1

Three-phase Cross-phase Voltage Sag Compensator to

Compensate Balanced/Unbalanced Voltage Sags and

Phase Outages in Distribution Systems

Ebrahim Babaei Mohammad Farhadi Kangarlu

Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran

EPEC 2011

2

Contents

 Introduction

 Proposed voltage sag compensator

 Proposed control method

 Simulation results

 Conclusions

3

  Importance of power quality for sensitive loads   Voltage sag as a common disturbance

  What is the definition of voltage sag?   Why voltage sags happen?   Why voltage sags are important?

 Preventing of the sensitive loads from the voltage sags is a very important issue.

  What are the solutions?  Solutions: Series-connected power electronic based devices such as

DVR, SSC, sag compensator, …

Introduction

4

Introduction

  Challenges: •  Increased cost and size •  Reduced compensation capability

(duration of compensation)

Second approach

Conventional compensator model

First approach

  Solution: Compensators based on the direct ac/ac converters

Typical compensator

5

  Advantages of compensators based on the direct ac/ac converters: •  Reduced size and cost •  Unlimited duration of compensation

  They can be divided into two groups: •  The topologies that are basically single-phase (can be extended to three-phase) •  The topologies that are basically three-phase topologies

  What is the problem about them? •  in single-phase topologies:

"  The compensation power in each phase is taken from the same phase (faulted phase) which worsens the fault condition,

"   Phase jumps during voltage sag cannot be compensated . •  in three-phase topologies:

"  The main part of compensation power in each phase is taken from the same phase (faulted phase) which worsens the fault condition,

"  Usually they use high number of power electronic switches

 Now, we are going to propose a new topology for voltage sag compensator based on direct ac/ac converter to cover the mentioned problems

Introduction

6

Proposed voltage sag compensator

Proposed cross-phase voltage sag compensator

7

Proposed control method

One phase of the cross-phase converter

Switching states of the cross-phase converter

Switching method of the converter

is the reference value of the injected voltage,

is the reference value of the load voltage,

is the measured grid voltage.

8

Simulation results

System parameters

The simulated system

9

0.5pu sag from 0.02 to 0.08sec

Simulation results

Three-phase balanced sag compensation

10

0.5pu sag from 0.02 to 0.08sec in phase a

with 30 degrees phase jump

Simulation results Single-phase unbalanced sag

with phase jump compensation

11 Compensation with the

proposed topology

0.35pu two-phase unbalanced sag with 20 degrees phase jump in phases b and c

Compensation with the single-phase structures extended to three-phase

Simulation results

12

Single-phase outage compensation (outage in phase b from 0.02 to 0.08sec)

Simulation results

13

  A new topology for direct converter based voltage sag compensator is proposed.

  The proposed topology is based on the cross-phase converter which the power of each output phase is produced using the other two input phases.

  As a result, the compensation power in each phase is taken from the other two phases leading to a better operation in the deep voltage sags and weak systems.

  This is important considering that most of the voltage sags are single-phase sags where one of the phases is faulted.

  The proposed topology can compensate balanced and unbalanced voltage sags accompanied by phase jumps.

  One-phase outage can be compensated by the proposed topology.

Conclusions

14