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FREQUENCY CONTROL DURING BLACK START OPERATIONSPowerPoint Presentation

FREQUENCY CONTROL DURING BLACK START OPERATIONS

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FREQUENCY CONTROL DURING BLACK START OPERATIONS

The Isochronous Governor Control Generator and QSE Remote Constant Frequency Control

Sydney Niemeyer: NRG Energy

Frequency Control During Black Start Operations is intended to provide basic dos and don’ts of frequency control during system recovery following a Black out. Review of Isochronous Governor Control of a Resource and Constant Frequency Control of an Island is included.

At the completion of this course of instruction you will:

- Identify the importance of understanding the amount of Primary Frequency Control available during Black Start and Island grid operations.
- Identify the amount of Primary Frequency Control available on an Island grid.

Continued:

- Identify the effects of cold load pick-up
- Identify the importance of one Isochronous Governor Control Resource per island.
- Identify the importance of having one QSE on Constant Frequency Control per island.

Definitions

- Area Control Error (ACE): The instantaneous difference between a Balancing Authority’s net actual and scheduled interchange, taking into account the effects of Frequency Bias, correction for meter error, and Automatic Time Error Correction (ATEC), if operating in the ATEC mode. ATEC is only applicable to Balancing Authorities in the Western Interconnection.

Definitions

- Area Control Error (ACE): Continued.
- Since ERCOT is not synchronously connected to any other Balancing Authority the net actual and scheduled interchange values are always zero.
- ERCOT does not correct for meter error in the measurement of actual interchange.
- ERCOT does not utilize ATEC.
- QSE’s on Constant Frequency Control (Flat Frequency Control) implement the ACE calculation exactly as ERCOT.

Definitions

- Automatic Generation Control (AGC): Equipment that automatically adjusts generation in a Balancing Authority Area from a central location to maintain the Balancing Authority’s interchange schedule plus Frequency Bias. AGC may also accommodate automatic inadvertent payback and time error correction.

Definitions

- Black Start Resource – A generating unit(s) and its associated set of equipment which has the ability to be started without support from the System or is designed to remain energized without connection to the remainder of the System, with the ability to energize a bus, meeting the Transmission Operator’s restoration plan needs for real and reactive power capability, frequency and voltage control, and that has been included in the Transmission Operator’s restoration plan.

Definitions

- Frequency Bias: A value, usually expressed in megawatts per 0.1 Hertz (MW/0.1 Hz), associated with a Balancing Authority Area that approximates the Balancing Authority Area’s response to Interconnection frequency error.
- Frequency Bias Setting: A value, usually expressed in MW/0.1 Hz, set into a Balancing Authority ACE algorithm that allows the Balancing Authority to contribute its frequency response to the Interconnection.

Definitions

- Frequency Bias Setting: (approved by NERC, not approved by FERC) A number, either fixed or variable, usually expressed in MW/0.1 Hz, included in a Balancing Authority’s Area Control Error equation to account for the Balancing Authority’s inverse Frequency Response contribution to the Interconnection, and discourage response withdrawal through secondary control systems.

Definitions

- Droop governor: a governor that changes the electricity generated proportionally to the change in electrical frequency. The slope of this change represents the ratio of electricity generated change to the frequency change.
- 100% electricity change for a 5% change in frequency is 5% droop.

- Isochronous governor: 1) a governor that maintains the same speed in the mechanism controlled regardless of the load. 2) The frequency of the electricity generated is “Flat” or constant and there is zero generator droop.
- RTU – Remote Terminal Unit

- The Black Start Generator and Frequency Control
- The next Start Generator and Frequency Control
- Sources of Primary Frequency Control
- Sources of Load Dampening
- Island Frequency Control

Black Start Resource

- Must operate with Isochronous governor in service.
- As electrical load is connected to the Black Start resource the governor responds to control the prime mover to maintain frequency to the frequency reference of the governor, 60 Hz.

Isochronous Governor Controller

Turbine actual speed = 3600 rpm

2 pole generator = 60.00 Hz

Comparator

Device

Increase Prime Mover if frequency is low, Decrease Prime Mover if frequency is high.

Reference input equal to 60.00 Hz

Isochronous Generator Controls to Target Frequency

60

- Load must remain within generator’s capability.
- The Isochronous Governor Controller does the rest.

