Some important consideration for the right sizing of Generators

Some important consideration for the right sizing of Generators are;

1. Voltage variation and frequency drop.

2. Motor starting capability

3. Harmonics injected by non linear loads.

What are linear loads & non linear loads?

The loads have a linear relationship between voltage and current. The current drawn will be Sinusoidal.

Non linear loads the relationship is non linear between the voltage and current. The loads draws discontinuous current in pulses, they distort voltage source supply. The metrics for non linear loads is Total Harmonic distortion (THD).

Non linear load drawing:

The current distortion Total Harmonic distortion (THD I)is the cumulative effect of the different order Harmonics normally the odd harmonics like the 3rd, 5th, 7th, 9th , 11th ….upto 50th. the 3rd is 150Hz (50 hzx3), 250hz ( 50 x5) and so on, Each order has different phase angles and sequence (Positive, negative and zero).

Now the distorted wave of each individual harmonics creates its own impedance on the line as ohms law applies (VX and cause voltage drops Ih). The ultimately distort the voltage depending on the number of switching devices.

The effect of non linear loads on the generator:

The Xd, Xd' and Xd" are mathematical quantities for evaluating the performance of a synchronous machine. As the Xd" decreases the generator will be able to handle more harmonic distortion caused by the harmonic load. Many generator specification in the industry specify 12%Xd" as a standard limit for the harmonic distortion. hence determining the distortion level is critical to selection of Generator and its performance in harmonic rich environment.

Adverse effect of non Linear loads on Generator:

Voltage distortion (THD V)is the combined effect of current distortion (THD I) and the alternator Xd" (sub transient reactance). many typical (linear Loads and Non linear ) loads face control issues because of distorted voltage supply. Water treatment Plants, datacenters, ARC furnace loads, datacenters and medical center require low sub transient reactance alternator, a common way to avoid issues of system Harmonics and voltage distortion THD V is emphasized more for generators sizing due to sizable impact of voltage distortion on electrical connected load and generator.

The THDI (current distortion) from the load might not have a huge impact on the VTHD in the system if the non linear loads are relatively smaller in quantity to the alternator.

Typically the led lighting of 100% THD I will have a voltage THD of just 4% as the relatively small current rating. But a six pulse drive of higher capacity with THD I of 45% can cause a voltage distortion of 15% even of the load is 50% of the alternator capacity. The consequence of higher harmonics can cause over heating in the generator, motors transformers and other electrical loads.

The generator can trip if the Power factor of big sized UPS with active front end rectifiers with power factor is 0.99 0r -0.99 as is the case with many such UPS, when the Generator comes on after power failure due to over capacitance and reverse flow of reactive energy into the generator (KVAR) a s the generator has the limitation if injecting the necessary reactive energy for maintaining stability, and hence trip.

Some of the serious Issue are as follows:

# Alternator Life is shortened because of thermal ageing of the insulation.

# Breakers may trip because of large voltage distortion.

# over frequency may be observed in the voltage regulator in the presence of higher harmonics.

Understanding Important Factors in sizing (case study):

UPS 150 KVA ( non Linear load) and 150 KVA linear load.

If no Voltage THD is mentioned one can follow the rule of the thumb 10%. THD I (current distortion) from the non linear loads.

System Xd" requirement = V THD/ THD I x Pulse for converter of UPS (6) x10 the Xd" with the UPS load will be =3.33%.

For F class insulated 300KVA 480 V alternator with 12% Xd" with UPS load will be 150/300x12% will be 6%, which is higher than3.33% . Hence the alternator is unacceptable.

But if the alternator is 600KVA 480 V with class f insulation and 10% Xd"the Xd" for the UPS system will be 150/600 x12% = 3% which is within the acceptable limit of 3.33%. Consequently the cost of the generator goes up.

Some suggestion to reduce alternator sizing and Capital Cost (Capex).

1. Use of UPS ( with minimum Harmonics) preferably a 12 pulse converter or Active front end converters.

2. VFD with minimum harmonics ( probable with active front end ) of Advanced Passive harmonic filter to mitigate Harmonics.

3. Use of Harmonic mitigation Transformer (HMT)

4. Balancing of single phase loads and reduction of zero sequence harmonics (3rd, 150Hz). Zero sequence impedance transformer connected in parallel to single Phase circuits.

Conclusion:

Optimizing the size of generator reduces capital cost (Capex) and operating cost (OPEX)reducing toxic gases buy reducing diesel consumption. Ultimately reducing carbon foot print. The performance of the generator and life span increases many times. 

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