SCO adaptive automation system - SmartLoad

     Operating effectively in a dynamic environment
 

     Maintaining production continuity despite loss of external power supply
 

     Sustaining key technology processes
 

     Reducing the cost of production interruption insurance
 

     Increase human safety
 

     The system can also act as an adaptive automation SCO
 

Adaptive automation system SCO - SmartLoad

The adaptive control system SCO SmartLoad is dedicated to industrial plants where generation sources are installed. The task of the system is, in case of a system failure, controlled separation of balanced islands, and then supervise their operation through the logic of classical SCO automation.

The essence of the problem

In the chemical industry, metallurgy, mining, power generation, heating, a sudden loss of power supply can lead to extremely serious failures, which in extreme cases may prevent further statutory activities of the company. Ensuring power supply reliability is one of the most important problems in such enterprises. The most commonly used solution in the systems "sensitive" to power failure is the installation of an independent power source in the internal network of the plant. However, there is a problem of "keeping" such a source in operation in case of a power outage of the basic system, due to the imbalance of active and reactive power produced with the power "consumed" by the loads. As a rule, such systems are not equipped with the necessary automatics enabling uninterrupted operation of generating systems in case of a failure in the power system. Very often outdated control systems of turbine sets do not tolerate sudden changes<s>y</s> of load and they cannot automatically correct dynamic parameters occurring during such disturbances. As a rule, in such cases the generator is turned off or switched to captive operation, especially that the previous solutions assumed that in case of disturbance in the power system (short-circuit, SPZ, blackout) the local sources of energy were immediately turned off to protect them from being again unsynchronized with the power grid.

One of the conclusions of the system operator aimed at preventing the aggravation of system failures is the necessity to update the settings and the method of operation of the SCO automatics, which in case of a failure in the power system is supposed to facilitate the control of the disturbance states by immediately turning off part of the load, which can be turned off without serious consequences in order to significantly relieve the distribution network. At present, this role is partially fulfilled by the classical automatic control system of automatic frequency offloading, the so-called SCO, whose task is to limit the effects of disturbances connected with sudden disturbance of active power balance in a given part of the network by detecting the decrease of frequency that occurs in such a case and to switch off loads that do not have any essential influence on the production process of an industrial plant. Due to the fact that the power of loads switched off by the SCO may depend on the frequency change rate, it is extremely difficult to predict the power of loads that will be switched off as a result of operation of this automation.

Therefore, in an industrial plant equipped with its own generating units, using an adaptive SCO - Smart Load automation system in the event of a system failure will significantly increase the chances of isolating and maintaining a power-balanced island. 

System structure

Elements of Smart Load SCO automation system:

• island operation detection system;
• power measurement system in all main bays of 6kV switchgear;
• communication system;
• main computing unit;
• output - switching off system;
• operator station

The automatics system has dispersed character with elements located in:

• all switchboards included in measurements and shutdowns;
• central electric control station 

Communication

Individual elements communicate with each other through optical fiber links and UTP links in Ethernet 100BaseFL and 100BaseT standard, additional connection is RS485 link with Modbus RTU protocol to the supervision and control system existing in the object. Via Ethernet links the central unit receives data from measuring systems regarding current values of voltages, currents and power in each bay of power distribution boards, also via Ethernet links the central unit gives the orders to switch off the fields assigned by it and communicates with the operator station, where the status of supervised system is visualized, as well as the status of automatics operation, current measurement values and the list of loads assigned to be switched off in case of failure. Using the operator station it is also possible to change the settings and to analyze the list of events and the list of alarms. For correct functioning of the algorithms that determine which fields should be switched off, the information on the current status of switches of the switchgear included in the automatic control system is necessary. In order to simplify connections of the automatics with the object, those data are obtained from the supervision and control system by means of the RS485 connection.

Adaptive load shedding algorithms

Protection relays that implement the df/dt, dU/dt criteria are used as islanding detection systems. An additional controlled parameter is the value and direction of active power in the fields supplied from 110kV/6kV transformers. In the case of simultaneous excitation of these criteria, outages are implemented to isolate the island, and the fields that have been determined by the continuous algorithm balancing the power in a stable state just before the occurrence of the disturbance.

Phase currents are connected from each field on the switchgear to the metering system, and phase-to-phase voltages are connected from the metering fields. The measuring systems measure continuously both active and reactive power in each bay and deliver these values several times per second to the central unit in order to execute the algorithms for selection of bays to be switched off. The measuring systems are simultaneously equipped with relay outputs, which are properly connected with control circuits of all bays in order to execute switching offs determined as a result of algorithm calculations.

The heart of the system is an algorithm analyzing the configuration of the system and determining the possible systems of island operation in a given network situation. On the basis of priorities established by the operator and current loads of the fields to be switched off, the fields are selected in such a way that in the created island operation systems there are loads which are the most important from the point of view of plant safety and that the sum of loads is as close as possible to the power of generators operating on a given island from the moment just before the disturbance. This algorithm is implemented on an industrial PC. For reliability reasons, a modern fanless Pentium Centrino class unit is used, which guarantees the maximum possible reliability of the algorithm.