57

63

Generator

Frequency

Hz

10 MW

20 MW Capacity

8 MW Output

300 MW Capacity

90 MW Output

Island 1

Black Start 1

Next Start D

10 MW

15 MW

House Load

Next Start 2

400 MW Capacity

12 MW Output

5 MW

Black Start A

60 MW Capacity

30 MW Output

Island 1:

420 MW Capacity

20 MW Load

Island 2:

360 MW Capacity

120 MW Load

100 MW

Island 1

- Next Start 2 generator is operating near minimum load.
- Unit is typically less stable.
- Risk of over-heating steam turbine from too little steam flow.
- Minimum flow is 2% to 5% of nameplate depending on manufacturer.

- Increase Island load if possible.

Island 2

- Should start next generator to balance load on generators. Increases reserves.
- Consider minimum operating conditions on each generator added to the island.
- Operating above minimum load on generator provides ability to place controls in AUTO at plant.
- Improves Primary Frequency Response delivery.

10 MW

Island 1

20 MW Capacity

8 MW Output

300 MW Capacity

70 MW Output

Black Start 1

Next Start D

10 MW

400 MW Capacity

70 MW Output

15 MW

House Load

Next Start 2

400 MW Capacity

12 MW Output

Island 2: Now with 3 generators on line. Prepare for Constant Frequency Control.

15 MW

Black Start A

60 MW Capacity

30 MW Output

Island 1:

420 MW Capacity

20 MW Load

Island 2:

760 MW Capacity

170 MW Load

100 MW

40 MW

Island Remote Constant Frequency Control

- QSE should have MW telemetry from each generator connected.
- QSE must have frequency telemetry from the island.
- At least one generator must be capable of remote dispatch (AGC).
- Black Start generator must be switched to “droop” governor control from isochronous control.

Preparation for Constant Frequency Mode

- QSE of the Island generators will not be following ERCOT Base Points.
- QSE will calculate Base Points for each controllable generator.
- QSE will calculate Island Area Control Error (ACE) using Constant Frequency (Flat Frequency) mode.
- Verify that Scheduled Frequency is set to 60.00 Hz.

Determine Island Frequency Response

- To calculate ACE of the island, the Frequency Response of the island must be estimated.
- ACE = (HzActual – HzScheduled ) * -10 * Bias
- Where Bias is the estimated Frequency Response of the island, a negative number.
- Determine MW capacity of all generators connected to the island that have a working “droop” governor.

Determine Island Bias

- Sum the island MW generation capacity.
- Divide the capacity by 30.
- The result is the estimated frequency response of the island in –MW/0.1 Hz.
- Capacity of frequency responsive generators = 760 MW.
- 760 MW/30 = 25.3 MW/0.1 Hz.
- Bias setting should be -25 MW/0.1 Hz.

Bias Setting and Load Dampening

- Synchronously connected loads provide some frequency response to the island.
- Synchronous AC motors on fans, pumps etc.
- Florescent lighting.

- Under normal conditions on the ERCOT grid, the frequency response of load on the grid can be estimated by multiplying the load (MW) by 0.00245 to get its MW/0.1 Hz contribution.

Bias Setting and Load Dampening Continued

- For a 30,000 MW load the frequency response provided by load dampening would be 30,000 * 0.00245 = 73.5 MW/0.1 Hz.
- During very cold weather with ERCOT load above 42,000 MW, the multiplier will reduce to 0.0015 since resistive heating load is not frequency responsive.

- During early stages of building an island, the load dampening contribution will be very small and can be ignored.

Know Your Generator Control System

- Some QSE Generator Control Systems require the Bias value to be entered as a positive number. It is converted to a negative value within the ACE calculation.
- It is simple to determine if you have Bias set correctly.
- If frequency is below target, the ACE value should cause generation to increase output.
- If frequency is above target, the ACE value should cause generator to decrease output.

Bias Setting Lower Better

- Initially set Bias lower than calculated until several load pickups have been observed.
- Cold Load Pickup will initially appear as 10 times the load increase but will decrease with time. It is better to under control (raise generation slowly) to avoid frequency oscillations as the cold load pickup will decay with time and aid frequency recovery to normal.

