Tripping-current substation batteries (part 2) --Industrial Electrical Power Systems

Construction of battery chargers

Battery charging is accomplished with sophisticated electronically controlled rectifiers that permit a battery to be continuously maintained on a floating charge and to recharge a discharged battery as fast as possible. It also has to handle the electrical load.

Chargers presently used in stationary applications are normally of the constant voltage type. Voltage adjustments can be made with precision to 0.01 of a volt per cell. This is necessary because floating voltage and equalizing voltage levels critically affect battery performance and life expectancy. Voltage level specifications are normally expressed to two decimal places, i.e. 2.16-2.33 V for a lead acid cell.

Proper specifications and correct adjustments of the battery charger are the most important factors affecting the satisfactory performance and life of the battery cells.

Voltage levels from the charger also usually serve the electrical load, so changes in charger voltage output affect the load.

Chargers are normally equipped to accommodate normal float voltages and the higher voltages for equalizing charges when required. As the charger DC output is rectified from AC there will be a ripple on the output unless smoothing techniques are employed.

Care must be taken to ensure that the maximum rms current value of the AC component does not exceed 7% of the battery capacity expressed in amperes. Failure to do this will result in a phenomenon called AC corrosion, where the negative peak of the AC component reverses the direction of the charging current, leading to corrosion and ultimate destruction of the plates in the cells.

Maintenance guide

Following are brief guidelines for maintaining the lead acid battery system, which are also applicable for nickel cadmium batteries.

• Check batteries and chargers regularly, at least once per month.

• Take specific gravity reading -- should be 1.215 minimum.

• Check electrolyte level - top up if necessary to keep level between its normal and upper limits. Use distilled water wherever possible. Impurities in normal tap water such as chlorine and iron tend to increase internal losses. Frequent topping up of electrolyte means excessive gassing brought about by overcharging.

• Check for gassing: This normally starts when lead acid cell voltage reaches 2.30-2.35 V per cell (1.30-1.35 for Ni-Cd cell) and increases as charge progresses. At full charge, most energy goes into gas, oxygen being liberated at the positive plate and hydrogen at the negative. Four percent hydrogen in the air may be hazardous. The room employing lead acid batteries must be well ventilated. However, the nickel cadmium batteries discharge very low hydrogen.

• Look between the plates for any signs of mossing or treeing. This is a build-up of a sponge like layer of lead on the negative plates, which can accumulate to such an extent as to bridge over or around the separators and cause a short circuit to the adjacent positive plate. This condition is usually an indication of overcharging.

• Also look for sediment build-up on the floor of the container. If this increases to the point where it reaches the bottom of the plates, it will short them out to cause failure. Overcharging can also accelerate the accumulation of sediment and shorten the useful life of the battery.

• Battery cells must be kept clean and dry to the extent that no corrosion, dust or moisture offers a conducting path to partially short-circuit the cell or contact ground.

• Finally, proper charging is the most important factor in battery service and life.

A short boost charge is beneficial on a regular basis to prevent stratification, to freshen the electrolyte and to equalize the cells. Ensure that the charger is working properly and that it’s operating in accordance with the manufacturers recommended settings.

• The VRLA batteries cannot be inspected visually like the flooded cells but their healthiness is ascertained by measuring the cell voltage with/without the charger input. Cells reaching end voltage after disconnection of charger comparatively faster than the other cells need to be replaced completely, as there is no question of changing the electrolyte.

• The VRLA batteries last longer if their ambient temperature is controlled to around 25 °C by use of air conditioning, as otherwise they may lose their charge more quickly.

• The flooded type lead acid batteries are normally prone for spill over of electrolyte and it’s highly recommended to keep these batteries in separate room with acid proof tiles to take care of the spill over conditions. The nickel cadmium batteries on the other hand don’t pose such major hazards though they are quite costlier compared to lead acid type.

Arrangement of DC supplies

For strategic switch boards it’s sometimes worthwhile to fit two trip coils to each circuit breaker to ensure positive tripping. Two batteries and chargers should then be installed to ensure the integrity of each tripping system, DC fail relays being installed on each panel to monitor the continuity of each supply. For breakers fitted with only one trip coil, a single battery and charger should be installed and trip coil supervision relays fitted to monitor each circuit.

Charger no. 1, Charger no. 2, Main protection TC 1 DCF Back-up protection TC 2 Alarm; Alarm DCF Close coil-- Spring rewind

++++ Arrangement of DC supplies with two trip coils for each circuit breaker

Grounding of DC supplies

It’s a normal practice to ground the tripping battery at one point to prevent it floating all over the place with respect to ground. One popular method is to ground the negative rail. It will be noted that in this case a solid link is used instead of a fuse on the negative side. This ensures that the negative is never lost. If it was, 'sneak circuits' become a distinct possibility causing many vexing problems, etc. The drawback with this system is that the first ground fault on the wiring could possibly create a short on the battery.

