Mobile Backup Battery Bank Streamlines Discharge-Testing

by Hugo Ordoñez Ruiz, Henry Barrios García and Ernesto Hernández Peña – Nuevo Pemex, Tabasco, Mexico

The Challenge

We wanted to find a way to reliably perform battery discharge-testing as stipulated by the IEEE as well
as NERC standard PRC-005-02 at the recommended intervals without jeopardizing the Nuevo Pemex
cogeneration plant’s integrity.
Located in the state of Tabasco, Mexico, Nuevo Pemex is equipped with two GE MS7001FA combustion
turbines, each with a single pressure stage HRSG. In cogenerating configuration, the plant generates 277
MW and 1.76 million lbs/hr of steam.

Plant photo here

Currently, Nuevo Pemex has 10 banks of batteries:

  • Two 125 VDC 1300 AH for DC distribution system A/B
  • One 125 VDC 1500 AH for Main UPS
  • One 125 VDC 600 AH for Secondary UPS
  • One 125 VDC 600 AH for DC systems for the gas turbine controls (PECC U1)
  • One 125 VDC 600 AH for DC systems for the gas turbine controls (PECC U2)
  • Three 125 VDC 300AH for the electrical S/S
  • One 125 VDC 190 AH in the fuel gas ERM

Battery banks provide the ultimate defense against a catastrophe. They supply backup power that
ensures the availability of the emergency lubrication systems for the turbine bearings, the hydrogencooled electric generator’s sealing system, and other critical equipment that cannot go off line for even
a brief period of time without grave consequences.
The capacity of such batteries can decrease significantly before reaching their life expectancy: high
ambient temperatures, dust, loose contacts, overload and other conditions all take their toll. It’s
therefore critical to check the batteries at regular intervals, and the only reliable way to measure their
capacity is discharge-testing. However, if we tested during routine operations, we would have to briefly
disable the battery bank, leaving the system unprotected by its backup power.
Unfortunately, battery banks are not typically designed with built-in redundant systems, and it would
require a huge investment to retrofit each of our 10 existing banks with its own backup unit.

The Solution

We put our heads together and came up with the idea of building a mobile backup battery bank that we
could use to provide redundant power while discharge-testing each battery bank – rather than taking
the plant off line or waiting until our next planned outage. To determine the capacity of the bank, we
based it on the highest ‘real’ consumers of all the installed banks (allowing for the fact that some of the
banks are oversized). We determined that a 125 VDC 600 AH bank would serve the purpose.

Figure 1 – Layout of mobile battery bank

We selected a metal container that measures 19 X 7 X 8 feet to accommodate the batteries (figures 1
and 2). The mobile bank is divided in two sections: one of them houses the rack of 60 SBS 900 batteries
that make up the 125 VDC 600 AH bank, and the other houses the charger.
The shaded zone in figure 1 is equipped with lighting, accessories, HVAC and instrumentation that meet
Class 1, Division 2, explosion-proof requirements. HVAC-1 is a 24,000 btu/hr capacity heating/cooling
unit; the TIs are two local temperature gauges; XE is a stand-alone gas monitor with highly visible LED
indicators, piezo horn and strobe lamp. EX is a 12-inch vertical extractor activated automatically by the
XE to extract explosive gases from the enclosure.
The unshaded zone at the bottom of figure 1 houses an AT-30 CPU-controlled battery charger; a
distribution panel (PB) that feeds all of the installed equipment; and HVAC-2, a non-explosion-proof
12,000 btu/hr heating/cooling unit.

Figure 2 – Mobile container

The Results

We came up with a cost-effective way to conduct discharge-testing without taking the plant off line or
waiting for our next planned outage. Our portable battery bank complies with the IEEE and NERC
standards for discharge-testing batteries in a safe, reliable way.