Using float current to detect open circuits on single station DC supply battery
On April 1st, 2023, the NERC standard TPL-001-5 will take partial effect in the United States. The FERC Order 867 is approving the updated standard addressing a protection system’s single point of failure. As it stands, non-redundant components of a protection system such as a station DC supply with only one charger and one battery string, is considered a single point of failure.
Adding a redundant DC supply system to eliminate this single point of failure for some substation owners may be difficult and, to some extent, impossible to do for various design constraints. The provisioning cost of this addition could exceed $50 000, making it difficult to obtain funding.
However, based on Footnote 13c, a non-redundant station DC supply can be excluded if it is monitored 24/7 for low voltages and open circuits, and reported to a control center. A retrofit solution to monitor low-voltage and open-circuit conditions may be considered too expensive for many; however, what if you could have access to a proven and reliable solution for around $700 per station DC supply battery?
Float current monitoring uses
As of today, all legacy battery chargers are equipped with built-in high and low float voltage alarm relays, which should be already wired to your substation’s remote telemetry unit and reporting 24/7 to the control center. As this feature is already in place, it will enable you to meet half the requirements concerning the “low voltage conditions”.
The second half is related to the “open circuit requirement”, which is the verification of battery continuity, a requirement you also can find in the PRC-005-6 standard; it is stated that battery continuity should be verified every 18 months. It is common knowledge that monitoring the float current is the best and foremost reliable method for verifying battery continuity as it proves that the battery charger is connected to the batteries, all intercell connections are satisfactory, and batteries are fully charged.
The float current is associated with any battery lead acid chemistry, such as Vented Lead-Acid (VLA), Valve Regulated Lead-Acid (VRLA), and Vented Nickel-Cadmium (NiCd) technologies. The float current flows through each cell/unit of a battery and maintains the cell/unit fully charged as long as a constant float voltage is present. Otherwise, the battery will self-discharge and run out of energy.
Depending on the battery technology and manufacturer specifications, you would typically apply a constant float voltage to the battery bank, somewhere around 2.25Vdc for VRLA and down to 2.17Vdc for VLA. The constant float voltage applied to the battery load terminals will produce a small charging current in the milliamp range. A good rule of thumb for VRLA is 10mA of float current for each 100Ah capacity. The float current will maintain the battery fully-charged until a discharge event occurs, at which time the entire substation DC load will be energized from the battery for a pre-determined reserve time. On the other hand, the discharge and recharge current could be 10 thousand times higher than the float current depending on the DC supply system designed.
As required in the PRC-005-6 standard, it is possible to periodically verify battery continuity manually. You could simply turn off the battery charger to measure discharge current using a clamp on the battery load terminals and verify that the current is flowing to substation DC load. The pass or fail result can be recorded in the maintenance form. However, what if you want to verify battery continuity on a 24/7 basis?
Using float current as a means to continuously monitor for open circuit/battery continuity is not as difficult and expensive as you may think. In fact, with the right tool, it is simple, practical and inexpensive – thanks to Multitel’s non-intrusive Float Charging Current Probe.
How does our float current monitoring solution work?
The FCCP is a patented and reliable solution with proven sensing technology that easily interfaces with your existing telemetry substation.
Why use Multitel’s float current monitoring solution
Here are some of the benefits of Multitel’s FCCP:
- Measure float current precisely with 1 mA resolution while enduring or not impacted by elevated discharge currents.
- Split core current transducer design enables non-intrusive retrofit installation which can be performed by your field technicians.
- Low current alarm user setting to provide open-circuit/battery continuity alarms.
- Smart high current alarms that prevent false alarms when the discharge and recharge current are present.
- The high current setting can signal the presence of an imminent VRLA thermal runway event.
- Provides two (2) relay contacts to interface with any RTU or SCADA telemetry systems.