Theory of uninterrupted DC power supply

overview

Uninterrupted DC power system in Power stations and substations providing power for control, signal, protection and automatic device, needs to be reliable. Under both normal conditions and emergency, it should ensure uninterrupted DC power supply to the DC load.

Battery

DC power system comprised of battery shows high level of reliability. The failure of entire battery is very unlikely because the whole battery banks' deterioration always begin in some individual cells, whose deterioration process is slow, easy to spot and easy to replace, thus extending the life of overall battery bank.

Many types of batteries are currently being used in the substation, among them valve regulated sealed lead-acid battery, acid-spray-proof lead-acid battery and nickel-cadmium alkaline batteries are the most common.

Valve regulated sealed lead-acid battery: Under rapid development since 1990s, the VRSL batteries overcome the shortcomings of acid-spray-proof lead-acid battery. it is widely used in the power system, with advantage such as sealed structure, no mist discharge, minimum needs of maintenance, high voltage on single cell, relatively low cost, long life expectancy, good discharge characteristics, easy installation, safe and reliable in use and takes up smaller space.

Acid-spray-proof lead-acid battery has been in the market for a longer time, these batteries have the advantage of allowing adding fluid under the operation, easy to monitor, long life expectancy, low prices, but also have the disadvantage of taking large space to install, producing hydrogen under operation and mist, causing pollution and complicated maintenance. So it is recommended to use Acid-spray-proof lead-acid battery in large and medium-sized power plants, 220kV and above substation and HVDC converter station, where 24 hours attendance are mandatory and larger space is allowed.

Nickel-cadmium alkaline batteries took off in the early 1980s, because its cell voltage is 1.2V, so the DC system needs more batteries, making maintenance complex. At the same time, it has the shortcomings such as discharge characteristics are declining steeper, with lower final voltage, higher cost, stronger memory effects and thermal derating.

Meanwhile, nickel-cadmium alkaline battery has the advantage of instantaneous large current discharge output, for example, high-rate battery maintaining final voltage of 1.1V, the initial discharge current can reach 10C5 ~ 12C5 up to 300ms. For substation with 110kV and below, its DC power normal and accident load are typically small, but if the electromagnetic switch coil are used in the circuit breaker, then bigger instantaneous currents are required and small-capacity nickel-cadmium alkaline batteries can satisfy the need.

The selection of the battery

Large and medium-sized power plants, 220kV and above substation and HVDC converter station should adopt Acid-spray-proof lead-acid battery or VRLA batteries.

Small power plants and 110kV substation should adopt the VRLA batteries, Acid-spray-proof lead-acid battery or middle-rate nickel-cadmium alkaline batteries.

35kV and below substation and power plant’s auxiliary plant should use VRLA batteries or high-rate nickel-cadmium alkaline batteries.

Number of sets of batteries

Power plants with three or more generating units and with the total capacity above 100MW, should install two sets of batteries; one set is recommended for other cases.

The 110kV and below substation should install one set of batteries; for some important 110kV substation should install two sets of batteries, when one set of battery undergo charging and discharging test, the other set can be put into operation to ensure uninterruptible DC power supply.

Classification of load

Classification by function

Control load: including electrical and thermal control, signal devices, relay protection, automatic devices and instrumentation. Control load are small in volume, wide in range, and big in number.

Power load: including high-power load such as DC lube oil pump motor, hydrogen seal oil pump motor, electro-magnetic operating mechanism in circuit breaker, AC uninterruptible power supply, remote power and communication devices, DC lighting and so on. Power load’s capacity determines the battery capacity and equipment needed. In substation, the main power loads are electro-magnetic operating mechanism in circuit breaker and DC lighting.

Classification by need

Often load: loads that should be reliably powered under every condition, including relays, signal, position indicator and pilot lamp or other DC electrical equipment, the general capacity can be taken as 1 ~ 2kW.

Accident load: loads that should be reliably powered under AC power outage, and can be divided into initial accident load, lasting accident load and random accident load, including emergency lighting load, the DC pump motors, uninterruptible power supply, remote power, communications standby power, signal and electrical protection devices.

Impact load: loads that last shortly but needs a large current, if happen in the first 1min accident, it is called the initial shock load, or random shock load otherwise. Including DC pump’s start-up current and closing current in circuit breaker.

System voltage level

DC system with only power load should adopt the 220V.

DC system with only control load should adopt the 110V.

DC system with both control load and power load could adopt 220V or 110V.

DC system with weak current control or signal should adopt 48V and below.

Adopting 220V system, can reduce the cross-sectional area of cable and conductor, saving non-ferrous metals. If the DC power only supplies control, protection, signal load, then 110V system can not only reduce battery cells’ number and saving space, which simplifies installation and maintenance work, but also reduce need of insulation levels, reducing the chance of intermediate relay disconnection and ground fault, thus making secondary equipment safer. However, because of voltage drop, cable cross-sectional area need to increase 4-fold, raising the amount of initial investment.

Charging device

There are currently two types of charging device: high-frequency switching charging device and thyristor-based charging device. High-frequency switching charging device are widely used whose output current is typically 5 ~ 40A. It is also small, light, automated, with high efficiency, easy maintenance, high reliability and advanced performance. Thyristor-base charging device use simpler circuit and its output current can be larger, making it cheaper. Its technical performance also meet the requirements of DC system, so its applications are also common.

Charging measure

Equalizing charge is an over-charging operation to eliminate the density and voltage imbalances accumulated during regular use. The measures generally use the first constant-current-limit-voltage process, and then constant-voltage-limit-current process.

Floating charged batteries needs an equalizing charge at least once a quarter. And batteries also need equalizing charge immediate after discharge under AC outage.

Floating charge is a measure that supply the DC load on the one hand, and compensate for battery self-discharge loss with a small current on the other hand. VRLA battery’s floating charge current is very small, approximately 1% of I10.

VRLA batteries need compensation in charging voltage with temperature, setting standard to 25 ℃, a -3mV/℃ is needed for each 2V cell to avoid the "Thermal runaway" effect.

Fresh out of factory VRLA batteries need initial charging, to ensure that the battery electrolyte even and active material fully transformed. Battery manufacturers will supply battery initial charging settings, similar to equalizing charge but with different value. Our professional staff will be dispatched to the scene to mix the electrolyte and setting initial charge for the Acid-spray-proof lead-acid battery and nickel-cadmium alkaline batteries, typically a deep discharge is needed immediate after initial charge, and multiple above charge-discharge loop operation might also be needed.

Furthermore, in order to avoid "Thermal runaway" effect, and warning about the dispersion of the battery voltage, battery charging voltage difference between any two cells should be monitor. For single 2V VRLA cell, under 2.25V of floating voltage, the voltage difference between the maximum and minimum must be less than 50mV. For 12V single cell, the limit is 300mV.

Bus voltage

To ensure safety and reliability, the DC bus voltage must be within the following range:

DC system with only control load: (85%～110%)Un.

DC system with only power load: (87.5%～112.5%)Un.

DC system with combined load: (87.5%～110%)Un.

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