Highlights from the High. Voltage Direct Current (HVDC) History. The transmission and distribution of electrical energy started with direct current. In , a. High-voltage direct-current (HVDC) transmission has advantages over ac With the advent of thyristor valve converters, HVDC transmission became even more. PDF | Dennis A Woodford and others published HVDC Transmission.
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Part of the Power Electronics and Power Systems book series (PEPS) HVDC Power Transmission Systems — Technology and System Interactions, K.R. This comprehensive reference guides the reader through all HVDC technologies, including LCC (Line Commutated Converter), 2-level VSC. transmission systems, this paper presents an overview of the status of HVDC systems in First commercial HVDC transmission, Gotland 1 in Sweden in
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. 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.
Since a 6-pulse version of the converter system is modelled, it is necessary to add tuned and harmonic ac filters at the kV ac commutation bus to cope with the generation of characteristic harmonics from the converter.
Other filters include a damped high-pass and a double tuned filter. A capacitor bank also provides the necessary reactive power consumed by the converter; this was appropriately reduced to account for the extra reactive power provided by the and harmonic ac filters.
The converter transformer is a grounded star-star transformer with its leakage impedance split equally between its primary and secondary windings. The saturation characteristic from the transformer has a knee level at 1. The dc system is composed simply of a smoothing reactor with its inherent resistance and a resistive load.
A shorting switch is included for applying a dc line fault test case. Complete data for the system modelled with EMTP is presented in the figure.
The measured dc current is compared to a current order and a current error signal is generated. The two HVDC lines share a transmission corridor connecting the city with the Nelson River hydropower projects in the far north. When built, they offered a more economical option than the more familiar high-voltage alternating current HVAC lines because of the long distance to the hydro generation, decreased power losses, and a smaller right of way. Unlike HVAC transmission technology, HVDC travels through the entire cross section of a conductor and needs fewer wires, enabling it to move energy over greater distances with less power loss.
HVDC also provides controlled power flows, contributing to grid stability. As costs come down for DC transmission, the business case improves for even shorter distances. HVDC links can connect two power networks operating at different frequencies. HVDC also has many challenges. For example, managing the loss of an HVDC link, which may constitute an extremely large power import into an area, can be difficult. The converters are costly and can generate harmonics, and multi-terminal HVDC systems require expensive communication systems.
Many utilities have a lack of familiarity with HVDC maintenance practices. For one, the integration of large-scale solar and wind generation is well suited for HVDC. Such generation is typically far from load centers, and increasingly many proposed wind plants are at remote offshore sites. An example is the Atlantic Wind Connection, a proposed undersea transmission cable running from New Jersey to Virginia that would deliver up to 6, megawatts of offshore wind energy.
The most recent major U. HVDC can help reduce the spread of such large-scale disturbances by providing a buffer between regions. EPRI research is helping to improve inspections and maintenance on energized HVDC lines, providing a scientific basis for using proper tools and setting minimum approach distances. The need to build can be offset by using HVDC to increase capacity on existing transmission lines.
The core of LCC technology is a semiconductor with a controllable switching-on action.