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TB 116 1997 SC 11/14 JWG 11/14.09 Guide for preliminary design and specification of hydro stations with HVDC unit connected generators.

Several technical and economical reasons strongly suggest that in certain HVDC applications it is of great advantage to simplify the rectifier station via a direct connection of hydro generating sets to 12 pulse converter groups. The proposed arrangement is referred to in HVDC literature as "Unit Connection".

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TABLE OF CONTENTS

1 INTRODUCTION

      1.1 General

      1.2 The Unit Connection Concept

      1.3 Adjustable Speed Operation of HVDC Unit Connected Hydro Stations

      1.4 Impact of Adjustable Speed Operation on Generating and Converter Station Equipment

      1.5 Summary of CIGRÉ JWG 11/14-09 Findings

2 THE HVDC UNIT CONNECTED HYDRO STATION

      2.1 Arrangement and Main Characteristics of HVDC Sending End Converter Station

            2.1.1 Conventional Connection

            2.1.2 Independent Generator-Converter Units (The HVDC Generating Unit)

            2.1.3 Group Connection

            2.1.4 Adjustable Series Connection

            2.1.5 Diode (uncontrolled) Rectifier Unit Connection

            2.1.6 Main Applications

      2.2 Operational Characteristics of HVDC Unit Connected Hydro Station

            2.2.1 Classification of Hydro-Generating Stations According to their Power/Energy Parameters

            2.2.2 Overview of Characteristics Specific to Stations Operating in the Adjustable Speed Mode

            2.2.3 Station Auxiliaries, Circuit Breakers, Control and Protection Equipment

      2.3 Economic Implications

            2.3.1 Sending end Converter Station Capital Cost Comparison

            2.3.2 Other Economical Impacts

                  2.3.2.1 Capital Savings if Designing for Adjustable Speed Operation

                  2.3.2.2 Long Term Economic Impacts of Adjustable Speed Operation.

3 ADJUSTABLE SPEED OPERATIONS OF HYDRO TURBINES

      3.1 Introduction

      3.2 Limits

            3.2.1 Output Limits

            3.2.2 Water Head Limits

            3.2.3 Speed Limits

      3.3 Gains

            3.3.1 Efficiency

            3.3.2 Cavitation at Fixed and Adjustable Speed

      3.4 Governing

      3.5 Basic Theory, Turbine Types and Characteristics

            3.5.1 General

            3.5.2 Basic Theory

            3.5.3 Potential Impact of Adjustable Speed on Different Turbines

            3.5.4 Operational Characteristics

      3.6 Practical Consequences of Adjustable Speed Operation

            3.6.1 Capital Costs, Overall Optimization and Savings in Civil Works

            3.6.2 Energy Gains and Environmental Benefits

            3.6.3 Efficiency, Cavitation and Water Head Variations

                  3.6.3.1 Operation at Rated Head

                  3.6.3.2 Operation at Low Heads

                  3.6.3.3 Operation at Higher Than Normal Heads

            3.6.4 Cavitation Control

            3.6.5 Margin for Correction of Undetected Design Flaws

            3.6.6 Reliability, Availability and Maintenance Costs

            3.6.7 Capability of the Generating Set and Generator Rating for Adjustable Speed

      3.7 Concluding Remarks

            3.7.1 General

            3.7.2 Special Turbine Designs

            3.7.3 Upgrading of Old Stations

4 SYNCHRONOUS GENERATORS IN HVDC UNIT CONNECTION

      4.1 Introductory Comments and General Assessment

      4.2 Commutation Reactance, Converter Reactive Load and Generator Fundamental Frequency Terminal Power Factor in Unit Connection

