
Hierarchical Protection for Smart Grids
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Persons
Zengping Wang and Jing Ma, North China Electric Power University, Beijing, China
Content
About the Author ix
Foreword xi
Preface xiii
Introduction xv
1 Basic Theories of Power System Relay Protection 1
1.1 Introduction 1
1.2 Function of Relay Protection 1
1.3 Basic Requirements of Relay Protection 3
1.3.1 Reliability 3
1.3.2 Selectivity 4
1.3.3 Speed 4
1.3.4 Sensitivity 5
1.4 Basic Principles of Relay Protection 6
1.4.1 Over-Current Protection 6
1.4.2 Directional Current Protection 6
1.4.3 Distance Protection 7
1.5 Hierarchical Relay Protection 9
1.5.1 Local Area Protection 10
1.5.2 Substation Area Protection 11
1.5.3 Wide Area Protection 12
1.5.4 Constitution Mode of Hierarchical Relay Protection 13
1.6 Summary 15
References 15
2 Local Area Conventional Protection 17
2.1 Introduction 17
2.2 Transformer Protection 18
2.2.1 Adaptive Scheme of Discrimination between Internal Faults and Inrush Currents of Transformer Using Mathematical Morphology 18
2.2.2 Algorithm to Discriminate Internal Fault Current and Inrush Current Utilizing Variation Feature of Fundamental Current Amplitude 30
2.2.3 Identifying Transformer Inrush Current Based on Normalized Grille Curve (NGC) 39
2.2.4 Adaptive Method to Identify CT Saturation Using Grille Fractal 50
2.2.5 Algorithm for Discrimination Between Inrush Currents and Internal Faults Based on Equivalent Instantaneous Leakage Inductance 57
2.2.6 A Two-Terminal Network-Based Method for Discrimination between Internal Faults and Inrush Currents 70
2.3 Transmission Line Protection 82
2.3.1 Line Protection Scheme for Single-Phase-to-Ground Faults Based on Voltage Phase Comparison 83
2.3.2 Adaptive Distance Protection Scheme Based on the Voltage Drop Equation 99
2.3.3 Location Method for Inter-Line and Grounded Faults of Double-Circuit Transmission Lines Based on Distributed Parameters 117
2.3.4 Adaptive Overload Identification Method Based on Complex Phasor Plane 134
2.3.5 Novel Fault Phase Selection Scheme Utilizing Fault Phase Selection Factors 148
2.4 Summary 172
References 172
3 Local Area Protection for Renewable Energy 175
3.1 Introduction 175
3.2 Fault Transient Characteristics of Renewable Energy Sources 176
3.2.1 Mathematical Model and LVRT Characteristics of DFIG 177
3.2.2 DFIG Fault Transient Characteristics When the Crowbar Protection Is Not Put into Operation 178
3.2.3 DFIG Fault Transient Characteristics When the Crowbar Protection Is Put into Operation 211
3.3 Local Area Protection for Centralized Renewable Energy 230
3.3.1 Connection Form of a Wind Farm and its Protection Configuration 231
3.3.2 Adaptive Distance Protection Scheme for Wind Farm Collector Lines 233
3.3.3 Differential Protection Scheme for Wind Farm Outgoing Transmission Line 239
3.4 Local Area Protection for Distributed Renewable Energy 248
3.4.1 Adaptive Protection Approach for Distribution Network Containing Distributed Generation 248
3.4.2 Islanding Detection Method 255
3.5 Summary 269
References 270
4 Topology Analysis 273
4.1 Introduction 273
4.2 Topology Analysis for Inner Substation 273
4.2.1 Characteristic Analysis of the Main Electrical Connection 274
4.2.2 Topology Analysis Method Based on Main Electrical Wiring Characteristics 275
4.2.3 Scheme Verification 278
4.3 Topology Analysis for Inter-substation 284
4.3.1 Static Topology Analysis for Power Network 285
4.3.2 Topology Update for Power Network 287
4.3.3 Scheme Verification 291
4.4 False Topology Identification 294
4.4.1 Road-Loop Equation 294
4.4.2 Analysis of the Impacts of Topology Error and Undesirable Data on Branch Current 296
4.4.3 Topology Error Identification Method Based on Road-loop Equation 300
4.4.4 Scheme Verification 301
4.5 Summary 315
References 316
5 Substation Area Protection 317
5.1 Introduction 317
5.2 Substation Area Protection Based on Electrical Information 317
5.2.1 Substation Area Regionalization 318
5.2.2 Typical Fault Cases 323
5.2.3 Scheme Performance Analysis 326
5.3 Substation Area Protection Based on Operating Signals 327
5.3.1 Setting Principle of Adaptive Current Protection 327
5.3.2 Supporting Degree Calculation Method 330
5.3.3 Substation Area Current Protection Algorithm 334
5.3.4 Scheme Verification 338
5.4 Summary 346
References 346
6 Wide Area Protection 347
6.1 Introduction 347
6.2 Wide Area Protection Using Electrical Information 347
6.2.1 Wide-Area Protection Using Fault Power Source Information 348
6.2.2 Wide-Area Protection Using Fault Network Information 358
6.2.3 Wide-Area Protection Suitable for Multiple Fault Identification 369
6.3 Wide Area Protection Using Operating Signals 375
6.3.1 Wide-Area Protection Based on Distance Protection Operational Signal 376
6.3.2 Wide-Area Protection Based on Current Protection Operational Signal 393
6.3.3 Wide-Area Protection Based on Virtual Impedance of Fault Component 406
6.4 Wide Area Tripping Strategy 419
6.4.1 Tripping Strategy Based on Directional Weighting 419
6.4.2 Simulation Verification 428
6.5 Summary 432
References 433
Appendices 435
Index 439
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]:
- Clear the faulty components from the power system automatically, quickly and selectively, and ensure that the non-faulty parts remain in normal operation.
- 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:
(1.1)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...
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