; assuming typical values for this, an overlap angle degrees is necessary, and more than two valves will conduct during the commutation period.
The switch turn-on time, however, is much smaller that the turnoff time of common power switches and does not impact on the commutation process in a significant manner.
Depending on which type of switch is in use in the bridge, the type of commutation technique feasible is shown in Table Forced commutation techniques [1,2,3] may be applied either on the highvoltage power side of the converter by means of auxiliary components i. For the commutation of the conventional thyristor converters, both circuitdependent and switch-dependent parameters are critically important. For the commutation of the HVDC converter with, say GTO devices, the circuit-dependent parameters are now less crucial since the devices can be treated as perfect switches within certain limits.
For these newer devices, self-commutation techniques are employed. As a result of the new devices i. A certain amount of confusion and misuse of these terms is apparent within the industry. These terms are defined below to clear misconceptions. In this guide, conventional thyristors are called circuit-commutated devices, and GTOs, IGBTs and other such devices are called self-commutated devices.
Artificial or Forced Commutation FC applies to both circuit-commutation using conventional thyristors, and self-commutation using GTOs and other devices. Although both circuit- and self-commutation techniques are examples of forced commutation techniques, the difference between circuit- and self-commutation is significant. These techniques are discussed below. To initiate commutation, the firing pulse from the outgoing valve is removed and an alternate incoming valve in the same row is triggered to take up the dc current.
During the commutation overlap period, the dc current is shared between the outgoing and incoming valves as a result of the leakage inductance of the transformer. Once current is transferred to the incoming valve, the reverse voltage across the outgoing valve is maintained for a time period equivalent to gamma angle ; the outgoing valve must be reverse biased for a period greater than the turn-off time of the device. During this period a small reverse current is drawn from the device to deplete the charge carriers within the pn-junction of the device.
The time difference between and is required to provide a margin of security for the device to achieve its voltage blocking capability. Typical valves of and are and respectively.
With line commutation, because of the direct dependence of the firing angle alpha to the ac voltage, it is only feasible to delay the firing angle; it is not possible to advance the firing angle with reference to the ac system voltage. This means that alpha can vary only from 0 to degrees; as is well known from converter theory, operation within these angles by a line commutated converter can only absorb reactive power from the ac system. Power systems are subject to disturbances, voltage regulation difficulties and harmonic pollution which cause commutation problems for such converters.
As a result, LC converters have difficulties to feed into weak ac systems and may take prohibitively long times to recover from disturbances. Furthermore, the ability of the LC converter to control reactive power is limited. These limitations can be overcome by the use of forced commutation employing either circuit- or self-commutation techniques.
An artificially generated voltage can be used to force commutate the valves. This artificially generated voltage is temporarily stored on a commutation capacitor until it is required to commutate the valve. This artificially generated voltage may be derived either from the following sources: The ac line voltage, whenever it is present, The dc line voltage, or An auxiliary voltage. Commutation circuits deriving their energy from any one or multiple of these sources exist.
The commutation circuit serves two distinct, but intertwinned roles: To provide the commutation voltage for the switching device, and To divert inductive load currents from the main switching device to another auxiliary switching device. The current diversion role, however, is sometimes not fully appreciated by utility engineers, especially in relation to circuit-commutated devices.
The significance of this current diversion role, however, becomes more apparent with the use of self-commutating devices.
The commutation capacitor could be either in series or in parallel with the main valve. Circuits of either type are feasible. It is noteworthy that all forced commutation circuits can be reduced to either one of these two types. General operating principles for these circuits are given below; specific examples of circuit which employ such techniques are provided in a later section.
When the main valve T1 is fired, load current is established in the commutation capacitor C and one phase of the equivalent load.
This equivalent load can be considered to be the resultant impedance of the load, ac filters and converter transformer. If no further circuit topology changes occur, the capacitor voltage will eventually become greater than the dc line voltage and the current will be reduced to zero.
Commutation is then completed. The circuit also employs an auxiliary valve CT1. To commutate the main valve, the auxiliary valve CT1 is fired. Assuming that the capacitor was pre-charged in the polarity indicated, the load current will be diverted into the parallel path formed by CT1 and C; this will turn-off T1.
