Compensating the Changing Reactive Power in the Medium-Voltage Grid in Stockholm

Sammanfattning: A decline in inductive reactive power has been noted by several DNOs around Europe for the last decade. Why this is happening is unknown. One of the DNOs is the Swedish DNO Ellevio AB that has seen large decreases in demand of inductive reactive power from their primary substations. The capacitor banks that are installed at the primary substations have historically been used to compensate the inductive load with capacitive reactive power. However, with a declining inductive load, the need for VAr compensation is diminishing. This thesis sets out to find out why Ellevio has been seeing a decrease in inductive reactive power and how the VAr compensation should be carried out in the future when the load is becoming more capacitive. The results from the thesis show that the decrease can be attributed to technological changes in lighting, motors and electronic loads as well as a change in consumer behavior. Lighting technology has gone from being a resistive and inductive load to a capacitive one. This is due to the phasing out of incadescent lightbulbs and fluorescent lighting with magnetic ballasts and being replaced by compact fluorescent light (CFL) with electronic ballasts and Light Emitting Diodes (LED). Furthermore, variable frequency drives are being used more frequently for controlling pumps and fans which results in the motor load being seen as capacitive from the grid side. The share of the electricity consumption going to power electronic loads, which tend to be capacitive, has also increased for the last decades. The phasing out of older technologies with newer ones results in the medium-voltage grid slowly becoming more capacitive.  An optimal sizing of capacitor banks was found for all the primary substations in Stockholm for three scenarios: compensating just the primary-substation load reactive power, as currently done; compensating also the primary-substation transformers; and compensating load and transformers with one transformer out of service. Furthermore, a comparison between controlling the capacitor banks from the primary substations to controlling them from the bulk supply point substation. The results indicate that the compensation of capacitive reactive power can be controlled in a more efficient way from the bulk supply point with a fewer amount of switches from the capacitor banks.