|Some black start units have never even been used during an actual black start, but all undergo annual testing to ensure fast and fluid responses of their instrumentation and controls.|
By Lindsay Morris, Associate Editor
In Stephen King’s 1994 made-for-TV movie “The Stand,” most of the human race is wiped out by a deadly virus. As a result, power stations are unmanned and Americans are left without electricity for months. That is, until a husband and wife team works engineering magic at a power plant, flipping the right switches to bring the entire grid back online.
Anyone familiar with the black start process knows that in real life, it doesn’t happen with quite so much Hollywood pizzazz. But black start is a remarkable process and the controls and instrumentation used during a black start must operate with the utmost precision and speed.
A black start unit is one that can start its own power without support from the grid in the event of a major system collapse or a system-wide blackout. In the U.S., every region within the North American Electric Reliability Corp. (NERC) has its own black start plan and procedures. Each region also designates certain plants as black start units. The controls used on a black start unit include a DC auxiliary support system, an ignition source, a gas turbine and a diesel generator.
Carlo Barrera, senior consulting engineer at PAL Turbine Services LLC, has overseen several conversions of gas turbines to have black start capabilities, including projects for Puget Sound Energy and Massachusetts Municipal Wholesale Electric Co. For the city of Gardner, Kan., PAL installed its own programmable logic controller for turbine control. At a later date, black start capability was incorporated and proved out using a load bank.
Barrera said the DC auxiliary support system is perhaps the most important part of the control system. The battery system must have enough capability to provide DC power for multiple start attempts in case the gas turbine fails to start or fire the first time. “The battery systems need to have the capability in reserve power for two or three firing attempts if a true blackout emergency happens, since gas turbines don’t always start on the first attempt in a blackout situation,” Barrera said.
When the loss of AC power in the grid is noticed on a black-start turbine, an undervoltage relay initiates the start of numerous DC motor-driven auxiliaries. Devices like the turbine lube oil pump, liquid fuel forwarding pump, atomizing air compressor, starting clutch, diesel starting motor and shaft turning ratchet all require DC power to operate. DC auxiliary support system suppliers include GE, Siemens and ABB.
Barrera said another control priority is an ignition source, because gas turbines require high-voltage spark plugs to ignite the fuel in the combustors. The main power source to the auxiliary motors is direct current, assuming the batteries are properly charged, so a method of creating AC power for the spark plugs is needed. This is typically done with a DC motor driving an AC generator, Barrera said. An alternative would be a DC rectifier bridge system.
Most of the time, the starting device for a unit with black start capabilities is an auxiliary generator rather than a diesel engine, Barrera said. Diesel engines can typically be used for units up to 35 MW, but larger frames need auxiliary generators, which are available through suppliers such as Caterpillar.
Once the black start gas turbine has reached complete sequence (also called full speed, no load), the system then must be able to close the main circuit breaker. A grid undervoltage relay senses that there is no power on the other side of the breaker but still allows it to close. Once the breaker closes, feeder breakers can then bring in the system load as the grid is slowly restored. It is important also to operate the grid at 60 cycles per second (60 Hz), Barrera said.
David Lucier, founder and general manager of PAL, said that many gas turbines manufactured by GE allow the operator two mode choices: droop or isochronous operation. During the black start process, the turbine must operate temporarily as an isochronous (constant frequency) generator, Lucier said. Later, as the system is restored and larger generators start and synchronize to the newly established grid, the black start turbine’s control can be reverted back to a “droop” style governor by the plant operators. This allows for load sharing between all generators on the restored power grid, Lucier said.
Black start procedures
While every region must have a black start plan in place, ERCOT, for example, has never used its black start plan, said ERCOT spokesperson Dottie Roark. That’s because ERCOT has never experienced a system-wide blackout; however, the region has black start units under contract to provide black start service in the event of a system collapse. In the case of a system-wide blackout, ERCOT also has black start procedures in place for restoring the system to a normal state as quickly as possible. For example, units must be able to start up on their own without support from the grid and be able to pick up their own internal load.
According to NERC, black start resources are compensated as additional reliability services in most electric wholesale markets. In the case of an ERCOT system-wide blackout, these units will be required to build a stable island with the ultimate goal of reaching synchronization points with other neighboring black start islands. Operators are to contact ERCOT when the islands are ready to be synchronized and ERCOT will coordinate frequency control. In the case of a blackout, ERCOT’s goal is to have 90 percent of load restored within 48 hours.
