Sonel PQM analyzers case study #11: No back-up power supply from the generator

Backup power supply is often a critical component of the electricity infrastructure in facilities where safety is fundamental (e.g. hospitals, server rooms) or which incur high loss costs (e.g. manufacturing processes). Internal combustion engine-driven generators are commonly used where power backup is essential. When properly selected and serviced on an ongoing basis, a generator is an effective source of power.

Description of the identified problem

A generator with a continuous apparent power of 124 kVA (and active power of 99 kW) provides emergency power to the fans and automation, which ensures the safe shutdown of the industrial furnace in the event of a mains power failure. During a mains power failure, the generator automatically starts up, yet without powering critical equipment. This results in the furnace cooling fans not being switched on.

Measurement tools used

• Sonel PQM-710 analyser
• Set of F-2A clamps (In=3000 A)
• Sonel Analysis software

Figure 1 Voltage and current waveforms with 10 ms averaging, no current flow after generator start-up

 

Figure 2 Voltage and current waveforms with 200 ms averaging, no start-up sequence of critical devices

 

Figure 3 Voltage and frequency waveforms with 200 ms averaging

 

PRELIMINARY CONCLUSIONS:

1. The first attempt to supply power from the generator failed (Fig. 1), as a too short delay was set between the start-up of the generator and the switching on of critical equipment.
2. After extending the delay time, the automatic start-up sequence of the other fans (fields no. 1 in Fig. 2 and Fig. 3) still did not function properly. Significant voltage and frequency fluctuations were observed due to significant inrush currents (Fig. 2).

An analysis of the preliminary findings was followed by a manual start-up sequence for all the required critical equipment to safely extinguish the furnace. The aim of this experiment was to verify that the generator power was sufficient to rule out this reason. Averaging every 200 ms was used to ensure precise measurements. Fig. 4 shows the waveforms of power consumption from the generator when switching on successive load points. The end result was the correct operation of all required appliances. The apparent power at the highest load reached 81.95 kVA, while active power was 53.03 kW. At steady state, the relevant powers were 48 kVA and 40 kW.

 

Figure 4 Three-phase apparent and active power waveforms with 200 ms averaging for the manual sequence.

 

With the averaging of the measurements every 200 ms, one can precisely see the voltage and frequency fluctuations during the inrush currents in Fig. 5. However, the operation of the equipment was not negatively influenced by these fluctuations.

 

Figure 5 Voltage and frequency waveforms with 200 ms averaging for the manual sequence

 

FINAL CONCLUSIONS:

1. The use of the Sonel PQM-710 analyser, with its short measurement averaging periods, provided a clear solution to the identified problem. 
2. First, it was shown that the generator's failure to supply power to the equipment was caused by a too short time delay between generator start-up and equipment start-up.
3. Next, the generator was demonstrated to have an adequate power reserve needed for critical equipment when the furnace was extinguished. During the manual start-up of the appliances, apparent power reached values of up to 81 kVA and active power up to 57 kW. Once all the required appliances were started up, apparent power stabilised at 47 kVA and active power at 40 kW. In no case were the generator's continuous powers exceeded.
4. The reasons for the lack of power from the generator should be sought elsewhere (e.g. poor settings of the generator switching automation, faulty connections or damage to the automation components).

RECOMMENDATIONS:

1. It is necessary to check the correctness of the connections in the switchboard of the appliances responsible for switching on critical equipment.
2. It is necessary to check whether the automation has been damaged.
3. The automation settings should be diagnosed.

 

Author:
Marcin Szkudniewski