Simulation of Advanced ELC with Synchronous Generator for Micro Hydro- power Station

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Simulation of Advanced ELC with Synchronous Generator for Micro Hydro- power Station

ANKITA GUPTA

¹ Alternate Hydro Energy Centre Indian Institute of Technology, Roorkee, India

Abstract- This paper presents an analysis and design of an Electronic Load Controller (ELC) for standalone synchronous generator suitable for micro hydropower stations. ELC is basically used to maintain the constant power at the generator terminals, which in turn helps to maintain the system frequency constant. The designed ELC consists of a controlled bridge rectifier-chopper system feeding a resistive dump load whose power consumption is varied through the duty cycle of the chopper. Proper design of rectifier, chopper and dump load is very important for trouble free operation of ELC. The ELC is designed and simulated in MATLAB using Simulink. This scheme can be efficiently used in micro hydropower stations.

Index Terms - Electronic Load Controller; Synchronous Generator; Micro Hydropower Station.

I.

I

NTRODUCTION

Micro hydropower stations are emerging as a major renewable energy resource today as they do not encounter the problems of population displacement and environmental problems associated with the large hydropower plants. They have been playing a great role to provide electricity to remote area especially in developing countries. Micro hydropower stations are defined as hydro electric system upto 100 kW power range.

Almost all of the micro hydropower stations are based on run-off-river type. The operating point of the generator is fixed such that it gives constant rated output at the rated conditions of voltage, current and speed, but the consumer load may vary. When the consumer load on the generator decreases, the turbine begins to accelerate and increases generated frequency. Similarly, an increase in consumer load on the generator causes deceleration in the turbine speed and the frequency decreases [1]. Now the variation in the consumer load connected is neutralized by the controller diverting the extra power to a dump load. ELC regulates the voltage and frequency of the generator through monitoring of consumer load variation and automatically dissipating any surplus power produced by the generator in additional load known as dump load, so that, the total output power of generator remains equal to its rated power.

Synchronous generator has some advantages over induction generator for such systems.

Synchronous generators can run isolated from the grid and produce power since excitation is not grid-dependent.

Also, synchronous generators are readily available in market, no excitation capacitors are required to provide reactive power, highly efficient, having in-built AVR for voltage regulation and using synchronous generator with ELC we can achieve good frequency regulation.

Literature has revealed that lots of work is done on ELC with asynchronous generator [2-7]. This paper therefore deals with the unexplored and relevant topic of Development of ELC for micro hydropower station using Synchronous generator of 50kW. The designed system consists of hydro turbine, exciter, synchronous generator, consumer load and ELC. In this case it is 50kW generator driven by a hydro turbine. The voltage output of the generator is regulated by AVR. The ELC is controlled rectifier type with chopper controlled dump load. Here IGBT is used as chopper. The system is designed in MATLAB Simulink and the results are analyzed.

II.

DEVELOPED SCHEME

A schematic diagram of the developed ELC system with synchronous generator is shown in “Fig. 1”. The system consists of 3-phase synchronous generator of 50kW driven by a constant hydro turbine. Since the input to the hydro turbine (i.e. Head and discharge) is assumed to be constant, so the output of hydro turbine is nearly constant, output power of the Synchronous generator must be held constant at all loads. Any decrease in load may accelerate the machine and raise the frequency levels to high value.

The power in surplus of the consumer load is dumped in the resistive dump load through an ELC connected at the terminals of the synchronous generator.

The ELC consists of a controlled bridge rectiﬁer in series with IGBT chopper and dump load (resistors). The duty cycle of the chopper is adjusted so that the output power of the generator remains constant.

ELC reacts so fast to load change which is not even noticeable unless a very large load is applied. The advantage of ELC is that, it is reliable and not having any moving part, so virtually maintenance free.

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Fig 1: Schematic Diagram of Developed ELC Scheme

III.