Main and B/U protection TC Charger TCS Close coil--Spring rewind - +

++++ Arrangement of DC supplies with one trip coil

++++ Grounding of DC supplies negative rail

A more secure system is the center-point high-impedance ground method, which utilizes a battery ground fault alarm relay (BEFAR). An ground fault on either the positive or negative rail will cause a very small current to flow up the neutral (0 V) connection. This will cause the BEFAR to operate to alarm and indicate which leg has faulted. The supply will however remain on but this condition should be attended to before a second ground fault occurs. A test button is also provided to check the relay is functional at any time, by offsetting this away from the center zero point.

++++ Center-point high-impedance grounding method of DC supplies

Trip circuit supervision

It’s necessary to monitor that the trip supply is available in a trip circuit to ensure that the system is continuously being monitored. This is some times achieved by means of 'fail-safe circuits', which ensure that there is no question of operating a system without monitoring.

Trip circuit supervision is a method, where the trip supply is continuously monitored, so that any break in the circuit is brought to the operators' attention. There are two possible types (a) supervision only when the breaker is in closed (in service) condition; (b) supervision irrespective of the breaker status. These are achieved by using breaker auxiliary contacts in the DC trip circuits as shown.

Lamp PR 52a

++++ Supervision while circuit breaker is closed Lamp 52b

++++ Supervision while circuit breaker is open or closed

++++ Supervision with circuit breaker open or closed with remote alarm

Reasons why breakers and contactors fail to trip

Breakers

1. Open-circuited DC shunt trip coil

2. Loss of circuit DC trip supply

- Trip fuse blown or removed or ...

- Trip MCB open

3. Loss of station DC trip supply

- Battery and/or charger failure

- Battery and/or charger disconnection

4. Burnt out DC shunt trip coil

5. Failure by open circuit of control wiring or defective relay contact

6. Breaker mechanically jammed:

- Trip bar solid

- Trip coil mechanically jammed

- Trip coil loose or displaced

- Broken mechanism

- Lack of regular/correct maintenance

- Main contacts welding

- Contacts arcing -- loss of vacuum or SF6 gas pressure.

Trip circuit supervision (TCS) provides continuous monitoring and gives immediate warning of conditions 1 to 5 before breaker is called upon to trip. Corrective action can then be taken before the event, to prevent breaker failure occurring.

Breaker fail (BF) protection covers all conditions but only highlights problem after a system primary fault occurs and the breaker has failed to clear.

TCS can be regarded as the fence at the top of the cliff whereas BF is the ambulance at the bottom, only operating after the event!

Contactors

(a) Latched type

- Suffer same failures as breakers

(b) Electrically held-in type

- Can be energized from battery source but not favored because of battery drain

- Usually energized from AC supply via built-in rectifiers

- Dip-proofing provided by capacitor across hold-in coil to delay drop out by 200-300 ms for transient dips.

Failures to open could arise from:

• Welding closed of normally open control contacts

• Accidental short-circuiting of normally closed contacts preventing de-energizing hold-in coil

• Welding closed of main contacts (old vacuum or air break types)

• Mechanism mechanically jammed.

Capacitor storage trip units

These units are intended for use in substations where no tripping batteries have been provided.

They can also be used as an alternative tripping supply for installations, which employ two-shunt trip coils per circuit breaker, one connected to the tripping battery and the other to the capacitor storage unit.

The unit comprises two capacitors to provide short-time storage of auxiliary energy, one to operate a protective relay whilst the other operates the circuit breaker trip coil. The device has three inputs, which may be fed from DC or AC sources. A test button is provided for checking the degree of capacitor charge, whilst an in-built signaling relay with a normally closed contact offers remote indication of discharged capacitors. ++++ shows the unit below.

Auxiliary voltage for the protective relay Tripping contact of the protective relay

++++ Capacitor storage trip unit

Points to watch for...

• Always endeavor to feed the unit from a busbar connected VT if possible as the first priority as this is the most secure supply.

• Only feed one relay and circuit breaker from each unit and check that the stored energy is sufficient to do the job required of it.

• When the storage device is fed from current transformers, it’s recommended that at least two phases are used.

• A set of special interposing auxiliary CTs should be installed between the main CTs and the storage device, to protect it from overvoltages caused by nearby short-circuits. They should be designed to saturate at about 200 V rms.

• However, they must also ensure that they are able to charge the capacitors from zero to maximum in two cycles to cover the case of 'closing onto fault'.

• Ratios and performance of the line CTs on light loads must also ensure that the capacitors can be charged rapidly.

• Care should be taken when installing these devices into CT circuits that they don’t overburden other relays and protection that may be connected to the same CTs. The recommended interposing CTs may well pose a problem in this regard.

• As the protection and tripping will depend entirely on the correct performance of the capacitor storage unit, it would be as well to consider allocating an exclusive dedicated set of CTs for this function.