Demand response

What happens if there is a mains failure during a demand response event?

Author: Jill Cox
9 Sep 2015

In a business relationship, like any relationship, there is a moment when you stare each other in the eye and ask the hard questions. In demand response (DR), the question is this:


“What happens if there is a mains failure during a Demand Response event?”


This is one of the obvious concerns that a site owner will raise during discussions about demand response. At its core, this is about how demand response affects operational or mechanical risk. “Am I putting my site at risk of blackout or risking my generators by participating in demand-side response"?

For Flexitricity, the question is a vital and often very fruitful stage in project appraisal. Every site is different, so answering this question can reveal much about current site operation. There are three main categories of site to consider.


Flexible load – Where the site will provide DR by load curtailment or load increase, then the effect of a mains failure is no different to its effect without DR. Perhaps the only consideration is whether a mains failure after a long DR event could put a process parameter – such as a critical temperature in a cold store – at risk. This is best handled by ensuring that process limits have adequate headroom, and by carefully choosing which loads participate.


CHP – On many sites, gas-fuelled combined heat & power (CHP) generation provides all or part of the electricity requirement. Here, DR means using space in operational schedules, or spare headroom capacity, for DR. But very few CHP generators are configured as standby generators, because they can’t deliver the rapid power variations that a site needs when it’s disconnected from the mains. Normally, protection systems will be set to trip the CHP if the mains fails. It doesn’t matter whether the CHP was running for demand response or according to normal site schedules when this happens.


Standby generation – Most of the complexity around mains failures is found at sites which use standby generators to provide DR. This is where Flexitricity’s engineers spend much of their time in site appraisal and development.


The first thing to note is that using a standby generator for DR makes it much more likely to succeed in its core role as an emergency power supply. According to one industry estimate, more than half of all standby generators in GB are unreliable. This comes from neglect and poor testing regimes. DR reverses this.

Preparing a standby generator for a DR role usually involves changing the way it interacts with the mains. There are two preferred configurations: full parallel and short-term parallel.

With permission from the local distribution network operator, generators can run at fixed output while the site remains connected to the mains supply. Excess electricity is exported from the site, or top-up electricity is supplied by the mains. This arrangement is known as a “full parallel” or “long-term parallel”. If the mains fails during a DR event, the generator will already be supporting site load.

When the mains fails, it’s vital that the generator only supports the local site, and doesn’t try to feed other nearby customers who have also been affected by the power cut. That’s what G59 protection is for. If the G59 scheme is carefully chosen, the site can continue with normal business with no interruption, and return to mains power as soon as the power cut is over.

At full parallel sites, step load capability is an important consideration – the site’s consumption at the moment of the power cut is unlikely to be matched to the output of the generator at the time, so the generator is going to have to adjust its power output quickly. This will be well within the generator’s capability if it’s been appropriately chosen for the site.

In other cases, the local network operator may only grant permission for the generator to run together with the mains for a period of minutes. This is called short-term parallel. A mains failure during a DR event will have no effect on the site, as the generator will already be supporting site load with the mains disconnected. Flexitricity’s defensive engineering principles ensure that at the end of the event, the generator will continue to support load until the mains supply is restored and healthy. Nevertheless, it’s also vital to design the scheme to cater for a mains failure during the short periods of synchronisation at the start and end of the event.

Flexitricity’s leadership in DR aggregation is in part attributable to the confidence that our customers have in our engineering principles. Our engineers tackle the big questions directly, our IT systems are designed to be robust yet flexible, and our 24-hour control room staff are trained to deal with the nuances of each site. We never stop investing in and developing our technology and our processes. And the more questions you ask us, the better we get.


Written by Jill Cox, Sales Operations Manager at Flexitricity



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