P

RINCIPLE

O

F

O

PERATION

The Synchronous generator–ELC system consists of a three- phase delta-connected generator driven by a micro hydro turbine and an ELC. Since the input power is nearly constant, the output power of the SEIG is held constant at varying consumer loads. The power in surplus of the consumer load is dumped in a dump load through the ELC.

Thus, Synchronous Generator feeds two loads in parallel such that the total power is constant, that is,

P

G

= P

C

+ P

D

where, P_G = Generated power of the generator (which should be kept constant),

P_C= Consumer load power, and P_D= Dump load power

The power dissipated in the dump load can be used for battery charging, water heating, cooking, etc.

IV.

D

ESIGN OF

ELC

FOR

S

IMULATION A number of Electronic Load Controller circuits are developed based on various methods, to dissipate the power in dump load, to obtain the balancing between the hydro turbine input and the generator output [1]. With the variation of consumer load the load controller has to change the effective dump load resistance, so that,

P

G

= P

C

+ P

D (1)

where,

P_G = Output power of synchronous generator P_C = Consumer load power

P_D = Dump load power

The power in dump load depends on the duty cycle of the chopper and is given as:

(

_dc

)

2 D

D

P S V R

 

(2)

where,

S = Duty cycle of chopper

V_dc = DC output voltage of thyristor bridge rectifier R_D = Dump load resistance

The rating of dump load resistance is given by:

(

_dc

)

2 D

G

R V

 P

(3)

The model of the 50 kW synchronous generator is designed for simulation. The calculation is based on assumption and it can be used to analyze the system and to understand the effect of using ELC on the micro hydropower plant. The model was designed to control the frequency of the system but the voltage is controlled by Automatic voltage regulator (AVR).

A. Generator Parameters

For simulation, the 3 phase synchronous generator model of 50 kW, 400 V, 50 Hz, 4-pole is considered. The generator is Salient pole type. The speed of synchronous generator is calculated as below:

120f 1500

N rpm

 p 

(4)

where,

N = synchronous speed of generator f = frequency of generated voltage p = no. of poles

B. Design of ELC

The rating of bridge rectifier and chopper switch depends on the rated voltage and power of the synchronous generator. The DC output voltage of controlled bridge rectifier is given as below:

3 3

^m

cos

dc

V V





  

(5) where,

α = firing angle

V_m = Input voltage at ELC terminal

The maximum value of Vdc occurs when α=0, and is given as,

3 3

_m

dc

V V



 

(6)

V V

_dc

 3 3  400  664 . 59



₍₇₎

(3)

ELC current is given as,

G dc

dc

I P

V

(8)

A I

_dc

75 . 57

59 . 661

50000 



(9)

The dump load resistance is calculated as :

2 2

( ) (661.59)

8.754 50000

dc D

gen

R V

 P   

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C. Design of DC Filter capacitor

When the AC signal passed through rectifier it would become an uneven DC. A filtering section is used to smooth out this uneven DC signal. Filters filter unwanted AC in the output of a rectifier. The Ripple factor for C- filter is given by:

1

4 3

_L

r

 fCR

(11) Where,

r = Ripple factor of C- filter f = frequency (in Hz)

RL = Resistance of dump load (in Ohm) or,

1 4 3 _D C

fR r



(12) Assume, the ripple factor is 15%,

1

3

2.1994 10 4 3 50 8.75 0.15

C   ^ F

   ₍₁₃₎

V.

S

IMULINK

M

ODEL OF

D

ESIGNED

S

CHEME The imulink model of the designed scheme is as shown in Fig 2. In this scheme, the ELC consists of a three-phase controlled bridge rectifier and an IGBT chopper. The AC voltage is rectified by means of a controlled bridge rectifier.

An electrolytic capacitor is connected across the bridge rectifier to filter out the ripples. The dump load in series with the chopper switch is connected across the DC link.

The dump load is designed such that, when the duty cycle of the chopper is unity, it should consume the rated output power of the generator.

