By Kevin Smith, Power Parameters engineering solutions manager
Irrespective of future renewable energy targets, the influence of increasing levels of wind and solar penetration is providing network engineers with changing criteria for protection, stability and power quality.
…offline monitoring of phasors and a host of power quality parameters at MV distribution levels is now very necessary if for no other reason than wind generator farms and large solar PV plant are being connected to distribution networks.
A decade ago, solar PV had no influence on medium voltage distribution networks. Now in South Australia, Northern New South Wales and Queensland low power factor can occur during high-insolation periods and, as a result, inverters are required by some distributors to accommodate lagging power factors of 0.9.
Low power factor caused by vigorous feed-in power is, as yet, not a serious issue in the overall operation of MV networks. However, at 30-plus per cent penetration, stability and protection issues require monitoring as the only solid basis for rejigging reactive power compensation, protection gear siting and levels of protection.
Harmonic generation from inverters as used in double-fed, induction generator (DFIG) and synchronous wind turbines, as well as from solar PV, make power quality an acute issue. Placing of harmonic traps and filters in a distribution network requires extensive network monitoring for predominant harmonics and predominant phase angle.
Stability of networks, in particular in high voltage transmission, is often controlled ‘live’ by phasor monitoring units (PMUs). Basically, PMUs monitor power flow direction and power swings with the important task of limiting the latter, thus maintaining stability. However, offline monitoring of phasors and a host of power quality parameters at MV distribution levels is now very necessary if for no other reason than wind generator farms and large solar PV plant are being connected to distribution networks.
This point was made during a recent visit to Australia by Willie van Wyk, managing director of CT Labs, a South African high technology company designing and producing power quality monitors in collaboration with the faculty of electrical engineering of the University of Potchefstroom. South Africa’s ESKOM distribution systems incorporate thousands of power quality monitors. The CT Lab monitors, the Vecto II and Impedo DUO are GPS-time synchronised, making measurements with a time precision of ±100 nanoseconds permitting correlation with PMU data.
Harmonic data is gathered in strict accordance with harmonic standard IEC61000-4-7 requiring implementation by means of low-pass filters with a 1.5s time constant on each one of the individual harmonics. The instruments also meet IEC61000-4-15 (flicker measurement standard). Power quality parameters include micro-synchrophasors, symmetrical components of phase voltage and current, power factor, predominant harmonics and phase angle, and amplitude and angle information of each harmonic (time-stamped with a one microsecond time resolution). Digital inputs can monitor the status of substation equipment. Two gigabit facilitate local and networked communication.
While in Australia, Mr van Wyk visited a number of major distributors and he reported a lively response to the possibilities seen by network engineers to improve visibility of their systems. Of particular interest to them is the study of overloads, and by having precise data gathered over long intervals, to set new tripping values that deliver both better availability as well power quality.