At the same time, the capacitor will charge up in the opposite polarity. This type of commutation circuit carries the load current only during the commutation period, unlike the series capacitor circuit above.
Minor variants of either of these two types of circuit exist for forced commutation purposes. Sometimes, just the removal of a gate bias voltage at the gate or base of the device may be enough to turn-off the device such as a MOSFET transistor. This type of commutation is termed self-commutation and will be successful with power circuits having purely resistive in phase currents; however, since power circuits usually have inductive currents to be commutated, the transfer of current to another valve in the same row may not be successful unless additional circuits having diverters are utilized.
The function of these diverters will be to temporarily divert the inductive load current to a capacitor, until the next incoming valve is able to pick up the current. The impact of such diverters has not yet been fully assessed by the industry. The principle of a current source GTO converter is shown in Figure a. The dc line current is maintained constant by the use of a large smoothing reactor The main valve T1 is on and load current is established in phase R of the load reactor The diverter capacitor is pre-charged in the polarity indicated.
In order to self-commutate T1, the turn-off pulse to T1 is applied, and at the same time, the next phase main valve T3 is turned on. The main valve T1 is instantaneously turned off, subject to the dc line current being diverted to valve T3. The current in valve T3 Figure b is composed of: Current in capacitor and phase R of the load; as the capacitor charges up in the opposite polarity, this component of current will gradually reduce to zero, Current being established in phase S of the load; this current will gradually increase to be equal to TLFeBOOK Forced Commutated HVDC Converters 5.
The principal of the voltage source converter is shown in Figure a. The circuit requires that the dc line voltage be maintained constant at the converter terminals.
This is achieved by having a large capacitor on the converter side of the dc smoothing reactor. In addition to free-wheeling diodes D1, D3 etc. To commutate valve T1, a negative pulse is applied to its gate while the next valve T3 is fired. Valve T1 is turned off instantaneously, since the diode D1 is able to freewheel the load current in phase R; this current will decay at a rate depending on the resistance and inductance of the load.
In the meantime, valve T3 is on and load current is being established in phase S of the load. For this comparison, it is assumed that the same type of converter is available at both ends of dc system; this may not be the case in practice.
Some of the major characteristics of systems with the two types of converters are listed in Table These 4 regions are divided into four quadrants Q1 to Q4. Theoretical and practical limits to these regions are defined in Table Provided an adequate ac supply is available, line commutated LC converter operation is possible in quadrants Q1 and Q2.
In Q1, the converter operates as a rectifier consuming reactive power from the supply. The practical alpha-min limit 5 degrees is required for the valves to have a forward-bias voltage before turning on.
In Q2, the converter operates as an inverter again consuming reactive power from the supply. Generally, two limits apply in this region i. The practical operational region for a LC inverter is shown as region X. To operate beyond region X into quadrant Q3 requires assistance from forced commutation.
Depending on the commutation technique, practical limits for the FCCs also exist. In quadrant Q2 this limit merges into LC inverter operation region X. With a series-capacitor type circuit, the low voltage limit extends further end of region Y1 depending on the size of the capacitor. Operation into region Z is possible with variant of the parallel-capacitor circuit which employs an auxiliary source for charging up the capacitor.
Operation into quadrant Q4 will be as a CC rectifier supplying reactive power to the ac supply. Hence, FCCs with self commutated devices will have practically no limits in its operation in all four quadrants. These two categories of FCCs will be discussed next. Although 6-pulse circuits are shown in the following examples for reasons of simplicity, it is understood that the more usual pulse configuration will be used for practical schemes.
This type of circuit may require an auxiliary supply for start-up purpose when feeding a dead load. A simplified equivalent circuit Figure shows the commutation principle. When valve T1 is fired, load current is established in capacitor and the T-phase of the equivalent load. This equivalent load can be considered to be the resultant of the load, ac filters and converters transformer.
Before this happens, valve T3 will be fired and load current in the S-phase will be established. Capacitor had a charge of the polarity indicated from a previous cycle. Firing T3 will turn-off T1. This procedure is then repeated for the next commutation cycle using the required firing order.
