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During transients and disturbances, the signal may temporarily lose synchronism with the signal. The dynamic characteristics of the GFU must be capable of restoring synchronism rapidly, and in a stable manner, with the commutation voltage as soon as the transient is over. Consequently, the firing pulses are vulnerable to harmonic pollution on the waveform.
Early attempts to use filtering techniques to alleviate some of these problems were not successful for operation with weak ac systems due to the introduction of phase shifts.
FACTS Controllers in Power Transmission and Distribution
Developments in tracking band-pass filters  which derive the fundamental frequency component of the commutation voltage with no phase shift may be useful in operation with weak ac systems.
However, the main disadvantage of IPC systems, which eventually led to their demise, was the generation of non-characteristic harmonics which caused harmonic instability problems. Two GFUs of this type have been described in the literature: 3. The characteristic feature of this method is that a dc input control signal to the VCO results in a change in the frequency of the VCO.
HVDC and Facts Controllers
A free running VCO generates a train of short pulses at a pulse repetition frequency directly proportional to the dc control voltage. A ring counter is used to separate the pulse train into 6 or 12 sets of pulses for a 6 or 12 pulse converter. An indirect method is used to synchronize the VCO output frequency to the ac supply frequency.
An error signal is derived from either the converter dc current or extinction angle controller as a feedback signal. When there is an error, the VCO will either speed up or slow down to correct for the error.
Both  and  use the dc current controllers for synchronizing the VCO when the converter is a rectifier; however, when the converter is an inverter, the VCO synchronizing is based on an extinction angle controller, although the method used by  is based on a predictive estimation of extinction angle. Due to the integral characteristic of the VCO, it is not possible to modulate the firing pulses on an individual basis; for this reason, an asymmetric firing unit is used in  to optimize dc power flow during unbalanced ac system faults.
The transfer function of this type of unit is therefore proportional rather than integral. To ensure the synchronism of the VCO output frequency with the ac supply frequency, a slower acting frequency error feedback loop is used. This type of GFU does not permit the modulation of firing pulses on an individual basis either.
Under steady state, the synchronizing voltage will be locked to the commutation voltage. In this case, and and the first term of eq. The second term is an unwanted ac component which has a frequency of under steady state. In order to extract the dc error signal and filter out the unwanted ac component, a low-pass filter having the transfer function is used.
The output is passed onto an integrator with a transfer function of The integrator output, is used to modulate the phase and frequency of a free-running Sine-Cosine Oscillator to generate the output signal Under steady state conditions, the feedback signal will be in phase and at the same frequency as the commutation voltage, Thus can be used as a stable pollution-free signal to derive the zero-crossover points to provide the timing reference points for the GFU.
Figure shows the waveforms of the conventional GFU shown in Figure It can be seen that the error signal, contains a dominant second harmonic ac component which is removed by a low-pass filter to reduce its impact on the overall system operation.
One is the cut-off frequency of the low-pass filter, and the other is the integrator time constant. The design objective is to achieve synchronization between and in the shortest time possible. One common design approach is to study the small signal model of the circuit and design the parameters in the frequency domain. Studies show  that the small signal model of the circuit in Figure can be represented as shown in Figure 33, where H s is the low-pass filter transfer function and variables in hats represent small signal quantities.
The loop transfer function is given in eq. TLFeBOOK Chapter 3 46 Figure also shows that the loop response speed, represented by the gaincrossover frequency, is to a large extent, limited by the cut-off frequency of the low-pass filter. There is a compromise in selecting If is too high, the ac component remains large and it will interfere with the system operation.
On the other hand, if is too low, the overall system response of the system will be very sluggish. Simulation studies show that a cut-off frequency around one fifth of the ac component gives satisfactory results. An error signal, derived using eq. 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.
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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.
Dc Power Transmission Technology 1. Introduction 1. Economics of Power Transmission 1. Technical Performance 1. Reliability 1. Application of DC Transmission 1. Description of DC Transmission System 1. Types of DC Links 1. Converter Station 1. Some Operating Problems 1.
Introduction 2. Line Commutated Converter 2. Analysis of Graetz Bridge Neglecting Overlap 2. Choice of Converter Configuration for any Pulse Number 2. Analysis of a 12 Pulse Converters 2. Effect of Finite Smoothing Reactor 2. Voltage Source Converter 2. Pulse Width Modulation 3. Analysis Of Hvdc Converters 3. Introduction 3. Analysis of Line Commutated Converter 3.
Lcc Bridge Characteristics 3. Characteristics of a Twelve Pulse Converter 3. Detailed Analysis of Converters 3. Capacitor Commutated Converter 3. Analysis of a Voltage Source Converter 4. Converter And Hvdc System Control 4.
General 4. Principles of DC Link Control 4. Converter Control Characteristics 4. Basic Characteristics 4. Modification of the Control Characteristics 4.
System Control Hierarchy 4. Firing Angle Control 4. Individual Phase Control 4. Control Hardware 4. Current and Extinction Angle Control 4. Starting and Stopping of DC Link 4. Energization and Deenergization of a Bridge Contents note continued: Start-up of DC Link 4.
Power Control 4. Higher Level Controllers 4. Stabilization of AC Ties 4. Emergency Control 4. Reactive Power Control 4. Subsynchronous Damping Control 4. Telecommunication Requirements 4. Control of Voltage Source Converter 5.
Coverter Faults And Protection 5. Introduction 5. Converter Faults 5. General 5. Commutation Failure 5. Arc Through 5. Misfire 5. Current Extinction 5. Short Circuit in a Bridge 5. Protection against Overcurrents 5. Overvoltages in a Converter Station 5. Disturbances on the AC Side 5. Disturbances on the DC Side 5. Overvoltage Caused by Internal Converter Disturbances 5.
Surge Arresters 5.
HVDC Power Transmission System
Protection Against Overvoltages 5. General Contents note continued: Overvoltage Protection in a Converter Station 5. Smoothing Reactor And Dc Line 6.
Introduction 6. Smoothing Reactors 6. DC Line 6. Corona Effects 6. DC Line Insulators 6. Transient Overvoltages in DC Line 6. Protection of DC Line 6. Detection of Line Faults 6. DC Breakers 6. Characteristics and Types of DC Breakers 6. Applications of DC Breakers 6. Monopolar Operation 6.
Ground Electrodes 6. Reactive Power Control 7. Introduction 7. Reactive Power Requirements in Steady State 7. Conventional Control Strategies 7.
Alternate Control Strategies 7. Forced Commutation Contents note continued: Sources of Reactive Power 7. Reactive Power Control During Transients 8. Harmonics And Filters 8. Introduction 8.
Generation of Harmonics 8. Characteristic Harmonics 8. Non-characteristic Harmonics 8.
Design of AC Filters 8. Criteria of Design 8. Types of Filters 8. Passive AC Filters 8. DC Filters 8. Passive DC Filters 8. Active Filters 8. Carrier Frequency and RI Noise 9. Introduction 9. Current Margin Method 9. Voltage Limiting Control 9. Decentralized Current Reference Balancing 9. Two ACR Method 9.Introduction 3. A Flexible Per Unit System The error between these two signals is then fed to a VCO to alter the frequency and phase angle of the synchronizing voltage such that this error is reduced to zero.
Login to add to list. Complete data for the system modelled with EMTP is presented in the figure. However, in a weak ac system, it is difficult to obtain a stable pollution-free signal. The transfer function of this type of unit is therefore proportional rather than integral.