Over the period of 2009 to 2011, ENELT Group Co. Ltd. performed tests of operating DC voltage systems and battery diagnostics at existing substations of branches of Setevaya Kompania OAO, and evaluated the actual residual battery capacity without interrupting continuous supply to DC consumers. The article contains main findings for the inspected storage batteries, root causes leading to deterioration of storage batteries and to decrease in battery capacity, as well as key factors affecting the battery lifespan.

 

Aleksandr GORYANSKY,
Head of Design and Engineering Department, ENELT Group Co. Ltd.

 

Lead-acid batteries with float chargers are used as a source of operating DC in electrical installations of substations.

A storage battery should retain nameplate capacity through its full life cycle, to perform core functions. To make it clear, each cell will hold the charge and put out capacity individually. That’s why the battery total capacity will be strongly affected by several "lagging" cells (those attaining the discharge limit faster than other cells). Battery discharge tests should be conducted periodically, to detect "lagging" cells in the battery, as well as to evaluate the battery capacity.

ENELT Group Co. Ltd. performs in situ tests of storage batteries at the substation without discontinuing uninterruptible power supply to consumers of DC power. A 150 or 300 Ah temporary battery consisting of sealed 12 V monoblocks is used to ensure uninterruptible power supply to consumers and routine switching during the test. This allows you to install a temporary battery pack in common production areas in close proximity to the DC board. The discharge test of storage battery is performed by currents of equal magnitude over the cycle, using a special discharge device.

Over the period of 2009 to 2011, ENELT Group Co. Ltd. performed tests of operating DC voltage system and battery diagnostics at 30 existing 110-500 kV substations of six branches of Setevaya Kompania OAO. See below for overall statistics for the inspected storage batteries (AB), main causes leading to deterioration of storage batteries and to decrease in capacity, and key factors affecting the battery lifespan.

As shown in the figure, 8 of 30 storage batteries inspected have performed well. At the time of tests, these storage batteries had capacity equal or close to 100%. No lagging cells or comments on battery were detected during tests.

Although nine of the storage batteries tested had a high residual capacity, lagging and faulty cells were detected in these storage batteries during the discharge test. These storage batteries require greater attention during operation; they also require monitoring of lagging cells, and periodic re-testing. If lagging cells continue to deteriorate, then they will need to be replaced or, at least, shunted, since they significantly reduce the total capacity of the storage batteries during discharge.

Storage batteries installed at 13 substations (out of all the inspected substations) need to be replaced because of the poor condition. Some of them failed the residual capacity test – at the discharge test, the capacity of these batteries was less than 80% of the original committed value. The other need to be replaced due to overaging and exceeding physical deterioration limit. We can't predict how these batteries will perform down the road. For instance, a sudden scaling of active- mass and destruction of battery plates were observed in some batteries during control discharges. These batteries are no longer usable and must be replaced.

MAIN FACTORS AFFECTING BATTERY LIFE:

Main points:

• Lead-acid battery is a chemical current source
• The functioning of chemical current source always involves electrochemical conversion of materials
• The functioning of chemical current source implies slow degradation
• Maximum service life is only possible under ideal operating conditions

Processes that limit battery lifespan:

• Corrosion of positive grids;
• Scaling of positive active-mass;
• Distortion (growth) of positive plates;
• Penetration of lead dendrites causing short circuits to arise between opposite plates;
• Self-discharge;
• Sulfatation (irreversible);
• Passivation of plates.

Corrosion of positive grid

Corrosion is an ongoing process that never stops. The positive grid corrosion means that the surface layer of the grid is oxidized to lead oxide (PbO2) – the same material as the active mass. When it comes to good alloys, the rate of this process under nominal operating conditions is 25-30 microns per year. If low- quality alloys are used or if there is no “extra” material, corrosion can destroy the grid after several years of operation.

Plate sulfatation is the formation of insoluble lead sulfate crystals with a characteristic cubic structure on positive plate. These crystals are much bigger than normal PbO2 crystals (see the upper section of the picture in the middle).

Passivation (degradation) of the negative active-mass means a decrease in capacity towards the end of the battery life due to changes in the microscopic structure of the spongе lead of the negative plate. This is due primarily to a decrease in the surface area of negative active-mass and, as a result, a decrease in the capacity.

