Tripping-current substation batteries--Industrial Electrical Power Systems

The operation of monitoring devices like relays and the tripping (overload) mechanisms of breakers require independent power source, which does not vary with the main source being monitored. Batteries provide this power and hence they form an important role in protection circuits.

The relay/circuit breaker combination depends entirely on the tripping battery for successful operation. Without this, relays and breakers won’t operate, becoming 'solid', making their capital investment very useless and the performance of the whole network unacceptable.

It’s therefore necessary to ensure that batteries and chargers are regularly inspected and maintained at the highest possible level of efficiency at all times to enable correct operation of relays at the correct time.

How a battery works?

A battery is an assembly of cells. Whether it’s used to make a call using cell phone or to trip a circuit breaker, every cell has three things in common: positive and negative electrodes and an electrolyte. Whereas some of the dry cell batteries drain out their energy and are to be discarded, a stationary or storage battery used in the switchgear protection has the capability to be recharged.

There are two types of batteries used in an electrical control system:

1. Lead acid type

2. Nickel cadmium type.

Both the above types can be classified further into flooded type and sealed maintenance free type. The flooded cell construction basically refer to the electrodes of the cell in the electrolyte medium, which can be topped up with distilled water as the electrolyte gets diluted due to charging and discharging cycles.

The batteries also discharge hydrogen during these cycles and it’s very necessary to restrict this discharge to less than 4% by volume to air, to avoid the surroundings becoming hazardous. The higher discharge of H2 in lead acid cells have resulted in the manufacture of sealed maintenance free or valve-regulated lead acid (VRLA) batteries.

Here the H2 discharge is restricted to be below the hazardous limit.

Nickel cadmium batteries are comparatively costlier though they are considered more reliable with lesser maintenance and lesser environmental issues that go with lead acid types. In addition, the hydrogen discharge in a nickel cadmium cell is comparatively less.

Hence, for conventional switchgear protection applications, sealed nickel cadmium batteries are not required. As such, the sealed nickel cadmium cells are only used for small battery cells used in modern electronic gadgets.

The rechargeable lead acid cells as used in switchgear/relay applications are generally of the Plante type and has an electrical voltage of 2V. The cell contains a pure lead (Pb) positive plate, a lead oxide (PbO2) negative plate, and an electrolyte of dilute sulphuric acid.

The nickel cadmium cell has an electrical voltage of 1.2V containing nickel compound (+) and cadmium compound (-) plates with potassium hydroxide solution as the electrolyte.

The following table briefly gives the advantages and disadvantages of nickel cadmium batteries over lead acid type, the most common types being used for protection application.

Advantages; Disadvantages-- Better mechanical strength --Easy maintenance --Long life Space and weight low; Low H2 discharge and no spill over issues Lower cell voltage (1.2 vs 2.0) More expensive Higher current consumption for charging Not recommended at higher ambient temperatures Higher distilled water consumption

+--+ Nickel cadmium vs lead acid cells

Discharging and recharging

When a load is connected, across plate terminals of a charged cell an electrical current flows and the lead and lead oxide start to change into lead sulphate. A similar phenomenon occurs with nickel cadmium cell. The result is the dilution and weakening of the electrolyte. It’s thus possible to measure the state of the battery's charge by measuring the electrolyte's specific gravity with a hydrometer.

The cell is recharged by injecting a direct current in the opposite direction using another source to restore its plates and electrolyte to their original state.

Application guide:

Plante Flat Plate Tubular Substations * Telephone exchanges * Mobile telephone exchanges * Emergency lighting * * Alarms * * Computer emergency * * * Engine starting * * Oil rings * *

+--+ Application for different electrode types

Life expectancy

Plante 25 - 30 years, Flat Plate 5 - 6 years, Tubular 10 - 12 years

Construction

For long life with very high reliability needed in places like power stations and substations, batteries are made up of cells of the kind named after Plante. ++++ gives typical construction of a lead acid battery.

++++ Typical construction of a lead acid cell

The positive plate is cast from pure lead in a form which gives it a surface twelve times its apparent area. The negative plate is of the pasted grid type made by forcing lead oxide paste into a cast lead alloy grid.

The positive and negative plates are interleaved and insulated from each other to prevent short circuits, and are mounted in transparent plastic containers to allow visual checking of the acid level and general condition.

Because of the high initial cost of Plante cells, specially designed flat plate cells have been developed to provide a cheaper but shorter-lived alternative source of standby power. Although this is the basis of the modern car battery, it’s totally unsuitable for switch-tripping duty because it has been designed to give a high current for a short time as when starting a car engine. Cells with tubular positive plates are also available but these are normally used to power electric trucks, etc., where daily recharging is needed i.e. frequent charge/discharge cycles.

