CHAPTER 1: INTRODUCTION
4. CHAPTER 4: VSC-HVDC PROTECTION SCHEME ON PSCAD
4.3. VSC-HVDC system model
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67 During steady state, station VSC1 and VSC4 were set to control voltage while VSC2 and VSC3 control the power flow. Figure 4-1 shows the system’s VSC single line diagram model with a zoomed view of VSC1 and its direct connections shown in Figure 4-2. In Figure 4-3 a schematic diagram with a detailed view of the inside of the converter unit is shown. In this case the rectifier converter unit is shown.
However, all converter units were made identical. The specifications for the VSC-HVDC system under study are given in Table 4-1.
Table 4-1: Specifications of MTDC VSC-HVDC model under study.
VSC-HVDC SYSTEM SPECIFICATIONS
Configuration 2-level
Symmetrical monopole
Grid Voltage VSC1 and VSC3 ±420 kV
Grid Voltage VSC2 and VSC4 ±500 kV
AC frequency VSC1 and VSC 4 60 Hz
AC frequency VSC2 and VSC 3 50 Hz
DC voltage 400 kV
Active power (P) 200 MW
Transformer ratings Star/star connection
Vprim VSC1 and VSC3 = 420 kV Vprim VSC2 and VSC4 = 500 kV Vsec = 230 kV
S= 1500 MVA
System AC filters q = 25
Freqbase = 60 Hz (VSC1 and VSC4) Freqbase = 50 Hz (VSC2 and VSC3) Q = 5 MVar
System phase reactors R = 0.015 Ω = 0.005 pu
L = 0.002 H = 0.26 pu
DC capacitance C= 300µF
68 Figure 4-1: Single line diagram of the 4-terminal VSC system developed.
Figure 4-2: VSC1 of the 4-terminal VSC system developed.
69 Figure 4-3: Rectifier side of VSC-HVDC scheme on PSCAD.
4.3.1. Voltage source converter unit
The developed system used series-connected IGBTs as switching devices to share the high blocking voltage [185]. A two-level converter was adopted for this test model. The PSCAD converter model is illustrated in Figure 4-3. For this converter topology, six switch valves were used. These contain several IGBT switches, and anti-parallel diodes to facilitate the bi-directional power flow of the converter [186].
The switches were controlled with PWM techniques to reproduce a sinusoidal waveform on the AC side.
For their own protection, during faults, the IGBTs were designed to self-block; the fault current is then forced to flow through the freewheeling diodes [102]. The converter unit was controlled using a technique known as a decoupled vector control, which includes the inner loop current controllers and outer loop controllers. This will be explained a bit further in this Chapter.
4.3.2. Transformer
The MTDC VSC-HVDC test model has been connected to the AC system through 50/60 Hz AC transformers. The main function of a transformer in the system was to adjust the voltage level of the grid to an appropriate level of the VSC station. In addition, the power transformer was used to provide a reactance between converter AC terminals and AC system [186], [70]. Its ratings are presented in Table 4-1.
4.3.3. Filters
Filters have been included on the AC side of the converter to limit the harmonic content of the converter current and voltage. The filters were located between the converter and the converter transformer. Their components were calculated using equation (4-1) to (4-3) [70].
2 n2 shunt
fV C Q
(4-1)
70 C
R q
0
(4-2)
2 0
1
LC (4-3)
Where C = capacitance, Qshunt= shunt reactive power, f = switching frequency (i.e. the rate at which the DC voltage is switched ON and OFF during the PWM process), 0= the tuned resonant frequency, Vn= the rated voltage, R = the resistance, q = quality factor and L = the inductance as given in reference [67], [187]. In the model, switching frequency is set as 1350 Hz, the shunt reactive factor Qshunt= 0.06 pu and the resonant frequency is 2f [67].
4.3.4. Phase reactor
The phase reactor acts as a filter for the harmonic currents generated by the converter’s switching. This aids in preventing rapid changes in polarity that can be caused from the valves switching, while it limits short-circuit currents. An additional purpose of the reactor was to permit independent and continuous control of active and reactive power, by controlling the voltage drop and the direction of the current flow through it. A common size for the phase reactor is 0.15 pu and was also chosen for this model [67].
4.3.5. DC-link capacitor
DC-link capacitors were installed to maintain the DC voltage for VSC operation. They were also tasked to filter the ripples that results on the DC-side of the system. The DC-link capacitance was calculated using the equation (4-4) [70]: -
2
2 _ DC
ref
DC V
C P
(4-4)
Where τ = time constant (is time needed to fully charge the DC-link capacitor to its base/rated power and rated voltage level), P_ref= reference power and VDC= DC voltage. In this test model, the DC-link capacitor was divided into two DC-link capacitor components which are connected to the neutral ground point of the VSC, therefore making the network one with the symmetrical monopole topology.
4.3.6. Transmission medium
Amongst its numerous advantages, VSC-HVDC technology is favoured for its ability to transmit power underground and underwater over longer distances.
71 Although both AC and DC cables can be motivated and are technically feasible, DC cables gives no technical limit to the transmission distance. The cables add no short-circuit power and enable improved reactive power balance due to the properties of the converters [188]. Most VSC cables are manufactured are either cross-linked polyethylene (XLPE) or mass impregnated insulation [67]. These consist of a copper or aluminium conductor surrounded by metallic sheath and plastic outer coating to protect the cable. For subsea cables an additional steel armouring is required to increase the cables strength and to protect it from harsh sea conditions. The cables shown in Figure 4-4 and Figure 4-5 were designed to be buried underground. These were set to be a 100 km long and designed to carry ± 400 kV and 200 MW.
Figure 4-4: Cable connections in PSCAD.
During design, the frequency dependent (phase) model option was selected. Figure 4-5 shows the cable configuration used. The two cables were set at a depth 1.5 m below ground and 0.5m apart from each other as shown. The frequency dependent (phase) model implemented in the system is recognised as the most advanced time domain model available in PSCAD [189], [190] when investigating transients.
Figure 4-5: Cable configuration.
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