# BASIC COMPENSATION METHODS & APPLICATIONS IN SOLAR PLANTS

This article gives a clear idea about the differences between active and reactive power – and also about their effects – and how to compensate them by basic theoretical devices. This also gives the preliminary concepts about reactive power generation. It is to clarify the existing misconceptions about them…

In many books we find that total network has two main quantity one current and second is voltage and both measured at same time can give power. This power has also two components as one cosine component that is active power and other is sine component that is known as reactive power – and obviously this sine and cosine have an angle between voltage and current. Here these two components behave differently as one is well defined in average value that is active power, and reactive power has average zero in all conditions whether system is in which state.

The fundamental definition of reactive power can be explained by first looking at the relationship between a sinusoidal voltage and current waveforms of the same frequency. Reactive power has its origin in the phase shift between these two waveforms. When a device consumes real power such that the voltage and current waveforms are in phase with each other, the device consumes zero reactive power. When the current defined “into” a device lags the voltage, it consumes reactive power The amount of reactive power consumed by the device depends on the phase shift between the voltage and current.

Now here it is important to understand how reactive power is produced or how it is differing from active power. Actually, when current passes through inductor it is not able to change instantaneously because inductor store current in its magnetic field, and same voltage is not able to change in capacitor instantaneously. So here one quantity is changing suddenly but second quantity flows as it is. So now the problem is that one quantity is some angle behind from first one. So now if we define instantaneous power, which is multiplication of current and voltage of circuit.

Consider a simple RLC series circuit, fig. 1, with resistance R, impedance L and capacitance C, supplied by an alternating power source with maximum voltage Vmax. The voltage and current at a point are expressed by:

Where  v=Vmax cosa(wt)…………………………..……(1)

i=Imax cosa(wt-q)………………………………(2)

Consider a simple LRC series circuit, fig. 1, with resistance R, impedance L and capacitance C, supplied by an alternating power source with maximum voltage Vmax. And maximum voltage Imax. So maximum instantaneous power can given as

P = v*i

= VmaxImaxcosa(wt)*cosa(wt-q) Now by giving simplification we can

get: P= (VmaxImax[cos(q)*(1+cos]a(wt))/2) +

(VmaxImax[sin(q)*sin]a(wt))/2)……………………..   (3)

It is customary to call the first part of equation (3) the instantaneous active power and the second part of equation (3) the instantaneous reactive power. There is a fundamental difference between the two powers that can be seen in fig. 2. The active power oscillates around an arbitrary average value while the reactive power oscillates around a zero average power as the average value of a cos and sin function is zero. This observation is valid under any conditions as long as the oscillations are sinusoidal.

It is not practical to work with instantaneous quantity, because it oscillates as frequency of supply, so we are using average value of oscillating quantity. Now, here average power of instantaneous power can be given as:

P=VI*cosq

Where V,I is RMS values. This quantity is obtained from averaging in time the first part of equation (3). Following the same method for the instantaneous reactive power, the average is zero and therefore is useless to introduce a quantity that is always zero. Another quantity is introduced to describe the instantaneous reactive power that is transferred to the network and this quantity is the maximum of the instantaneous reactive power,

Q=VI*sinq

Which is exactly equal to the second part of equation (3).

Here we can also summarize all discussions by following:

Active power

Instantaneous power = (VmaxImax[cos(q)*(1+cos]a(wt))/2

Average active power P=VI*cosq

(Simply called as Active power)

Maximum instantaneous active power is generally ignored.

Reactive power

Instantaneous power= (VmaxImax[sin(q)*sin]a(wt))/2

Average reactive power is assumed to be of zero value.

Maximum instantaneous power

Q=VI*sinq

(Simply called as Reactive power)

Here Active power is flowing around average magnitude line. while Reactive power has average zero that means in first half cycle how much power is flowing in one direction in second half cycle direction of the power go in reverse direction. That gives average power zero but still reactive power has some instantaneous value. That indicate flow of ENARGY in case of reactive power.

Here if we say there is loss in reactive power then the loss is taking place in instantaneous value of reactive power. Now actually reactive power can be simply understood from the following example:

Here as shown when we throw a ball from A to B person, get two components :

• Force component (cos in fig.)
• Transmitting component (sin in fig.)

