1
Basic Theories of Power System Relay Protection

1.1 Introduction

As the first defence line to ensure the security of a power grid, relay protection is very important for the fast isolation of faults and the effective control of fault expansion [1,2]. However, in recent years, with the continuous integration of large-scale renewable energy sources, the structure of modern power grids has become more and more complex, and more and more problems in traditional relay protection have been exposed, such as difficulties in backup protection setting and cooperation, unexpected changes of grid structure or operating conditions which may cause protection to malfunction or refuse to operate, and cause major load transfer which easily leads to cascading, tripping and even blackout, etc.

To solve these problems, the smart grid hierarchical relay protection system was proposed. Hierarchical relay protection is based on the smart grid [3] and information sharing technology, and is composed of bay level protection, substation area protection and wide area protection. Bay level protection, which is also called local area protection, aims to realize primary protection for components in the substation through independent and decentralized configuration. Substation area protection aims to realize backup protection for components in the substation by centralizing the information of components to the substation host computer. Wide area protection aims to realize local backup protection and remote backup protection between substations through the interaction of information between relevant substation protection units.

In this chapter, first the basic theories of power system relay protection are introduced, the functions and basic requirements of relay protection are summarized, and the basic principles of relay protection are illustrated. Then, the composition mode of hierarchical relay protection is analysed in detail. The cooperation between local area protection, substation area protection and wide area protection and the protection range of each are discussed, laying the foundation for subsequent chapters in this book.

1.2 Function of Relay Protection

A power system is an energy transmission network composed of various electrical devices corresponding to electric power production, transformation, transmission, distribution and use, which are connected according to certain technical and economic requirements. Generally, the devices through which the electric power flows are called the primary equipment of the power system: for example, the generator, transformer, circuit breaker, bus, transmission line, compensation capacitor, shunt capacitors, shunt reactors, motor and the other power consumption equipment. The devices for the monitoring, measuring, control and protection of the operating state of the primary equipment are called the secondary equipment of the power system. Through voltage and current transformers, the high voltage and large current signals of the primary equipment are converted in proportion to low voltage and small current signals for the secondary equipment [4].

The operating state of a power system can usually be described by the operating parameters. The main operating parameters include active power, reactive power, voltage, current, frequency and the angular difference between emf phasors. According to different operating conditions, the operating state of a power system can be divided into normal state, abnormal state and fault state.

When a power system is in normal operation, the primary equipment and main operating parameters are all within the allowed deviation ranges, and the power system can operate continuously to provide electric power. However, when a disturbance occurs in a power system, the balance of the main operating parameters will be broken, and the power system operating state will change.

After a power system has been disturbed, then, according to the degree of disturbance, two circumstances may result. One is that the power system transits from the original stable state to a new stable state, the deviation of operating parameters from normal values remaining within the allowed ranges - for example, an increase or decrease of load, or the regulation of the prime mover - and the system could continue in normal operation. The other is that when a fault occurs in a power system, the operation of the system will change dramatically, resulting in local failure of the power system, electrical equipment and normal power supply to electricity users, even global failure.

If there is a fault state, and no special measures are taken, it is difficult to restore the system to normal operation, which could have a major impact on industrial and agricultural production, national defence, construction or the lives of ordinary people. Many types of fault can occur in a power system, including short circuit, phase disconnection and successive occurrence of multiple faults. The most common and most dangerous faults are various forms of short circuit, including three-phase short circuit, phase-to-phase short circuit, two-phase grounding fault, single-phase grounding fault, and motor and transformer winding turn-to-turn short circuit. In addition, there may be disconnection of one phase or two phases and complex faults such as some of the above faults occurring in succession.

Since the devices in a power system are connected to each other, a fault on one device will soon affect the other parts of the system. Thus, the time to clear the faulty device must be very short, sometimes even as short as tens of milliseconds, i.e. a small number of cycles. In such a short period of time, it is impossible for the operating staff to identify the fault and clear the faulty device. Automatic devices are needed to do that, i.e. relay protection devices.

A relay protection device is an automatic device installed on the components of the whole power system, which can respond to various faults or abnormal operating states of electrical components in the designated area quickly and accurately, and operate within the preset time limit to issue tripping signals to the circuit breaker. The term 'relay protection' generally refers to the relay protection technology or relay protection system composed of various relay protection devices.

The basic tasks of relay protection are [5-8]:

1. Clear the faulty components from the power system automatically, quickly and selectively, and ensure that the non-faulty parts remain in normal operation.
2. Respond to the abnormal state of the electrical devices and issue signals to inform the duty personnel, or automatically make adjustments, even issuing tripping commands.

When a power system is in normal operation, relay protection does not operate, it simply monitors the operating state of the power system and the components. Once a fault or abnormal operating state is detected, relay protection will quickly operate to isolate the fault and issue a warning to ensure the safety of the power system. Relay protection plays an important role in ensuring the safe operation of the system and its power quality, and preventing the expansion and occurrence of faults.

1.3 Basic Requirements of Relay Protection

Technically, relay protection which operates to trip switches should meet four basic requirements, i.e. reliability, selectivity, speed and sensitivity. These four basic requirements are the important criteria to analyse, evaluate and study relay protection.

1.3.1 Reliability

The reliability of relay protection refers to the capability of the relay protection to operate reliably when a fault occurs within the protection range, without any refusal to operate, and not malfunctioning in any case where protection should not operate.

Reliability is the basic requirement of relay protection. It depends on the design, manufacture and operational maintenance levels. To ensure reliability, protection schemes with performance that meets the requirements and with simple principles should be used. Reliable hardware and software with anti-jamming capability should be used to form the protection device. In addition, there should be essential automatic detection, locking and warning measures, with convenient setting, debugging and operational maintenance.

An important index for evaluating the reliability of relay protection is the correct operational rate of relay protection, which is calculated as follows:

where Rc is the correct operational rate of relay protection, kcorrect is the correct operational times of relay protection, ktotal is the total operational times of relay protection, which includes the correct operational times, malfunctioning times and refusing-to-operate times.

1.3.2 Selectivity

Selectivity means that a particular fault should be cleared by the protection of the faulty device itself, and only when the protection or circuit breaker of the faulty device refuses to operate will protection of the adjacent device or the breaker failure protection be allowed to clear the fault. Thus, fault clearance can be limited to the minimum range, and the safe operation of the non-faulty part of the system is guaranteed.

To ensure selectivity, apart from using a time delay to make the backup protection and primary protection of a line cooperate correctly with each other, the correct cooperation between the backup protection of adjacent components also...