            4.2.1 Nature of the Fundamental Frequency Converter Reactive Load in Unit Connection

            4.2.2 Specification of the Generator Nominal Fundamental Frequency Power Factor

      4.3 Generator Reactance

            4.3.1 Fundamental Frequency Generator-Converter Interaction

                  4.3.1.1 Commutations

                  4.3.1.2 Variation of Reactance with Speed

                  4.3.1.3 Typical Values of Commutation Reactance for 12 Pulse Unit Connections

                  4.3.1.4 Non Standard Generator Subtransient and Transformer Leakage Reactances

                  4.3.1.5 Damper Windings

                  4.3.1.6 Variation Due to Machine Load

                  4.3.1.7 Influence of Dampers Resistance

            4.3.2 Generator Operational Inductances over a Range of Speed

                  4.3.2.1 Hydro Generator Operational Inductances

                  4.3.2.2 Hydro Generator Frequency Response

                  4.3.2.3 Generator Subtransient Reactance as Affected by Operating Conditions

                  4.3.2.4 Generator Harmonic Reactance in Adjustable Speed Operation

      4.4 Generator Excitation

            4.4.1 Auxiliary 3 phase Generator

            4.4.2 Fully Independent Auxiliary Generators

      4.5 Cycling and Fatigue

            4.5.1 Fatigue Aging due to Adjustable Speed Operation

            4.5.2 Stator Windings

            4.5.3 Electromechanical Effects of Converter Harmonics on Large Hydro-Generators

      4.6 Insulation Stresses

            4.6.1 Rotating Machine Insulation Degradation in Power Electronic Applications

                  4.6.1.1 Repetitive Impulse Voltage

                  4.6.1.2 Insulation Problems of Rotating Machines

                  4.6.1.3 Insulation Degradation of Form Wound Windings for Large Machines

                  4.6.1.4 Partial Discharges Inception Voltages

            4.6.2 Insulation of Unit Connected Generators

      4.7 Generator Losses

            4.7.1 Windage Losses

            4.7.2 Iron Losses

            4.7.3 Stray Losses in Stator Copper

            4.7.4 Losses on Pole Shoes and Damper Windings

      4.8 Generator Rating

            4.8.1 Generator Rating at Fixed Speed

            4.8.2 Generator Rating at Adjustable Speed

            4.8.3 Generator Voltage

            4.8.4 Generator Inertia Parameters (GD2)

      4.9 New Zealand Field Tests of Unit Connection Operation

            4.9.1 TransPower Benmore Station

            4.9.2 1993 Test Conditions and Results

            4.9.3 1995 Test Conditions and Results

      4.10 Concluding Remarks

            4.10.1 Concluding Remarks on Cycling and Fatigue

            4.10.2 Insulation Aging of HVDC Unit Connected Generator

            4.10.3 Concluding Remarks on HVDC Unit Connected Generators Design and Rating

5 SENDING AND RECEIVING END STATION: GENERAL EQUIPMENT AND REQUIREMENTS

      5.1 Introduction

      5.2 Unit Connected Rectifier Station

            5.2.1 Operating Parameters

            5.2.2 Converter a.c. Voltage Uv

            5.2.3 Converter d.c. Voltage Ud

            5.2.4 Operation with Adjustable Speed

                  5.2.4.1 Generator EMF

                  5.2.4.2 Valve Voltage

                  5.2.4.3 Angle of Overlap

            5.2.5 Some Additional Points

                  5.2.5.1 Tap Charger Range under Adjustable Speed

                  5.2.5.2 HVDC Line Efficiency Estimates

      5.3 “Generator-Converter” Transformer

            5.3.1 On Load Tap Charger

            5.3.2 Transformer Ratings

            5.3.3 Operation with Adjustable Speed

            5.3.4 General Guidelines Concerning the “Generator-Converter Transformer” Specification

      5.4 Smoothing Reactors

      5.5 d.c. Filters

            5.5.1 Conventional Filtering Scheme

            5.5.2 Alternative Filtering Schemes

      5.6 Valve Bridges

            5.6.1 Fundamental Considerations

            5.6.2 Rectifier Station Valve Requirements

            5.6.3 Considerations of Valve Ratings for Unit Connected Stations

            5.6.4 Rectifier Station Auxiliaries

            5.6.5 Circuit-Breakers

            5.6.6 Control and Protection Equipment

      5.7 Inverter Station

            5.7.1 Receiving end Requirements

            5.7.2 Implications of Receiving end Station Current Control

      5.8 Insulation Requirements

            5.8.1 Introduction

            5.8.2 Arrester Protection Scheme

            5.8.3 Continuous Operating Voltage

            5.8.4 Temporary Overvoltages and Switching Surges

            5.8.5 Fast Transient Overvoltages

            5.8.6 Protection Levels and Test Voltages

