Addressing industrial power quality problems

July 2nd, 2018, Published in Articles: Energize

Industrial  power quality solutions generally consist of devices to deal with individual problems, and if more than one problem needs to be dealt with, more than one device is required. The unified power quality conditioner (UPQC) is designed to cope with multiple PQ issues.

Power quality problems experienced at the grid connection point of industrial systems include current harmonics, low power factor, resonance and high frequency interference. Problems experienced at the load side include voltage harmonics, voltage sag and spikes, and well as conducted interference. Most existing solutions, such as DSTATCOM and DVR systems, tend to deal with one problem only, such as harmonics, and treat grid and load problems separately. Static shunt compensators suffer from source voltage oscillations, while, static series compensators cannot operate in nonlinear load condition correctly [1].

Based on these two compensator device characteristics, the unified power quality conditioner (UPQC) was invented in 1998 [3]. It can operate in both nonlinear load condition and source voltage oscillations to improve current and voltage specifications [2]. The UPQC can deal with multiple problems at the same time and offers an integrated means of dealing with power quality problems, especially in industrial applications. In recent years, most of the attempts to improve the performance of this device were focused on new control techniques to overcome to previous control schemes drawbacks.

Fig. 1: UPQC configuration [1].

UPQC basics

The most complete configuration of hybrid filters is the unified power quality conditioner (UPQC), which is also known as the universal power quality system (UPQS). the UPQC is a multifunction power conditioner that can be used to compensate a wide variety of voltage disturbances of the power supply, to correct voltage fluctuation, and to prevent harmonic load currents from entering the power system. It is a custom power device designed to mitigate the disturbances that affect the performance of sensitive and/or critical loads. The UPQC has shunt and series compensation capabilities for harmonics, reactive power, voltage disturbances, and power-flow control. A UPQC consists of two voltage-source converters with a common DC link (Fig. 1).

The two converters are driven by a controller, which generates the control signals to drive the converters. One converter is connected in series through a transformer between the source and the critical load at the PCC and operates as a voltage-source inverter. The other converter is connected in shunt at the PCC through a transformer and operates as a current-source inverter. The active series converter compensates for voltage supply disturbances, performs harmonic isolation, and damps harmonic oscillations. The active shunt converter compensates for load current waveform distortions and reactive power, and performs the DC link voltage regulation.

Converters

The converters consist of a 3-phase bridge bidirectional arrangement of switches, which could be IGBTs. The power for the bridge is drawn from the storage capacitor. a typical arrangement is shown in Fig. 2.

The DC link and storage

The key component of the UPQC is the DC link and storage, which provides the power for the switches in the active power filters. Other active power devices such as Statcom and DVR also use storage, but the UPQC combines the storage of both the series and shunt filters into one unit. The DC storage takes the form of a capacitor, which can absorb or supply energy to the units. In compensation process, the DC side voltage will change because the UPQC compensates the active power and the losses of switches, etc. If the DC voltage is not the same as the rating value, the output voltage of the series active filter will not equal to the compensation value. The compensation will not correct. It is the same with the shunt active filter. A  DC voltage regulator used to generate a control signal to keep the voltage constant. It forces the shunt active filter to draw additional active current from the network.

Fig. 2: Typical converter configuration [1].

Operation

The operation can be understood by considering the series and shunt components individually.The operation of the device can be understood by considering the operation of the two separate arms, in terms of the operation of the equivalent individual devices.

Shunt active power filter (APF) operation

In shunt operation which acts as a voltage source converter, the active filter injects current at the point of load coupling to produce a distortion free current as seen by the supply. This cancels out the “harmonic” content of the load current presented to the source. Static shunt compensators, such as DSTATCOM and APF, are mainly intended for conditioning the current flowing from a load into the network. The shunt active power filter provides the current and the reactive power  compensation(as the system needs). It acts as a controlled current generator that compensates the load current to force the source currents drawn from the network to be sinusoidal, balanced and in phase with the positive-sequence system voltages.

Series APF operation

The series active power filter provides the voltage compensation. It generates the compensation voltage that, synthesised by the PWM converter and inserted in series with the supply voltage, forces the voltage at the PCC to become sinusoidal and balanced.

Fig. 3: Basic concept of shunt active power filter operation [1].

In series operation active power filter introduces a voltage in addition to the supply voltage that produces an undistorted sine wave, of the required value at the point of load connection.

The controller

The functions of the controller are twofold:

  • To analyse the current and voltage waveforms and determine the “error” or difference between the waveform and a “ perfect” reference waveform.
  • To generate signals to the converter with sufficiently fast response time to allow “instantaneous” correction of the error.

To perform these functions on a single phase circuit is fairly simple but to do the same on three phases simultaneously is a complex operation.  Where the UPQC is used on three phase equipment, the distortion on each phase is likely to be similar, making the operation much easier. On three phase distribution feeds, the distortion on each phase can differ significantly and analysis and correction becomes more difficult.

There are many techniques used for analysis, but all rely basically on comparing the sampled waveform with a reference or “perfect” waveform. The difference between the systems lies in how the perfect waveform is generated. While the UPQC configuration is fairly simple, the complexity lies in the controller. The controller is the heart of the UPQC system. The UPQC is an active filter and as such requires an active power filter controller for operation.

