School of Electrical Engineering & Telecommunications ENGINEERING @ UNSW 20 14 EET & UNSW e~óÇÉå táííáÖ òPPPPRNU pìéÉêîáëçêW gçÜå cäÉíÅÜÉê ^ëëÉëëçêW RáÅ~êÇç ^ÖìáäÉê~ Thin AC Converters for Utilities Abstract Higher reliability targets push network operators (NOs) to adopt mesh networks, but high power demand requires NOs to increase asset utilisation. These conflicting goals push NOs to adopt flexible AC transmission system (FACTS) devices. Using dual virtual quadrature sources and pulse width modulation switching, a “thin” controllable network transformer (CNT) can be developed with numerous advantages over existing FACTS devices. In this thesis, such a CNT is modelled in Simulink, and its behaviour verified. A control system and harmonics filters are developed, and its capabilities are assessed in general networks and in a case study on the NSW grid. Thin AC Converters FACTS devices have been readily available for decades, yet have experienced poor adoption. “Thin” AC converters are a more attractive option because: • They have no bulk energy storage components, • They feature “fail normal” behaviour, • They can utilise existing grid assets, and • They are fractionally rated compared to the load. A Controllable Network Transformer Pulse width modulation (PWM) switching techniques can be used to provide phase control when the “dual virtual quadrature sources” (DVQS) method is used. DVQS involves adding an odd harmonic to a signal so a phase shifted signal can fit within the original signal’s envelope, as in Figure 1. Figure 1: signal (b) cannot be synthesised using conventional PWM techniques with input (a). By adding (c) to (b), signal (d) is produced, which can be synthesised. Using DVQS PWM switching techniques, a controllable network transformer (CNT) can be created from an existing transformer and a thin AC converter, to provide phase control and therefore power flow control, by quickly changing between the highest and lowest tap, with duty cycle D = k 0 + k 2 •sin(2π•100•t) Figure 2: a network transformer augmented with a thin AC converter, to create a controllable network transformer (CNT). Assessing the Technology This thesis involved modelling the converter and determining its capabilities in terms of: • Controlling power flows on the network, and • Maintaining and improving power quality. It was shown to be useful for balancing line currents, reducing circulating currents, mitigating flicker, sags, and swells. Figure 3: summary of network application results Case Study on the NSW Grid The findings were validated by modelling a portion of the New South Wales transmission network, the Newcastle–Tomago ring. This ring suffers from circulating currents, imbalanced parallel line currents, and flicker from upstream sources. Figure 4: simplified representation of the Newcastle– Tomago transmission network. Line Current Balancing Feeders 98R and 98N share load unequally and in the opposite ratio to their ratings. By splitting the bus at Tomago and using a CNT, or by installing a series CNT on 98N, their combined rating can be increased. Figure 5: results of the line current balancing simulations on the Newcastle–Tomago network. Power Flow Control CNTs installed at Tomago were capable of reducing power flow through 98R/N by over 14%, or 28MVA. Power Quality Impacts CNTs installed at Newcastle reduced injected 5Hz arc furnace flicker from Pst = 2.2 to 0.9. Conclusion A controllable network transformer was modelled in Simulink, and its behaviour verified. It was shown to be successful in controlling power flow and mitigating power quality issues, both in general networks and within a NSW grid case study. 1 10 100 0 50 100 Line Length (km) | S | (MVA) Power Flow Control 1 10 0 50 100 Step Frequency (Hz) Flicker Reduction (%) Flicker Mitigation Sag and Swell Mitigation reduction in sag and swell magnitude 10% 10% TACC Tx Bypass Tap 1 Tap n Tap ₂ V V Newcastle Tomago Beresield 98R 98N 9NA 330kV 132kV 132kV 330kV
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School of Electrical Engineering & Telecom
munications
ENGINEERING @ UNSW
2014
EET&U N S W
e~óÇÉå=táííáÖ=òPPPPRNU=pìéÉêîáëçêW=gçÜå=cäÉíÅÜÉê=^ëëÉëëçêW=RáÅ~êÇç=^ÖìáäÉê~
Thin AC Converters for Utilities
Abstract Higher reliability targets push network operators (NOs) to adopt mesh networks, but high power demand requires NOs to increase asset utilisation. These conflicting goals push NOs to adopt flexible AC transmission system (FACTS) devices. Using dual virtual quadrature sources and pulse width modulation switching, a “thin” controllable network transformer (CNT) can be developed with numerous advantages over existing FACTS devices. In this thesis, such a CNT is modelled in Simulink, and its behaviour verified. A control system and harmonics filters are developed, and its capabilities are assessed in general networks and in a case study on the NSW grid.
Thin AC Converters FACTS devices have been readily available for decades, yet have experienced poor adoption. “Thin” AC converters are a more attractive option because: • They have no bulk energy storage components, • They feature “fail normal” behaviour, • They can utilise existing grid assets, and • They are fractionally rated compared to the load.
A Controllable Network Transformer Pulse width modulation (PWM) switching techniques can be used to provide phase control when the “dual virtual quadrature sources” (DVQS) method is used. DVQS involves adding an odd harmonic to a signal so a phase shifted signal can fit within the original signal’s envelope, as in Figure 1. !!!!!!!!!!!!!!!!
Figure 1: signal (b) cannot be synthesised using conventional PWM techniques with input (a). By
adding (c) to (b), signal (d) is produced, which can be synthesised.
Using DVQS PWM switching techniques, a controllable network transformer (CNT) can be created from an existing transformer and a thin AC converter, to provide phase control and therefore power flow control, by quickly changing between the highest and lowest tap, with duty cycle D = k0 + k2•sin(2π•100•t) !!!!!!!!!!!!Figure 2: a network transformer augmented with a thin
AC converter, to create a controllable network transformer (CNT).
Assessing the Technology This thesis involved modelling the converter and determining its capabilities in terms of: • Controlling power flows on the network, and • Maintaining and improving power quality. !It was shown to be useful for balancing line currents, reducing circulating currents, mitigating flicker, sags, and swells. !!!!!!!!
Figure 3: summary of network application results
Case Study on the NSW Grid The findings were validated by modelling a portion of the New South Wales transmission network, the Newcastle–Tomago ring. This ring suffers from circulating currents, imbalanced parallel line currents, and flicker from upstream sources. !!!!!!!!!!!Figure 4: simplified representation of the Newcastle–
Tomago transmission network.
Line Current Balancing Feeders 98R and 98N share load unequally and in the opposite ratio to their ratings. By splitting the bus at Tomago and using a CNT, or by installing a series CNT on 98N, their combined rating can be increased. !!!!!!!!!!
Figure 5: results of the line current balancing simulations on the Newcastle–Tomago network.
Power Flow Control CNTs installed at Tomago were capable of reducing power flow through 98R/N by over 14%, or 28MVA.
Power Quality Impacts CNTs installed at Newcastle reduced injected 5Hz arc furnace flicker from Pst = 2.2 to 0.9.
Conclusion A controllable network transformer was modelled in Simulink, and its behaviour verified. It was shown to be successful in controlling power flow and mitigating power quality issues, both in general networks and within a NSW grid case study.
110100
0
50
100
Line Length (km)
| S |
(MVA
)
Power Flow Control
1 100
50
100
Step Frequency (Hz)
Flic
ker R
educ
tion
(%)
Flicker Mitigation Sag and Swell Mitigation
reduction in sag andswell magnitude
10%10%
TACCTx
BypassTap 1Tap n
Tap ₂"
V# V$
Newcastle Tomago
Beres!ield98R
98N
9NA
330kV 132kV 132kV 330kV
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