# A Review of Harmonic Detection, Suppression, Aggregation, and Estimation Techniques
## 4 Harmonic Aggregation
The aggregation factor at low-order harmonics is more significant than that of high-order harmonics; the aggregation factor is high at harmonics while low at inter-harmonics. Figure 8 shows the 10 wind turbine park models used as a case study for the aggregation model presented in [49]. The Monte Carlo simulation is performed for the aggregation of harmonic emission from wind parks; the study found that aggregation was different for low-order, high-order, and inter-harmonics. Aggregating current harmonics in low voltage (LV) networks is one of the critical aspects of harmonic analysis. The characteristics of residential customers' harmonic current emission and propagation depend significantly on the number of users [50]. The aggregation (as well as the summation) is addressed while performing the summation of harmonic currents by taking different combinations of load devices [51]. Energy-efficient LED lamps comprise a considerable portion of the lighting load in modern buildings, and their harmonic profile depends on the internal filtering circuit [49]. These energy-efficient lamps are categorized into four types, depending on the current waveform. Researchers have found that the impedance of cables (connecting the devices to PCC) and the thermal stability of the equipment (loads and measurement devices) affect the harmonic fingerprint of loads [52; 53]. The magnitude and phase of current harmonics are small and precise values, so measurement accuracy is critical [15]. In [15], authors have devised an accurate measurement setup that takes into account the uncertainty related to the system and measurement device. Accurate summation can be carried out when the phase angles involve the aggregation procedure and magnitudes [54]. Figure 9 represents the block diagram of the setup used in [54]; the relationship between the harmonic current variation and the number of lamps has been observed for high and low-order harmonics. As the high-order harmonics phasors have more spread on complex plane compared to low-order harmonics, the deviation in aggregated sum and the actual measurement of the high-order harmonic will be more significant than that of low-order harmonics. The reason behind this deviation is the phenomenon of harmonic cancellation. Harmonic cancellation is more visible when several load types are connected simultaneously at the point of common coupling (PCC) and the number of loads is connected. Current harmonics from energy-efficient lamps and electric vehicle (EV) loads are measured individually and combination-wise in [43]. After the accurate aggregation of load harmonics, it was established that the cancellation effect is directly related to the number of loads connected at PCC and is more visible in high-order harmonics. The IEC 61000-4-30 [55] standard clarifies harmonic aggregation and synchronization of each measurement interval's start and end time. Data collected during the standard measurement interval is challenging to process as the power system's operation changes continuously due to the integration of renewables, especially EV loads; therefore, ref. [56] presents the method of the reduced data set that aids the continuous assessment and  
Figure 8: Ten wind-turbine park configurations. Figure 9: Measurement setup. aggregation of harmonics. The large-scale integration of EVs poses new challenges for distribution system operators, so the model proposed in [57] discusses the impact of the aggregation interval; the results indicate that 10 min of aggregation will lead to a significant deviation (>30%) in terms of maximum current harmonic magnitude, as illustrated in Figure 10.  
Because of the impact of current and future loads on the propagation of harmonics in the network, harmonic aggregation models are necessary to accurately assess distortion in the system. In [58], a method for creating a stochastic aggregation model is described.