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ERLPhase Power Technologies

Application for A Sub-harmonic Protection Relay

Outline

• Introduction

• System Event at Xcel Energy

• Event Analysis

• Microprocessor based relay hardware architecture

• Sub harmonic detection process

• Principle of sub harmonic detection

• Operations/Minute detector

• Results and discussions

• Conclusions

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Introduction

• Wind power based generation is growing rapidly throughout the world

• Since wind farms are generally located far away from the load centers – series compensation is used for economical reasons

• Sub Synchronous Resonance (SSR) is a well known phenomena in series compensated system and many mitigation techniques exist

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Introduction..

• Sub harmonic oscillations due to wind generation interaction (controls/system) impose new challenges to today’s electrical grid.

• An event on Xcel Energy grid involving the interconnection between a wind generator and a series compensated transmission system lead to the development of a protection relay to detect frequencies in the range of 5 HZ to 25 HZ, later expanded to 40Hz, to provide back up protection to special protection schemes installed around the series compensated line.

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Introduction...

• Sub harmonics is defined as the harmonic frequency below the system frequency (60 Hz or 50 Hz)

• Natural frequency of series compensated transmission system is defined by

Where,

fn = natural frequency (Hz)

fsys = system frequency (60 Hz or 50 Hz)

Xc = series capacitor reactance

XTOT = sum of all the system inductive reactance (transmission line, transformer, generator sub transient reactance

Note:

For compensation between 20% to 80%, fn < 60 Hz which is sub synchronous frequency

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fn= fsys √Xc /XTOT

System electrical dynamics

Generator electromagnetic

dynamics

current

voltage

Electrical self excitation or Induction generator effect (IGE)

Introduction….

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Introduction…..

• SSCI –Sub Synchronous Control Instability

– Interaction Between Power Electronics Devices (Wind Turbine, HVDC, SVC etc.) and Series Compensated Transmission System.

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Power Electronic Devices

Series Compensated Transmission

System

Control

Signals

Introduction…...

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System electrical dynamics

Generator electromagnetic

dynamics

Current

Voltage

Turbine mechanical dynamics

Electromagnetic

torque

Shaft speed

Torsional interaction

Capacitor fault induced voltage

Generator electromagnetic

dynamics

Current Turbine

mechanical dynamics

Electromagnetic

torque

Mechanical Torque Amplifications

Mechanical torque

System Single Line Diagram

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Event Description

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Event Description

Event Description

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Event Description

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Event Description

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1- Breaker 1 & 2 opened for

regular system switching

procedure

2- CT1,CT2, and W start feeding

radially through series capacitor

3- Tripped the CT generator unit

Line 1 345 kV

345 kV

45 MVA

System Event

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CT1, CT2 - Combustion turbine generators

W - Wind generators – as a single unit

15 MW (20% of total generation)

To the

System

To the

System

Event Analysis

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9 Hz & 13 Hz dominant sub harmonics

High speed recording of 3 phase currents

captured by the DFR

Event Analysis

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Slow speed (swing ) recording of one of the phases

Studies and Conclusions

• Radial connection of generation to the system would lead to sub synchronous frequency oscillation.

• Study was conducted to determine the impact of series capacitors installation on existing generators in the region.

• The study concluded that only the combustion turbines and the wind farm at Lakefield generation were impacted.

• Special Protection Scheme (SPS) was needed to bypass the capacitor under some system configuration.

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Effect of Series Capacitors

On Combustion Turbine

• The study concluded that the CT generator units are stable under base (normal) condition but with some potential for SSR under N-1 contingency.

• Since conclusion depended heavily on the actual mode shape of the turbine-generators and damping due to load, it was decided to determine the load damping and mode shape through actual measurement on one of the units.

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Effect of Series Capacitors

On Wind Farms

• Study concluded that the wind farm will be affected by what is known as SSR induction generation effect.

• In the case of Xcel Energy, it was determined that this can be mitigated by bypassing the series capacitor on loss of LFD-LAJ line.

• In addition, a study performed by Xcel Energy also concluded that failure to bypass should be backed up by suitable SSR protection that can recognize the electrical instability and trip the wind farm as a last resort.

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Special Protection Scheme

A special protection scheme was designed to bypass the series capacitor for the following conditions:

At Lakefield Generation substation:

– Any relay operation on the LFD-LAJ line

– Breaker configuration at Lakefield Generation that leads to radial connection of wind farm or CT Generators

At Lakefield Junction substation

– Any relay operation on LAJ-LFD line

– Any configuration of breakers at Lakefield Junction that leads to opening of the line towards Lakefield Generation.

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Special Protection Scheme

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Second Event Analysis

• Phase currents of wind generators connected to the system during the second switching event had sub synchronous frequencies even when the series capacitor was not in service.

• The conclusion was to develop a back up protection device to detect sustained oscillations.

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Need for Backup Protection for Wind Farm

• None available at that time to detect sub synchronous frequencies.

• Specification/Requirement:

– Ability to detect any frequencies from 5-25 HZ with 1 HZ resolution.

– Set points based on nominal, fundamental and THD frequency ratio.

– Set points based on individual current or voltage input as well as summation of more than one current input.

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Is this the only event that ever occurred?

• There was an event in 2009. Reference NERC Lesson Learned Sub-Synchronous Interaction between Series-Compensated Transmission Lines and Generation. Published July 26,2011.

http://www.nerc.com/files/LL_45_Sub-Synchronous_Interaction.pdf

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Description of the 2009 Event

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NERC Conclusiones

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4000 Series

Large LCD allows for better metering display.

