Eskom reaps returns on DPI investment

Eskom, Africa`s largest power generating corporation, has eliminated costly voltage dips, which were once an ongoing problem, through a system-wide dip-proofing project.

Prior to 1990, Eskom experienced many trips in its large power stations during voltage depressions on the auxiliary supply system. The dips, introduced by faults on the auxiliary system or the station`s interconnected high voltage transmission network, interrupted the operation of auxiliary plants, causing units to be tripped.

A fault in the high voltage yard could lead to trips at more than one unit, referred to as multiple unit tripping (MUT). An investigation into MUT sources indicated that more than 50 percent of these incidents were caused by short circuits in the high voltage yard.

Due to small amounts of reserve on the Eskom interconnected power system (IPS), loss of more than one 600-MW unit could result in severe deviations in system frequency. The MUTs were a major risk to secure operation of the IPS.

In 1990, Eskom introduced protection against voltage dips at its large power stations. The auto tripping and reclosing voltage dip-proofing system on the switchgear boards was removed and voltage dip-proofing inverters (DPIs) were installed. The auto reclose systems were unreliable, difficult to maintain and not effective for all types of dips, a spokesman said.

Factors influencing the utility`s decision to try DPIs included:

A relatively strong supply and short fault clearing times at power stations excluded possibility of motors stalling when held on the system during a voltage dip, stresses to drives as well as the power supply system are reduced when holding contactors closed,

DPIs were simple to introduce, maintenance free and did not require complicated studies during the design phase, disturbance to the generating process is minimized when holding contactors close, enhancing the probability of the unit riding through the dip.

Cost effective

Before dip-proofing was introduced, a voltage dip related trip of a 600-MW unit occurred every 34 days, costing approximately (US)$30,000 to restart the unit and (US) $43,000 to replace generation.

Since the installation of DPIs, no voltage dip related trip or MUT has occurred, saving the utility approximately (US)$3.42 million. The voltage dip-proofing project cost (US)$1.39 million, and gave the utility an investment return time of approximately one year.