Cold Load Pick-up

- When adding load remember that cold load pickup is 10 times the steady state load and takes up to 30 minutes to decay.
- 1 MW pick up looks like 10 MW and will cause frequency to drop to 59.96 from 60 Hz with an island Bias of 25 MW/0.1 Hz.
- 3 MW pick up looks like 30 MW and will cause frequency to drop to 59.88 Hz.

Frequency Control in Constant Frequency Mode

- Will NOT be similar to having the Black Start Generator on Isochronous control.
- Frequency movement will be much greater.
- Frequency recovery to schedule will be dependent on the ramp rate of the generator(s) on AGC.
- Sustained Primary Frequency Response from all generators connected is critical for stable frequency control.

Keep Operating Margin

- The QSE should closely monitor operating margins on all the AGC generators.
- If Up or Down margin gets low, manually adjust non-AGC generators to give AGC generators more room to operate.
- When moving other generation, consider the ramp rate of the AGC generators to avoid excessive ramping demands.

Prepare for Tying Islands Together

- Synchronize smaller island to the larger island. (Smaller in MW load.)
- Before synchronization, determine MW capacity of incoming island that will not be on Constant Frequency Control.
- Calculate new Bias Setting of combined area by adding capacity of frequency responsive generators to existing island.

Bias Setting For Two Islands Connected

- Existing Island on Constant Frequency Mode capacity: 1260 MW.
- Incoming Island on Isochronous governor control by the Black Start generator. Capacity 420 MW.
- Combined capacity: 1680 MW.
- Bias Setting: 1680/30 = -56 MW/0.1 Hz.
- Be sure to include only generation capacity that is frequency responsive.

20 MW Capacity

8 MW Output

300 MW Capacity

70 MW Output

Island 1

Black Start 1

Next Start D

Sync Point

10 MW

400 MW Capacity

70 MW Output

15 MW

House Load

Next Start 2

500 MW Capacity

90 MW Output

400 MW Capacity

12 MW Output

Island 2

15 MW

Preparing to synchronize Island 1 to Island 2.

Black Start A

60 MW Capacity

30 MW Output

Island 1:

420 MW Capacity

20 MW Load

Island 2:

1260 MW Capacity

260 MW Load

100 MW

130 MW

Synchronizing Two Islands

- In the smaller island, take the Isochronous Black Start generator out of Isochronous governor control and place it in droop mode.
- At Sync point, observe island voltages and match appropriately.
- Observe Sync scope and adjust generation in smaller island to achieve a slightly “fast” rotation of the scope. Close the breaker at the proper phase alignment.

Connecting Two Islands When Both are on Constant Frequency

- The process is the same as the isochronous generator.
- Take the smaller MW load island (incoming generation) off Constant Frequency mode.
- Field operator at Sync location should have contact with QSE of the incoming island to direct generation up or down to achieve proper synchronization.

- Only one QSE remains on Constant Frequency mode after synchronization.

20 MW Capacity

8 MW Output

300 MW Capacity

70 MW Output

1 Island

Black Start 1

Next Start D

Sync Point

10 MW

400 MW Capacity

70 MW Output

15 MW

House Load

Next Start 2

500 MW Capacity

90 MW Output

400 MW Capacity

12 MW Output

15 MW

Synchronized

Black Start A

60 MW Capacity

30 MW Output

Island:

1680 MW Capacity

280 MW Load

Bias Setting: -56 MW/0.1 Hz

100 MW

130 MW

After Connecting Islands

- QSE that remains on Constant Frequency mode adjusts Bias Setting to account for the additional frequency responsive generation.
- Communicates through ERCOT for adjustments in generation operated by the other QSE.
- Close additional tie lines to strengthen island connections.

20 MW Capacity

8 MW Output

300 MW Capacity

70 MW Output

1 Island

Black Start 1

Next Start D

10 MW

400 MW Capacity

70 MW Output

15 MW

House Load

Next Start 2

500 MW Capacity

90 MW Output

400 MW Capacity

12 MW Output

15 MW

Synchronized

Black Start A

60 MW Capacity

30 MW Output

Island:

1680 MW Capacity

280 MW Load

Bias Setting: -56 MW/0.1 Hz

100 MW

130 MW

One Generator on Isochronous Governor Control per Island or

One QSE on Constant Frequency Control per Island

Only one bossTwo Isochronous Governor Controls Active on One Island

- Each generator will control to a slightly different frequency.
- The generator controlling to the higher frequency will increase output.
- The generator controlling to the lower frequency will decrease output.

