System Restoration through Black Start Resources at Hydropower Stations
System Restoration through Black Start Resources at Hydropower Stations
A total or partial shutdown of the national electricity transmission system is usually an unlikely event. However, if it happens, we are obliged to ensure that there are contingency arrangements in place to restore electricity supplies in timely and orderly ways. Black start is a process of restoring an electric power station or a part of an electric grid to operation on its own without relying on the external network, when there is major system collapse or system-wide blackout.
In general, the electric power used within the plant is provided from the station’s own generators. If all of the plant’s main generators are shut down, station service power is provided by drawing power from the grid through the plant’s transmission line. However, during a wide-area outage, off-site power supply from the grid will not be available. In the absence of grid power, so-called black start needs to be performed to bootstrap the power grid into operation by restarting those individual power stations before they can get the grid powered up again.
To provide a black start, some power stations have small diesel generator, normally called the black start diesel generator(BSDG), which can be used to start larger generators, which in turn can be used to start the main power station generators. Often hydroelectric power plants are designed as the black-start sources to restore network interconnections. A hydroelectric station needs very little initial power to start (just enough to open the intake gates and the excitation of the generator field coils), and can put a large block of power on line very quickly.
Normally, all power stations need electrical supply to start up, but with a total electricity blackout, there’s no electricity to restart the system. That’s why the reboot procedure is called ‘Black Start’ and it’s one the most important in power system restoration.
2. Black start Restoration Plan
“Black start resource” means a generating unit(s) or aggregated generating facility 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.
Restoration plan needs to assure that reliability is maintained during restoration and priority is placed on restoring the interconnection. The generic task of restoration includes determination of system and equipment status, preparation of plants and networks for systematic restoration, re-energization of the network and system rebuilding. The procedures for developing an effective restoration plan include formation of a qualified planning team, review of relevant system characteristics, formulation of assumptions regarding blackout scenarios, agreement on restoration goals, development of strategy and tactics, validation of the plan, training and documentation.
Increasing exposure of power systems to extensive blackouts is consequence of heavier system loading and recent almost revolutionary changes in industry structure. The impact of prolonged blackout on the public, on the economy, and on the power system itself makes rapid effective restoration very important. An effective restoration plan reduces an impact of an outage on customers and on the economy of the affected area while reducing the probability of damage to equipment.
A total, nationwide grid blackout may be unlikely, but that doesn’t mean it’s not prepared for. Tests need to be carried out by operator regularly to assess how long it would take to restart an individual generation unit at a major power station, bring it up to capacity, and make sure that any Black Start would run smoothly. Further, system operator shall need to develop, maintain and implement a restoration plan approved by its reliability coordinator.
3. Need of a Black Start Generator?
As modern society becomes increasingly dependent on electricity supply, consequences caused by large blackouts become more and more serious. We need black start resources to be setup online as:
- The actual operation of power system at home and abroad show that new equipment and technology can make power system to operate safely and steady, but impossible to avoid blackout occurrence.
- Most of our transmission and distribution system is of radial network and we are facing many more outages so black start capability is the most important one in our scenario.
- Our system load distribution pattern is more scattered with many customers on rural areas due to which black start capabilities on site can be very beneficial.
- The decision support system for black-start can help dispatchers to establish black-start schemes and restore the power system as soon as possible.
- To achieve the re-synchronization of parts of the total system which have become out of synchronism.
4. Black Start and Off-grid Mode Operation Provisions in PPA
Every independent hydropower project has to setup black start resources at their plant as per provision of Power purchase agreement (PPA) signed between NEA and IPPs so that power plant can be operated locally even in case of national grid outage. Also, it will be easier for system operator to coordinate with number of power island networks than coordinate with many number of single power station to bootstrap national grid system into operation.
Some of the provisions in the PPA are as follows:
- “During system outage or as per the requirement of system, the dispatcher may ask power station in charge to black start the unit and feed local load in off grid mode as per the provisions made in article 7 of PPA. Therefore, company shall keep power station in ready condition at all times to execute a request from the dispatcher to black start the unit and operate power station in off grid mode.”
- “Dispatcher shall instruct power station to black start the unit and operate power station in off grid mode only under the following conditions:
- local electrical island is formed with a load not exceeding the available capacity of power station.
- local dispatcher has credible information that restoration is going to take time.”
- “Whenever a black start or off grid mode operation is required from the power station, dispatcher may instruct the company or power station in charge for the same. The instruction may be either verbal or in writing. the verbal instruction, if issued shall be endorsed in writing as soon as possible but not later than two business day. however, before issuing such instruction dispatcher shall make all necessary arrangement including formation of an isolated network by use of the isolating devices at various points in local distribution networks and arrangement of suitable load. Adequate safety measures will be taken into consideration.”
- “Power station in charge shall black start the power station as soon as the instruction from dispatcher is received. In the event of any problem to execute the instruction of black start, company power station in charge shall promptly shall notify the dispatcher of the same.”
- “Power station in charge shall operate power station in off grid mode with or without the support of other power stations to the extent technically possible, if so instructed by the dispatcher.”
- “Power station in charge shall not be required to operate the power station in off grid mode beyond the available capacity of the plant as declared by the company.”
- “With the coordination of dispatcher, it will be the responsibility of power station in charge to maintain the voltage and frequency during off grid mode operation.”
5. Black Start Test Procedure:
Under normal conditions load will be supplied through generators G1, G2 and G3 being coordinated through national grid operator called load dispatch center (LDC) setting up communication channel to each generator maintaining system operation healthy. But during national grid failure, communication setup with LDC will cutoff so as to operate system, there should be local system operator (LSO) in operation online to setup communication schedule with generating station so that during a black start event, LSO will instruct to start up main generator(s) for carrying out initial energization of sections of the national electricity transmission system and distribution network, and support sufficient demand to create and control a stable ‘minigrid’. Then system will come into online as per load adjustment timely so that local system can be operated in minigrid mode and this minigrid will be synchronized to national grid under the instruction of LDC.
