Power system triad of reliability, security and quality - how these are different, correlate but need specific focus...

Reliability, security and quality are three important attributes of product or service that power networks deliver to their customers. Each being important, adds to the metrics of performance of electricity supply utilities. At times used interchangeably by some industry people but they are distinct and at the same time somewhat interlinked in the ways they need to be addressed.

1. How "reliable" - can we trust utility will keep on supplying electricity in desired quantum as and when needed, so sort of resource adequacy across time scales.
2. How "secure" - if there is a problem with system, can we trust network will recover and full or part blackouts will not be there for a "credible contingency", so reserves adequacy.
3. What "quality" - does utility give a supply that is appropriate for the equipment that we want to use it for, so all three have to be responsible, network planner, consumer and equipment manufacturer.

Reliability is more like across the timeline, if the utility has tied up enough generation to cater to its consumer base and would it be able to meet the temporal demand patterns that its consumers will put onto the system. Of course driven by forecasts and impacted by accuracy of forecasts (net demands on the system, as it has become more relevant post high penetration of embedded solar roof tops!!). It's sort of bottoms up, you start from bottom and expand the area and in the process helped by load diversity.

Security is more like short term, various time scale reserves that need to be provisioned to handle a credible contingency e.g. failure of biggest unit in the network or tripping of lines, typically evaluated based on N-1 scenario. Things like interconnections to adjoining control areas, possibilities of optimizing reserves, electricity market provisions for transaction of reserves make the things bit complex.

It's a sort of urgent and important relation between security and reliability!!

Quality is both now and across the timeline e.g. voltage dips, swells, rapid changes, very short term harmonics evaluation and extending to weekly or annual evaluations of attributes as defined in standard EN50160 or IEEE519 and with very precise measurement methods again defined in measurement standards.

While these are three different attributes addressing three different aspects of electrical utility operation and services but they correlate (particularly short term reliability and security) in several aspects and different in some other, some classic examples are below

1. Inadequacy of short term "reliability" reserves may mean reduced system "security" and maybe reflected in failure in power "quality", frequency conditions as stipulated in EN50160 (real observation in an island country system power quality measurements)
2. Interconnections help improving "reliability" through interregional transfers for inadequacies and balancing, in distributing and optimizing reserves across control areas, helping "security" also but this also means power "quality" impact across control area (fault in one control area leading to voltage DIP in an adjoining area)
3. Harmonic limiting provisions of IEEE519 are more about quality and impact on network assets and consumer equipment's but do not directly relate to security or reliability

Lately, paradigm in which these factors are evaluated has changed a lot and so has the urgency to address and it's ranking in priority.

Reliability was always a topic top of the agenda with short, medium and long term resource adequacy assessment but in a totally different paradigm than what we have as of now. It was more about having enough supplies of fuel (coal, gas and nuclear to the extent it was in resource mix and hydro to the extent it was available), timely build up of adequate generation assets and network build/augmentations. Compare it to what we have as of now, it's more complex with a renewed interest in demand response and flexibility that has probably become an important part of resource adequacy assessment. We are having much closer look at distribution networks, pilots and business models are being undertaken to explore in how far DER's and consumer assets and behavioral changes can help.

Security is probably the most interesting attribute as of now, maybe for the speed at which it has got into everybody's attention because of the changes that are happening in resource mix which has changed the entire thought process (or shall we say brought new perspectives!!). With kind of and location of generation assets that got into system, IBR's, DER's, storage and lot of synchronous assets retiring, entire system dynamics are changing. New assets are reducing grid inertia but faster response times are possible with IBR's, so you can really operate reduced inertia grids, subject to programmed performance of IBR's. Way these new assets respond to disturbances and mitigation measures to ensure that security is not risked (existing interconnection norms being refined, new standards are being formed!!).

Quality was the topic that gained more interest over past 3-4 decades as load types changed that were more vulnerable to supply issues and paradox was at the time partly impacted supply quality. We had power quality assessment practices adapted in distribution networks and it evolved (CIGRE publication 396 e.g.). Standards like EN50160 (or some variations of that) and IEEE519 became common references. Separating from quality in industrial networks where it was always extremely important and focused, quality in utility networks was in different stages of implementation in different geographies, well practiced in most of the developed economies but getting established in developing world.

So, while power quality was just starting to get the attention that it deserved and was expanding in newer markets, but, with major flux in resource mix driven by decarbonization efforts, security and reliability (and sustainability) became such hot topics (more for the topics of discussions than real because in many developing markets resource mix has not yet changed that drastically) that probably quality got a little behind in attention e.g. networks where it was being done (developed economies), it continued in the ways it was being done and remained an important attribute in new changes. But networks where it was just getting started, suddenly reliability and security took center stage.

All these three are topics worth several books in themselves and its's only touching on the surface.

While the broadly sequence as of now goes like, reliability + security passing through quality gate in developed economies, in developing economies it's reliability + security followed by quality (quality, do they consciously care??). But we have to understand that it's an equilateral triangle and all the three aspects have to be focused because that's the kind of grid needed to support growth, economic prosperity and well being.