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1. Introduction

2. The Series APF in Harmonic Cancellation Mode

3.The Series APF as a Reactance Compensator

4.  The Series APF in Harmonic Isolation Mode

5. PSpice Simulation of a Series Active Filter in Harmonic Isolator Mode

6. Differences in DC Side Control between Series APFs and Shunt APFs

 [ This article was prepared for a two-weeks short-term course Department of Electrical Engineering, NIT Calicut offered to working professionals from  Industry and Teachers of Engineering  in June 2003. This HTML version was uploaded on 20^th March 2004]

1. Introduction

In Shunt Active Power Filtering ,the Inverter injects harmonic currents required for elimination of harmonics in the source current and injects it at the node where the load is connected. The current drawn by the Inverter is forced to contain a small in-phase sinusoidal component in order to draw enough active power from source to supply losses in the APF and to maintain the D.C side capacitor voltage constant. Series APF is the dual of Shunt APF.

In Series APF the Inverter injects a voltage in series with the line which feeds the polluting load through a transformer. The injected voltage will be mostly harmonic with a small amount of sinusoidal component which is in-phase with the current flowing in the line. The small sinusoidal in-phase (with line current) component in the injected voltage results in the right amount of active power flow into the Inverter to compensate for the losses within the Series APF and to maintain the D.C side capacitor voltage constant. Obviously, the D.C voltage control loop will decide the amount of this in-phase component.

Depending upon the location of Series APF, nature of bus voltage and nature of load the purpose of injecting harmonic voltage in series with the line can be one of the following.

(i) In this case the distribution bus (say 11 kV) is polluted and has non-negligible harmonic content. It is required to clean up this voltage before it reaches sensitive loads. Essentially we want to remove the harmonic content in the voltage at the distribution substation before it is fed into a feeder supplying harmonic-sensitive loads. The bus voltage corruption may have been due to harmonic current generating loads upstream. However, the Series APF is not aimed at that harmonic generation problem; but is applied to protect other chosen loads from the already present harmonics in the source bus. In this mode ,the Series APF senses the bus voltage and line currents and injects the right amount of harmonic voltages in series with the line in such a way that the voltages after the filter will be harmonic free and clean.Fig.1 shows the power circuit and control blocks of a Series APF working in this mode. This mode of operation can be termed Harmonic Cancellation Mode since the Series APF in this mode cancels the harmonics present in the source voltage before it gets to the load. 

(ii) In the second context, a Series APF is used to help a shunt connected Passive Filter in diverting the harmonic currents generated by a non-linear load. Tuned LC filters are supposed to have zero impedance at the tuning frequency. However, they will have non-zero value due to losses in the inductor. Hence, the tuned filter shares the harmonic current with the line and source impedance instead of absorbing it entirely. Moreover, the filter is easily detuned with ageing of components and degradation in capacitors. In addition, changes in system frequency make the filter detuned. If the filter is detuned, the harmonic current generated by the non-linear load will flow in the source path partially thereby reducing the filtering effectiveness of the Passive Filter.One way to increase the effectiveness of the Passive Filter and make it absorb all the harmonic current is to insert a high impedance in series with the line (source) before the load. Of course, this high impedance should be there only for harmonic current flow and it should go to a zero value for fundamental current flow.
The Series APF in this mode of operation, senses the harmonic current flow in the line and forces the Inverter to inject (through a transformer) a harmonic voltage proportional to this current in series with the source in direction to oppose the current flow-in short the Series APF simulates a high resistance in series with the line for harmonic currents alone. With this high resistance in the source side, the Passive Filter is forced to absorb all the harmonic current generated by the load even if it has a non zero impedance at the harmonic frequency due to detuning.

Of course, the load voltage will become distorted if the filter impedance is not zero. Moreover, a single tuned LC filter will take care of only one harmonic component. It needs multiple LC filters to handle all the major harmonics. All the LC sections will derive benefit from the same Series APF.If there are harmonic components which the Passive Filters can not absorb without distorting load bus voltage beyond acceptable levels, they will have to be permitted to flow into the source by Series APF presenting a low or zero resistance those frequency components. The Series APF used in this mode of operation is called Harmonic Isolator since it isolates the source bus at h frequencies from the polluting load.

