The US Electrical Grid is a complex integrated structure that is under continuous development. To ensure that reliable development matches the growth in demand, the North American Electric Reliability Corporation (NERC) developed the Standard, TPL-001-1 System Performance Under Normal Conditions. The objective was to establish the basic performance requirements for both generation and transmission. The -1 release of the standard was approved by the Federal Energy Regulatory Commission (FERC) on July 6th, 2012.
How to meet NERC TPL-001-5 Compliance
Like many standards, TPL-001 is under continual review and updating; the current release is TPL-001-5. In this release, Table 1 was updated to identify in Paragraph 13 a list of non-redundant components that need to be taken into account or corrected in the planning process. Paragraph 13 (c) identifies the single station DC supply associated with the protective functions required for “Normal Clearing” as a non-redundant component. This must be updated to a fully redundant DC Power system in all future network expansion.
If an existing DC system is checked for both DC Voltage (charger fail) and Battery Continuity every 24 hours, reporting any change in status to a location that is manned on a 24/7 basis, it will be compliant. This exemption is necessary in order to accommodate a large majority of existing control houses that do not have the wall or floor space for a duplicate DC Power System.
24/7 Battery Monitoring
There are a number of ways in which this monitoring can be implemented. If the utility is using battery monitoring that is compliant with PRC-005-6, including the measurement of DC voltage and using ohmic or float current measurements to verify battery continuity, as long as any change in status is sent to a location that is manned on a 24/7 basis, then the system will also be TPL-001-5 compliant.
For locations where no separate DC power system monitoring exists, then the alarm circuits within the existing charger will typically be the way in which changes to the DC power system status is being reported on a 24/7 basis. The existing charger alarm system will identify and report DC voltage failure but will require additional sensors that will need to be integrated with the charger to identify an open circuit battery.
While this type of solution will comply with the standard, it is simply a patch to cover up the limitations of the existing power system. After all, is the duplication of the existing DC power system design the optimum solution? Based on history, probably not.
Modular Switch Mode Battery Charger Technology
Coincidentally, over fifty years ago, the Communications industry faced similar challenges, and the result was the development of the modular switch mode power system. This enabled high frequency switching circuits for the actual AC to DC converter section of the charger to be reduced in size. For example, a charger that was once wall or floor mounted to facilitate large utility transformers could now be reduced to a shoebox-sized plug-in unit.
The capacity of a modular charger could now be governed by the number of slots available into which the converter units could be plugged in and then managed by a single controller. Redundancy no longer required two completely separate DC power systems, and the concept of a single, fully integrated N+1 configuration became the standard. This is where, by adding one additional converter to the number of converters required to support the load, you achieve full redundancy within a single charger.
In the US, switch mode chargers are now the charger of choice for all communications applications. However, acceptance within the utility industry has been much slower, in part because the environmental conditions at some locations require the use of convection cooled chargers, while the majority of available switch mode chargers are fan cooled.
Battery Continuity Testing for Switch Mode Chargers
Depending on the manufacturer, over the years, the switch mode charger controller has gained other capabilities to create a comprehensive DC power systems controller. One feature that has become a standard is the ability, on a routine basis, to reduce the charger voltage to below the open circuit voltage for a short period of time in order to place the battery under load. This is to ensure that the battery will support the operational load if required.
Should the battery be open circuit, the system voltage will now be at the same voltage as the charger, which will continue to support the load and then return the charger to the original voltage setting with no risk to the operational load. Clearly, this would be fully compliant with the battery continuity requirements of TPL-001-5.
A Fully Compliant Substation Charger Solution
Finally, if you don’t have full battery monitoring, then the optimum design for a substation charger to meet full compliance with the charger requirements in TPL-001-5 will include:
- Switch Mode Design – This will provide full charger redundancy in the same wall space
- Convection Cooled – No fans increase reliability and reduce maintenance in dusty areas
- Modular Design – Eliminates a single point of failure with multiple modules
- Battery Test – The ability to validate battery availability is key to meeting NERC compliance
