The proliferation of digital technologies in power distribution systems has given rise to advances on two fronts. “Digitally enabled” devices are on offer today that pack more and more functionality into edge devices like switchgear and circuit breakers.
Meanwhile, advanced centralized protection and control (CPC) schemes are available that roll up multiple devices—of any vintage—via merging units to provide protection and control as well as fault monitoring and event recording across larger distribution systems. There is a role for both.
Complexity Abounds
If you look at a typical electrical grid, you might find electromechanical, solid state, or first- and second-generation microprocessor relays, or even a mix of all of the above. Different manufacturers, different communications protocols, and non-standard customizations used to meet particular utility or customer needs all add complexity.
At the same time, a range of larger forces are at work, and these all carry implications for protection and control strategy. These include things like the addition of distributed energy resources (DERs), including storage, a shift from radial to ring configuration, and the conversion of overhead distribution lines to underground cables.
These trends are aimed at improving reliability and resiliency. Grid modernization calls for additional and/or new protection and control functions to adequately monitor and protect the evolving grid, and that’s where centralized protection and control comes in.
The CPC Concept
The basic idea behind centralization is to move protection and control from multiple bay-level devices to a single central processing unit. This is possible thanks to the IEC61850 protocol for substation communications, which allows us to share information between bays or up to a CPC device at the substation level where we can apply modern computing capabilities to software-defined protection and control functions. These functions work as individual application packages, so new elements can be added while the device is installed, with no hardware changes.
CPC architecture comprises two approaches. Centralized P&C schemes use a stand-alone merging unit (MU) in each bay that is connected to a CPC device. The unit converts analog values like voltage and current at the primary equipment to digital and passes them to the CPC unit where the data is analyzed using on-board algorithms. The CPC can then send control actions back down via the merging unit to the edge equipment.
Hybrid P&C scheme (HPCS) also rely on the open protocol, specifically relays supporting IEC 61850-9-2LE publishing that act as intelligent merging units (IMUs). These devices are capable of performing some analysis right at the edge, or for algorithms that require data from more devices, they can send their information to the CPC. IMUs are able to do more on their own in part because they can see what’s going on in other places within the substation and can communicate directly with their peer devices. Such smart relays can also function as a backup to the CPC unit.
HPCS offers grid operators options. They can roll up older relays to a dedicated MU that is in turn connected to the CPC device, or they can opt for newer relays with broader functionality that are still part of a CPC scheme. The former can be a good choice from a cost perspective since MUs typically cost less than the relays they support, thus allowing the asset owner to get more out of their investments.
CPC vs HCPC
The list of benefits for centralized protection and control is long and compelling. Greater flexibility, more functionality (without new hardware), better user interfaces including remote access, and the ability to introduce advanced P&C functions are just a few. CPC also offers functional, physical, and communication redundancy for the protection and control scheme in the substation, and the CPC unit can also be used as a centralized disturbance, fault monitoring, event recording system replacing traditional fault monitoring systems.
Hybrid architecture brings the best of both de-centralized and centralized approaches. Greenfield applications can be planned with IMUs for the P&C system at each bay level while the CPC unit would be placed at the central level to act as the fault monitoring device.
Reluctance to move to a centralized approach often centers on concerns about communications latency, lack of redundant communications and/or the lack of a CPC unit in the given site. For CPC, the modifications to the protection device might cause downtime for the substation if the device needs to be taken out of use. A redundant unit can provide protection while main is offline for config changes, updates, testing.
The choice between CPC and hybrid schemes must be made on an individual, project-by-project basis. Grid owners should ask themselves questions like:
- How many low-voltage devices need to be monitored/controlled?
- How much monitoring/control functionality is required?
- Do you need switchgear or are boards and panels adequate?
- Does the facility owner already have assets in place that can be leveraged (e.g., SCADA to connect LV devices)?
- Is the installation likely to undergo expansions or changes, or will it remain in place for the foreseeable future?
There is no one-size-fits-all approach, but the availability of advanced digital relays, merging units and CPC devices that leverage the IEC61850 standard gives utilities and industrial grid owners options they didn’t have before. Facilities like data centers are now beginning to realize the value of CPC and utilities are going one better by starting the process of virtualizing their P&C systems, bringing monitoring, analysis and control into the control room. Given the advantages—and critically, the flexibility—that centralized protection and control brings, it’s likely we’ll see broader adoption of these technologies in the near future.
