Technical Articles
The electrical grid is an enormous source of power and can be referred to as infinite bus. There are innumerable loads connected to the grid and a change in any of those loads accounts to only a small fraction of the total grid capacity. The effect of load changes are less on the grid, in terms of stability of supply voltage. It can be called a 'stiff' power source or having high inertia. Diesel generators (DG) sets, however, are used as a backup supply or even permanent power source in some systems. As any change in load connected to the DG set accounts for a considerable percentage of its total capacity, it can easily affect its output voltage. DG responds to any small change in current demand immediately, even at the expense of output voltage. It means DG set has low inertia and can be called 'soft' power source.
Another difference between grid and Diesel Generator sets is that DG sets have a very high source impedance that can be as high as 25-30%, as compared to transformer impedance of 3-5%. Hence, the short circuit breaking capacity of the Short Circuit Protection device (SCPD) used under the grid is higher as compared to SCPD used for loads connected with a DG set. It is important to note that harmoric distortion is a function of source impedance. As the steady state impedance in case of DG set is very high, presence of even small amount of I-THD creates large V-THD when it travels through the high impedance.
Let us consider an example with a total load current of 1000 A in which 700 A is the nonlinear current. Here, nonlinear loads are drives with harmonics of say 55%.
V-THD can be calculated considering transformer impedance of 5% in case of grid and DG impedance of 30% in case of Diesel Generator.
For the said % I-THD, approximate values of % V-THD are indicated as follows:
| Source (%Z) | % I-THD | %V-THD |
| Electricity Grid (5%) | 38.5 | 1.92 |
| Diesel Generator (30%) | 38.5 | 11.5 |
Therefore, it can be clearly seen that due to higher source impedance in case of DG set, the same I- THD leads to a higher levels of V-THD.
Due to the differences in the two sources, DG sets behave differently in presence of harmonics. This article discusses about the impact of harmonics on Diesel Generators and the ways to mitigate the effects.
Some important effects in Diesel Generators in the presence of harmonics are:
1) Flat topping
As discussed at the outset, Diesel generator is a soft source, which means it cannot sustain sudden changes in load very well. Majority of the nonlinear loads in industries are variable frequency drives (VFD), which consist of a rectifier, a DC bus and an inverter. The capacitors of VFD's DC bus pull the current when the incoming sine wave voltage is higher than the DC bus capacitor voltage, which happens near the peak of the incoming AC Voltage. When the total VFD loads account for a significant percentage of the genset capacity, such current drawn at the peaks will cause a sudden increase in demand from the DG.
The DG being a "soft" source responds to this current demand immediately at the cost of output voltage. The voltage at DG output will drop. The AVR starts regulating and brings the voltage to the required level. But before the instant when voltage reaches the required level, the DG voltage is already clipped, this is called "flat topping". The voltage waveform occurs to be flat as seen in the following graphs:
As can be seen from the graphs, flat topping of the voltage waveforms increases with the increase in generator loading. Flat topping can reduce the actual system peak to peak voltage to levels as low as 70% of the RMS voltage. This reduction creates an excessive flow of current through the system which can lead to burn out of machines.
2. Automatic Voltage Regulator malfunction
Presence of voltage distortions implies defective functioning of one of the most critical equipment for a generator, which is the Automatic Voltage Regulator (AVR). AVR senses any change in the voltage levels due to changes in load and adjusts the excitation of the generator accordingly to maintain the same level of voltage output. In the presence of nonlinear loads, distortion in voltage supply and flat topping adds additional burden to the Automatic Voltage regulator (AVR) of the genset as it tries to compensate the voltage drop by increasing the voltage. In the presence of harmonics, the AVR won't be able to sense the voltage changes accurately due to highly distorted voltage wave. As the excitation won't be as per the correct requirement, the output voltage of the generator would suffer which leads to instability.
3. Losses and overheating
Diesel Generator includes an IC Engine working as the prime mover for the generator. Engine generates a lot of heat, which gets amplified in the enclosed environment of the Genset. Hence, heating in Diesel Generators is a bigger issue as compared to heating in transformers. Harmonics being high in frequency cause high frequency flux change in the stator leading to heating of the stator core. Increased current in the system also lead to copper losses in the armature, which in turn lead to further overheating. High harmonic currents also lead to eddy current losses in the rotor. Increased losses of various kinds degenerates the machine efficiency and life. Other effects include skin effect and insulation failure.
4. Vibrations
Certain Harmonic currents which have a negative phase sequence, on interacting with the magnetic field generate torque in opposition to the machine rotating torque. A resultant pulsating torque is generated which leads to vibrations in the machine. It is detrimental to the machine life.
Generator manufacturers recommend derating the Diesel Generator in presence of harmonics in such a way that the total non-linear loads under a genset should be less than 40% of its total capacity (as recommended by most genset manufacturers; may change from manufacturer to manufacturer). For better understanding, consider the following example.
Case 1: Suppose the DG rating is 1000 kVA
Total Current = (1000 * kVA) / (√3kVAr) = (1000 * 100)(√3 * 440) = 1312.16 A
Hence, total nonlinear current should be less than 0.40 x 1312.16 = 524.86 A
Case 2: If the nonlinear current increases in the system as a result of increased drive loads to say 700A
Then the DG should be sized as per current of (700 x 100) /40 =1750 A
DG in this case should be rated at (√3 x V x I)/1000 = (√3 x 440 x 1750)/1000 = 1330 kVA
Result: In this case, increase in nonlinear loads has increased the size of the generator by 25%.
Derating the DG set is not a permanent solution. Increasing the DG size leads to increased costs and increased fuel consumption. This remedy is neither feasible nor economical.
To avoid malfunctioning of AVR under high harmonic levels, a harmonic filter can be incorporated in its sensing circuitry, but this option is quite expensive and doesn't deal with the presence of harmonics.
Many DG manufacturers do not recommend the use of capacitor banks with DG. Even when capacitor banks are used to maintain the power factor of 0.8, which is the optimum power factor for majority of DGs, presence of harmonics in the system will lead to premature failure of capacitors. Life of capacitors connected to DG set would be less than life obtained when they are connected to the grid for the same current harmonics level (this is due to higher voltage harmonics at the same given current harmonics).
In order to limit the levels of distortion, detuned filters can be used along with 525 V capacitors for preventing harmonic amplification. But as V-THD is high in this case, use of normal reactors is not recommended as there are high chances of reactor burnout. In such systems with high harmonic distortion, special reactors for high V-THD withstand should be used. Best solution for getting rid of the effects of harmonics in systems with DG is by eliminating the harmonics using an Active Harmonic Filter. Active filter senses the exact amount of harmonic levels in the system and eliminate their presence by pumping anti-harmonics.
It is important to note that as soon as we connect AHF near to the load, the load which was earlier seeing the high source impedance due to DG set will now see a very low impedance due to AHF. Hence, there would be high I-THD circulating between the AHF and the load. In such a case the sizing of the AHF needs to be done considering a higher safety factor. Sizing of AHF under a transformer is done with a safety factor of 20%, whereas for AHF under DG systems, a safety factor of 35% should be considered.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
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