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HVDC System
The final example described in this section illustrates modeling of a high-voltage direct current (HVDC) transmission link [1]. Perturbations are applied in order to examine the system performance [2]. The objectives of this example are to demonstrate the use of the Universal Bridge block and the Three-Phase Transformer (Three Windings) block in combination with Simulink blocks in the simulation of a complete pole of a 12-pulse HVDC transmission system. The electrical part representing the AC network is built using three-phase blocks. The Discrete 12-Pulse HVDC control system is a generic control available in the Discrete Control Blocks library of powerlib_extras.
Description of the HVDC Transmission System
Open the psbhvdc12pulse model and save it as case5 in order to allow further modifications to the original system. This system is shown in Figure 2-27.
A 1000 MW (500 kV, 2 kA) DC interconnection is used to transmit power from a 500 kV, 5000 MVA, 60 Hz network to a 345 kV, 10000 MVA, 50 Hz network. The AC networks are represented by damped L-R equivalents with an angle of 80 degrees at fundamental frequency (60 Hz or 50 Hz) and at the third harmonic.
The rectifier and the inverter are 12-pulse converters using two Universal Bridge blocks connected in series. Open the two converter subsystems to see how they are built. The converters are interconnected through a 300 km line and 0.5 H smoothing reactors. The converter transformers (Wye grounded /Wye/Delta) are modeled with Three-Phase Transformer (Three-Windings) blocks. The transformer tap changers are not simulated. The tap position is rather at a fixed position determined by a multiplication factor applied to the primary nominal voltage of the converter transformers (0.90 on the rectifier side; 0.96 on the inverter side).
From the AC point of view, an HVDC converter acts as a source of harmonic currents. From the DC point of view, it is a source of harmonic voltages.
The order n of these characteristic harmonics is related to the pulse number p of the converter configuration: n = kp ± 1 for the AC current and n = kp for the direct voltage, k being any integer. In the example, p = 12, so that injected harmonics on the AC side are 11, 13, 23, 25, and on the DC side are 12, 24.
AC filters are used to prevent the odd harmonic currents from spreading out on the network. The filters are grouped in two subsystems. These filters also appear as large capacitors at fundamental frequency, thus providing reactive power compensation for the rectifier consumption due to the firing angle 
. For = 30 degrees, the converter reactive power demand is approximately 60% of the power transmitted at full load. Look under the AC filters subsystem mask to see the high Q (100) tuned filters at the 11th and 13th harmonics and the low Q (3), or damped filter, used to eliminate the higher order harmonics, e.g., 23rd and up. Extra reactive power is also provided by capacitor banks.
Two circuit breakers are used to apply faults on the rectifier AC and DC sides.
The rectifier and inverter control systems use the Discrete 12-pulse HVDC Control block of the Discrete Control Blocks library of powerlib_extras.
The power system and the control system are both discretized with the same sample time Ts.
Define parameter Ts = 50e-6 in your workspace before starting the simulation.
| | Speed Regulation Dynamic Performance | Frequency Response of the AC and DC Systems | |