HVDC Network: Wind Power Integration and Creation of Super Grid

Giddani. Kalcon, G. P. Adam, O. Anaya-Lara, G. Burt and K. L. Lo University of Strathclyde, Glasgow, UK

grain.adam@eee.strath.ac.uk

Abstract—This paper investigates the possibility of using HVDC networks to create a large power pool that may facilitate the following objectives: increased connection of renewable power to the ac network without the need for bulky storage systems; improve the resiliency of the ac networks attached to this power pool against power and frequency instability problems during local loss of generation or major load; and enable power trading between these regional networks in real time. In this investigation, a generic HVDC network comprising four terminals connected to three strong ac networks and one weak ac network (mainly, wind farm based on fixed speed induction generators) is simulated under two scenarios. First case, where the wind farm power output varies, and variable power injection at the terminals of the converters that control active power. The second case has tested the resiliency of the HVDC network during loss of the ac network at one of the converter station.

Keywords─Active and reactive power control; HVDC network; modular multilevel converter; and two-level converter

I. INTRODUCTION With the recent developments in the voltage source

converter (VSC) technologies for HVDC systems, particularly on reduction in semiconductor losses, increased power ratings and operating voltage, potential elimination of ac side filters, and development of better approaches for handling of the ac and dc side faults, the concept of dc grids has become attractive in many applications [1-7]. Specially, for operation of power systems with high penetration of renewable power generation, and asynchronous connections of regional/or national power systems for grid reinforcement or power trading facilitation. As voltage source converters are able to reverse power flow without the need to reverse the dc link voltage polarity, the VSC has been favoured over the current source converter for any practical realization of dc grids in the recent future. This mainly due to the relative simplicity of the implementation as number of converter terminals increases [2, 4, 8].

The main attractive features of the VSC-HVDC network for applications such as connection of large offshore wind farms, ac network reinforcement, and creation of a large power pool to facilitate power trading in real time, and improve system stability are:

a) Operation independent of ac network short-circuit ratio.

b) Inherent leading and lagging reactive power generation capability within each converter station.

c) Allows power exchange between different regional ac networks, while each ac network maintains its autonomy.

d) Contributes limited current during ac side faults, this feature has made voltage source converter more resilient to ac faults than conventional synchronous generation.

However, vulnerability of the VSC-HVDC network to dc side faults has prevented its practical realization during the last decade due to the technology gap in the isolation mechanism of the dc fault (absence of reliable dc circuit breaker). This vulnerability has been reduced by recent introduction of two-switch modular multilevel converter (M2C) to HVDC systems[4, 9]. Because with M2C, the magnitude of the current that converter switches may experience during dc side faults is significantly reduced. Further reduction in this vulnerability may be achieved with modern voltage source converters with inherent dc fault reverse blocking capability[4-5].

This paper investigates the feasibility of creating a large power pool using HVDC networks to facilitate connection of large offshore farms and power trading in real time between large regional ac networks, without comprising system reliability and security of supply. Also such solution may allow high penetration of renewable power into power systems without the need for energy storage systems (for power levelling or other power quality issues). The validity of this concept has been tested on a four-terminal HVDC network using simulations in Simulink.

II. TEST SYSTEM Fig. 1 shows an HVDC network comprising of four

terminals modelled with neutral-point clamped (NPC) converters, and controlled using sinusoidal pulse width modulation (SPWM), and a 2kHz switching frequency. Each NPC uses two dc link capacitors of 300µF with neutral point ground to form bipolar HVDC systems. Converter VSCWF is connected to a 400MW wind farm. Converters VSC1 to VSC3 are connected to ac networks 1, 2 and 3. The dc grid comprises of four links, each 150km and modelled by a lumped pi-section. Converters VSCWF, VSC1 and VSC2 control active power and ac voltage magnitude at buses BWF, B1 and B2. Converter VSC3 regulates the dc voltage at 300kV pole-to-pole

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(±150kV) and the ac voltage magnitude at B3. Further details on voltage and power levels are depicted in Fig. 1, and power

flow directions indicated are assumed positive.

Fig. 1: Four-terminal HVDC network used as a test system for grid integration of offshore wind farms, and implementation of a transnational dc grid for power trading with real-time dispatch capability

III. SIMULATIONS To demonstrate the viability of using HVDC networks to

form a power pool for accommodation of large wind farms with variable output powers and possible power trading in real-time between different regional ac networks, the test system in Fig. 1 is started with both wind farm converter VSCWF and ac network 1 converter (VSC1) injecting 300MW into the power pool (dc grid), and converter VSC2 imports 200MW from the power pool. At time t=2s, the wind farm power output is gradually increased from 300MW to 400MW, while converter VSC1 reduces its injected power to the dc grid to zero (allowing VSC1 to operate as a static synchronous compensator, regulating the ac voltage at B1). At the same time the power imported by converter VSC2 is gradually increased from 200MW to 300MW. At t=4.5s, the wind farm power output is decreased gradually from 400MW to 200MW and converter VSC1 is commanded to import 200MW from the dc grid. The imported power by VSC2 is reduced from 300MW to 200MW. The results obtained from this case are shown in Fig. 2. Figs. 2a and 2b show the active and reactive power that the converter stations exchange with the wind farm and the three regional ac networks 1, 2 and 3. Figs. 2c and 2d show the power flow in the dc grid. It can be observed that the converter terminal connected to ac network 3 that regulates the dc link voltage acts as slack bus for the whole system, to maintain an active power balance between the ac and dc sides. Figs. 2a, 2b and 2e show that during variations of the active power exchange between the converter stations and the ac networks, including the wind farm, the converter stations are able to maintain constant voltage magnitudes at the points of common coupling using appropriate reactive power exchange with the

ac sides. Fig. 2f shows the variation of the converter dc link voltages with respect to converter station VSC3 that regulates the dc voltage at 300kV, as powers exported and imported to/from the power pool vary. These results illustrate the potential usefulness of such HVDC network in dispatching power to different ac networks in real-time, and to accommodate large amount of unscheduled power from wind farms without the need for any storage systems.