Voltage and capacity

The nominal voltage is 2 V per lead acid cell, i.e. a 110V battery will have 55 cells. On discharge, the recommended final voltage at which the discharge should be terminated depends on the discharge rate. This is shown in discharge curves (e.g. the final voltage for the 3hr rate of discharge is 1.8 V).

++++ Typical battery discharge curves --

Capacity

The capacity that can be provided by a cell varies with the discharge rate as indicated in the capacity curves shown above. The capacity of a battery is defined in terms of ampere hour (AH) related to 5 h or 10 hr duty. It refers to the capacity of the battery to supply a load current over a period, until it reaches its pre-defined final cell voltage. After this time, the cell has to be recharged to again feed a load. For example, in case of lead acid batteries, the acceptable final cell voltage could be as low as 1.70 V. But it’s common to define the capacity of the lead acid batteries for different cell voltages like 1.75, 1.80 and 1.85 V. Accordingly, the discharge curves of a battery vary showing comparatively higher time to reach the lowest acceptable cell voltage.

+--+ gives the current that can be drawn from a battery depending upon the 10 h rating.

+--+ Capacity variation of a lead acid cell with load current -- Time in hours--Capacity in %--Final cell voltage--Current in % of 10h rating

The above table typically refers to a cell, which can supply 100% of its rated amperes for 10 h at the end of which it reaches an end voltage of 1.85 V. The cell will reach 1.85 V if 100% rated current is continuously drawn for 10 h. Alternatively, if the current drawn is 600% of its rating, the cell will reach 1.75 V at the end of 1 h itself. Hence, while designing the capacity of the cell, proper margins should be taken into account based on the nature of loads and the likely currents to be drawn over a cycle.

Capacity is also affected by ambient temperature. The lower the ambient temperature, the capacity will be comparatively higher.

Battery charger

In a protection system, it’s necessary that the control DC voltage shall remain constant for as much time as possible, so that the system works without interruption. Hence, the batteries are normally kept on charge continuously by a battery charger. The charger is a rectifier, which produces a slightly higher voltage compared to the nominal cell voltage of a battery. The main power source is derived from the normally available AC source, which is rectified by the charger. Typical connection.

Constant voltage charger Load Battery +, -

++++ Charger/battery/load connection

Here the battery is a combination of multiple cells connected in series to get the nominal DC tripping/control voltage required for the operation of relays and breakers and could be from 24 to 220 V, depending on the loads and the capacity requirement.

Trickle charge

Trickle charging is a method of keeping the cells in a fully charged condition by passing a small current through them. The correct trickle charge current is that which does not allow the cell to discharge gas and does not allow the specific gravity to fall over a period. The cell voltage will be approximately 2.25 V for lead acid cell and 1.35 V for nickel cadmium cell.

Float charge

Float charging is keeping the voltage applied to the battery at 2.25 V per lead acid cell or 1.35 V for nickel cadmium cell, i.e. maintaining a constant voltage across each cell. This method is usually adopted in conjunction with supplying continuous and variable DC loads from the charging equipment, as would typically happen for a substation battery.

The loads in a substation normally comprise of small continuous load consisting of pilot lamps, relays, etc., and momentary short time loads of comparatively high values such as those for circuit breaker tripping and closing operations, motor wound springs and so on.

Since the charger, battery and load are all connected in parallel, the continuous load is carried by the charger at normal floating voltage and the battery draws its own maintenance current at the same time. Any load that exceeds the charger capacity will lower its voltage slightly, to the point where the battery discharges to supply the remainder. If there should be a complete power failure the battery will supply the entire load for a period depending on the AH capacity and the load, until AC power is restored and then automatically starts being recharged. Typical float currents will be in the range of 30-50 mA per 100 AH of rated capacity, increasing to about 10 times towards the end of the battery's life.

Specific gravity

A simple hydrometer reading indicates the state of charge in a cell. A fully charged cell will have a specific gravity reading of between 1.205 and 1.215. As a rule of thumb, in a lead acid battery, Open-circuit volts = specific gravity + 0.84 Thus, the open-circuit voltage of a cell with a SG of 1.21 will be 2.05 V; one with an SG of 1.28 will be 2.12 V.

Recharge

The ampere-hour efficiency of the cells is 90%; therefore, on recharge, the amount of recharge required is equal to the discharge in ampere-hours plus 11%. On recharge, the voltage increases and reaches a saturation value as the charge proceeds. The highest voltage reached with the finishing rate of charge flowing is 2.7 V per lead acid cell. It’s possible to recharge a cell by limiting the voltage of the charging equipment to a much lower value than 2.7 V per cell, 2.4 V per cell being the minimum desirable value. This will result in an extended recharge period, as the battery will automatically limit the charge current irrespective of the charger output.