Power supply is critical for many sectors. We are not only referring to interruptions in production, but for many companies, a power cut or a failure in the main system can be really critical if an auxiliary generator set is not available, as is the case in hospitals, data centers, or financial institutions.

To adequately guarantee this emergency supply, a generator set must be correctly sized from the beginning, i.e., design criteria must be followed that allow us to adapt each set to the expected supply needs, also taking into account specific energy peaks or medium and long term growth forecasts.

So, what are the design criteria for a high-performance generating sets for critical installations?

## Generating set power

The power of a generator set is defined in the ISO 8528-1 standard, which indicates the application, ratings and performance for alternating current generator sets driven by reciprocating internal combustion engines. Thus, according to this standard, the power of a generating set is “the electrical power available at the terminals of the set to supply the loads, excluding that required for the auxiliary services of the set”.

This power is expressed in KW, at nominal frequency and with inductive power factor 0.8. In addition, it is given under certain environmental conditions:

- ISO 8528-1 standard conditions: 100 kPa (150 m), 25º, 30% RH.
- Electra Molins standard conditions: 1000 m, 40º C, 30% RH

At Electra Molins we apply stricter conditions so that our generating sets can be adapted to a wider range of situations. Furthermore, according to the intended use of the generating set, ISO 8528-1 defines four power categories:

## 1. Continuous Power COP

Allows a constant load for a limited time. It is normally applied to cogeneration gensets.

## 2. First Power PRP

Allows variable load for a limited time, and continuous operation in normally off-grid installations.

b=0,7a

*Source: ISO 8528-1*

## 3. Limited-time running power LTP

Allows a constant load (maximum 500 h/year). It is normally applied to gensets operating in permanent parallel with the grid to cover peak shaving.

## 4. Emergency stand-by power ESP

Allows variable load (max. 200 h/year), and is suitable for emergency power failure.

## 2. Operating class

The operating class is also defined by ISO 8528-5, and determines the operating limits according to the tolerance of the loads to voltage and frequency variations of the unit.

Class G1 | Class G2 | Class G3 | Class G4 | |

Steady-state frequency variation | ≤2.5% | ≤1.5% | ≤0.5% | Client |

Frequency transient drop on sudden load connection | ≤-15% | ≤-10% | ≤-7% | Client |

Frequency transient time | ≤10s | ≤5s | ≤3s | Client |

Steady-state voltage variation | ≤+-5% | ≤+-2.5% | ≤+-1% | Client |

Maximum transient voltage drop at sudden load connection | -25% | -20% | -15% | Client |

Management transient time | <10s | <6s | <4s | Client |

For generator sets for critical installations, the PRP power category from the previous point and the G2 operating class from the table apply, unless there are different specifications.

## 3. Load characteristics

In the case of high performance generator sets for critical installations, the following characteristics are defined for the loads:

- Frequency: 50 Hz or 60 Hz.
- Number of phases: three-phase or single-phase.
- Number of conductors: 3F+N, 3F without neutral. Artificial neutral.
- Voltage: 400/230 V at 50 Hz, 480/277 at 60 Hz, MV>=1000 V.
- Maximum total load: S kVA, P kW.
- Regime: Stable and transient start-up regime must be considered.
- Connection sequence: We must consider the worst case, if all loads occur at the same time or staggered, and if in the case of staggered there is a preset order or we can choose the most favorable one. Normally the large loads are connected at the beginning and the UPS at the end. For gensets with turbocharged engines, the first load step is between 40% and 50% of the total power of the genset.
- Load factor: The maximum percentage of its rated power consumed by a load once the connection transient is overcome. By default, it will be FC=100%.
- Simultaneity factor: The maximum percentage of loads operating at the same time. By default, it will be FS=1.
- A reserve of at least 10% must be provided for future expansions.

## 4. Environmental conditions

At Electra Molins we apply standard conditions of 1000 m, 40º C and 30% RH, more restrictive than those required by ISO-8528. If the required conditions are different, a derating is applied according to the engine and alternator manufacturer.

Example of alternator derating (NIDEC, Leroy Somer)

**Ambient temperature**

ALTITUDE | 25 ºC | 40 ºC | 45 ºC | 50 ºC | 55 ºC | 60 ºC |

0 to 1000 m | 1.045 | 1 | 0.97 | 0.94 | 0.91 | 0.88 |

1001 to 1500 m | 1.01 | 0.97 | 0.94 | 0.91 | 0.88 | 0.85 |

1501 to 2000 m | 0.98 | 0.94 | 0.91 | 0.88 | 0.86 | 0.83 |

2001 to 2500 m | 0.95 | 0.91 | 0.88 | 0.86 | 0.83 | 0.8 |

2501 to 3000 m | 0.91 | 0.87 | 0.84 | 0.82 | 0.79 | 0.77 |

Source: NIDEC

Example of engine derating (Baudouin engine)

Source: Baudouin

## 5. Starting time

In generator sets for critical installations, the starting time is between 10 and 15 seconds from the start command until the voltage and frequency are within limits. It is important to foresee starting aids, such as the heating resistor in the engine cooling circuit, supplied by auxiliary services.

## 6. Load connection sequence / first admissible load step

The genset has limited power compared to the mains, so the connection and disconnection of loads causes frequency and voltage transients. Therefore, we have to consider the load steps to limit the transient drops of V and F within the admissible limits of the chosen ISO 8528-5 operating class:

First step: switch on after the start-up time has elapsed.

Second step, the rest after at least 5 seconds have elapsed.

Example 1: Response of a 715kVA (572kW) LTP unit

First load step: 429kW (75%)

Power factor =1

Motor: Baudouin 6M33G715/5E2

Alternator: Leroy Somer 047FC

Source: Electra Molins test bed

Example 2: Response of a 1650kVA (1320kW) LTP generator set

First load step: 726kW (55%)

Power factor =1

Motor: Mitsubishi S12R-PTTA2

Alternator: Leroy Somer 50.2L8

In this case, the first allowable load step depends on the motor-alternator combination.

Source: Electra Molins test bench.

## 7. Non-linear loads

The voltage harmonic distortion (THDU) caused by non-linear loads on the genset must be taken into account.

- Maximum THDU with linear loads: 5%.
- Permissible THDU with non-linear loads: 20%.

If THDU values exceed 20%, it is recommended to oversize the alternator.

### 8. Minimum steady state load

Operating the genset with too low a load can cause problems in the engine, so operation with less than 30% of the rated power connected is not recommended.

In conclusion, when calculating the genset power, it is not enough to know the rated power of the loads to be supplied. It is also necessary to consider factors such as the environmental conditions of the site, the sequence of connection of the loads or the presence of non-linear loads that may cause unwanted voltage harmonics. The correct assessment of these factors will determine the most suitable genset for each installation.