Evaluation of the Performance of Back-to-Back HVDC Converter and Variable Frequency Transformer for Power Flow Control in a Weak Interconnection B. Bagen, D. Jacobson, G. Lane and H. M. Turanli Manitoba Hydro, Winnipeg, Canada Presented By: David Jacobson 2007 PES General Meeting 24-28 June 2007, Tampa, FL USA
Presentation Outline Background and Introduction Evaluation Models Results and Discussions Summary and Conclusions
Background and Introduction
The Green Power Corridor East-west power grid connecting Manitoba-Ontario- Quebec- Newfoundland and Labrador First part of the corridor considers to bring clean Hydro resources in Northern Manitoba to Ontario The studies described in this paper is related to the idea of Winnipeg Route Option
Nearly all electricity generated is from water power and 75% is produced by five major generating stations on the Nelson River. Power from the Nelson River generation is transmitted by approximately 900 km of HVDC transmission to the major load center in Winnipeg The Manitoba system is interconnected to the neighboring provinces of Saskatchewan and Ontario and to the US states of North Dakota and Minnesota by 12 tie lines
Manitoba North Dakota 2 Minnesota Great Lakes Loop Northwest Ontario 2 Michigan Southern Ontario Quebec New York Existing 200 MW tie between Manitoba and Ontario was put into service in 1972. As a result of this interconnection a transmission loop around the Great Lakes was created. Conventional phase shifting transformers were provided to control the power flow can-us-boundaries
500 kv HVDC 840 km ( longer for western route option ) Conawapa 500 kv AC Line 1410 km To date, there are several proposed transmission and generation developments associated with increasing transfer reserve to Ontario that may be approved by Manitoba Hydro. 1. 400 MW point-to-point TSR to Ontario without new transmission 2. Additional 1600 MW pointto-point TSR to Ontario with new 1410 km 500-kV transmission. Riel 500 kv AC Line 135 km Lakehead Back-to-Back HVDC or VFT Porcupine Hanmer This paper is focused on performance assessment of the VFT compared with Back-to-back HVDC for the total 2000 MW high capacity transfer to Ontario (Winnipeg Route Option)
Evaluation Models
2002 MAPP model series using PSS/E Version 26 on Unix platform Load: 2012 summer off-peak for Manitoba and 2012 summer peak for Ontario 500 kv AC transmission: for HVDC option: 1600 MW back-to-back HVDC bipole and two 500 Mvar, 500 kv series capacitors. A 200 Mvar, 500 kv AC filter on the Manitoba side and a 100 Mvar, 500 kv AC filter on the Ontario side For VFT option: 1600 MW VFT consisting of 16 parallel 100 MW units. Approximately 50% of total MW rating reactive compensation
Results and discussions
Steady State Performance (Power order versus net reactive power ) Reactive Power Consumption (MVAR) 350 Back-to-Back HVDC 300 VFT 250 200 150 100 50 1000 1100 1200 1300 1400 1500 1600 Power Order (MW) Reactive power consumption increases with the power order increase for both schemes The VFT consumes less reactive power than the back-toback HVDC system
Steady State Performance (Device size versus net reactive power ) Reactive Power Consumption (MVAR) 500 450 400 350 300 250 200 150 100 50 1000 1100 1200 1300 1400 1500 1600 Device Size (MW) Back-to-Back HVDC VFT Reactive power consumption decreases with increase in back-to-back HVDC converter size mainly due to increased reactive power supplied from larger AC filters Although the effective reactance of the VFT decreases with increase in its size, reactive power consumption increases due to increased power flowing through the device reactance The VFT consumes less reactive power than the backto-back HVDC system
Transient Stability (Real power response to a Manitoba side fault) Initial transient recovery of the VFT is faster Real Power (MW) 2000 1500 1000 500 0 Back-to-Back HVDC VFT 0 1 2 3 4 5 Time (s) The back-to-back HVDC system, however, provides faster and smoother recovery to pre-disturbance power level The initial fast and then oscillatory response of the VFT is beneficial in stabilizing the overall network. A VFT acts like a finite impedance and allows power to swing naturally between the two systems
Transient Stability (Ontario side voltage response to a line fault) Per Unit AC Voltage 1.2 1.1 1 0.9 0.8 0.7 Back-to-Back HVDC VFT 0 1 2 3 4 5 Time (s) Similar conclusions can be drawn from the voltage response Conclusions can be drawn from the studies that both alternatives meet the relevant stability criteria and could be implemented
Real Power (MW) 2000 1500 1000 500 0 Controlled Power Change (Power order reduction ) Back-to-Back HVDC VFT 0 1 2 3 4 5 Time (s) The power changes in response to a control initiated 45% power order reduction The back-to-back HVDC system provides faster response The VFT, however, provides better natural damping capability due to device inertia HVDC has limit on power order reduction. The VFT, however, does not have a similar reduction limit as it is fully bidirectional through zero. The VFT allows power through the tie to deviate from its setpoint. This actually helps damp power swings in the network. Simulation results show that critical dynamic voltages are more likely to reach their lower limit (0.82 p.u.) while the VFT is able to hold these voltages at a higher level following a disturbance.
Controlled Power Change (Tie line power response to the power order reduction) Tie Line Real Power (MW) 2200 2100 2000 1900 1800 1700 Back-to-Back HVDC VFT 0 1 2 3 4 5 6 Time (s) Power changes on the existing 500 kv Manitoba-US tie line due to 45% power order reduction with no internal Manitoba DC reduction VFT provides smoother response to this sudden power surge due to its slower response. This may be beneficial to stabilize the system in some cases Manitoba Hydro would, however, be implementing a special protection method to eliminate significant Manitoba-US power changes due to sudden power changes on the Manitoba to Ontario tie
Summary and Conclusions
Two power flow control concepts for a weak AC link are proposed. The performances of these schemes are compared based on steady state and dynamic considerations. Both schemes are viable in controlling power flow. The VFT is superior to the back-to-back HVDC converters based on reactive power consumption in these studies. The VFT provides faster initial response while back-to-back HVDC provides faster recovery to pre-disturbance level. Back-to-back HVDC system responds faster and may require less reduction than the VFT for power order reduction. The VFT however, provides better natural damping capability. Back-to-back HVDC has limit on power order reduction. The VFT, however, does not have a similar reduction limit as it is fully bidirectional through zero. VFT also has the advantage of holding voltages at a desired level following a disturbance. The selection of a preferred option therefore depends largely on the economics involved.
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