Factors underlying the deterioration of storage batteries

The VAZP type floating chargers in use do not provide the required charge voltage regulation and the required DC voltage ripple (which shortens the battery lifespan), are low-power and have had exceeded their useful life. The poor constant-voltage regulation is aggravated by the human factor. Output voltage of VAZP floating chargers varies depending on AC input voltage, and the operational staff is forced to constantly adjust the output voltage of the device. However, in real life, it seems to be impossible to accurately adjust the output voltage of the VAZP floating chargers due to the high wear or malfunction of adjusting potentiometers. VAZP floating chargers do not meet the DC ripple requirements for the quality of power supply for modern batteries and relay protection microprocessing terminals. Hence, it is impossible to power DC consumers from chargers without connection to storage battery.

Another serious issue directly related to the operation of storage batteries is the lack of automatic temperature compensation of charge voltage. Operating temperature conditions of storage batteries have a significant impact on the battery lifespan. When the temperature of the batteries deviates from the nominal value, in order to maintain the full state of charge, minimize corrosion and sulphatation processes, the voltage of continuous float charge must be adjusted to a coefficient = -3.3 mV/0C per each battery cell. So, for example, when the temperature decreases by 100 °C, for a battery consisting of 104 cells, the voltage of continuous float charge must be adjusted by 3.43 V, and when the temperature increases by 100°C, the voltage must be lowered by 3.43 V. For the above reasons, it is impossible to track and make such adjustments manually.

The constant battery undercharging and, as a result, a decreased storage battery lifespan is caused by the wrong number of cells when replacing the SK type battery. The reason is the difference in the continuous float charge voltage for SK type batteries and modern batteries types such as OPzS, OGi, Vb, and GroE. The continuous float charge voltage for SK type batteries is 2.15-2.2 V/ cell, therefore, a 108 cells battery (main cell group) was used to power RZiA circuits. That said, continuous float charge voltage (control cabinet busbar voltage) was 232-235 V.

Currently, manufacturers of up to date low- maintenance batteries prescribe a continuous float charge voltage of 2.23 V/ cell. Therefore, to maintain the same group of 108 low- maintenance batteries in a fully charged condition, a control cabinet \ power cabinet busbar voltage 108 * 2.23 = 240.84 V is required – this value is at the limit of the maximum allowed range for protection relays supply voltage. Typically, the substation staff will not allow an increase in the control cabinet/power cabinet busbar voltage over 240 V, resulting in a constant undercharging of 108 cells storage batteries, increased plate sulphatation and, inevitably, in capacity loss and service life decrease.

That’s why, in replacing a storage battery, you should consider the charging voltage change and select the battery cell number based on permissible voltage range for DC consumers in all battery operating modes. The optimal cell number for the installed storage battery (main battery group) is 104 cells. In this case, the continuous float charge voltage of the storage battery should be 104 * 2.23 = 231.92 V.

Moreover, to ensure maximum service life and quickly get the storage battery back in working order after emergency and operating discharges, it is best to perform a rapid charge with high voltage. For most modern rectifier chargers, this function is performed automatically after resumption of auxiliary power supply of a substation. In this case, the rapid charge voltage is 104 * 2.3 = 239.2 V – this value cannot exceed < Unom. +10%.

For 16 of 30 substations inspected, the operating DC voltage system was used with additional storage battery "tail" cells, that is, the load that consumes current continuously (control and signaling circuits) was powered from the main part of storage battery. Instantaneous load (supply circuits of high- voltage circuit-breaker actuators) is powered from the entire storage battery. However, these substations use single-channel rectifier chargers (VAZP, PNZP) connected to the entire storage battery. To prevent overvoltage on additional "tail" cells, rheostats or other ballasts are connected to them to provide load current comparable to the load current of the main group. The use of ballasts leads to increased "wear" of storage battery "tail" cells. Sometimes, "tail" cells can miss recharging, while ballasts remains connected to the storage battery cells. In this case, there is a risk of a deep or complete discharge of "tail" cells, which results in failure of these storage batteries.

ENELT Group Co. Ltd. has acquired a highly skilled team and considerable experience in performing a complete scope of works involved with reconstruction of 35-750 kV operational DC voltage systems, in accordance with the standard STO 56947007-29. 120 40 041-2010 "Operational DC systems of substations" approved by FGC UES.