It is required to carefully monitor the operation of the island due to small stiffness in islands. While adding loads, care should be taken to ensure step by step addition keeping in view load characteristics, i.e., variation of load with respect to voltage and frequency and stiffness of island.
The current PPA practices to test black start facilities in a hydropower station are described below:
- In this test generating units are run without grid supply.
- Transmission line is charged upto distribution substation and also local feeders are supplied by increasing local load successively and machines were run for few minutes.
- Various electrical parameters like generator output power KW, KVAr, current, frequency fluctuation, excitation, power factor are recorded and also for delivery side different parameter like MW output, voltage, current are to be measured.
6. Case Studies of Black start, An International Experience
In October 1987, there was a regional Black Start due to powerful hurricane in UK that hit the south of the country. The storm damage left Kent and Sussex which disconnected from the National Grid but through Black Start contingency plans, Kingsnorth Power Station restored power to the area and it ran independently, cut off from the rest of the Grid, until repairs enabled it to be connected up again.
The way it worked was actually relatively simple: using smaller power sources to start ever bigger ones, scaling up and up until the entire country was powered up. The black start resources were used as the backbone of an ‘island network’ – a network operating independently of the national grid. The generating units matched the speed and frequency to create normal grid conditions and to restore supplies fast locally. Finally, the affected area’s ‘island networks’ were hooked up to each other so electricity was distributed around the country with reliability and stability.
7. Regulation Regarding Black Start
The North American Electric Reliability Corporation (NERC) adopted a revised Emergency Operations and Preparedness (EOP) reliability standard EOP-005-2 titled, “System Restoration from Black Start Resources”. This report describes the development of a practical application tool, Optimal Black Start Capability, which supports decision-making of power system restoration in an interactive manner. Since EOP-005-2 imposes requirements for system restoration from existing black start resources, transmission operators must evaluate whether the existing Black Start capabilities in their system are sufficient to meet the new requirements. Furthermore, it is expected that some black start units will be retired in the next few years due to emerging environmental regulations. Therefore, it is important for system operator to understand the new requirements of black start resources as installing new Black Start generating units enhances the black start capability of a power grid further It enables system restoration planners to increase the efficiency of the restoration process since system restoration depends upon the available black start capability. However, the benefits of additional black start capabilities need to be evaluated and quantified based on optimal locations and capacity amounts for installation of new black start units as it is systematic solution to ensure for reduction of the total system restoration time and increase the total system generation capability during the system restoration period so that the costs of installation can be justified so as to fully comply with system restoration plan requirements.
System operators need to update the system restoration plan when certain changes occur in the system condition and to evaluate the system performance during the restoration process. Also they have to verify through analysis of actual events, steady state and dynamic simulations, or testing, that their restoration plan accomplishes its intended function, such analysis, simulations or testing shall verify:
- The capability of Black Start Resources to meet the dynamic capability to supply initial Loads, the Real and Reactive Power requirements of the Cranking Paths as “cranking path” means a portion of the electric system that can be isolated and then energized to deliver electric power from a generation source for enabling the startup of one or more other generating units or aggregated generating facilities.
- The location and magnitude of loads required to control voltages and frequency within acceptable operating limits.
- The capability of generating resources required to control voltages and frequency within acceptable operating limits.
The operation of Black Start is important from both sides generation and distribution as well because it minimizes the generation loss as well as increase the reliability of the distribution system around the vicinity of the powerhouse. There is clearly mentioned about the provision of Black Start resources in PPAs signed with the private sector developers to be installed at every generating station. Most of generating station has Black Start resources installed at their powerhouse but still they are not able to operate plants in island mode in case of national grid failure. So as to implement provision of Black Start in real life scenario there should be some tasks need to be accomplished by both grid operator and generating station. From grid operator side there should be a LSO to look after operation of local system in case of national grid failure and also to co-ordinate with national grid for system restoration. Also LSO need to update system restoration plan regularly to evaluate whether the existing Black Start capabilities in their system are sufficient to meet the new requirements of system or not. From generating station side, they should setup communication system to LSO so that on the event of national grid failure generating stations can communicate with LSO to bring system online.
Operating Black Start resources in case of system collapses also helps NEA to reduce the claims of IPPs due to non-supply of their power during such periods. IPPs also may be benefited by operating the Black Start resources in isolated mode as in case of energy already meeting 80 % of contract energy they cannot claim for non-supply of energy but if they are able operate plant in isolated mode they can assure payment of remaining 20 % of energy by supplying to local loads as stipulated in PPAs. Hence every power station should maintain Black Start resources for healthy operation of the system in case of grid failures. Further, there is huge need of the trained operating personnel so that they could operate their Black Start resources as per system requirements as envisaged in the PPAs.
1. Alberta Reliability Standard System Restoration from Black Start Resources EOP-005-AB-2 .
2. M. M. Adib-I IRD Corporation, L. H. Fink Power System Restoration Planning IEEE Transactions on Power Systems, Vol. 9. No.1, February 1994.
3. PES Task Force Report: Power System Restoration -the Second Task Force Report; IEd TZL4NS.v.PWRS-3, n.1, 1988, pp.118-126.
4. Omid Palizban, Student member, IEEE, and Kimmo Kauhaniemi Dept. Electrical Engineering and Energy Technology University of Vaasa Vaasa, Finland “Microgrid Control Principles in Island Mode Operation.”
5. Drax report about the most important back up plan.
6. Black Start from Wikipedia.
7. Various PPAs signed between NEA and IPPs.