2. The Series APF in Harmonic Cancellation Mode

Fig.1 shows a Series APF in this mode. The source V_S and inductance L_S represent the Thevenin's equivalent of the power system behind the distribution bus. V_sb is the source bus voltage which contains harmonics. Lline represents the line inductance of the feeder feeding the load bus.V_lb is the load bus voltage.The Series APF injects V_i in series with the line as shown.Single-Phase topology is considered in the interest of simplicity.

The line current I is sensed by a CT and converted to electronic level and is fed into a PLL-Counter-EPROM-DAC type sine wave generator.This block generates unit amplitude pure Sine wave which is in phase with the feeder line current.It also outputs a unit amplitude Cosine wave.The harmonic content of the source bus voltage (and thereby the voltage that the APF must inject into line) can be found out by subtracting the fundamental component of bus voltage from the total bus voltage.This requires the extraction of fundamental component.

The sensed bus voltage is multiplied with unit Sine and Cosine respectively to extract the fundamental orthogonal components of voltage.The products are integrated for a cycle duration and the value of integrals is noted by a sample and hold mechanism at the end of cycle period.After sampling the integrators are reset and allowed to perform the integration for the next period.The sampled values will give the amplitude of in-phase and quadrature (with respect to line current) fundamental components of voltage.These amplitudes are multiplied with appropriate sinusoidal templates and added to re-construct the net fundamental component of voltage and then it subtracted from the total bus voltage to get the harmonic content.

The D.C side capacitor voltage will discharge down to zero unless sufficient power is drawn from the line to meet the losses in the Inverter.This power is drawn by injecting a fundamental in-phase component against the line current flow. The amount of this component is decided by a PI controller which monitors the D.C bus voltage.

The Inverter in the Series APF carries the full line current (after transformation). But the voltage generated at the output has only a small fundamental component and has mostly harmonic components (usually 5^th,7^th,11^th etc.).Therefore the a.c side power in the inverter will be at 4^th,6^th,8^th harmonic etc. Thus, the reactive power flow into the D.C. side capacitor is at those frequencies and the ripple across the capacitor can be made small due to high frequency nature of these power components. Also, the kVA rating of Inverter and D.C side capacitor will be decided by the harmonic content in the voltage and the maximum line current.

The control system design considerations for the D.C. voltage control loop has been described already described in other contexts (SVC,Shunt APF,PWM Rectifier etc.) and need not be repeated here. The crucial control block in this application is the harmonic content calculator. The calculator has to ensure that the output from it does not contain any in-phase component. If it contains that the capacitor will either discharge fast or overcharge and in order to limit the change in capacitor voltage before the voltage control loop can act ,it will be necessary to use large valued capacitor. The calculator will ensure that there is no fundamental component in its output under steady conditions. But under transient conditions (change in line current, changes in bus voltage etc.), it may output fundamental component. The value of D.C side capacitor will be decided almost entirely by the dynamic response of this Calculator block.

The most difficult thing about a Series APF is to protect it. Note that it is in series with the line and has to carry all the load current (fundamental plus harmonic, if any) .Moreover,the fault currents also will pass through it. It is not enough to shut down the Inverter based on fast over current sensing because if the Inverter is shut down the transformer primary goes open and secondary imposes a large impedance in series with the line. A Series APF is to be shorted to take it out of service i.e. it has to be taken out of line and a sort path has to be put in its place. This can call for fast acting static transfer switches.

The Series APF effectively cancels the entire harmonic content of source bus voltage. Now what if the load at the load bus is non-linear ? The harmonic currents drawn by the load via the line will flow through the Thevenin's impedance of the source (L_s) and produce further harmonic voltages at the source bus. But these also will get cancelled by the Series APF.i.e. the Series APF makes the harmonic impedance to its left side zero. Hence the harmonic impedance of the line plus source is reduced by Series APF and any shunt filter put at the load bus will have to compete with a lower harmonic impedance in the harmonic current sharing process. In particular ,if this Series APF is installed at the load bus i.e. after the line ,a passive shunt filter (like capacitor or tuned LC etc.) is going to be completely useless and all the harmonic current will go into the line,thereby corrupting the voltage received by other loads which are not protected by a Series APF.Of course it is possible to install a Shunt Active Filter at the load bus to cancel the harmonic currents taken by load.

