The three subsystems interact through voltage. The battery sets the voltage during a discharge, the charger sets it during normal operation, and the protection scheme depends on that voltage being high enough to operate the trip coils and low enough not to stress them. Coordination is a voltage question, and it is answered by measuring the voltage at the trip circuit rather than at the battery terminals.

The three subsystems and how they interact

The battery provides the stored energy. Its capacity determines how long the DC system can support the load with the AC supply lost, and its internal resistance determines how much the terminal voltage falls as current is drawn.

The charger maintains the battery in a charged state and supplies the standing load. Its regulation behaviour determines the voltage the system sees during normal operation, and its current limit determines how quickly it can restore the battery after a discharge. Its response to the loss and restoration of the AC supply determines whether the transition happens without a disturbance to the DC system.

The protection and control scheme consumes the energy. Trip coils, protection relay supplies, indication, alarms and communication equipment all draw from the same system, and their operation depends on the voltage remaining within a range during a fault, which is the moment at which the battery is likely to be at its lowest state and the load is at its highest.

Battery capacity as the baseline

The battery commissioning test establishes the capacity the system can rely on, and it should be performed at a defined rate to a defined end voltage with the individual cell or block voltages recorded. The result becomes the reference for every later test.

The per-cell record is the part that gives the test its long-term value. A string that delivered its rated capacity with all cells tracking together is a string that will be easy to monitor; one that delivered the same figure with two cells reaching the end voltage early is a string whose weakness is already documented.

The test conditions belong in the record: the rate, the end voltage, the temperature at the start and through the discharge, and the state of charge before the test. A capacity figure without those conditions cannot be compared with any later test, which is how a battery commissioning record becomes unusable within a few years.

Charger performance and regulation

The charger test covers the float and boost voltages, the transition between them, the current limit, and the regulation behaviour as the load changes. Each of those affects the battery: a float voltage that is too high shortens the battery life through overcharging, and one that is too low leaves the battery undercharged and unable to deliver its rated capacity.

The AC changeover test is the one that most often receives less attention than it deserves. The charger should respond to the loss of the AC supply by allowing the battery to carry the load without a discontinuity, and to its restoration by returning to normal regulation without a voltage excursion that could disturb the connected equipment.

Recording the charger’s behaviour through a real changeover, with the DC voltage logged at a resolution fine enough to show the transient, produces evidence that a steady-state voltage reading cannot. The log belongs with the commissioning record because it establishes the behaviour the operating team will rely on for the life of the substation.

Protection and alarm coordination

The coordination question has two parts. The first is that every protective device operates at the DC voltage the design assumes, and the second is that the trip circuits operate at the lowest voltage the DC system is expected to reach during a discharge.

The first part is verified by measuring the operating voltage of each device and comparing it against the design. The second part is verified by testing the trip circuit at a reduced supply voltage, which is the check that reveals voltage drop in the wiring, undersized conductors, or a distribution arrangement that places the trip supply downstream of a section that is not the battery.

Alarm thresholds need coordination in the other direction. A low-voltage alarm that operates below the level at which the protection still functions informs the operator of a problem they can act on; one that operates above that level produces nuisance alarms that eventually lead to the alarm being ignored.

Load profile verification

The load profile is the current the DC system draws under normal conditions and under the conditions the design assumes during a fault. Measuring it during commissioning establishes the figure the battery capacity has to cover and identifies loads that were added without being reflected in the design.

The profile should be measured rather than estimated. A clamp measurement of the total standing load and of the individual circuits identifies the largest consumers and reveals standby loads that are energised but not accounted for. The difference between the design figure and the measured figure is a finding that belongs in the handover package.

The trip currents deserve separate measurement. A trip coil draws a current that is much larger than the standing load, and the duration for which it draws it determines the voltage depression at the battery. Measuring the battery voltage during a trip test establishes whether the system remains above the level at which the protection operates, which is the coordination question in its most direct form.

Commissioning tests in sequence

The sequence that produces the most reliable result starts with the battery and works outwards. The battery capacity test establishes the baseline and, because it discharges the string, it requires the charger to be proven able to recharge it afterwards.

The charger test follows, including the changeover behaviour and the recharge following the capacity test. Once the battery and charger are established, the distribution system and the loads are verified, followed by the protection operating voltages and the trip circuit test at reduced voltage.

The sequence matters because each stage depends on the previous one. A trip circuit tested while the battery is partially discharged produces a result that describes the state of charge rather than the circuit, and a charger tested before the battery has been established cannot be judged on how well it restores the string.

Recording as-built settings

The as-built settings for the charger, the alarms and the protective devices should be recorded as a set rather than individually. Their values interact through the voltage range the system has to operate across, and a record that lists them separately conceals whether the set is coordinated.

The record should also capture the distribution arrangement: which circuits are supplied from which section, where the battery connects, where the charger connects and where the trip supplies are taken from. That arrangement determines the voltage each load sees, and it is not always the arrangement shown on the design drawing.

Where the arrangement differs from the design, the difference should be recorded as a deviation with its justification. A deviation that is accepted informally during commissioning becomes an unexplained difference in the as-built record, and its consequences appear the first time the system is tested under fault conditions.

Handover documentation for operations

The handover package should contain the capacity test result with the per-cell data and the test conditions, the charger test records including the AC changeover log, the as-set protection and alarm thresholds with the measured operating voltages, and the load schedule with the measured currents.

It should also contain the as-built DC single line diagram, the maintenance intervals with the basis for each, and the identification of the instruments and methods the operating team will use for the periodic tests. Where a capacity test interval has been set from the design duty, the duty calculation belongs in the package.

The commissioning record then becomes the reference for the maintenance programme. The related methods are described in the accompanying articles on battery load bank testing and on impedance screening, and the requirements that the surrounding installation has to meet are set out in IEC 61936-1 for high-voltage installations and IEC 60364-1 for low-voltage installations, with the stationary lead-acid battery requirements in IEC 60896-21 and IEC 60896-22 and the measuring relay requirements that the protection equipment is judged against in IEC 60255-1. The research background for DC system reliability in substations is published by EPRI, and utility practice is coordinated through CIGRE study committees.

Ground fault detection on the DC system is a separate commissioning item that deserves its own test: the arrangement should be verified by applying a known insulation fault and confirming that the detection responds at the correct level and identifies the correct pole. Instruments for that test are grouped with the other DC system equipment, including the battery ground fault tracer. The full range is grouped on the DC system and battery testing hub.