In the final part of our ‘Complexity of Renewables’ series, we look at why storage assets need to be more comprehensively considered in security standards to reflect their dual role as non-synchronous generators or, when charging, as a load on the network. PSC’s Graeme Hutchison defines the new standards that are needed.
Check out the other topics here, or scroll to the links at the bottom of the page.
New security standards for a new energy landscape
Inevitably, the ever-increasing deployment of renewable energy sources is changing the energy landscape in many ways. The non-dispatchable nature of renewable resources in power systems leads to an inherent need for storage. But how do we ensure that the security standards we use with grid-connected storage solutions are fit for purpose?
While a storage system in charging mode appears to the network as a load, the social and economic consequences of an interruption to its connection differ from those caused by an interruption to consumer loads.
For this reason, one logical solution is for storage connections to be treated as a separate section of any Security Standard. In addition, the definition of load groups for load security should not include storage systems when they are in charging mode.
The dual role of storage systems
We already know that the intermittent nature of renewable generation means that significant volumes of energy storage are necessary for the correct functioning of the network. When exporting, this appears to the network as non-synchronous generation (NSG), with all the consequent considerations of infeed loss risk for frequency response. However, storage would appear on the network when charging as a load.
Consequences of loss of load
In a conventional AC power system, the consequence of the loss of a load following a fault is a rise in frequency. In future grids – which may be dominated by power electronics – the consequences will depend on the control configuration of converters and other devices.
If the power imbalance in the local network is transferred to a remote interconnected AC network – either by using a common frequency reference for all inverter-generated grids or other means – then a loss of load will lead to a rise in frequency. However, if the local frequency is maintained regardless of the power imbalance, the consequence will be a rise in voltage.
In either case, the network must be designed so that an appropriate response and reserve are in place to contain the consequence within acceptable limits.
Recognizing outfeed loss in security standards
As power systems continue to evolve rapidly, now would be a prudent time to define two terms that address two types of risk: Normal Outfeed Loss Risk (NOLR) and Infrequent Outfeed Loss Risk (IOLR).
The definition of a NOLR could easily be based on the frequency control response and active power reserves for frequency events. These are already required to ensure frequency levels are kept within defined values for the most probable events that disconnect loads or storage connections in their charging state.
In practice, the NOLR would correspond to the size of the largest storage facility that can be connected so that a single circuit breaker controls it. Alternatively, it could be controlled by a combination of circuit breakers opening together to clear a fault at a single point.
The IOLR would, as its name suggests, be based on less probable events that exceed the values defined for the NOLR. This would correspond, for example, to the loss of a complete storage installation at a single site.
Security standards fit for purpose
A frequency or voltage rise (depending on the detailed control logic for inverter-formed grids) will occur in response to a loss of load, whether this consists of charging storage units or end-user load. However, there are additional requirements for the end-user load to meet the load’s security expectations.
These requirements would not be appropriate for energy storage facilities, where the power exchange with the network is much more likely to be managed as an operational tool by the system operator. So, the PSC recommendation is that these requirements for storage connections and end-use load should be addressed separately in different sections of the associated security standard.
Ultimately, the security standards we apply should reflect the practical realities of any power system. In power systems incorporating renewable sources of generation, storage is an essential element. Security standards should reflect this by adopting NOLR and IOLR definitions that are separate from standards applied to end-use loads.
The complexity of renewables series links:
Part 1: System performance metrics
Part 2: Control interaction
Part 3: Dynamic equivalents
Part 4: Fault ride through
Part 5: Reactive power
Part 6: Frequency response
Part 7: Power quality
Part 8: Transient stability
Part 9: Protection