reactive power-voltage control of inverter based...

Reactive Power-Voltage Control of Inverter Based ResourcesDouglas Brown, Siemens PTI

Unrestricted © Siemens 2020

Unrestricted © Siemens 2020November 2020

Overview

Discussion of reactive power and voltage control of Inverter Based Resources

• Reactive Power Requirements

• Implementation

• Performance Targets

• Modeling of reactive power-voltage controls for planning and operating studies

• Examples

See companion paper for references.


Active and Reactive Power

Active power is the power transmitted to loads and transformed into mechanical energy, heat, or light.

Reactive power is used to establish and maintain the magnetic field in electrical equipment.

The active and reactive power supplied must equal demand plus losses. • Shortage of active power will cause frequency to decline. • Shortage of reactive power will cause voltage to decline.


Inverter Based Resources

• Wind turbines, solar PV inverters and battery energy storage inverters are asynchronously connected to the grid

• Partially or completely interfaced through power electronics• Non-synchronous generators are also referred to as Inverter-Based Resources


Reactive Power Requirements for Non-Synchronous Generators

FERC Order No. 2003 (2003)• Established standard procedures for interconnecting generators larger than 20 MW• Generators required to provide ±0.95 power factor at POI• Exempted wind plants from reactive power requirement

FERC Order No. 661 (2005)• Established standards for interconnection of large wind plants• Preserved reactive power exemption for wind plants unless system impact study

showed capability was required for reliability• Exemption did not apply to non-wind technologies

FERC Order No. 827 (2016)• Newly interconnecting non-synchronous generators required to provide dynamic

reactive power within the power factor range of ±0.95


FERC Order 827 Requirements for Non-Synchronous Generators

Power Factor Range• Dynamic reactive power within the power factor range of 0.95 leading to 0.95 lagging

Point of Measurement• Reactive power requirement is measured at the high side of the generator substation

Dynamic Reactive Power Capability• Dynamic reactive power capability of the inverter or other dynamic reactive power

devices• Static reactive power devices to make up for losses

Real Power Output Level• Generating Facility is required to meet the reactive power requirements at all levels

of real power output


Plant Controls

• Plant-level controller

• Inverter-level controllers

• OLTC or DETC on the main power transformer

• Plant-level capacitors and/or reactors


Plant-Level and Inverter-Level Controllers

Plant-level controller• Controls voltage at POI or POM• Distributes voltage or reactive power setpoints to each inverter

Inverter-level controllers• Inverters inject reactive current in response to command from plant controller• Many inverters also provide fast control of terminal voltage


Performance Requirements: Plant Capability Curve

Order 827 requires plant provide reactive power within the power factor range of 0.95 leading to 0.95 lagging at all levels of real power output

Critical test points are usually at rated output and near 0 MW output.

Plant shown at right requires switched capacitors when operating above 90% of rated output.

Many wind turbines and solar inverters cannot control voltage at zero active power output unless this option is purchased by the Generator Owner.

Plant Reactive Power Capability at POM


Reactive Power Requirement as a Function of Voltage

Order No. 827 does not specify a voltage range for the reactive power requirement.

Consider power factor requirements at different system voltages.

System voltage affects the reactive capability of the inverter as well as the need for reactive power from the plant.

Pay special attention to capability within VAR-002-4.1 voltage schedule tolerance band.

Australian Energy Market Operator Reactive Power Requirement


Disturbance Performance Targets

• Response of non-synchronous generators is determined by the controls programmed into the inverters and plant level controller

• NERC Reliability Guideline1 provides performance targets for small and large disturbances

• Plant-level controller provides primary response to small disturbances

1. NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance


Disturbance Performance Targets

• Inverter-level controllers provide primary response to large disturbances


Generic Models for Positive Sequence Stability Analysis

Generic models for renewable energy plants are available as standard library models in commercial planning software platforms• Intended to provide a representation of dynamic electrical performance at POI• Intended for analyzing electrical phenomena in the frequency range of zero to 10 Hz• Applicable for systems with a short circuit ratio of 3 or higher at POI