Unique front panel USB and Ethernet ports provide easy and fast access to settings and set up.

Navigation controls allows for and easy experience through maintenance, service and view menus.

Programmed target LEDs provides tripping information facilitating crew personnel.

Optical ports ready for IEC-61850 Goose.

Fast processor and hardware platform.

S-PRO – Sub-Harmonic Protection

S-PRO – Sub-Harmonic Protection

Inputs and Outputs

• 4 sets of 3-Phase CT inputs (<0.25 VA) – 5 A RMS – nominal (1 Amp nominal version available)

– 15 A RMS - maximum continuous

– 100 A for 1 second - maximum full scale without distortion

– 400 A for 1 second - maximum thermal rating

• 2 sets of 3-Phase VT inputs (<0.15 VA) – 69 V RMS - nominal

– 138 V RMS – maximum continuous

– 207 V RMS for 10 seconds - maximum thermal rating

• 9 digital inputs, externally wetted

• 14 programmable output contacts, plus 1 relay inoperative output

S-PRO – Sub-Harmonic Protection

Rear Connections

3U Unit

4U Unit - Future

S-PRO – Sub-Harmonic Protection

Protection

• Sub-Harmonic Detector – Frequency Range 5 – 25 Hz

• Fundamental Frequency Protection functions — IEEE devices 27, 50LS, 59

• Ring bus capability

• ProLogic — 24 control logic statements

• 8 setting groups with setting group logic

Sub Harmonic Detection

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Trip or Alarm: = max (f2, f3, f4, f5, f6, f7) >

Lset

Operations/Minute Detection

53 Return

Sub Harmonic Detection Logic Diagram

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Results and discussions

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Calculating Relay Settings

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Current response to sub-synchronous resonance

Calculating Relay Settings

64 Sub-harmonic magnitudes (RMS). Return

Sub-Harmonic Current Nominal Ratio

• Calculated by taking the highest sub-harmonic current observed for the 5Hz - 40Hz divided by the nominal input current setting (1A or 5A).

• Sub-harmonic current levels can vary depending on the topology of the power system and the type of event. It is recommended that different contingencies be considered.

• In the case under consideration, the highest sub-harmonic current observed during the event was the 11th harmonic with a magnitude of 172 primary amps with a current transformer ratio of 400:1. The relay current inputs were 5A inputs.

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Sub-Harmonic Current Nominal Ratio

• Applying the nominal ratio setting, we have the following:

• Set nominal ratio > 6%

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Sub-Harmonic Current Fundamental Ratio

• Calculated by taking the highest sub-harmonic current between 5Hz - 40Hz divided by the fundamental current value (50Hz or 60 Hz). This ratio is calculated as follows:

• The fundamental current observed in the record was 1300 primary amps with a current transformer ratio of 400:1. The fundamental ratio then becomes:

• Set fundamental ratio > 10%

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Total Sub-harmonic Distortion

• This setting takes into account every sub-harmonic from 5Hz - 40Hz by taking the square root of the sub-harmonics currents squared.

• The record in slide 40 shows a captured value of total sub-harmonic distortion (TSHD) between 160% and 165%. A TSHD setting greater than 100% would be appropriate.

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Operations per Minute

• Due to nuisance cases, and as shown in slide 32 an operations per minute setting should be applied.

• For this case, a minimum of 30 op/min is required for any of the other sub-harmonic settings to take effect.

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Sub-Harmonic Voltage Nominal Ratio

• Calculated by taking the highest sub-harmonic voltage value seen in the 5Hz – 40Hz frequency range divided by the secondary nominal voltage level 69V

• For the case under consideration, the 11th sub-harmonic had the highest magnitude. Using the PT ratio of 3000:1, the nominal ratio is:

• Nominal ratio = 3%

• Set > 2%

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Sub-Harmonic Voltage Fundamental Ratio

• Calculated by taking the highest sub-harmonic voltage value and dividing it by the fundamental voltage value. This ratio is calculated as follows:

• Nominal ratio = 3%

• Set > 2%

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Overcurrent and Overvoltage

• Overcurrent and Overvoltage Setting for the fundamental shall be determined by means of a power system study that should reveal the current and voltage levels that may appear in the system during an event.

• The overcurrent settings shall be coordinated with the other overcurrent relays being used to protect the transmission line.

• The overvoltage settings shall be coordinated with the regional reliability center requirements and the minimum insulation requirements for the equipment.

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Conclusions

• The event captured at the Xcel Energy Utility lead to the development of a new microprocessor based sub harmonic protection technique.

• With the increase use of wind generators feeding HV and EHV utility networks, it is necessary to ensure that sub harmonic oscillations are monitored, and that the electrical grid is protected from any resulting detrimental effects.

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Conclusions ..

• The performance of the sub-harmonic protection technique was successfully tested using a Real Time Digital Simulator (RTDS)

• The tests demonstrated that the solution is capable of performing reliably the following functions:

– Nominal sub-harmonic detection

– Fundamental sub-harmonic detection

– Total sub-harmonic detection

– Operations/Minute detection

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Conclusions ...

• Digital fault recorders can be used to capture osciollography files that can be analyzed to determine levels of sub-harmonics during a SSR event

• Using graphical tools similar to those shown in this presentation, protection and control engineers can easily determine appropriate relay settings for sub-harmonic protection relays

• There are also software tools that can be used to simulate the power system to determine possible SSR phenomena for various power system topologies

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