The output power of the synchronous generator is kept constant by the ELC.

The change in speed of generator (change in frequency) corresponding to the change of load is measured. This is compared with a reference frequency, which is taken as proportional to the rated frequency of the generator. A controller is used to process the error between feedback and reference frequency signals. The error frequency is fed to a controller as shown in “Fig. 2”.

The output of the controller is compared with a saw- tooth carrier waveform to result in a PWM signal to alter the duty cycle of the chopper.

The saw-tooth waveform is defined as [7]:

m st

p

V A t

 T

(14)

where,

A_m= amplitude of the saw-tooth carrier waveform t = time in m-sec

T_p= time period of the saw-tooth PWM carrier wave

The controller output is compared with the saw-tooth carrier waveform and output is fed to the gate of the chopper switch (IGBT). The switching logic is as follows:

If Vst > Vo, then S=1 If Vst < Vo, then S=0

Where, S is the switching function used for generating the gating pulse of IGBT of the chopper of ELC.

Figure 2: Simulink model of Designed scheme

D. Cases consider for analysis of designed scheme For analysis of ELC with synchronous generator, we considered three cases with synchronous generator of output power 50kW and varying consumer load demand as shown in table 1.

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TABLE 1: DIFFERENT CASES FOR SIMULINK MODEL ANALYSIS

Case No. Consumer load in kW

1 50

2 30

3 40

VI.

RESULTS AND DISCUSSUION

The simulation circuit is simulated in MATLAB Simulink, after simulation we get the results as shown in

“Fig 3, 4 and 5”, case 1, 2 and 3 respectively. The oscilloscope output shows the Excitation Voltage, Rotor Speed (in pu), Mechanical Power Output of Turbine, 3- Phase Current output of generator, Error in Frequency and Power outputs for all the three mentioned cases.

A. CASE 1: Generator output is 50kW and Consumer load demand is 50kW

Figure 3(a): Excitation voltage

Figure 3(b): Rotor Speed of Synchronous Generator (in pu)

Figure 3(c): Mechanical Power Output of hydro turbine

Figure3 (d): 3-Phase Current Output of Generator

Figure 3 (e): Error in Frequency

Figure 3(f): Output power of generator and power across consumer load and dump load (P_G = P_C + P_D)

B. CASE 2: Generator output is 50kW and Consumer load demand is 30kW

Figure 4(a): Excitation voltage

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Figure 4(d): 3-Phase Current Output of Generator

Figure 4(e): Error in Frequency

Figure (f): Output power of generator and power across consumer load and dump load (PG = PC + PD)

C. CASE 3: Generator output is 50kW and Consumer load demand is 40kW

Figure 5 (a): Excitation voltage

Figure 5(d): 3-Phase Current Output of Generator

Figure 5(e): Error Frequency

Figure 5(f): Output power of generator and power across consumer load and dump load (PG = PC + PD)

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VII. CONCLUSION

An Electronic Load Controller with synchronous generator using a three-phase controlled bridge rectifier and IGBT as chopper converter has been modeled in MATLAB Simulink, for controlling the frequency of micro hydro turbine under varying load conditions. The controller is modeled and the results are analyzed after simulation. The designed ELC is achieving its objective to control the frequency. This ELC can be fabricated and used efficiently in any micro hydropower plant.

R

EFERENCES

[1] B. Singh, S.S.Murthy, M.Goel and A.K.Tandon, “A Steady State Analysis on Voltage and Frequency Control of Self-Excited Induction Generator in Micro- Hydro System”, International conference on power electronics, Drives and Energy Systems, pp.1-6, 2006

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2, pp.842-847, 2003

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[4] B.Singh, G.K.Kasal and S.Gairola, “Power Quality Improvement in Conventional Electronic Load Controller for an Isolated Power Generation”, IEEE Transactions on Energy Conversion, Vol. 23, No. 3, pp.

764-773, 2008.

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