An advantage of the SmartLoad system is the ability to dynamically change the loads to be switched off. Fields intended to be switched off by the "active" algorithm are selected in such a way that after switching them off they maximally balance the power generated and consumed for the separated island. The work of the active algorithm can be divided into several processes carried out simultaneously, these are:

• analysis of the current network configuration;
• determination of islands that may form;
• performing power balance for each of the potential islands;
• determination of fields to be switched off in case of island formation;
• detection of island operation in the system and determination of the size of the resulting island;
• "cutting" the island from the system by opening appropriate switches;
• switching off loads on the basis of the table of exclusions prepared directly before the failure for the specific island;
• supervision of the island created in this way by the classical SCO algorithm with the possibility of current manual assigning of priorities of exclusions

The most important condition for the success of such an algorithm is the speed of its reaction to network disturbances. Undertaking fast and accurate outages in the case of island formation reduces the depth of the disturbance, and thus increases the stability of operation and the probability of maintaining a fragile equilibrium in the new system. In the construction of the system, 10s was assumed as the maximum time for determining the fields to be switched off after a change in the network configuration, and a maximum of 1s as the time from detection of the island to making the appropriate outages.

The algorithm determines "to switch off" loads multi-criteria for each potential island.  It takes into account factors such as active and reactive power balance, priorities of consumers and the smallest number of outages. After optimizing the potential outages, this information is sent to the execution unit. The task of this unit is to monitor the voltage in a given node of the network and detect the state of islanding operation. For this purpose, the under-voltage and under-frequency criteria, the rate of change of these parameters and the criteria for the direction of power flow in the node are used. After the controller interprets the disturbance as "island operation", appropriate shutdowns are performed immediately to balance the power in the isolated island. This action of the active algorithm allows to keep its own energy source "alive" with maximum success.

In contrast to this action, the standard SCO algorithm does not provide power balancing after islanding, which is its main drawback. The reason for this is the rigid assignment of outages to individual SCO stages, which does not allow for control of the power being shut down.

Possibility of extending the system with lower level switchgears

The system's coverage of the lower level switchgears allows to prioritize more loads, and thus to reduce individual quanta of switched-off power. This increases the system's resolution, which translates into more precise balancing of the island. It also makes it possible to make more "surgical cuts" to the grid, leaving power to the important, "non-energy consuming" parts of it related to control. The information from lower levels of the network also allows optimizing the operation of the ATS automatics, consisting in the selective selection of only the important switches that are likely to be successful.

The possibility of extending the system with the acquisition of information from deep within the power system is desirable in order to detect island operation more unambiguously. Additional information about the status of switches and about the power dissipation and voltage levels in the immediate vicinity of the company's power point serves this purpose.

Information from the system operator

The most far-reaching extension is the acquisition of current information from the system operator concerning the state of the network, in particular information about large developing system faults, separation of wide islands, power deficit, etc. This allows for an earlier reaction of the service, preparation of an appropriate operating system, and a possible safe stoppage of production.

Example of application

Below there is an example of Smart-Load application, which is installed in a large metallurgical plant, which has two main double-section 6 kV switchgears (GPZ-ty), supplied by five 110kV lines, through five 110/6 kV transformers. The plant has its own thermal power station, with five generators, three of which are constantly maintained in operation. The branch switchboards supplied from the main GPZs are equipped with 3 supply fields, and the most important switchboards are equipped with automatic power supply switching, guaranteeing the necessary continuity of their supply.

In the situation of deprivation of power supply to the important switching stations supplying the metallurgical equipment, there is a serious probability of damage to this equipment, which would involve considerable financial losses and costly long-term shutdown of the plant, therefore the plant staff puts special emphasis on maintaining such a system of operation that the switching stations are simultaneously supplied from the NPS through 110/6 kV transformers and from the operating generators. In case of lack of power supply from the NPS, the Smart-Load automatic control system identifies the fact of loss of power supply from the network and switches off the circuit breakers in the power supply bays from the side of the power system in order to prevent other consumers connected to 110 kV network from being supplied by the operating generators.

Then the system performs such shutdowns that in the island created as a result of its operation the difference between the power supplied and received was as small as possible. A very important element that must be taken into account is the fact that, as a rule, operating configurations of the 6kV network in the plant are optimized in terms of load uniformity, which makes it difficult to permanently maintain such a network configuration so that within the sections on which the generators operate the powers produced by the generators and consumed by the loads are similar.

After the selection of power-balanced islands, the next task of the automation system is to supervise the operation of the created island systems. In the case of island operation of generators, there is a high probability of further disturbances related to power surges resulting from the operation of process system automatics as well as disturbances in the operation of the turbine sets themselves. At this stage, the discussed automatics performs the function of a classical SCO which, in the case of frequency decrease inside the island system, performs subsequent switching off in order to maintain the operation of the "island" for a maximum time - i.e., until the power supply from the 110kV network is restored or until the emergency safe shutdown of the technological lines in the plant.