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**Design Considerations for Diesel Generator Systems**
1. **Design Concepts for Diesel Generators**
1.1 **Required Conditions for Design**
a. **Meteorological Conditions**: The rated capacity of the diesel generator is tested at 40°C. As long as the ambient temperature does not exceed this value for extended periods, the unit can be designed based on its rated output.
b. **Altitude Conditions**: At higher altitudes, the performance of the diesel generator may degrade due to reduced air density. It is generally not recommended to operate diesel generators at high altitudes under full load.
c. **Water Quality**: Hard water can cause scale buildup in the cooling system, reducing heat transfer efficiency and potentially leading to overheating. In areas with hard water, it’s essential to implement a water softening system for the circulating water.
d. **Load Capacity**: Accurate load calculations are necessary to meet current regulations. For example, critical loads in primary power systems should be considered separately, and the generator's capacity should be determined based on the largest expected load, including fire protection and essential services.
1.2 **Types of Diesel Generator Usage**
a. **Standby Power Station**: Used during normal operations.
b. **Emergency Power Station**: Designed for critical loads like fire-fighting equipment, which rarely operates but must be available when needed.
c. **Backup Power Station**: Activated during power outages to maintain essential functions.
1.3 **Preliminary Design**
- Estimate the diesel generator’s capacity based on the total transformer capacity or building area. For instance, a 4,000 kVA transformer might require a 500 kW generator, while buildings over 10,000 m² may use 10–15 W/m² for estimation.
- Create a main line schematic, determining whether the generator will operate in standalone or parallel mode, and whether critical and fire loads will be on separate bus sections.
- Plan the layout of the generator room according to standards such as JGJ 16-2008 for civil building electrical design.
2. **Construction Drawing Design**
- This stage involves more detailed calculations and precise specification of components like buses, motors, switches, and equipment.
- Draw the layout of the generator room, including installation dimensions, inlet and outlet cabinets, busbars, trenches, and grounding systems.
- Include vertical distribution diagrams showing the number of generators, their capacities, and how power is distributed to terminal boxes.
2.1 **Load Capacity Selection During Construction**
- When selecting the generator size, consider both fire load and non-fire critical loads separately. The larger of the two values should be used to determine the generator’s capacity.
- A more practical approach is to base the selection on the total of fire and non-fire loads, ensuring both normal and emergency power needs are met. However, this often results in a larger, more expensive system.
2.2 **Diesel Generator Loads**
- Typical loads include fire pumps, fire alarms, lighting systems, emergency elevators, sewage pumps, and other essential equipment.
- Emergency lighting should be provided in control rooms, switchrooms, stairwells, and other public areas to ensure safety during power failures.
2.3 **Capacity Calculation**
- Follow the guidelines from JGJ 16-2008 for accurate capacity determination.
- Ensure the generator operates within economic limits (around 75% of rated power) to avoid excessive fuel consumption and wear.
- Avoid running the generator below 50% of its rated power for long periods, as this can lead to carbon buildup and increased failure rates.
2.4 **System Design for Diesel Generators**
- The generator room is typically located in the basement, with one or more units installed. They can run individually or in parallel.
- During emergencies, the system must automatically switch to the diesel generator, ensuring power is restored within 15 seconds.
- An emergency busbar section is required to isolate critical loads and maintain reliability. The phase sequence of the generator must match that of the main power supply.
2.5 **Applicable Environment**
- The generator must start within 15–30 seconds after a power failure. For critical loads requiring faster response, an online UPS system may be added.
- The generator should support short-term overload (up to 25%) and have a quick-start capability for multiple attempts within a short time frame.
2.6 **Fuel Tank Configuration**
- A daily fuel tank should hold enough fuel for 3–8 hours of operation. For larger volumes, an external storage tank is recommended.
- Fuel consumption varies with load: 0.115 l/kW·h at 50%, 0.2 l/kW·h at 75%, and 0.3 l/kW·h at 100%.
- Example: For two 600 kW generators running at 75% load for 3 hours, the daily tank should be around 0.9 m³. For 24-hour operation, a 7.5 m³ buried tank would be suitable.
2.7 **Control Requirements**
- The control panel should include protection against short circuits, overloads, and voltage fluctuations.
- Monitoring systems allow for real-time data tracking and fault alerts.
- Communication interfaces (e.g., RS485) enable integration with building automation systems.
- A 24V battery pack ensures reliable starting, with sufficient charge for at least six starts.
- The generator must provide stable voltage and frequency, with minimal variation under load changes.
2.8 **Grounding**
- Three types of grounding are required: working, protective, and anti-static.
- The neutral point of the generator should be directly grounded, with proper interlocking to prevent circulating currents.
- All metal parts and fuel system components must be properly grounded.
- Grounding systems should be shared with the building’s overall grounding network for consistency and safety.
2.9 **Additional Design Considerations**
- Short-circuit current is typically around 10 times the rated current.
- Motor sizes should not exceed 25% of the generator’s capacity to avoid excessive starting loads.
- Ensure the generator room has proper ventilation, noise control, and access for maintenance.