Sizing DC Systems for Switchgears: Comprehensive Guide to Battery and Charger Selection

Introduction to Switchgear:

What is Switchgear?

• Switchgear refers to the combination of electrical disconnect switches, relays, lighting, controls, fuses, or circuit breakers used to control, protect, and isolate electrical equipment. It acts as the backbone of power distribution systems.
• It involves large panels of electrical distribution circuit breakers that distribute power to a facility or grid, ensuring safe and efficient power transmission and distribution.

Why is Switchgear used?

• Switchgear is used to de-energize equipment to allow maintenance work and to clear faults downstream, minimizing downtime and ensuring worker safety.
• It facilitates the fixing of power lines and handling of breakers that are too large to flip manually, enabling quick restoration of power in case of disruptions.

Types of Switchgear Applications:

• Medium Voltage (MV)
• Low Voltage (LV)
• Paralleling

Medium Voltage (MV)

• MV switchgear provides utility-level protection and typically serves medium to large-scale facilities, industrial plants, and utility substations. It is designed to handle higher voltage levels, typically ranging from 1kV to 38kV.
• These systems often follow an 8-hour load profile, reflecting typical operational patterns and providing sufficient backup capacity for extended outages.

LV Switchgear

• LV switchgear offers building-level protection and is commonly found in commercial and residential settings. It operates at lower voltage levels, typically ranging from 120V to 480V, and provides protection for individual circuits or equipment within a building.
• LV switchgear is essential for ensuring safety, controlling power distribution, and protecting equipment from overloads and faults.

Paralleling

• Paralleling switchgear is used to synchronize and distribute power from multiple sources, such as generators or utility feeds, to ensure continuous and reliable power supply. It allows for seamless transfer between different power sources and provides redundancy for critical applications.
• These systems typically involve two or more gensets and follow load profiles ranging from 2 to 8 hours, depending on the specific requirements of the application.

The Battery’s Purpose:

• Function: Batteries play a crucial role in providing DC power to switchgear equipment during outages, ensuring uninterrupted operation and protecting critical loads.
• Individual Batteries: Best practice involves installing individual batteries for each load/application to prevent single points of failure and ensure redundancy.
• Backup Duration: The duration of backup provided by batteries depends on their ampere-hour (Ah) capacity, which determines the amount of energy they can store and deliver during outages.
• Battery Loads: Battery loads include trip current, close current, spring motor rewind/charge current, and continuous loads such as relays, meters, control circuits, PLCs, and lighting.

IEEE Standards:

• IEEE 1115: Recommended Practice for Sizing Nickel Cadmium Batteries.
• IEEE 485: Recommended Practice for Sizing Large Lead Acid Batteries.
• IEEE 1189: Recommended Practice for Selection of Valve Regulated Lead Acid Batteries.
• Sizing Practices: These standards provide guidelines and best practices for sizing and selecting batteries based on specific application requirements and environmental factors.

Building Load Profiles:

• Components: Switchgear load profiles comprise four main components: trip, close, spring motor rewind/charge, and continuous loads.
• Duration: Different durations apply for Ni-Cd and Pb-acid batteries, with minimum sizing durations of 1 second and 1 minute respectively.
• Load Sequencing: Load sequencing during outages significantly impacts battery capacity, requiring careful consideration and analysis to ensure optimal performance.

The Voltage Window:

• Design Consideration: Batteries operate within a designed voltage window, which is crucial for maintaining their performance and longevity.
• Upper Limit: The upper limit of the voltage window allows for battery equalize/boost charging to optimize charging efficiency.
• Lower Limit: The lower limit of the voltage window ensures maximum usage during discharge, preventing over-discharge and damage to the battery.
• Typical Voltage Windows: Typical voltage windows for standard nominal voltages are provided, but adjustments may be necessary based on the specific equipment connected to the battery.

Temperature Factor:

• Influence on Performance: Battery capacities and discharge ratings are influenced by temperature, with correction factors applied accordingly.
• Calculation: Temperature correction factors are applied to adjust the battery's capacity and discharge ratings based on the operating temperature. These factors ensure that the battery can deliver the required performance even under extreme temperature conditions.

Other Factors to Consider:

• Design Margin: Design margin allows for future load growth or unknowns in the load list, ensuring that the system can accommodate potential changes in requirements.
• Aging Factor: Aging factor accounts for the degradation of battery capacity over time, ensuring that the battery can maintain its performance throughout its service life.

