Abstract— Power quality is the major concern in today’s electrical system. Mainly, the power quality
problems are endlessly observed in distributed power system. The voltage
quality is the major feature in distributed electrical system. Voltage quality
can be adequately improved in distributed systems by using some custom power
electronic devices such as Dynamic Voltage Restorer (DVR). The Dynamic Voltage
Restorer (DVR) is fast, flexible and professional solution to voltage sag and
voltage swell crisis. The DVR is a power electronic based device that provides
three-phase controllable voltage output, which adds to the provider voltage
during sag event, to restore the load voltage to pre-sag states and when
voltage swell occurs DVR injects 1800 out of phase voltage into the
power line. The simulation is based on designed PI controller for mitigating
the voltage sag and swell. The transfer function is first obtained to design PI
controller. Two 11 KV feeders are used. DVR can also be designed for protecting
the whole plant with loads in some MVA range. The DVR can be able to restore
the load voltages within few milliseconds.

 

Keywords — custom power device, PI
controller, dynamic voltage restorer, voltage sag, voltage swell.

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Ashish Doorwar

Asst. Professor

 

Dept. of
Electrical Engineering, AITS

Rajkot, India

[email protected]

 

voltage sags are not compensated because of the
limited time period of mechanical switching. The transformer tap changing can
be used but tap changing under load is very much complex and pricey. Here,
power electronics technology had played an essential role in power flow control
of an electrical energy. Also for the electrical power system, the FACTS
devices are more often developed because of its extraordinary performances and these
devices have the ability to mitigate power quality problems such as voltage
sag, swell, harmonics, current quality etc.

 

The definitions of voltage sag
and voltage swell can be highlighted in the following table 5.

 

TABLE I

 

VOLTAGE SAGS AND SWELLS CRITERIA

 

Disturbance
Type

Voltage

Duration

 

 

 

Voltage sag

0.1 –
0.9 pu

0.5 cycles- 1 min

 

 

 

Voltage swell

1.1 –
1.8 pu

0.5 cycles – 1 min

 

 

 

 

I.       
INTRODUCTION

A s an electrical energy plays a fundamental role in today’s modern life.
It is very important to handle this immense energy with supreme care.
Especially, the thermal power plant, nuclear 
power  plant  and 
hydro  power  plant 
took  the responsibility to supply
this immense electrical energy. This electrical energy can be well utilized by
industrial, customer, commercial, military, etc. applications and the demand of
this energy is increasing gradually. Thus, the control and use of this

 

energy must be limited by certain limits.

 

Actually, the electrical devices
are very much sensitive and became less tolerant towards the power disturbances
such as voltage sags, swells, harmonics etc. voltage sags/dips are considered
to be the most severe disturbances when industrial equipments are considered.
Some of the industrial equipments are very sensitive that these are unable to
handle a very low deviation in voltage. Voltage sags can cause brutal damage in
these equipments. Now, the voltage support at the load end is very much
required and it can be achieved by injecting the reactive power at the load point
of common coupling.

 

Some of the useful conventional
method is to utilize the mechanically switched capacitors – SVC – which are
connected in shunt. The reactive power can be injected by mechanical switching
the capacitors as per the requirement of the electrical systems. But the
disadvantage is that some of the high speed transients cannot be compensated by
this way and also some

For example, the FACTS for
transmission systems, the term custom power pertains to use of power
electronics controllers in particular distribution systems in order to mitigate
various power quality problems. FACTS devices can boost the power transfer
capabilities and improve the stability of the system. The custom power ensures
that the customers get pre-specified power quality and reliability of the power
supply. The pre-specified power quality may include the following
specifications of the parameters: low phase unbalance, low flicker at load
point, no power interruptions, low THD (total harmonic distortions), and
overvoltage and under- voltage within specified limits etc.

 

There are many types of Custom
Power devices. Such as : Active Power Filters (APF) 7, 11, Battery Energy
Storage Systems (BESS), Distribution static synchronous compensators (DSTATCOM)
7, Distribution Series Capacitors (DSC), Dynamic Voltage Restorer (DVR) 7,
Surge Arresters (SA), Super conducting Magnetic Energy storage(SMES), Static
Electronic Tap Changers (SETC), Solid-State Transfer Switches (SSTS), Solid
State Fault Current Limiter (SSFCL), Static VAR Compensator (SVC), Thyristor
Switched Capacitors (TSC),and Uninterruptible Power Supplies (UPS) 1.

 

II.     
DVR BASIC CONFIGURATION

 

DVR is a series connected custom power electronic
device. It has other features such as to compensate for fault current,

harmonic reduction etc. Especially for the
distribution electrical set-ups, voltage profile is so much important So it is
necessary to improve the voltage profile mainly in distribution networks. The
DVR is an excellent in mitigating the voltage sags and swells in very precise
manner. It has main five components.

 

a)      
The
booster transformer

b)      
The
harmonic filter (L-C filter) unit

c)      
Voltage
source inverter

 

d)      
DC energy
storage unit

e)      
Control
unit

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig.1  Basic DVR Power Circuit

 

Fig. 1 1 shows the basic
principle setup for DVR in which control block is the main concern for
designing DVR.

 

A.  The booster transformer

 

The designing of
transformer is very crucial because it faces saturation, overrating,
overheating, cost and performance which are very important. The injected
voltage may consist of fundamental, desired harmonics, Switching harmonics and
dc voltage components 2. The transformer is designed as per the requirement
i.e. this well designed transformer is mainly used in distribution electrical
networks. If the transformer is not designed properly, the injected voltage may
saturate the transformer core which can cause the improper operation of the DVR
1.

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