The capacitor derives its energy directly from the dc line. The circuit employs one capacitor and two commutating CT1 and CT2 per 6-pulse bridge.
This type of circuit does not rely on an auxiliary supply for start-up purposes when feeding a dead load. Load current into the equivalent load is established by firing valve T1. To commutate T1, valve CT1 is fired.
At the same time, the capacitor will charge up in the opposite polarity, in readiness for the next commutation when valve CT2 will be fired. The circuit Figure relies on charging the commutation capacitor directly from the dc line. A more economic two-valve version of this circuit is shown in Figure ; this version requires a charge reversal cycle on the capacitor which imposes time restrictions on circuit operation for high frequencies only.
For power frequency operation, this version is quite feasible. The dc current is maintained constant by use of a large smoothing reactor. In Figure are shown the position of three ac-side capacitor diverters and in delta-configuration a star-configuration for these capacitors id also feasible.
For example, consider the case of valves T1 and T2 conducting, and the commutation of valve T1 and transfer of current to valve T3. The valves are GTO based and can be self-commutated by the application of control pulses. For the commutation of valve T1 and transfer of current to valve T3, the capacitor will temporarily take over the current in the transformer inductance of phase R, until valve T3 and phase T is fully able to establish the current. The dc capacitor diverter will also assist in the transfer of the dc current from valve T1 to T3.
The dimensions of the three ac side diverter capacitors are functions of the product clearly, a lower value of the inductance will help in reducing the size of the capacitor. The design of the circuit should also consider the natural resonant frequency of the circuit which would interfere with the operation of the converter.
The diverter capacitors would also tend to reduce the harmonics generated by the converter. The circuit requires the dc voltage to be maintained constant at the converter. This is achieved by having a capacitor on the converter side of the smoothing reactor. In addition to free-wheeling diodes D1 to D6 are required across each valve, to assist in the current diversion during commutation of the main valves.
The capacitor will also reduce the dc harmonics generated by the converter. Additionally, the capacitor will provide protection from line surges. In order to take advantage of the fast switching capability of the GTOs, pulse width modulation PWM techniques can be utilized to reduce the low-order harmonics generated by the converter; this will reduce the ac filter cost.
Baron and G. PAS - 87, No. Sood and J. Canadian Electrical Association, Spring meeting Gole and R. Jotten and W. Tam and R. June , Turnali, R. Menzies and D. McMurray and H. Power Electronics, Int. Holmgren, G. Asplund, S.
Valdemarsson, P. Hidman, U. Jonsson, O. Sadek, M. Pereira, D. Brandt, A. Gole and A. Jiang and A. Meisingset, A. Gole, R. Burton, O. Eide, R. Schettler, H. Huang and N. Gole, M. One of the earliest studies of this configuration was first carried out in the s . A more detailed investigation into the operation of this circuit was reported in .
In the late s and early s, studies with the parallel capacitor version were reported [3,4,5,6].
However, due to practical limitations with the valve ratings, these studies did not lead to any actual installations. However, in the early s, the CCC configuration  was resurrected again due to a number of reasons: Problem of voltage ratings of valves became less of a constraint financially due to increasing ratings and decreasing valve costs , Management of reactive power and high performance harmonic filtering could be dealt with independently due to the development of the continuously tunable ac filter and active filters, and Utility demands for operation with increasingly weaker ac systems has meant that commutation with LCCs has become much more unpalatable.
Thus, this capacitor is in series with the leakage impedance of the transformer and the main valves. This has a two-fold effect: The capacitor provides a forced commutation facility to the main valves as explained earlier in another chapter , and The capacitor compensates for the leakage inductance or reactive power demand of the converter transformer. Sizing of the commutation capacitor, therefore, becomes a very important criteria as it impacts on the above two effects.
A too-small capacitor will cause a large overvoltage across the capacitor and valves , and not compensate sufficiently for the leakage inductance to result in a lagging current drawn from the ac bus. A too-large capacitor will result in low overvoltages and over-compensate for the demanded reactive power and might even draw a leading current from the ac system.
