Transmission System Benchmark and VSC Analysis

Q1. Benchmark of the transmission system. Models of instantaneous quantities shall be used where appropriate. [30 marks]

(a) Calculate the reactance and resistance of power lines of AC and CB in ohms. [3]

(b) Calculate the active and reactive loads at both Bus B and C in MW and MVar. [3]

(c) Use simulation case studies to evaluate how the variations of loads may affect the voltage profiles at both B and C (keep the power factors unchanged). [6]

(d) Suppose the location of Bus C can be shifted along the power line between Buses A and

B. Use simulation case studies to evaluate how the location of Bus C may affect the voltage profiles (all buses with full loads). [6]

(e) Use simulation case studies to evaluate the impact of an additional shunt capacitor bank at Bus C on the voltage profiles under full load condition. [6]

(f) Conceptually design two reactive power compensation schemes to provide the service of local voltage regulation for Buses B and C so both voltages are within the range of 0.95 p.u. and 1.06 p.u. at all time. [6]

Please provide the schematic diagram(s) of the design and how they are incorporated into the benchmark network;

Please provide the control diagram(s) to demonstrate the prospected control;

Please provide necessary descriptions to support the diagrams;

Please provide comments on the strengths and weaknesses of the two schemes;

Numerical specifications and detailed modelling are not required.

Q2. VSC system analysis: [30 marks]

A VSC converter station is to be integrated at Bus C at a capacity of a 500 MVA. The equivalent impedance between the AC terminal of the 3-phase bridge circuit and the point of connection at Bus C is 0.01+j0.2 p.u..

For simplicity, please use the following assumptions in the following tasks in Q2:

• The DC voltage stable and adequate for the pulse width modulation of VSC;

• Modelling and analysis are carried out in per unit if not otherwise required;

• The effect of switching harmonics is negligible;

• The main circuit of VSC is a 2-level bridge.

Before incorporating the VSC into the AC transmission system model of Q1, accomplish the following tasks:

(a) Calculate the rated current of the VSC in amps; [3]

(b) Calculate the equivalent AC impedances of the VSC based on 110 kV. [3]

(c) Provide the mathematical model of a VSC in synchronous reference frame (dq), where the d-axis is aligned with the vector of the voltage at the point of connection. [3]

The dynamics of VSC current and the voltage at the point of connection;

The constraint of real power exchange between the AC and DC transmission system.

(d) Briefly explain the motivation of employing dq reference frame in the control of a 3- phase VSC. [3]

(e) Use an appropriate diagram to illustrate a standard 3-phase Phase Lock Loop (PLL) and briefly explain its role in VSC control. [3]

(f) Based on the initial specifications, provide the waveform of the voltage at Station C in dq and abc reference frame. [3]

Describe how the d-axis is aligned with the voltage vector according to the waveforms;

Quantitatively indicate the correlation between the waveform in dq and abc reference frames.

(g) Based on the dq reference frame used in (f), present the waveform of the current flowing out of VSC. [3]

Briefly explain how the currents are related to the active and reactive power of the VSC.

(h) Use a block diagram to illustrate a closed-loop current control strategy in the dq reference frame. [3]

Appropriate scheme of current saturation should be included;

Please provide necessary descriptions;

Simulation waveforms are not required.

By incorporating a VSC model into the transmission network of Q1, accomplish the following:

(i) Validate the proposed closed-loop current control based on the model established in Q 1 with a current step order of 0.7 p.u..

Present the waveforms in per unit;

Please present appropriate evidence of waveform. [3]

(j) Design a VSC based dynamic voltage control scheme that can keep voltages at B and C always within the range between 0.94 p.u. and 1.06 p.u. and verify the effectiveness. [3]

The block diagram of the schematic should be provided with descriptions;

The full range of loading condition must be considered;

The electromagnetic dynamics must be considered .