For steady state disturbances, such as harmonics, flicker and unbalance, two categories of indices can be distinguished given their use:
• Indices for planning levels: internal quality objectives;
• Indices for voltage characteristics: external reporting system performance. In reporting voltage characteristics, two level of indices can also be distinguished:
• Site indices: the performance at a specific site;
• System indices: the performance of a system.
Recommended indices for planning levels are similar to those given in IEC 61000-3-6 and 61000-3-7 with the difference that one or more of the recommended indices can be used. Maximum values are not rec- ommended however due to the possibility of their being inflated by transients (instead 99% values may be used). In the case of flicker, 10-min values should be sufficient for planning purposes. Because of the simi- larity of effects caused by harmonics and voltage unbalance (thermal effects) the working group recom- mends using similar indices for both.
For voltage characteristics/site indices, considering practical reasons such as allowing easy comparison of voltage characteristics between LV-MV and HV-EHV and also for having common monitoring methods, the WG recommends indices for HV-EHV that are similar to those given in EN 50160.
The recommended system index is the value of the site index not exceeded for a high-percentage of sites, for each individual index and parameter. Examples of high-percentile sites are 90, 95 and 99%. The choice of the percentage of sites for the system index a matter of agreement between the system operator and the regulator.
Objectives for planning levels are similar to those already given in IEC 61000-3-6 and IEC 61000-3-7 for harmonics and flicker. For flicker, it is also recommended to weight the planning levels at HV-EHV by tak- ing into account the transfer coefficient between EHV, HV, MV and LV where flicker sensitive lighting equipment is connected. It is noted that planning levels should remain indicative values that can be adapted to different system characteristics and circumstances.
Objectives recommended for voltage characteristics at HV-EHV are primarily based on the measurement results collected by the WG and summarized in chapter 3. For instance, for 5th harmonic voltage character- istic at HV-EHV a value of 3% is recommended as most surveys showed results between 2,5% and 3%.
For voltage dips, the basic characteristics are the retained voltage and the duration for each dip and it is recommended to calculate these according to IEC 61000-4-30. Site indices can be presented in a number of different ways as in the form of a voltage-dip table, as a contour chart, using SARFI indices, etc. Other details of the characterisation method used, such as the reference voltage to calculate the remaining volt- age, the time and phase aggregation method, etc., should be declared. Detailed indications are also given on time aggregation methods.
Voltage dips system indices can be calculated from the value not exceeded by a high percentage of sites (ex.95% of sites) the actual percentage of sites is to be agreed with the system operator. Weighting factors can be introduced to take into account the sites not monitored and the difference in importance between different sites. It is also recommended, particularly at HV-EHV, and in non-solidly earthed systems, that
voltage dips measurements use the phase-to-phase voltages which give an image that is closer to what the end-users equipment see.
At this stage, it not possible to give appropriate objective values for any of the voltage-dip indices. Indices can only be used to identify typical levels of disturbances for different types of sites and as such a feed- back for improvements towards the network operator.
For long interruption indices, recommendations for MV systems are to refer to IEEE 1366 which rec- ommends indices such as SAIFI, MAIFI, SAIDI, CAIDI.
And for HV-EHV transmission systems, three categories of interruption indices are presented in order to take into consideration different reporting requirements describing different aspects of interruption per- formance, i.e:
• Connection Point Interruption Performance (CPI);
• End-Customer Load Interruption Performance (CLI);
• System Interrupted Energy Performance.
It is generally recommended that companies select one of the three philosophies for system reporting pur- poses and use the set of indices defined for the selected philosophy.
The first philosophy is addressing the need, in fully liberalised electricity markets, for the interruption per- formance of a transmission company to be described and reported separately from the (negative or posi- tive) impact of the networks on its customers. Connection point interruption indices address this aspect by describing the reliability and availability of all individual interfaces between the transmission system and the networks of its various customers.
The second philosophy is addressing the need for transmission system reporting performance on the same basis as often used by distribution system operators. Thus End-customer interruption performance indices describe the reliability of the supply for transmission-caused events to actual end customers (i.e. taking into consideration the effect of the underlying distribution system). This requires information from Distribution customers on the number of customers affected, and the duration (particularly in the case of stepped restoration).
The third philosophy is actually similar to the second one in that the actual impact on end-customer inter- ruption indices is approximated using the estimated energy not supplied (EENS in MWh) and the size of the load interrupted (MW), so avoiding the need to gather detailed information about the distribution cus- tomers.
Objectives for long interruptions for site or system indices cannot be given due to the large difference in network topologies (radial vs. meshed, location of generators) and operational conditions (environment, geography). The dominant benchmarking approach for Transmission companies is therefore based on his- torical performance of the company itself. Use of the proposed indices will facilitate future benchmarking of utilities that have similar networks and operating environments.
Finally, this report should also send a message to equipment manufacturers to provide monitoring equip- ment able to measure the different power quality indices recommended in this report, in particular the need to calculate daily and weekly percentile values from 3-second or 10-minute values, without necessarily storing unmanageable quantities of raw data. It is equally important that monitoring equipment integrates the flagging concept introduced in the recent IEC 61000-4-30.