Isochronous Governor Controller

Small conversion errors can exist at several locations.

1. Resolution of actual frequency, turbine speed.

Turbine actual speed = 3600 rpm

2 pole generator = 60.00 Hz

Comparator

Device

Increase Prime Mover if frequency low, Decrease Prime Mover if frequency high.

3. Resolution of the comparator circuitry/logic.

Reference input equal to 60.00 Hz

2. Resolution of reference frequency, is it an analog signal or digital.

Gen A operating at 20 MW output and on Isochronous Control. Controlling to 60.008 Hz

Gen B switched to Isochronous Control and controlling to 60.002 Hz.

Gen B operating at 15 MW output and on Droop Control.

Why Only One QSE on Constant Frequency Mode Controlling to 60.008 Hz

- Just like having two Isochronous governor control generators active on the same island, Constant Frequency mode will generally result in undesired actions.
- The problem is that neither control system will measure the exact same value for the frequency.
- Neither control system will measure the exact same value for the frequency reference.

QSE Constant Frequency Mode Controlling to 60.008 Hz

QSE A on Constant Frequency Control

Island ACE = (HzActual – 60.00) * -10 * Bias

Island Frequency

Bias

ACE

RTU

RTU

Target Frequency

60.00 Hz

Gen 2 MW and/or AGC Telemetry

Gen 1 MW, Hz & AGC Telemetry

QSE AGC Control Loop Controlling to 60.008 Hz

QSE’s AGC Generator

Converts Digital Count to Engineering Units (Hz) and sends Control Signals to AGC Generator based on ACE.

4

Converts 13.8 KV to 120 VAC

Potential Transformer

1

QSE Generation Control System

Frequency Transducer

Converts 120 VAC to madc or vdc

2

Converts madc or vdc to a Digital Count and transmits to QSE when requested.

RTU

Data Request

3

Data Reply

Generator Control

Summary Controlling to 60.008 Hz

- The Isochronous Governor Controlled generator will manage frequency of the island.
- Don’t ask it to do more than it is capable of doing.

- One Isochronous generator per island.
- One Constant Frequency QSE per island.
- Consider Generator minimum operating level before connecting to the island.

References Controlling to 60.008 Hz

- NERC Glossary: http://www.nerc.com/pa/Stand/Glossary%20of%20Terms/Glossary_of_Terms.pdf2

- Initially during the first energization of an island grid, frequency control in Real Time will be managed by the ___________________.
- ERCOT Load Frequency Control (LFC) system.
- QSE on Constant Frequency Control.
- Resource on Isochronous Governor Control.
- Resource on Droop Governor Control.

- When a QSE initially begins controlling the island grid and places its Control System on Constant Frequency Control, they should set their Bias in their ACE equation to ________.
- 699 MW/0.1 Hz, the present ERCOT Bias setting.
- 698 MW/0.1 Hz, 1% of ERCOT Forecast Peak Load.
- Total MW Capacity of the island being controlled divided by 30.
- Sum of the frequency responsive MW Capacity of the island being controlled divided by 30.

- Cold load pick up will initially appear as _____ times the load increase but will decrease with time.
- 5
- 7
- 9
- 10

- Before a QSE places its Control System on Constant Frequency Control and begins controlling island frequency it must have _____________________.
- ERCOT Frequency and ability to control one Resource connected to the island on AGC.
- QSE Control System frequency and ability to control one Resource connected to the island on AGC.
- Frequency of the island and ability to control one Resource connected to the island on AGC.
- Resource on Isochronous Governor Control and one Resource on AGC, frequency input not needed.

- When synchronizing two islands where both islands have one Resource on Isochronous Governor Control, what is the correct procedure in synchronizing the two islands?
- Take both units off Isochronous Governor Control after synchronization to maintain stability, and leave the largest Island on Isochronous Governor Control.
- Take the unit in the smallest Island off Isochronous Governor Control and place in droop prior to synchronization and leave the largest Island on Isochronous Governor Control.
- Take the unit in the largest Island off Isochronous Governor Control and place in droop prior to synchronization and leave the smallest Island on Isochronous Governor Control.

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