3. The Series APF as a Reactance Compensator

In the last section ,it was pointed out that the harmonic voltage calculator has the in-phase and out-of-phase sine waves available to it in order to arrive at an estimate of net fundamental component in the voltage. The injected voltage is harmonic plus a little in-phase fundamental component required to draw the loss power. Now,if the injected voltage reference is made to have a sinusoidal component which is in quadrature with line current , the Series APF will absorb or deliver fundamental frequency reactive power. It becomes a series reactive power compensator or equivalently it becomes a reactance at fundamental frequency. Series APF in this mode can provide series capacitor/inductor compensation to the line along with harmonic cancellation. The required voltage reference can be obtained using the Cosine wave template already available in the control system. Of course, the kVA rating of Inverter and other components will have to be suitably chosen.

This series compensation capability can be made use of in two ways. In one case, it can be controlled in such a way that it injects a fundamental voltage in quadrature with the line current in proportion to the current magnitude. In that case, the Series APF becomes a fixed reactance value at fundamental frequency - usually capacitive. The application of series capacitors in the transmission lines to improve power transfer capability,system stability and voltage regulation is well known. Series APF can implement this series capacitor compensation as explained above.

A second way in which the fundamental Var compensation capability of Series APF can be employed is by using it to regulate the voltage after the Series APF location at a pre-decided value. This can be done by sensing the voltage magnitude downstream ,comparing it with a set value,processing the error in a PI Controller and using the error to multiply Cos (w t+180⁰) .The product is added along with the harmonic reference coming from the Harmonic Content Calculator to form the net reference signal for the PWM Inverter.Thereby the value of effective capacitive reactance at fundamental frequency simulated by the Series APF is varied to maintain a constant amplitude a.c voltage at a point after the Series APF.

In both ways of implementing the Var compensation action,it is possible to derive additional advantage from Series APF in the form of a fault current limiting reactance (provided the Series APF has high overload capability for short duration).When the sensed line current indicates the occurrence of a fault(either by p.f angle or by its magnitude) the Series APF fundamental reference can be shifted from -Cos w t to +Cos w t. Then the Series APF will simulate an inductive reactance and thereby limit the fault current.Series APF is used for this function too in practice. 
 

4. The Series APF in Harmonic Isolation Mode

Fig.2 shows a Series APF in harmonic isolation mode where it serves to isolate the source from the harmonic currents drawn by the load.it does this by simulating a high resistance in series with the line for harmonic current flow. Only one tuned passive filter is shown at the load bus and it is assumed that the load draws one harmonic component predominantly. Otherwise more tuned filters are needed at the load bus.

The source current I_s is sensed and a pure sine wave is in phase with it is generated by the PLL subsystem. The sine thus generated is used to extract the harmonic content in the source current using the orthogonal decomposition method which has been described in the last section. The extracted harmonic component of I_s is multiplied by a gain K and that (along with the small fundamental component needed to draw the loss power) is given as the reference to the PWM Voltage Source Inverter.Thus,the Inverter injects a harmonic voltage which is proportional to the harmonic current into the line ,thereby simulating a resistance of value K ohms in the line (only for harmonic current flows).

Now, the harmonic current drawn by the load has two parallel paths to choose-through the filter and through the line which appears as a high resistance now. It chooses filter path predominantly even if the filter is slightly detuned. Thus, the harmonic current into the source is reduced to very low levels.

Similar action takes place in the case of harmonic content in the source. The high resistance simulated by the Series APF will absorb all the harmonic voltages (for which there are passive filter branches at load end) present in the source bus and isolate the load from supply side harmonics. This is a welcome feature since in the absence of Series APF, the tuned filter would have drawn large currents from source if there were source side harmonics. This would have led to overloading of the filter and would have called for parallel tuned LC section in series with the line to isolate the series tuned filter from supply side harmonics.

With a series tuned LC filter,there is always a chance of system resonances due to parallel resonance between line/source inductance and filter components. The Series APF in the resistance emulation mode will damp these resonances well and will avoid dangerous harmonic amplification.