However, a too-large capacitor also has a cost penalty associated with it. This design criteria maintains the cost of the valves at a reasonable level. Since increases with this results in an increase of the dc voltage. This is in direct contrast to the case of a conventional LCC working at minimum extinction angle control where the dc voltage decreases with increase in This feature, therefore, results in a positive inverter impedance characteristic for the CCC providing improved dynamic stability Figure The capacitors are protected against overvoltages by parallel ZnO varistors.
The LCC serves the same need by means of switchable shunt capacitor banks. Since the CCC provides reactive power compensation Q proportional to the load current of the converter, the need for switchable banks is eliminated. Consequently, ac filters are needed only for harmonic filtering; so their design can be optimized for this duty alone.
TLFeBOOK Chapter 6 In the past, one of the problems of high-performance ac filters which were sharply-tuned using small capacitor banks was to keep them in tune while being subjected to daily component and frequency variations. This problem was resolved by use of a continuously tuned reactor which is controlled by a dc current fed into a control winding mounted perpendicular to the main winding, enabling continuous adjustment of its inductance and thus continuously tuning the filter branch.
This permits the rated dc voltage to be kept low to achieve the rated dc power. A low dc voltage rating is beneficial for a compact modular valve housing as air clearances can be kept small.
Each valve module contains two single valves, i. The thyristor valves are suspended from the ceiling and easily accessible for maintenance purposes. The surge arresters across the valves are also included in the housings. This arrangement can be considered as an amalgamation of the CCC and LCC configurations, and the results obtained from steady state and dynamic performances confirm this.
The advantage of this configuration is that the converter is a standard LCC. In addition, the series capacitors can be controlled similar to a thyristor controlled series compensation TCSC scheme Figure b. The dc systems in either case are rated at kV, 1. Extinction Angle Characteristics: One advantage of these topologies is that the series capacitors assist in the commutation process.
Thus the apparent extinction angle viewed from the kV ac bus bar can approach very small, or even negative values depending on the size of the selected series capacitor. The apparent extinction angle is the electrical angle corresponding to the time at which the valve turns off to the positive zero crossing of the corresponding apparent TLFeBOOK Capacitor Commutated Converters for HVDC Systems commutation line-line voltage on the ac bus bar.
The actual extinction angle is larger because the real commutation voltage is the sum of the line-line ac bus bar voltage and the series capacitor voltages. The small value of results in an improved power factor and diminishes the requirement for shunt reactive power compensation.
Because the voltage on the series capacitor actually increases with dc current, the natural tendency for the extinction angle on an increase in dc current is to increase.
This is the converse of the situation for the conventional converter in which an increase in dc current decreases thereby bringing the converter closer to its commutation failure limit. In these cases, a sudden lowering of inverter ac voltage, say due to a remote ac fault results in a sudden increase in dc current. The current controller on the rectifier has a negligible effect on this over-current which is primarily due to a discharge of the cable capacitance. The probability of commutation failure is reduced due to the natural tendency for increase with increasing dc current.
Theoretical relationships found in [1,4] of the real extinction angle as a function of dc current for the CCC and CSCC options are plotted Figure 65 assuming a control mode of constant It is noted that the extinction angle increases with dc current and that either option gives essentially the same result. Maximum Available Power: If the dc system is operated in the power-control mode, points on the power curve beyond the MAP point are unstable.
In fact, for the given system short circuit ratio, the rated operating point would be past the stability limit of 1. This stability limit is increased to 2. Converter Valve Voltage Stress: The valve voltages in the case of the CCC are higher than those for a conventional bridge [1,4]. The converter itself is of the conventional type in the CSCC option.
However the steady state voltage on the converter ac filter bus is higher kV than the rated voltage kV of the system bus. However, for operation with higher ac bus voltages, when the slightly different tap changing regimes for the two options are taken into account, the CCC option is seen to require a somewhat larger valve voltage rating. Harmonics and Filtering: Since the ac filters are required only for harmonic elimination and not for reactive power support, the MVAr rating of the filter is reduced to very small values, which results in a very narrow passband.