ERCOT contracts every two years for black start units and holds an annual drill with the black start unit providers to test the black start procedures. ERCOT has 15 black start units under contract for 2010 to 2011, including the Lower Colorado River Authority’s (LCRA’s) Wirtz Dam near Marble Falls, Texas, and LCRA’s Buchanan Dam near Burnet, Texas.
A former plant manager of one utility within ERCOT, which requested to be unidentified due to NERC Critical Infrastructure Protection (CIP) standards, said that nominal 50 MW gas generators are used for one of its black start units. If the plant goes dark, the generators kick in automatically, which provide critical service to various plant systems. Next, the gas turbine control house is powered up through a number of transfer switches, some automatic and some manual. Once the turbine is running, it must be synchronized to a dead bust.
In starting a unit from a true blackout condition, it is essential to assure there is absolutely no other power entering the control and protection systems – that the system is at a “dead bust” with no power, Barrera said. Otherwise, equipment could be damaged and personnel could be injured.
Once online, the utility can start to bring load back on. The gas turbine is started up in isolated droop mode, which allows the plant to set the frequency based on the governors on the gas turbine. Once the station is in droop mode, a couple of different scenarios could occur. If one of the larger steam units was online and tripped when the system went black, it would need to be stabilized. If a steam unit doesn’t need to be stabilized, then one of the larger motors would be added for the purposes of giving more stability to the generator.
Once transmission is started, the plant is hit with a large megawatt increase and the generator may not be able to handle that much of a swing, so stability is essential. At the point that the plant reaches stability, synchronization with other neighboring black start islands can begin.
Noteworthy black start
One of the most remarkable black starts occurred during the Northeast Blackout on Nov. 9, 1965, which affected Ontario, Connecticut, Massachusetts, New Hampshire, Rhode Island, Vermont, New York and New Jersey. At the time, Southampton, New York housed a 12 MW GE gas turbine at a unit owned by Long Island Lighting Co. (it is currently owned by U.S. National Grid).
“This little unit is credited for bringing back Long Island and feeding power to New York City after the Northeast Blackout,” Lucier said. “It had the capability of getting a steam plant nearby up and running.” The entire black start process took about eight minutes, Lucier said. Equally noteworthy is the fact that this 12 MW turbine is still in operation.
While some black start units have never demonstrated their capabilities during an actual black start, they all undergo annual testing to ensure fast and fluid responses during system-wide blackouts. NERC and regional standards ensure that units and personnel are trained for bringing the grid back online and quickly as possible, and that black starts occur in a safe and fluid fashion.
Black start operations must be conducted in compliance with NERC Critical Infrastructure Protection (CIP) standards. Black start resources are linked to the CIP EOP-005-2 standard, and any cyber asset that is essential to the operation of a black start resource is a “Critical Cyber Asset” by definition,” according to NERC.
Once a black start unit and its resources are defined as a “Critical Cyber Asset,” it must meet all the requirements of CIP standards CIP-003 through CIP-009. These standards include:
- Assessing and documenting at least annually the processes for controlling access privileges to protected information.
- A thorough personnel risk assessment.
- Maintaining a procedure for securing dial-up access to the Electronic Security Perimeter(s).
- Documenting and implementing the technical and procedural controls for monitoring physical access at all access points to the Physical Security Perimeter(s) 24/7.
NERC-CIP standards only come into play on machines that are connected by a routable protocol to the outside world (digital units). Some black start units, such as hydro units, are still analog. Therefore, those units would not fall under NERC-CIP standards.
The NERC-CIP-003 through -009 are available online at NERC.com.—LM
SERC-sponsored Black Start drill
In November 2010, SERC sponsored a restoration drill at the Entergy headquarters in Jackson, Miss., that required utilities to demonstrate their black start capabilities. Participants included Alabama Power, Entergy, Gulf Power, Mississippi Power, Power South, Southern Company and South Mississippi Electric Power Association. A scenario was developed that placed part of the SERC region in islanded conditions, in addition to abnormal system configuration.
The drill scenario consisted of a system event or multiple system events that caused the SERC Region to either blackout or separate. Companies were faced with one or more of the following conditions on their system:
- Partially blacked out – required to integrate black start into restoration
- Partially blacked out with load or generation connected to a neighboring area – required to integrate black start into restoration
- Partially blacked out with no internal generation available – required to obtain outside help.
During the drill, each entity was required to restore their system utilizing their designated Black Start Plan or modified plan based on the drill system conditions. All restoration activities were required to be conducted in accordance with NERC, SERC and company guidelines.—LM