Voltage Stability Analysis Of Distribution Network

ABSTRACT

Voltage Stability is a serious concern of today power system for its secure and reliable operation. Power system stability is de- pendent mainly on the degree of maintaining the synchronism of the whole system. Out of the all blackouts round the globe the primary reasons for the blackouts are observed to be voltage collapse. There is a need to perform studies to ensure that the reliability of the power system is maintained at all system condition and its different operating horizons. This paper analyzes the voltage stability of a system by finding a continuum power flow solutions with base load and obtaining steady state voltage stability limit the critical point. The in-between result of the process is used to determine voltage stability index and further to identify portion of the system prone to such voltage collapse.

CHAPTER ONE

1.0                                                        INTRODUCTION

A power system is an interconnected system composed of generating stations, which convert fuel energy into electricity, substations that distribute power to loads (consumers), and transmission lines that tie the generating stations and distribution substations together. According to voltage levels, an electric power system can be viewed as consisting of a generating system, a transmission system and a distribution system.

The transmission system is distinctly different, in both its operation and characteristics, from the distribution system. Whereas the latter draws power from a single source and transmits it to individual loads, the transmission system not only handles the largest blocks of power but also the system. The main difference between the transmission system and the distribution system shows up in the network structure. The former tends to be a loop structure and the latter generally, a radial structure.

The modern power distribution network is constantly being faced with an ever-growing load demand. Distribution networks experience distinct change from a low to high load level everyday. In certain industrial areas, it has been observed that under certain critical loading conditions, the distribution system experience voltage collapse. Brownell and Clarke [1] have reported the actual recordings of this phenomenon in which system voltage collapses periodically and urgent reactive compensation needs to be supplied to avoid repeated voltage collapse.

Literature survey shows that a lot of work has been done on the voltage stability analysis of transmission systems, but hardly any work has been done on the voltage stability analysis of radial distribution networks. Jasmon and Lee and Gubina and Strmchnik have studied the voltage stability analysis of radial networks. They have represented the whole network by a single line equivalent. The single line equivalent derived by these authors is valid only at the operating point at which it is derived. It can be used for small load changes around this point. However, since the power equations are highly nonlinear, even in a simple radial system, the equivalent would be inadequate for assessing the voltage stability limit. Also their techniques do not allow for the changing of the loading pattern of the various nodes which would greatly affect the collapse point.

In this paper, a new voltage stability for all the nodes is proposed for distribution networks. It is shown that the node at which the value of voltage stability index is minimum, is more sensitive to voltage collapse. Composite load modelling is considered for voltage stability analysis. It is also shown that the load solution with feasible voltage magnitude for radial distribution networks is unique.

1.1                                         BACKGROUND OF THE PROJECT

HILE planning and operating today’s power systems, the voltage stability is of major and growing concern. The Power transmission utility requirements have changed considerably after the deregulation of the power industry. These changes had brought a considerable unacceptable poor quality power, which is apparent by continuous increase in sophisticated generation, transmission, distribution and service industries. The modern society where the prime energy source is electricity, the user does not tolerate power outages and other disturbances that impact their conveniences or life style. Social, environmental, right of way costs are aggregated by potential problems that hinder the construction of new transmission lines. Introduction of the deregulated energy market has further stressed the transmission grid because of maximized financial returns with minimum investment and deliver energy at a reasonable cost to the ultimate customer. One of the major problems associated with the today stressed system is voltage instability or voltage collapse. Voltage collapse is a process, which leads to a reduction of voltage in a significant portion of a power system. The tripping of trans- mission or generating equipment often triggers voltage col- lapse. [1]

Voltage stability is the ability of a power system to main- tain voltage irrespective of the increase in load admittance and load power resulting in control of power and voltage. The process by which voltage instability leads to the loss of voltage in a significant part of a power system is called voltage collapse. The ability of a power system to operate not only in stable conditions, but also to remain stable following any reasonable contingency or adverse system change is termed a voltage security.
A system enters into the unstable state when a disturbance (load increase, line outage or other system changes) causes the voltage drop quickly or to drift downward and, and automatic system controls fail to improve the voltage level. The voltage decay can take a few seconds to several minutes. Voltage stability or voltage collapse has become a major concern in modern power systems. In deregulated market conditions, a power system is set to operate at its maximum operating limits to better utilize existing facilities. This kind of system cannot withstand for any network outage. So, it is important to study the system behavior in the case of prolonged overload or any other system disturbances. [1]

1.2                                             OBJECTIVE OF THE PROJECT

Voltage stability analysis is important in power system in order to maintain the power system equilibrium. This research focuses on the voltage stability analysis of the power system feeders at distribution and transmission systems.

1.3                                                 SCOPE OF THE PROJECT

The voltage stability analysis is carried out on the basis of single feeder comprising of two-bus system using ABCD line parameters. The feeder’s voltage in distribution and transmission system has been monitored at lagging and leading load conditions for various power factors. Hence, the active power-voltage (PV) curve and reactive power-voltage (QV) are used as tools to monitor the voltage stability at feeders. The voltage critical, voltage regulation, voltage gap and line current are monitored at each load power factor.

1.4                              CAUSES OF VOLTAGE STABILITY PROBLEMS

1. High reactive power consumption at heavy loads
2. Generating stations are too far from load centres.

• We have observed this from simulation of a case by increasing the length of the PI section transmission line.

1. Source voltages are too low.
2. Poor coordination between various control and protective systems.

1.5                                           TYPES OF VOLTAGE STABILITY

Voltage stability deals with the ability of a power system to maintain acceptable voltage levels at all buses in the system in any condition whether it is normal or during disturbance. A heavily loaded system enters a state of voltage instability due to a sudden large disturbance or a change in system condition. It causes a progressive and uncontrollable decline in voltage. The main factor causing voltage instability in any power system is the inability of the system to meet its sudden growing demand for reactive power.

The two different approaches available as a tool to analyse the voltage collapse problem in a system are:

(a) The static approach and

(b) The dynamic approach.

(a) Static methods involve the static model of power system components. These methods are important when the power system is in operation and planning stages, in-order to prepare an adequate fool proof plan for meeting the power requirements during different types of contingencies arising during its operation.

(b) The dynamic methods use time domain simulations to re- veal the voltage collapse mechanism such as why and how the volt- age collapse occurs. Dynamic methods analyze the effect of dynamic loads, on load tap changes (OLTC), generator over excitation limiters (OXL) on voltage collapse. [1]

In most of the cases, the system dynamics affecting voltage stability are quite slow. The static approach effectively analyzes most of the problems. It can examine the viability of a specific operating point of the power system. In addition, static analysis method provides information such as sensitivity or degree of stability and involves the computation of only algebraic equations. It is much more efficient and faster than dynamic approaches. The static analysis approach is more attractive than the dynamic method and well suited to voltage stability analysis of power systems over a wide range of system conditions.

Dynamic analysis provides the most accurate indication of the time responses of the system. Therefore, Dynamic analysis is extremely useful for fast voltage collapse situations, such as loss of generation and system faults, especially concerning the complex sequence of events that lead to the instability. However Dynamic simulations fail to provide information such as