The effect of VSC-HVDC Control on AC System Electromechanical Oscillations and DC System Dynamics
Journal: IEEE Transactions on Power Delivery
Publication Date: 30 August, 2016
Department of: Electrical and Electronic Engineering
Managing the electrical transmission networks of the future
To achieve a sustainable, low-carbon electrical energy supply, the utilities are making substantial changes to the electrical network. Increasingly, high voltage DC (HVDC) transmission is being used to send more power in a given ‘corridor’ and over longer distances than is possible with AC transmission. But the effect of the operating point of a voltage source converter HVDC link and its control have potentially substantial effects on both the oscillatory behaviour of the existing AC network as well as the new DC side-network dynamics. For the full, flexible capability of VSC HVDC to be exploited, the effects of these controllers and their interactions with AC system responses need to be properly characterized.
Researchers at the University of Manchester are identifying the best methods for controlling such systems and developing advanced models to more accurately test system behaviour. Although initially applied to a generic two-area model, now, tests have been extended using a newly developed large-scale more realistic network with detailed AC generator and HVDC models. Both modal analysis and transient stability analysis have been used to highlight tradeoffs between candidate VSC-HVDC power controllers and to successfully characterize the electromechanical performance of the integrated AC/DC model.
- Transmission is the bulk supply of electrical energy from the power station where it is generated to the load centres (e.g. city) where it is used. A distribution network then transfers the electrical energy to the individual consumers.
- Most electrical networks, both transmission and distribution use Alternating Current (AC). This changes polarity 50 times per second (in the UK, the USA uses a frequency of 60 times per second). The alternating polarity allows the use of electromagnetic transformers which allow easy voltage conversion between two voltage levels. Electricity is used generated and used at low voltage for safety, but is transmitted at high voltage to reduce losses.
- AC lines require energy to set up the magnetic and electric fields which allow current to flow. AC requires that these lines are charged and discharged at the network frequency. Over long distances this charging and discharging can limit the power which can be sent down the line. Under these circumstance High Voltage Direct Current (HVDC) is used. This requires high power electronics at either end to convert between AC and DC (with some small power loss) but then allows efficient transmission of power over very long distances.
- AC power generators synchronise themselves to the power network. Large power changes can ‘disturb’ this synchronisation causing generators to ‘oscillate’ against the power network. Such oscillations are generally designed to be damped over time (to settle down). The addition of new technology has the potential to reduce this oscillation or reduce damping.