Here actually sine components do not contribute in the force that is experience by person B. Means just cos component will reach to person-B and sine component helps in transmitting power from A to B and also give controlling action for the force. The same concept is used in actual system also. Reactive power is not increasing the power magnitude but it helps in control voltage and as the active power is transmited from one end to other-end.

Now consider two more parameters as force on ball and speed of ball. This two can be related with voltage and current in electrical language respectively. Means as to control direction of the ball, we have to control angle of throw which basically gives speed and force on ball. The same in electrical angle between voltage and current gives this controlling action.

Basic of compensation

Till we discuss about Reactive power, but afterword we are going to discuss about compensation. So all problems are caused due to difference between angle of voltage and current. So in compensation we basically try to correct that angle as correctly as our requirement by capacitors and inductors.

Need for reactive power compensation

The main reasons for reactive power compensation in a system are:

• Increased system stability
• The voltage regulation
• Reducing losses associated with the system and
• To prevent voltage, collapse as well as voltage sag
• Better utilization of machines connected to the system

Reactive power supply is essential for reliably operating the electric transmission system. Inadequate reactive power has led to voltage collapses and has been a major cause of several recent major power outages worldwide. Dynamic capacitive reactive power supplies were exhausted in the period leading up to the blackout. A voltage collapse can take place in systems or subsystems and can appear quite abruptly. Continuous monitoring of the system state is therefore required. And also indicate that reactive power planning and dispatching play an important role in the security of modern power systems.

Basic principle for reactive power compensation

Shunt compensation

Fig 4,5 shows the principles and theoretical effects of shunt reactive power compensation in a basic AC system, which comprises a source V1, a power line, and a typical inductive load. Fig. 4 shows the system without compensation and its associated phasor diagram. In the phasor diagram, the phase angle of the current has been related to the load side, which means that the active current Ipis in phase with the load voltage V2, Since the load is assumed inductive, it requires reactive power for proper operation and hence, the source must supply it, increasing the current from the generator and through power lines. If reactive power is supplied near the load, the line current can be reduced or minimized, reducing power losses and improving voltage regulation at the load terminals.

This can be done in three ways: 1) with a capacitor; 2) with a voltage source; or 3) with a current source. In Fig. 5, a current-source device is being used to compensate the reactive component of the load current. As a result, the system voltage regulation is improved and the reactive current component from the source is reduced or almost eliminated. If the load needs leading compensation, then an inductor would be required. Also, a current source or a voltage source can be used for inductive shunt compensation. The main advantage of using voltage- or current-source Var generators (instead of inductors or capacitors) is that the reactive power generated is independent of the voltage at the point of connection.

Series compensation

Var compensation can also be of the series type. Typical series compensation systems use capacitors to decrease the equivalent reactance of a power line at rated frequency. The connection of a series capacitor generates reactive power that, in a self-regulated manner, balances a fraction of the line’s transfer reactance. The result is improved functionality of the power transmission system through:

• increased angular stability of the power system;
• improved voltage stability of the system;
• optimized power sharing between parallel circuits.

Like shunt compensation, series compensation may also be implemented with current- or voltage-source devices, as shown in Fig. 4, also with the reference angle in V2, and Fig. 6 shows the results obtained with the series compensation through a voltage source, which has been adjusted again to have unity power factor operation at V2, However, the compensation strategy is different when compared with shunt compensation.

In this case, voltage Vcomp, has been added between the line and the load to change the angle of V2’, which is now the voltage at the load side. With the appropriate magnitude adjustment of Vcomp, unity power factor can again be reached at V2. As can be seen from the phasor diagram of Fig. 6, Vcomp generates a voltage with opposite direction to the voltage drop in the line inductance because it lags the current Ip. As was already mentioned, series compensation with capacitors is the most common strategy, on this platform, the main capacitor is located together with overvoltage protection circuits.

