Implementation in the PSCAD/EMTDC environment

In the following, the PSCAD/EMTDC implementation of the LVDC distribution network is presented. To analyse the transient behaviour, the power quality and protection of the system are simulated by using the electromagnetic transient simulation program PSCAD/EMTDC. The objective of the models is to study the transient behaviour, harmonic propagation and protection issues of the network.

The initial model of the LVDC distribution network prototype was implemented and verified by measurements (Vornanen et al., 2009). The model consists of a front-end transformer rectifier, a 200 m long bipolar DC network and one phase customer inverter with load. The transformer rating and winding voltages correspond to the laboratory transformer. The rectifier is a half-controlled thyristor bridge. The sizes of the DC capacitors correspond to the laboratory setup. The single-phase PWM inverter model is based on ideal switches, and thus, switching is included in the model.

The field installation of a public LVDC distribution network is based on three-phase inverters; the description of research platform is given in Chapter 3. To proceed with offline simulations, the model of the actual network research platform with full transient details is developed. In the model, the inverter modulation scheme is sine-triangle PWM with third-harmonic injection (THIPWM). Being different from the space vector PWM used in prototype inverters, this modulation method provides uniform results and is easier to implement.

Rectifier model and implemented functions

The rectifier model is based on commonly known principles. The power electronic components of the rectifiers, that is, the thyristors and the diodes are two-state resistive switches from the PSCAD main library. The model applies a start-up/restart logic:

above 60 % of the grid voltage the rectifier is on, while below 50 % of the grid voltage the rectifier is off and resets the delay angle for soft restart. Further, there are two work modes in the model, normal and charging, and the mode is changed by the hysteresis logic. If the DC voltage is above 89 % of the nominal, the work mode is normal. In the normal mode, two 30-degree firing impulses spaced by 30 degrees are supplied to the thyristor gates. If the DC voltage is below 87 %, the work mode is charging. In the charging mode, one 30-degree firing impulse is supplied to the thyristor gates. There are similar functions in industrial rectifiers, for example in the ABB ACS 600 DSU.

Customer-end inverter model and implemented functions

The CEI model in the PSCAD environment consists of a three-leg-pair IGBT bridge, an LC filter and an isolation transformer. The model allows full-detail simulation of high-frequency switching events. The solution time step is set to 5 us. The IGBT bridge components are two-state resistive switches from the PSCAD main library. The LC filter model is simple, made from basic linear electrical components; inductance and capacitance. The three-phase, two-winding transformer is a PSCAD main library component, and it is based on the classical modelling approach. The IGBT bridge modulation technique implemented in the model is THIPWM. In order to test the behaviour of the network during significant events and faults, additional functionalities (under/overvoltage protection and AC voltage droop) are implemented in the inverter model (Table 2.4). These correspond to actual customer-end inverter functionalities.

Table 2.4. CEI functionality implemented in the model.

Fault/Event Action Definition Prototype Model

DC undervoltage AC voltage

droop U_dc < 610 V U_ac – 15 % U_ac – 15 % DC overvoltage Protection Udc > 780 V Shutdown,

Restart Modulation Off DC undervoltage Protection U_dc < 520 V Shutdown,

Restart Modulation Off

Customer load model

To study the power quality and the harmonic flow in the customer load network, a complex load is modelled as individual components:

· Resistive load,

· Non-linear

- Single-phase diode bridge with a capacitive filter - Three-phase diode bridge with a capacitive filter and

· Induction motor.

The power ratings and configuration of the load are determined by the case under study.

Large-scale average model of the LVDC network installation

The MV distribution network branches, which can be replaced by an LVDC distribution network, have much more customers than the pilot case network. For example in this work, networks with 50–110 end-customers on MV/LV branches were calculated.

Again, further constraints for the simulation case can be imposed by the fact that simulations with microsecond-scale steps can be time consuming and require much more memory than the simulation environment or computer can provide. Therefore, to be able to simulate longer periods and larger networks than in the pilot case, the time step of the solution has to be increased. This will disable the option of high-switching-event simulations, but low-frequency harmonic analyses will remain accurate. In this case, a three-phase inverter average model, where the inverter is modelled as a controlled current source, is used to model the LVDC network load.

MV network model

The medium-voltage (MV) network is modelled by the following components: a 110 kV voltage source model, a 110/21 kV 16 MVA transformer and a 20 km Al/Fe 85/14 pigeon line model. The source fault level is 1000 MVA and the R/X ratio is 0.1. The supply frequency is 50 Hz. The MV network model is similar to the Finnish overhead power network models for rural and urban areas developed by the University of Vaasa and VTT Technical Research Centre of Finland. The models are described in detail in (Kauhaniemi and Ristolainen, 2005). These models can be used with the developed LVDC network models as sources for further studies.

In the network PSCAD model, which is used in the simulations, the transmission line is modelled using a PSCAD-coupled pi section transmission line component. The component parameters were set according to Table 2.5.

Table 2.5. Transmission line parameters (Kauhaniemi and Ristolainen, 2005).

Resistance, Ω/km Inductive reactance, Ω/km

Capacitive reactance, MΩ*km

Positive-sequence 0.337 0.354 0.3183

Zero-sequence 0.487 1.885 0.5218