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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      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


      2.1 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

         Capital Savings if Designing for Adjustable Speed Operation

         Long Term Economic Impacts of Adjustable Speed Operation.


      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

         Operation at Rated Head

         Operation at Low Heads

         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


      4.1 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


         Variation of Reactance with Speed

         Typical Values of Commutation Reactance for 12 Pulse Unit Connections

         Non Standard Generator Subtransient and Transformer Leakage Reactances

         Damper Windings

         Variation Due to Machine Load

         Influence of Dampers Resistance

            4.3.2 Generator Operational Inductances over a Range of Speed

         Hydro Generator Operational Inductances

         Hydro Generator Frequency Response

         Generator Subtransient Reactance as Affected by Operating Conditions

         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

         Repetitive Impulse Voltage

         Insulation Problems of Rotating Machines

         Insulation Degradation of Form Wound Windings for Large Machines

         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


      5.1 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

         Generator EMF

         Valve Voltage

         Angle of Overlap

            5.2.5 Some Additional Points

         Tap Charger Range under Adjustable Speed

         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


      6.1 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


      7.1 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

         Generator Load Rejection

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

         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


      8.1 Introduction

      8.2 Generator Modeling

            8.2.1. Representation of the Generator

         Two Axis Model

         Three Phase, Two Axis Model

         Finite Element Model

            8.2.2 Modeling Details

         Parameter Prediction for Equivalent Circuit Models

         Finite Element Modeling Aspects

         Theory of Finite element Modeling

         Modeling Rotor Motion

         Calculation of Winding Voltages and Currents

         Modeling of the Rectifier Bridge

      8.3 Station Modeling for Studies of Reservoir Design

            8.3.1. The Simulation Program as an Analysis Tool

         Hydraulic Reservoir Subsystem

         Turbine-Generator Subsystem

            8.3.2 Operation at Fixed Speed

            8.3.3 Operation at Adjustable Speed

         Representation of the Hydraulic Turbine Object Function

         Choosing the Number of Generating Units

         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

         Transient Circuit Analysis

         Finite Element Solution

            8.4.6 Concluding Remarks on Finite Element Modeling

      8.5. Conclusions