The overvoltage protection is a key design factor as the capacitor bank has to withstand the throughput fault current, even at a severe nearby fault. The primary overvoltage protection typically involves nonlinear metal–oxide varistors, a spark gap, and a fast bypass switch. Secondary protection is achieved with ground mounted electronics acting on signals from optical current transducers in the high-voltage circuit. Independent of the source type or system configuration, different requirements have to be taken into consideration for a successful operation of Var generators. Some of these requirements are simplicity, controllability, dynamics, cost, reliability, and harmonic distortion. The following sections describe different solutions used for Var generation with their associated principles of operation and compensation characteristics.

Remark

Through the above discussion, we are able to understand the basic about the reactive power and also about basic compensation principle, and the recent trend is going on IGBT, IGCT, but still, one has to know what is actually reactive power – so that one is able to compensate and also control flow of the reactive power.

Case Study for 10MW GNFC project located at Charanka

VAR/Reactive Power control application on industrial level

Plant owner :- M/s. Gujarat Narmada Valley Fertilizer & Chemicals Limited

Plant Location:

Nearest Village                   Charanka

District                               Patan

State                                  Gujarat

Latitude                              23.912021o N

Longitude                           71.199093o E

Prime Contact Person           J. H. Trivedi

Phone No. (Landline)          02642 – 202324

Phone No. (Mobile)            +91 9825806850

E-Mail                               jhtrivedi@gnfc.in

Objective

VAR control in PCU:  GNFC will have to pay charges for import of reactive energy from grid as per GETCO-Tariff-Order. The estimated charges are Rs. 19 lakh per annum. It is very essential to make use of VAR control feature available in PCUs. Some PCUs are having VAR control feature for day time when power generation in ON and also during night time when power generation is OFF.

Policy Ceiling for the Reactive Power Use

Ref: Reactive Energy Charges as per Gujarat Energy Transmission Corporation Limited

Truing up for FY 2019-20 and determination of ARR and tariff for FY 2021-22

Reactive Energy Charges for all renewable sources, i.e., Wind, Solar, Biomass, Bagasse, Mini-hydel, MSW, etc., at the same rate as approved in Order dated 26th March, 2020 in Case No. 1837 of 2019. After considering the submission of the Petitioner, the Commission decides to continue with the existing Reactive Energy Charges and approved the charges for FY 2021-22 as shown in the following Table:

Looking to the above policy scenario, it is very essential to control the drawal of reactive energy from grid to minimize the reactive energy drawal penalty.

Reason for Reactive Energy Import

Excessive Inductance in the path of the PCUs to the metering end (i.e. Transformers, Harmonic Filter present in PCUs, Transmission line etc.).

Improper Leading and lagging Power factor in PCUs/Grid Network.

VAR control (Reactive Power Control) feature

The reactive power is a key component for maintaining voltage stability of the grid and supply the active power in the grid, which can be done by phase-shift control of the PCUs. This can be done by SCADA operator/external communication.

There are three mainly methods to control the reactive power by PCUs:

• Auto tunning of power factor based on grid conditions
• Fixed Power factor
• Fixed amount of reactive power.

By obtaining any one from above, we can control the VAR/reactive power.

Case studies and comparison of various solar power plants

• 10MW GSFC Solar Plant Without VAR Control
• 10MW GNFC Solar Plant with VAR Control

Conclusion

Looking at the above table, we can conclude that:

GNFC’s 10 MW Solar Power plant has VAR control feature in PCUs and it is operated through SCADA – due to that GNFC has imported less Reactive energy while GACL doesn’t have the VAR control feature due to that reactive energy import of the plant is more and GACL has to pay more charges than GNFC.

Once the import of the reactive energy exceed more than 10% of active energy then the rate is 0.5 Rs/KVARh instead of 0.1 Rs/KVARh.

Patil Rakesh is a B.E. in (E&TC). Dy Manager GERMI, Having Extensive experience in the
renewable energy sector

Nisargkumar Dave (M.Tech) completed his M.Tech in Electrical Engineering in 2017 and B.E in 2014, Interest includes Power system, BESS integrated with RE, Power converters
and Matlab Simulations for various power system and BESS case studies and Project management of RE. Coordinated various prestigious projects like 700 MW Raghaneshda
UMSPP and Prepared various DFR and DPR.