To demonstrate the effectiveness of the HVDC network in improving the transient stability of the overall system during a major fault in one of the ac sides, the test system in Fig. 1 is subjected to a loss-of-grid at converter station VSC1 by opening the ac circuit breaker connecting VSC1 to the ac network 1 at time t=1.5s and reclosing at t=3.5s. During this period, the wind farm injects constant power into the dc grid, and the ac network 2 imports 200MW from the dc grid and then this import increases gradually to 320MW at t=3s. It can be observed that the loss of ac network 1 has no significant effect on the overall transient stability of the system (see Fig. 3). This effect is reflected as an active power mismatch and handled mainly by the converter terminal that regulates the dc link voltage and operates as slack bus (Fig. 3b). The wind farm and the ac network 2 remain unaffected (Fig. 3a). A similar situation will exist with the loss of the wind farm or of any converter terminal that controls active power. Based on these results, such HVDC network may be a conceptual starting point for the proposed European Super Grid, as the ac fault in any of the regional ac network may not propagate into other regions. However, with the present converters control objectives the system in Fig. 1 is vulnerable to overall system

collapse when the converter station that controls the dc link voltage is lost. Such situation can be avoided by assigning more than one converter station to regulate the dc link voltage. In this case, the active power control flexibility of the system can be maintained by incorporation of active power-dc voltage droops within the dc voltage controller of each converter station.

IV. CONCLUSIONS This paper investigated the viability of using voltage source converter HVDC networks to facilitate integration of large offshore wind farms and connection of regional/or national

power networks into large super grid. Based on the discussions presented in this paper, the following conclusions are drawn:

• Creation of power pool using HVDC system may eliminate the need for storage systems for load levelling in systems with high penetration of renewable power.

• Power dispatch in real time may be possible (i.e. improved power trading between national grids).

• Improvement in the resiliency of the ac networks connected to this power pool against power and frequency stability issues.

a) Active and reactive powers at busses BWF (wind farm exchange with

the dc grid) and B1 (Grid 1 exchange with dc grid)

b) Active and reactive power exchange between grids 2 and 3, and the dc

grid.

c) Power flow in the links between converter stations VSC-WF and

VSC-1; and VSC-WF and VSC-3.

d) Power flow in the links between converter stations VSC-1 and VSC2;

and VSC-2 and VSC-3.

e) Voltage magnitude at wind farm bus BWF and points of common

coupling B1 through B3

f) Voltage magnitude across the dc link of different converter stations

Fig. 2: Key waveforms to illustrate the use of HVDC networks to connect different ac networks and as a power to accommodate large offshore/onshore wind power

a) Active and reactive power that converters VSCWF and VSC1 exchange with the wind farm and ac network 1

b) Active and reactive power that converters VSC2 and VSC3 exchange with the ac network 2 and 3

c) Power flow in the dc grid (power pool) d) Voltage magnitude across the dc link of different converter stations

Fig. 3: Key waveforms that demonstrate the ability of the VSC-HVDC network to tolerate loss of ac grid from one of the converter stations.

V. REFERENCES [1] G. P. Adam, et al., "Modular multilevel inverter: Pulse width

modulation and capacitor balancing technique," Power Electronics, IET, vol. 3, pp. 702-715, 2010.

[2] M. P. Bahrman and B. K. Johnson, "The ABCs of HVDC transmission technologies," Power and Energy Magazine, IEEE, vol. 5, pp. 32-44, 2007.

[3] R. Feldman, et al., "A hybrid voltage source converter arrangement for HVDC power transmission and reactive power compensation," in Power Electronics, Machines and Drives (PEMD 2010), 5th IET International Conference on, 2010, pp. 1-6.

[4] G. P. Adam;, et al., "Network Fault Tolerent Voltage Source Converters for High-Voltage applications," presented at the IET 9th International Conference on AC and DC Power Transmission systems, London, UK, 2010.

[5] M. M. C. Merlin, et al., "A New Hybrid Multi-Level Voltage-Source Converter with DC Fault Blocking Capability," in IET ACDC2010, London,UK, 2010.

[6] R. Marquardt, "Modular Multilevel Converter: An universal concept for HVDC-Networks and extended DC-Bus-applications," in Power Electronics Conference (IPEC), 2010 International, 2010, pp. 502-507.

[7] B. Jacobson;, et al., "VSC-HVDC Transmission with Cascaded Two-level Converters," presented at the CIGRE 2010, 2010.

[8] A. Hagar and P. W. Lehn, "A scalable multi-input multi-level voltage sourced converter," in Electrical and Computer Engineering, 2009. CCECE '09. Canadian Conference on, 2009, pp. 265-268.

[9] N. Flourentzou, et al., "VSC-Based HVDC Power Transmission Systems: An Overview," Power Electronics, IEEE Transactions on, vol. 24, pp. 592-602, 2009.