The APF controller output is applied to the input or control inputs of the inverters. The efficiency of a good UPQC system depends solely upon its controlling algorithm. The UPQC control strategy determines the current and voltage reference signals and thus, decides the switching times of inverter switches, so that the expected performance can be achieved. Derivation of the controlling signals for the active filter involve complex 3-phase matrix modelling, transformation and analysis and will not be discussed in detail here.

There are numerous techniques that have been developed to control UPQC systems. The main control strategies which have been introduced are divided into two categories: Time domain techniques and frequency domain techniques. Time domain techniques such as instantaneous power theory (PQ theory), constant source instantaneous power,  synchronous reference frame control and sinusoidal source current technique have been introduced to control active power filters [3].

Nevertheless, whereas a low pass filter is used in their algorithms, time delay in their performance makes their response inaccurate and disrupted for compensating high order harmonic components. Moreover, these algorithms assume that the harmonic components of each phase are identical to the harmonics of other phases, so these algorithms can’t be used to compensate any disturbances in distribution systems because in practice, the harmonics of three phase currents are not identical.

Fig. 4: series voltage compensation [1].

Frequency domain techniques are based on the Fourier transform analysis of harmonics, and the FFT algorithm has been developed to extract the components which should be compensated by the UPQC. This algorithm is very simple from an operation point of view and it is free from drawbacks which were mentioned for time domain techniques. If the harmonic components of each phase are not identical to the harmonic of other phases, this algorithm can operate correctly.

All methods used extract the difference between the actual current and voltage waveforms and an ideal reference (error signals) and use this to generate the control signals for the active power filters. The difference between the methods lies in the effectiveness of responding to rapid changes such as spikes and glitches, and the amount of distortion which can be corrected.

Instantaneous active and reactive power (PQ theory)

In power systems, the electric power can be defined as the rate at which electric energy is transferred per time unit through a given cross section of a transmission line. The instantaneous power concept emerges when the time period observed during the energy flow becomes infinitesimal, representing the power at each instant. The theory is the set of power definitions, the explanation of its properties, the relationship among these concepts and their physical interpretations, combining mathematics, physics and technology models [4].

The PQ theory is based on instantaneous power as defined in the time domain. It can be applied to 3-phase systems with or without a neutral conductor. It is valid not only in the steady state but in the transient state. The PQ theory considers  the 3-phase system as a unit, not as the sum or superposition of three single phase systems. The system uses the Clarke transformation from abc to αβ0 co-ordinates and defines instantaneous power on αβ0 co-ordinates. The instantaneous power is calculated to yield the following quantities.

Fig. 5: Shunt active filter in a three-phase power system [5].

Where:

p = the instantaneous real power

q = the instantaneous imaginary power

po = the instantaneous zero sequence power

Fig. 5 shows an example of the shunt compensator of the type used and Fig. 6 illustrates the block diagram of the calculation process [5].

From the measured values of the phase voltages (Va, Vb, Vc) and load currents (Ia, Ib, Ic), the controller calculates the reference currents (Ica*, Icb*, Icc*, Icn*) used by the inverter to produce the compensation currents (Ica, Icb, Icc, Icn).

Synchronous reference frame control method

The conventional SRF method can be used to extract the harmonics contained in the supply voltages or currents. For current harmonic compensation, the distorted currents are first transferred into two-phase stationary coordinates using (α−β) transformation (the same as in PQ theory). After that, the stationary frame quantities are transferred into synchronous rotating frames using cosine and sine functions from a phase-locked loop (PLL). The sines and cosine functions help to maintain the synchronisation with supply voltage and current. Similar to the PQ theory, using filters, the harmonics and fundamental components are separated easily and transferred back to the (abc) frame as reference signals for the filter. The conventional SRF algorithm is also known as (dq) method, and it is based on (abc) to (dq0) transformation (park transformation), which is used for active filter compensation.

Fig. 6: calculations for a constant instantaneous power supply strategy [5].

Developments in control methods

Both the PQ and the SRF methods require complex operations and have drawbacks in operation. Most research is focussed on alternative methods, such as Kahlman filtering, the FFT, artificial neural networks (ANN),  and other derivatives such as space vector modulation. The fundamental configuration of the device, however, remains the same.

References

[1] P Prasad, et al: “ Unified Power Quality Conditioner (UPQC) With Storage Device for Power Quality Problems”, International Journal Of Engineering And Science, Vol. 3, Issue 8.
[2] G Juanjin: “Unified power quality conditioner (UPQC): the principle, control and application”, Journal of Scientific Research, 2010.
[3] M Forgan: “A Fast Control Technique for UPQC Control System Using Improved FFT Algorithm”,19th international power systems conference, 2004.
[4] A Montanari: “Enhanced instantaneous power theory for  control of grid connected voltage sourced  converters under unbalanced conditions”, PHD Thesis, University of Manitoba.
[5] J Afonso, et al: “Active filters with control based on the PQ theory”, IEEE Industrial Electronics Society Newsletter, vol. 47, number 3, September 2000.

Send your comments to energize@ee.co.za