Battery Sizing:

Load Profile Analysis:

Before sizing the battery, it's crucial to analyze the load profile of the switchgear system. The load profile typically consists of four main components:

• Trip Load: The instantaneous load experienced during the operation of trip mechanisms in the switchgear. This load can be simultaneous, sequential, or mixed, and its duration varies depending on the type of battery.
• Close Load: The load experienced during the operation of close mechanisms in the switchgear. Similar to trip load, it can be simultaneous, sequential, or mixed, with different durations for different battery types.
• Spring Motor Rewind/Charge Load: This load occurs during the spring motor rewind or charge process, usually following a sequential pattern. The duration of this load is critical for determining the battery size.
• Continuous Load: The steady-state load that the battery must sustain during normal operation. This includes powering relays, meters, control circuits, PLCs, lighting, and other auxiliary equipment.

Equations for Battery Sizing:

To determine the required capacity of the battery, we can use the following equations:

1. Total Load Capacity (TLC): TLC=Trip?Load+Close?Load+Spring?Motor?Rewind/Charge?Load+Continuous?Load
2. Effective Battery Capacity (EBC):

EBC=TLC/Discharge?Factor

1. Adjusted Battery Capacity (ABC): ABC=EBC×Design?Margin×Aging?Factor×Temperature?Correction?Factor

Example Calculation:

Let's consider a medium voltage switchgear system with the following load profile:

• Trip Load: 50 A for 1 second
• Close Load: 30 A for 1 minute
• Spring Motor Rewind/Charge Load: 20 A for 6 seconds
• Continuous Load: 100 A for 8 hours

Using the equations above and assuming a discharge factor of 0.85, a design margin of 15%, an aging factor of 1.25, and a temperature correction factor of 0.95, we can calculate the required battery capacity:

Total Load Capacity (TLC):

TLC=50×1+30×60+20×6+100×8

TLC=50+1800+120+800

TLC=2770?Ampere-seconds

Effective Battery Capacity (EBC):

EBC=2770/0.85

Adjusted Battery Capacity (ABC):

ABC=3258.82×1.15×1.25×0.95

ABC=4546.51?Ampere-seconds

Therefore, the required battery capacity for this medium voltage switchgear system is approximately 4546.51 Ampere-seconds.

Battery Charger Sizing:

Important Considerations:

When sizing a battery charger for medium voltage switchgear applications, several factors must be taken into account to ensure optimal performance and reliability. These factors include:

• Continuous Load: The charger must be capable of supplying sufficient charging current to replenish the energy consumed by the continuous load of the switchgear system. It is essential to accurately determine the continuous load to size the charger appropriately.
• Battery Type: Different types of batteries, such as nickel-cadmium (Ni-Cd) and lead-acid, have specific charging requirements. The charger must be compatible with the battery chemistry and designed to provide the correct charging voltage and current profiles to ensure efficient and safe charging.
• Battery Ah Capacity: The capacity of the battery, expressed in Ah (ampere-hours), indicates the amount of charge it can store. The charger must be sized to provide the necessary charging current to replenish the battery's capacity within the required recharge time.
• Altitude: Altitude can affect the performance of the charger due to changes in atmospheric pressure and temperature. It is essential to consider altitude derating factors when sizing the charger, especially for installations at higher elevations.
• Design Margin: A design margin is typically applied to the charger sizing calculation to account for uncertainties in the load profile, variations in battery performance, and future expansion of the system. The design margin ensures that the charger has sufficient capacity to meet the system's requirements under various operating conditions.

Example Calculation:

To illustrate the process of sizing a battery charger for medium voltage switchgear, let's consider an example scenario:

• Battery Type: Pocket Plate Nickel-Cadmium (Ni-Cd)
• Battery Ah Capacity: 100 Ah
• Required Recharge Time: 8 hours
• Continuous DC Load: 12 amps
• Design Margin: +10%
• Altitude: Less than 3000 ft.

Step 1: Determine Recharge Factor (RF):

• According to the manufacturer's specifications or industry standards, the recharge factor for Pocket Plate Ni-Cd batteries is typically provided. For this example, let's assume an RF of 1.40.

Step 2: Determine Altitude Derating (AD):

• Since the installation is at an altitude of less than 3000 ft., there is no altitude derating factor applied in this example.

Step 3: Calculate Charger Current (C):

The formula for calculating the charger current (C) is as follows:

C=(((AH×RF)/RT)+CL )x DM x AD

Where:

• C = Charger Current (in amps)
• AH = Battery Amp Hours
• RF = Recharge Efficiency Factor
• RT = Required Recharge Time (in hours)
• CL = Continuous Load (in amps)
• DM = Design Margin (as a decimal)
• AD = Altitude Derating Factor (if applicable)

Conclusion:

Proper sizing of the battery charger is critical to ensure the reliable operation and longevity of the battery system in medium voltage switchgear applications. By considering factors such as continuous load, battery type, capacity, altitude, and design margin, engineers can select the appropriate charger to meet the system's requirements and ensure optimal performance under various operating conditions.