Design of new load shedding special protection schemes for a double area power system

INTRODUCTION

Major power system breakdowns have been occurring historically in interconnected electric grids. In recent years economical restrictions in the installation of new power system components especially transmission facilities, which may be a consequence of power system deregulation, has considerably increased the frequency and intensity of power system blackouts (1), (2).

The following factors play important roles in the recently occurred blackouts:

* Incorrect operation of protective systems, especially distance relays (3).

* Voltage instability (4-8).

* Inefficient design or lack of Under Frequency Load Shedding system (4-9).

* Lack of under voltage load shedding system (4-8).

* Inefficient vegetation management strategies (5), (7), (10).

In this paper the third item, i.e. UFLS system, is considered and studied. UFLS scheme could be categorized in a class of protective systems known as system protection schemes, special protection schemes or wide area protection schemes.

UFLS scheme is one of the most generally used system protection schemes in power systems. Preliminary work on this issue started in 1950's. However it was accepted by most utilities after the 1965 northeastern blackout (12). UFLS systems are conventionally designed to prevent excessive frequency decline, following the outage of some generating units, by preserving the balance of generation and consumption. Whenever frequency of the system falls below a predetermined value, some part of system load will be dropped after a time delay.

In most of the recently occurred power system blackouts two or more islands emerge as a consequence of the disturbance (4-9). Any island which is capable of preserving the balance of generation and consumption may survive, otherwise it will cascade to blackout.

UFLS scheme, as a type of special protection scheme, may help the island preserve balance and prevent cascading outage. Conventional UFLS schemes are designed to protect the system against normal power deficiencies as in the case of one or two generating unit outage or sudden increase of load. Conventional UFLS scheme reaction is usually insufficient in the case of cascading outages, since in this case islands usually face excessive power deficiency. Therefore, in the case of large imbalances between generation and consumption UFLS scheme can not generally perform appropriately since it has not been designed specifically for these types of events. Adaptive UFLS schemes might be able to prevent complete system blackouts in the case of large imbalances.

In conventional under frequency protection design the only measured parameter of the system, involved in decision making, is frequency. Whenever frequency of the system falls below a predefined threshold, a portion of system load is disconnected in a few steps. Although this type of under frequency load shedding is simple and easy to implement, especially with electromechanical relays, it suffers from some disadvantages. The most important disadvantage of this scheme is its lack of adaptability. In other words, regardless of the severity of disturbance, settings of the under frequency load shedding schemes are constant. This lack of adaptability may result in either overshedding or undershedding in different situations.

Several conventional and adaptive UFLS schemes have been proposed in the literature (13-21). However, most of them have not proposed an effective method to prevent system blackout in the case of cascading outages.

From the above discussion it is concluded that a special protection scheme is helpful to preserve the integrity of islands following large disturbances in power systems. The special protection scheme may be a kind of adaptive UFLS system, especially designed for this purpose.

In order to detect large imbalances several methods may be used by under frequency relays. In this work two methods are proposed and analyzed:

* Using rate of change of frequency.

* Using the state of tie line circuit breakers.

Rate of change of frequency may be a suitable parameter to enhance the adaptability of under-frequency relays (18-21). This method could be implemented in different ways, some of which are proposed and tested on an actual network in this paper. Using this load shedding method, faster reactions could be obtained for major system failures. This way, system blackouts could be better controlled.

In order to verify ability of the proposed methods, Iranian interconnected grid is simulated appropriately. Khorasan, the greatest province of Iran, is considered to be connected to the grid through a double circuit 400kV tie line. The Iranian network is an interconnected grid with about 500 HV buses and 30000 MW generation. Khorasan HV network is a 72 bus system with about 2500 MW generation.

The power system under study is modeled as accurately as possible in this work. Distributed and dynamic model is used in this study. Generators, transmission lines, power transformers, Automatic Voltage Regulators (AVR), governors, tap changers, loads and under frequency relays are accurately modeled. A suitable special protection scheme for this double area power system is proposed in the paper and its performance is evaluated.