            5.8.7 Insulation Coordination Studies

      5.9 Simplified Steady State Analysis for Preliminary Considerations

6 CONTROL AND PROTECTION FUNCTIONS

      6.1 Introduction

      6.2 Basic Control Functions

            6.2.1 Quantities to be controlled

            6.2.2 Steady State Active Power

            6.2.3 Machine Speed

            6.2.4 Generator Voltage

            6.2.5 Direct Current

            6.2.6 Direct Voltage

            6.2.7 Receiving end Quantities

      6.3 Dynamic Control Functions

            6.3.1 Definitions and Scope

            6.3.2 Dynamic a.c. Voltage Control

            6.3.3 Damping of Electro-Mechanical Oscillations

            6.3.4 Suppression of Instabilities at Higher Frequencies

            6.3.5 Dynamic Interactions with the Power Plant

      6.4 Transient Control Functions

            6.4.1 Definitions and Scope

            6.4.2 Switching Transients without Faults

            6.4.3 a.c. System Faults

            6.4.4 d.c. overhead Line Faults

      6.5 Protection Functions

            6.5.1 Protection Philosophy

            6.5.2 Differential Protection

            6.5.3 Overcurrent Protection

            6.5.4 Overvoltage Protection

            6.5.5 Equipment Protection

            6.5.6 Generator Circuit-Breakers

      6.6 Diode-Equipped Rectifier

            6.6.1 General

            6.6.2 Steady State and Dynamic Control Functions

            6.6.3 Transient Control Functions

            6.6.4 Application of d.c. Circuit Breakers

            6.6.5 Conclusions

      6.7 Forced Commutated Inverter

            6.7.1 General

            6.7.2 Forced Commutation

            6.7.3 Forced Commutated Current source Inverter

            6.7.4 Other Types of Forced-Commutated Converters

            6.7.5 The Capacitor Commutated Converter (CCC)

            6.7.6 Conclusions

7 OVERALL PERFORMANCE OF HVDC UNIT CONNECTED STATIONS

      7.1 Station Capability

            7.1.1 Steady State

            7.1.2 Transient Capability

      7.2 Arrangements Capabilities: Limitations of Paired, Series Connected Arrangements

      7.3 Operational Characteristics

            7.3.1 Unit Connected Stations with Thyristor Bridges

            7.3.2 Operational Characteristics of Stations with Diode Bridges

      7.4 Dynamic Behavior

            7.4.1 Dynamic Behavior of Unit Connected Stations with Thyristor Rectifiers

                  7.4.1.1 Generator Load Rejection

                  7.4.1.2 Extinction of d.c. Line Faults By the Unit Connected Sending End

                  7.4.1.3 Station Faults

            7.4.2 Dynamic Behavior of Unit Connected Stations with Diode Rectifiers

      7.5 Conclusions

            7.5.1 Operational Characteristics of Stations with Thyristor Bridges

            7.5.2 Independence from Speed Oscillations Swings

8 MODELING ASPECTS RELATING TO HVDC UNIT CONNECTION

      8.1 Introduction

      8.2 Generator Modeling

            8.2.1. Representation of the Generator

                  8.2.1.1 Two Axis Model

                  8.2.1.2 Three Phase, Two Axis Model

                  8.2.1.3 Finite Element Model

            8.2.2 Modeling Details

                  8.2.2.1 Parameter Prediction for Equivalent Circuit Models

                  8.2.2.2 Finite Element Modeling Aspects

                  8.2.2.3 Theory of Finite element Modeling

                  8.2.2.4 Modeling Rotor Motion

                  8.2.2.5 Calculation of Winding Voltages and Currents

                  8.2.2.6 Modeling of the Rectifier Bridge

      8.3 Station Modeling for Studies of Reservoir Design

            8.3.1. The Simulation Program as an Analysis Tool

                  8.3.1.1 Hydraulic Reservoir Subsystem

                  8.3.1.2 Turbine-Generator Subsystem

            8.3.2 Operation at Fixed Speed

            8.3.3 Operation at Adjustable Speed

                  8.3.3.1 Representation of the Hydraulic Turbine Object Function

                  8.3.3.2 Choosing the Number of Generating Units

                  8.3.3.3 Simulation of Power Plant Operation

      8.4 Application to System Studies

            8.4.1 Equivalent Circuit Model of the Generator with Full System Representation

            8.4.2 Generator Models Validation

            8.4.3 Adjustable Speed Mode Long Term Operation Modeling

            8.4.4 Finite Element Model Integrated with Simplified Circuit Representation

            8.4.5 Finite Element Model Integrated with Transient Circuit Analysis Program

                  8.4.5.1 Transient Circuit Analysis

                  8.4.5.2 Finite Element Solution

            8.4.6 Concluding Remarks on Finite Element Modeling

      8.5. Conclusions