## 2 Comparison between Active and Passive Networks
### Traditional Planning, Design, and Operation of the Distribution Network  
For distribution networks to fulfill their intended function, they have to be managed appropriately, and this role is carried out by the distribution network operator (DNO)and normally regulated by a national regulatory authority (NERSA). Thus, according to a European Commission directive (Article 25(1), Directive 2009/72/EC), the DNO is \"responsible for ensuring the long-term ability of the system to meet reasonable demands for the distribution of electricity and for operating, maintaining and developing under economic conditions a secure, reliable, and efficient electricity distribution system in their area with due regard for the environment and energy efficiency.\" And according to the South African Distribution Code: System Operating Code (NERSA, 2014), the DNO (referred to in the document as the \"Distributor\") is mandated to \"so operate the distribution system as to achieve the highest degree of reliability, to coordinate voltage control and demand control, and to monitor (system) security to assure safe, reliable, and economical system operation\". From the two excerpts above, it could be stated that the DNO is tasked with operating the electric distribution supply system to maintain the security, reliability, and high quality of supply and achieve the above-stated goal in an efficient, economical, and (environmentally) sustainable manner. Network security and reliability of supply are two closely related measures of performance, as they both characterize the network's ability to meet load demand at any time and without interruption [23]. The nature of the distribution network load is such that it varies considerably and in a largely unpredictable pattern over time (daily and/or seasonally), and network security requires that enough capacity is maintained in the system not only to meet the peak demand but also to provide for uncertainty in generation availability as well as unpredicted demand increase. Reliability of supply also requires that supply interruptions (typically due to faulty conditions on the network) be limited both in extent (i.e., number of affected customers) and duration. Several performance indices are employed in evaluating the reliability of supply, prominent among them being the loss of load probability (LOLP), specifying the probability (or risk) of the available generation capacity being insufficient to meet peak demand, loss of load expectation (LOLE), reserve margin and expected energy not served (EENS). In the case of disturbances leading to supply interruption, the following performance indices are commonly used as a measure of the actual extent of the impact on customers' system average interruption frequency index (SAIFI), a measure of the amount of supply interruptions at a given voltage level over a certain period, system average interruption duration index (SAIDI), indicating the average duration of the supply interruptions (that are expressed by the SAIFI), customer average interruption duration index (CAIDI), quantifying the average duration of the supply interruptions for a given customer type. These indices are mainly used in network planning, particularly to determine the generation capacity required to deliver the desired level of supply reliability as expressed through one or more of the indices. Following network security and reliability of supply, the quality of supply is another very important aspect of distribution network operation. In the NER Directive on Power Quality [23], power quality is defined as \"all waveform variations from the ideal 50 Hz sinusoidal waveform (including dips and interruptions)\". This definition of power quality is made concerning the quality of the voltage waveform (in terms of the magnitude, distortion, imbalance, and fluctuation), as well as voltage events (such as dips, sags, interruptions, and rapid voltage changes), as further specified in the directive.