By David Gale, Fuseco Power Solutions business development manager
Power reticulation is increasingly affected by high penetration of grid-tied solar PV. Enforcement of only very limited standards by comparison to Germany with a high proportion of solar capacity, for example, is now showing up in problems of phase imbalance and low power factor at distribution transformers.
The need for power factor control as well as phase balancing can be met by employing a solid-state, pulse width modulated (PWM) VAR compensator suitable for pole or pad mounting. Unlike capacitor-based power factor correction, which does not allow for phase balancing, the PWM VAR compensator technology provides for leading as well as lagging power factor compensation. Another important advantage is its ability to respond virtually instantaneously to changes in feed-in energy because of the sudden appearance of cloud cover.
Requirements enforced on grid-tie inverters in Australia, other than for electrical safety, are restricted to anti-islanding, frequency stability, voltage limits, harmonics and DC injection (the bulk of systems being direct coupled). Some supply authorities impose power factor restrictions but this begs an important question – does anybody actually check these limits are observed in individual installations?
Phase imbalance comes about because of the scattered nature of PV installations, and most being single-phase. Householders make a decision to buy, and it’s ‘the luck of the draw’ as to which phase their system will be feeding into. This can easily upset any more or less balanced approach previously taken for connections from local distribution transformers.
The ability for power factor control in grid-tie inverters, where it is a feature (in some European manufactured equipment), is limited by the VA capacity of the DC link. The bulk of inverters basically connect with a power factor of unity or very close to that. Ability to deal with phase difference between voltage and current is a natural outcome of the freewheel diodes connected across the inverter switching transistors but it is not controllable.
Grid-tie systems supplying a large proportion of feed-in during high insolation will therefore cause power factor to deteriorate dramatically, or force the supply authority or company to provide reactive compensation. The alternative is to face voltage regulation issues. These problems can be avoided by correcting power factor at the distribution transformer. A low weight, small physical size solution is offered by the Sinexel SVG VAR compensator, which will also provide phase balancing.
Low switching loss, high temperature semiconductors combined with pulse width modulation (PWM) and control algorithms based on Park-Clarke transformations, providing a simple orthogonal vector presentation of three-phase vectors as the basis for control, are the backbone of the Sinexel SVG VAR compensator. In flexible AC transmission system (FACTS) controllers, the technology has been well established.
The Sinexel SVG three-level compensator, utilising PWM inverter control technology, is designed for LV power factor correction and has important advantages over capacitor based systems, chiefly: infinitely fine control of compensating reactive current; per phase compensation; leading as well as lagging compensation; rapid dynamic response; line voltage independent compensating reactive current; and phase balancing at the LV secondary of distribution transformers.
The use of PWM inverter control technology with lagging or leading correction of load current from -90° to +90° provides a unified method, which can only be approximately simulated by a switched capacitor–inductor paralleled system. Because of its dynamic range and speed of response, the technology is highly suited to power factor correction in areas where there is a high proportion of solar PV installations.