To keep the filter in tune for frequency or component variations, one option is to have a continuously tuned filter ; another option is to use active filters . Unlike the conventional case, neither option requires filter bank switching for variations in the load over the full range of operation which simplifies the switchyard design.
On account of the smaller overlap angles that results because of the additional commutation voltage provided by the capacitors, the dominant current harmonics and generated in both options are typically higher than that for the conventional dc installations.
Load Rejection Over-voltages: This is particularly so for a weak ac system in which the equivalent impedance of the ac system is large .
For the test system, studies indicate that the magnitude of the load rejection over-voltage for the CCC and CSCC are approximately the same 1. Typical faults were applied to the two options -- the LCC and CCC -- in order to evaluate the recovery performance of the two systems . No special controls were modeled in this exercise. Figure shows a dc voltage, b dc current and current order, and c inverter ac bus voltages for the LCC and CCC options respectively.
The fault is applied at 0. After an initial over-current, the dc current is brought to zero due to the VDCL action taken at the rectifier end. The nature of the recovery, i. The LCC option exhibits a slightly under-damped transient during the fault, but has a smooth recovery.
The CCC option is more damped during the fault period, but has a small oscillation during the recovery. One of the shortcomings of the LCC when used in a long dc cable system is that ac side voltage reductions can cause the dc cable to discharge, transiently increasing the dc current in the inverter.
The natural instantaneous effect of a current increase is a loss of commutation margin and hence an increased probability of commutation failure. The CCC and CSCC options have the opposite tendency, in that the instantaneous response to a current increase is an increase in the extinction angle. The dc system is modeled as a long cable, which would discharge into the inverter because of the resulting reduced ac voltage. Results are shown in Figure The CCC option easily rides through the disturbance without suffering a commutation failure with full power recovery within ms.
During the fault, a second harmonic current is observed in the dc current; this is a characteristic of an unbalanced fault of this type.
Valve Short Circuit Over-current: For rectifier operation with the CCC option, the series capacitor significantly reduces the valve short circuit current. In the CSCC option, the series capacitor is not directly between the filter bus and the converter valves, and consequently the short circuit current is larger than in the CCC option.
Nevertheless, as shown in Figure , the magnitude and duration of this current is still smaller than that of the valve short circuit current in a conventional converter.
On the other hand, the CCC option in rectifier operation exhibits a smaller valve short circuit current. Since the Argentinian system is at 50 Hz while the Brazilian system at 60 Hz, a BB frequency converter installation was necessary.
Furthermore, since the short circuit levels at the converters were TLFeBOOK Chapter 6 low, a CCC option was selected to provide the enhanced stability for the ac systems due to the forced commutated converters. A second interconnection of another MW is under construction.
A single line diagram and an aerial view of the installation is shown in Figure Since this installation provides a ground breaking departure from conventional BB stations, some of its unique and innovative aspects are discussed next.
This allows the transformer and valve current ratings to be optimized for lower cost. On the other hand, the voltage across the commutation capacitor results in higher peak voltages across the valves. The capacitors themselves need to be protected from overvoltages by means of Zn0 varistors across them. Since the series capacitor reduces the overlap angle due to compensation of the leakage inductance of the transformer, switching voltage stresses and losses are reduced.
The elimination of the switches for reactive power compensation equipment simplifies the design, layout and space requirements of the ac switchyard. Since no ac breakers or disconnects, apart from the energizing purposes Figure are needed, system reliability is improved.
This means that the ac filters need to be high performance units to optimize the costs. By contrast, conventional band pass filters have to be equipped with damping resistors to give a broad characteristic to allow them to perform within the frequency and component value variations Figure Therefore, switching banks are not needed.
A stable control design is achieved due to the high linearity of the rate of change of inductance with dc current, as shown in the figure. The control loop Figure requires measurement of the ac bus voltage and the filter current to derive the phase angle between them.
The control loop then controls the dc current to obtain zero phase shift between the harmonic voltage and current. Since no physically moving parts Figure are required for changing the inductance, the equipment enjoys high reliability and limited maintenance requirements. The filter reactor is the component that controls the tuning of the filter.
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