REGC Renewable Energy Generator/Converter Model

Represents the generator/converter interface with the grid

Inputs• Real and reactive current command from

REEC model

Outputs• Real and reactive current

injection


REEC Renewable Energy Electrical Control Model

Represents the electrical controls of the inverters

Variety of REEC models to represent controls associatedwith different prime movers

Inputs• Active and reactive power reference

from REPC model• Terminal voltage and inverter output power

Outputs• Real and reactive current command to REGC model

Operating modes selected using the flags PfFlag, VFlag and Qflag

REEC_A Reactive Current Control (Active Current Control not Shown)


REPC Renewable Energy Plant Control Model

Represents the plant controller

Inputs• Voltage• Frequency• Plant output

Outputs• Active and reactive power

reference to REEC model

Operating modes selected using the flag VCFlag, RefFlag

Model PLNTB is similar and includes constant power factor mode

Reactive Power Control (Active Power Control not Shown)


REPC Voltage Regulation

In voltage control mode, VCFlag used to select line drop compensation or droop response.

Reactive droop provides a reactive power setpoint based on the voltage error

• Ensures coordinated control among resources in close electrical proximity

• Droop is the voltage error that causes the reactive power output to go to the maximum value

• Graph shows 3% reactive droop on 0.95 power factor base

3% Reactive Droop, Qmax = 33% of Prated


Reactive Power Control Options

Flags in the REEC and REPC models can be used to simulate various combinations of plant-level and inverter-level controls

Common combination are shown below


Wind Power Plant Example

• 100 MW power plant

• POM connected to POI by short 161 kV transmission line

• Plant controller set to regulate voltage at POM to 1.02 per unit

• WTG have local coordinated Q/V control


Wind Power PlantSwitching 21 MVAr Reactor at POI

Terminal voltage falls and local V control quickly increases WTG reactive power output.

Local Q control starts to reset WTG reactive power

Around 2 seconds, WTG starts to increase reactive power output as a result of the plant controller changing the REEC reactive power reference to restore POM voltage

Plant-Level V Control & Local Coordinated Q/V Control


Wind Power PlantSmall Disturbance Reactive Power-Voltage Performance

• Small disturbance performance evaluated using techniques similar to those for MOD-026-1

• Wind plant model checked by applying a 2% step to the plant reference voltage

• Performance exceeds NERC Guideline targets

2% Step Applied to Reference Voltage


Wind Power PlantLarge Disturbance Reactive Current-Voltage Performance

• Wind plant model checked by applying a permanent fault at POI to drive voltage at WTG terminals to 0.50 per unit

• Performance exceeds NERC Guideline targets

Response to Fault at POI


Solar PV Power PlantSmall Disturbance Reactive Power-Voltage Performance

• 100 MW solar PV power plant• Plant controller set to regulate voltage• Inverters have local coordinated Q/V control

• Model checked by applying a 2% step to the plant reference voltage

• Performance does not meet NERC Guideline targets

2% Step Applied to Reference Voltage


Conclusions

Reactive power-voltage control requirements relevant to inverter based resources• FERC Order No. 827 reactive power requirement• NERC VAR-002-4.1 requires that generating facility operate in automatic voltage control

mode and maintain the voltage schedule provided by the Transmission Operator

Performance requirements and guidelines• IEEE working group P2800 is drafting a standard that will establish recommended

interconnection capability and performance criteria for inverter-based resources• NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance

Plant models should be verified• Ensure that FERC Order No. 827 requirements are implemented correctly by the

Generator Owner• Check dynamic performance using NERC guideline


Douglas BrownSenior Manager, Power System ConsultingSiemens Power Technologies International

10900 Wayzata BoulevardMinnetonka, MN 55305

952.818.2227

[email protected]

mailto:[email protected]

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