Selection of neutral point earthing method in diesel engine generator

With the rapid development of information construction, the dependence on information system and data integrity is more and more high. The data center widely uses diesel generator as the backup power system of the public power grid. The neutral grounding mode of 4stroke 6cylinder diesel engine is closely related to the reliability, insulation level, over-voltage protection, relay protection and automatic device configuration of power supply and distribution system in data center.

Neutral grounding

The system neutral point grounding is divided into 6kV ~ 750kV power system neutral point grounding and 220V / 380V low-voltage distribution system neutral point grounding. Neutral grounding refers to the neutral point of secondary coil of transformer. The neutral grounding system of LV distribution system is divided into IT neutral ungrounded system, TN neutral directly grounded system and TT neutral directly grounded system.

According to the requirements of Code for Design of Overvoltage Protection and Insulation Coordination of Ac Electrical Equipment (GB / t50064-2014) for effective grounding of neutral point, the ratio of zero sequence reactance to positive sequence reactance of the system shall be positive and not more than 3. The ratio of the equivalent zero sequence resistance of the system with the neutral point grounded through low resistance to the zero sequence reactance of the system shall not be less than 2, the grounding resistance value shall be 10-20 ohms and the general grounding fault current shall be 100-1000A, which can quickly cut off the single-phase grounding fault, improve the system safety level and reduce personal injury and death accidents.

Grounding of one unit

Code for Design of Data Center (gb50174-2017) mentions that one of them shall be grounded when multiple diesel generator units operate in parallel. Refer to the unit grounding diagram in 15D202-2 "design and installation of diesel generator set" for details. The neutral point grounding mode of data center public network system and standby power supply high-voltage diesel generator system has influence on the power frequency stress voltage of low-voltage device.

Selection of grounding mode

Code for Design of Data Center (GB50174-2017) stipulates that the neutral point grounding mode of 3kV～10kV standby diesel generator system shall be determined according to the common power grounding mode and single-phase grounding capacitance current value of the line. When the common power supply adopts the non-effective grounding system, the neutral point grounding of high speed 6cylinder diesel engine system should adopt the non-grounding system. When effective grounding system is adopted for common power supply, ungrounded system or low resistance grounding system can be adopted for neutral point grounding of diesel generator system. When the neutral point of diesel generator system is grounded without grounding system, grounding fault alarm shall be set. When multiple diesel generator units are running in parallel and the low resistance grounding system is adopted, one of them can be grounded.

When the neutral point of the high-voltage diesel generator set in the data center is ungrounded, high resistance grounded and arc suppression coil grounded, the power supply reliability and the insulation level of the equipment are high. The reliability of direct grounding and small resistance grounding is low and the insulation level of the equipment is not high. The neutral point grounding mode of low-voltage 4stroke electric starting diesel engine system of 1kV and below should be the same as that of low-voltage distribution system.

Code for Design of Data Center (GB50174-2017) mentioned that the neutral grounding mode of powerful 12cylinder diesel engine system should be consistent with that of common power system. The 10kV Distribution Station of data center is generally designed with public network as normal power supply.

The design of 10kV distribution station in data center must be based on the system grounding of power system to design the primary line and secondary line of high-voltage distribution system. According to the requirements of the current national standard design code, the neutral point coil grounding mode should be adopted and the feeder line should be equipped with over-current protection. When the neutral point is grounded through the low resistance system, the over-current protection installed on the feeder and the zero-sequence current protection shall be installed on the incoming line and the feeder circuit. If the neutral grounding mode of the high-voltage diesel power generation system is the same as that of the public power grid, the relay protection system does not need to be changed to facilitate the conversion of the normal power supply and the standby power supply system. The neutral point of high voltage 4stroke turbocharger diesel engine system is grounded by high resistance, which needs to be determined by technical and economic comparison.