by Vincenzo Balzano, ABB
Faults and outages always have been features of the power network. As well as inconveniencing customers, unplanned outages can cost major utilities several million dollars a year in regulator fines.
In recent years, outage frequency has increased with the rise in grid-connected renewable energy. There is growing demand for improved fault handling, which has the opportunity to reduce costs and improve planning and efficiency for utilities.
One smart grid objective is to improve service continuity by recognizing, locating and isolating faults as quickly as possible while rerouting power through alternative circuits to restore power to as many customers as possible.
Failures are inconvenient, but they also lead to significant costs and affect utility resource planning, efficiency and profitability. The fines and penalties from overseeing bodies such as regulators or public ombudsmen are a major motivator to avoid faults altogether or restore power quickly.
Regulators use two primary types of measurement to evaluate unplanned outages as part of utility performance: System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI). These form the basis of whether government agencies or public utility commissions (PUCs) will levy fines and the level of those fines.
SAIDI is the annual outage duration per customer. It considers the duration of outages and length of time that customers are without power. The clock starts ticking on SAIDI when an outage exceeds a set maximum time after the customer has called to report power failure.
SAIFI concerns the frequency of unplanned outages. It counts the annual number of interruptions per customer. Every outage longer than a set time counts toward SAIFI.
Dividing the duration index SAIDI by the frequency index SAIFI produces an overall reliability measure called the Customer Average Interruption Duration Index (CAIDI). On the basis of these measures, utilities set aside seven-figure budgets every year to pay regulatory fines.
By putting in place better management of faults and outages, utilities can improve their performances on these measures, reduce fines and improve their profitability.
Two approaches can help utilities tackle faults and outages: fault detection isolation and restoration (FDIR) and logic selectivity.
FDIR is focused on increasing grid reliability to decrease the duration of unplanned outages. Its benefits are improved customer service, an enhanced reputation for reliability and greater revenues as customers are connected for longer. Greater reliability reduces restoration costs and the level of fines and lawsuits from long outages.
Logic selectivity drastically reduces the number of outages and their durations. It isolates faults and reroutes power without affecting customers who are not directly affected by the faults. A logic selectivity system may require investment in primary equipment and communication network infrastructure.
For example, primary equipment might include circuit breakers and IEC 61850-enabled protection in secondary substations or pole-mounted reclosers. These would need to be used with a high-performance communication network with low latency to ensure messages are sent and responses made without delay.
Either FDIR or logic selectivity triggers remedial actions to restore power on a peer-to-peer basis with switchgear closing circuits in unison or through centralized control across the distribution grid.
To monitor and rectify grid outages, intelligent grid automation equipment is necessary. Experienced operators know that a one-size-fits-all approach does not work.
It’s helpful to use a model to describe the four functional levels of automation (see Figure 1):
- Level 1 is the most basic solution. It provides monitoring of the entire secondary substation, as well as current, voltage and energy measurement on the low-voltage side.
- Level 2 enables fault isolation through control of medium- and low-voltage primary apparatus. FDIR is achievable at Level 2 through wireless control devices.
- Level 3 features accurate current, voltage and energy measurement on the medium-voltage side. Intelligent electronic devices (IEDs) and instrumentation at this level can enable management of power flows, which becomes more important as distributed generation is added to the grid.
- Level 4 is the most technically complete and enables logic selectivity. The sophisticated protection and control can accurately isolate the fault branch in milliseconds by opening the adjacent circuit breakers, as opposed to minutes when taking the FDIR approach.
Logic selectivity requires high-speed communication and benefits from IEC 61850, which can perform peer-to-peer communication, ensuring the same message is received by multiple devices within tens of milliseconds (see Figure 2). When a failure occurs, the protection relays near the fault communicate with one another and trigger a selection algorithm only to the substations immediately upstream and downstream of the fault to open the appropriate breakers.
This technique contains the outage to the fault-affected area, minimizes the number and duration of outages on the secondary grid and protects a utility’s SAIDI, SAIFI and CAIDI indices.
Vincenzo Balzano is smart grid program manager for ABB power products in Dalmine, Italy.