INTRODUCTION

Good question, and one your insurance company or investors are likely to ask and want documentation on.

The heart of the safety system that allows for an orderly and safe generating unit shutdown is the station battery and associated DC system. The generating unit will sustain major or even catastrophic damage if the station battery and associated DC system fail to perform as specified in the system design criteria.

How can you determine if your generation plant can safely survive an unplanned or emergency trip without risking plant damage? By conducting properly designed and performed battery capacity tests at the appropriate frequency and by conducting proper battery maintenance in accordance with the appropriate IEEE standards. We’ll illustrate why capacity testing of the station battery and associated DC system is essential to safe generation plant operation and how to properly perform such tests.

WHY CAPACITY TEST?

Capacity testing of stationary battery systems has been a key component of comprehensive preventative maintenance programs since the first version of IEEE 450 was issued in 1972. Today, capacity testing is required by NERC Standard PRC-005-2 and a key component of IEEE standards 450-2010, 1188-2005, 1106-2005. Properly conducted capacity tests are the only method of determining and quantifying stationary battery discharge performance. Internal ohmic measurements, (impedance or internal resistance measurements), while valuable, do not measure battery/cell capacity and are not a replacement for capacity tests.

Properly designed and performed capacity tests will prove that the battery can, or cannot, support a worst-case trip condition, as well as determine how much useable life is left in a battery system thus allowing for planning and budgeting stationary battery replacement. Capacity testing also provides baseline performance data, aids in identifying manufacturing defects, installation deficiencies, and incipient problems not detectable by other means.

DESIGNING THE CAPACITY TEST

The capacity test design is crucial to prove the battery will support worst-case emergency loads and measure battery capacity. This is because batteries can produce different measured capacities at different discharge rates. This effect becomes more pronounced as the battery ages. We need to design a test that encompasses the battery duty cycle to prove the battery will, or won’t, support the connected emergency loads and measure the battery capacity. In essence, we want to design a test that objectively and repeatably measures battery capacity as it is used in a specific plant or generating unit.

A capacity test is basically a constant load discharge of the battery to a specified end voltage. Unfortunately, generation plants don’t have a constant DC load during an emergency; they have a complex DC load profile. So, a Type -1 “Modified Performance Test” is designed, as defined in IEEE/ANSI Std. 450-2002 (the latest version of Std. 450 is 2010).

As shown in the table “Typical Load Profile”, the highest loads occur during the first minute of an outage as the emergency pumps start, circuit breakers trip and other devices operate. The load drops after the first minute as the battery has to run only the emergency pumps and various controls over a time period that usually stretches for hours.

Typical Load Profile
Period Time (in minutes) Current Amps
1 1 1200
2 29 500
3 150 350
4 59 275
5 1 400

The most critical portion of this load profile is the first minute. That’s because all of the emergency pumps start during these 60 seconds. It won’t matter how long the battery can run the pumps if the battery can’t supply the energy required to start the emergency pumps. Therefore, the test must require the battery to supply the first minute’s currents, but only for one minute. A battery capable of supporting this load profile, is capable of supporting 1,200 amps for 58 minutes to an end voltage of 1.75 volts per cell. Why not just test the battery at this rate? Because this would be more severe than the duty cycle and would likely result in premature replacement because high rate capacity generally falls off earlier in battery life than low rate capacity.

The first minute of the test should require the battery to supply 1,200 amps. This proves the battery can start the emergency pumps while supporting other loads. We prove the battery can run the emergency pumps and other loads for the required time and also measure the battery capacity by examining the load profile and the battery manufacturer’s discharge ratings.

We then find a discharge rate that envelopes the remainder of the duty cycle. In this case, the next-highest current is 500 amps and the total duty cycle is four hours. The battery manufacturer’s published discharge ratings show that this battery is rated to support a load of 504 amps for four hours to an end voltage of 1.75 volts per cell.

This means the “Modified Performance Test” will require the battery to supply 1,200 amps for one minute and then transition to the battery manufacturer’s published four-hour discharge rate to 1.75 volts per cell (504 amps). This test covers the entire battery duty cycle, proving that the battery can, or cannot, support the worst-case emergency loads and measures battery capacity.

This is the first post of a two-part series on the role of capacity testing in proving a battery will support worst-case emergency loads and measuring battery capacity. Coming up in Part 2 of this two-part series, we will cover Test Equipment, Test Procedure and Test Frequency. The second post will appear here on our blog in the next few days.

Authors: Michael P. O’Brien is Technical Services Manager and Bryan Dardar is Vice President Stationary Service for Nolan Power Group.