It is possible to combine series capacitor compensation along with harmonic isolation in this system by suitably modifying the reference signal to the PWM Inverter.

5. PSpice Simulation of a Series Active Filter in Harmonic Isolator Mode 
    (Also called Hybrid Active Filter)

The PSpice Simulation diagram (using Microsim Design Lab 8.0) for a Single Phase Series APF in Harmonic Isolation mode is given below. The inverter was modelled as an ideal controlled voltage source. A half-controlled thyristor converter is used as load.

The simulation run results for a pure sinusoidal source and thyristor load is shown below in Fig. 4.The harmonic calculator takes one half cycle to calculate the harmonic content properly, till then it outputs all the input as the harmonic content ; this explains why the inverter had to inject maximum (limited to 50V) in the beginning.This will lead to a large active power outflow from the DC Side of Inverter and will require a suitably sized capacitor to hold the voltage against such outflow (or inflow) of active power.The filter is seen to take a large leading reactive power – expected since passive filtering is practically possible only along with passive capacitor reactive compensation.The value of inductor required for harmonic filtering alone (without fundamental leading reactive power) will be impracticably high.The source current, though more or less sinusoidal, shows high frequency content.This is so since the load current has high frequency content , but the passive filter offers low impedance path only for a few harmonics.Thus the current sharing ratio between the Series Inverter equivalent resistance (40 ohms in the simulation) and filter impedance is adversely affected at high frequencies – leading to more of high frequency currents flowing to source side and consequent appearance of high frequency harmonic content at the load terminal voltage.

The simulation results for a distorted source containing 10% fifth harmonic is shown in Fig. 5 below. Now the source current is distorted perceptibly – since the inverter has to absorb all the source fifth harmonic across it. However the load voltage is more or less sinusoidal with a little h.f content which is due to the h.f content in load current as eaplained above.Thus it can be seen that the Series APF handles all those load current harmonics and source voltage harmonics for which there are tuned passive filter structures at load bus well. And it can not handle the harmonic components for which there is no low impedance path across load bus.The distortion reported in this simulation will be on the optimistic side due to the neglecting of switching frequency filter of the series inverter.

6. Differences in DC Side Control between Series APFs and Shunt APFs

Both Shunt and Series APFs with self-sustained DC bus (i.e a Capacitor holding DC Voltage constant without any AC-DC Converter to help it to do so) control their DC Side voltage by drwaing a small amount of active power from ac side to supply the losses in the inverter.In the Shunt APF this is done by a PI Control loop on the DC Voltage injecting an active current component into the reference current of the APF. Correspondingly a similar control loop will inject a sinusoidal voltage component which is in phase with the line current to draw/supply the required adjustment power.In the Shunt APF case the loop gain of this control loop will be directly proportional to the bus voltage magnitude and hence reasonably constant.But in the Series APF case the loop gain of voltage control loop is directly proportional to the fundamental current amplitude in the line i.e the load current and hence is widely variable with line loading level.This is a major problem with the design of this control loop – a loop which is well damped under low load conditions will either be unstable or will be highly oscillatory under full load conditions.

If the function of Series APF is only harmonic cancellation or isolation (and not load voltage regulation or series reactive compensation) an easy solution to this control problem will be to replace the DC Side with a small rated single phase diode rectifier or a Battery with a Charger.In fact this is how such installations are made in practice. The rating needed of such a converter will be very small and usually is about 3-5% of the line full load capacity.The rating of Series Inverter itself will be about 10-20% of the line capacity depending on the amount of source and load side harmonics and on the extent of detuning and quality of the passive filters.

Note :- The Series APF Systems described here can be applied in three phase systems too.Usually Series APFs in three phase systems make use of three single phase inverters feeding a Y-Open Y transformer and in this case the control strategy described here can strightwaway be applied on a phase by phase basis.Only that three PLL systems are not needed. A single PLL locked onto first phase along with suitable EPROM storage can generate the six required unit sinusoidal templates.Also, there are a variety of algorithms available for harmonic content extraction which may have stndard implementations in DSP hardware.One of those can replace the harmonic extraction procedure described here (but not with much advantage in performance !)
 
 

Download the PSpice Schematics File Described in this Article (Microsim Design Lab 8.0)