Technology for Discrete & Digital Communication of the Relay-controlled Protection Equipment with the Central Control System and the Power Consumers’ Connected Equipment

NE OJSC
Mark Tikhomirov, chief manager
Anatoly Neelov, head of the chief managers’ sector, R&D
Sergey Staroded, head of R&D, PhD in Technical Sciences

This article describes a technology used for transformation of the protective equipment from discrete communication to a digital one with the expansion of functions being realized by protection and control systems for transferring of the processed data to the offshore facility Central Control System. It provides an assessment of impact of monitoring facilities used for control of the state of the electrical equipment and the electric power system upon the total situation when a ship is navigated.

When ship electric power systems are operated under normal conditions, they guarantee the performance capability for the main equipment, habitability and endurance facilities, and when using the electric propulsion these systems are ranked as an important component that provides propulsion and course-keeping qualities of the ship. Some certain independent damage control facilities are limited by power and running time and can be mainly and additionally used for survival of the crew but not the ship therefore.

The functionality of shipboard electric power systems on a modern ship is a key issue for the navigation thereby, and its importance for the ship in whole is not less than the hull strength. Power supplying of drives of the electric propulsion system is related to the special group that forms a part of the 1st category of electrical receivers and interruption of their supply in case of mains interruption from one of its sources may be acceptable for the period required for restoration of supply. The probability of such event must be minimized thereat. It is provided by the shipboard electric power system’s structure and the reliability of its components, but these issued are independently solved. The article describes a technology used for transformation of the protective equipment (through wire communication) from discrete communication to a digital one with the expansion of functions being realized by protection and control systems for transferring of the processed data to the offshore facility Central Control System

Relay-based protection and automation facilities installed in the Main Switchboard monitor instantaneous parameter values (current and voltage vectors and amplitudes) from the source to the consumer when the electrical equipment of the shipboard electric power system is operating. Additional characteristics that evaluate the operability envelope of units forming a part of the ship’s equipment are computed on the basis of measured current and voltage values.

Automation devices in the Main Switchboard form an integral part of ship’s automation equipment complex that includes the propulsion and positioning control system, engineering and process-dependent facilities control systems. The shipboard electric power control system receives commands from other control systems and sends generalized directives to automatic subsystems in the Main Switchboard for arrangement of conditions required for generation and transmission of electric power. Layered distribution of control and adjustment functions is considered to be the only reliable method for localization of failures and malfunctions in control systems applied for complex objects. Implementation of back-up layered control circuits improves the resistance to some failures in control systems but it doesn’t exclude occurrence of emergency situations in the power part of the electric power system. To isolate faults in the system used for generation, distribution and transformation of electric power, the structure for overlapping of electric protection zones and their intercommunication with local protections of units, arranged on the basis of other principles, is created.

Measures taken by all developers of the system are quite similar to each other. They are back-up and self-diagnostic check. Taken into account the automation level growth and the tendency to reduce participation of a man in a local control of the ship’s equipment, the role of monitoring in the distributed system of the electric power control system and its automation facilities is dramatically increasing.

Equipment state monitoring units do not substitute unit protection and diagnostic systems, but can establish interference between processes taking place in units, control subsystems, in distribution networks and in the environment around them. Monitoring facilities for operating, pre-alarm and alarm values of controlled parameters gathered and registered by protection and control facilities are to be arranged in a unique net first of all. A task, which is traditionally solved by a man, namely, making decision about using of the unit in the overload mode, if a provision of a run and manoeuvring in accordance with the operating conditions or mandatory application of another concrete pieces of equipment is required in the emergency situation, can be transferred to hardware/software and network links of monitoring facilities.

Choice between comfort and safety must be automatically made in favor of safety not only at the moment of alarm data receipt, but it is to be predetermined being based on a state trend and a considerable increase of deterioration or when the situation is reaching a permissible value limit trying to exclude its occurrence.
The generalized structure for behavior of technical system units can be presented by dividing the area of state of the electric power system into three parts:
• А – normal operation permissible in accordance with parameters for all units;
• B – change of a parameters group towards undesirable states (inadequate performance of units and lowering of the resource);
• C – change one or a group of parameters towards alarm values after which the protection activates.

When the state of the equipment complex is determined as that related to A zone, monitoring system operation can be resulted in gathering of the statistic information for its further generalization at the ship’s home port.

If an integral tendency for a part of the equipment complex to the B zone is detected, the adjusting objective aimed at improvement of parameters is to be generated. In relation to the electric power system, it can be expressed in reloading or incomplete use of operating generators, overloading of transformers or converters, that can be temporarily permissible for a period grounded by technicians, but the situation is to be further improved. Estimated time to the critical event is a command for the high-level control system to change the mode that is not a direct command for realization of the «relay» algorithm, but one of many criteria for solution selection performed by this control system. One of the most important objectives of the monitoring system is a generalization of data received from different unit control systems. When one of protections that controls its own parameter set, is activated (for example, critical decrease of oil pressure, that unavoidably leads to a local accident and emergency stop of the unit) in the adjacent control system, the signal for load reduction is to be generated and power supply may be interrupted. There is a great number of such logic connections.

Therefore, the expansion of the emergency situation originated from one of the causes must be excluded, i.e. exclusion of development of emergency aftermath. Transmission of information to the next (top) control level will allow to make a decision about balancing of such event prior to its impact to the whole system of electric power distribution in the electric power system.

When the possibility for overstepping of limits of the C-zone is detected, i.e. protections start operation and irreplaceable destructions appear, processes aimed at localization of alarm conditions and prevention of inadmissible decrease of life-critical parameters in local parts of the electric power system that provide power supply of essential consumers being used (or prepared for their further involvement) must be started.

Microprocessor-based relay protection and automation distributed controllers, active and passive keys, data acquisition and storage devices are used in marine electric power systems. All these devices are microprocessor-based units, the term «intellectual electronic devices» is used for their definition in publications.

Integration of protection, monitoring and automation facilities in the Main Switchboard of the electric power system is considered as one of the ways for reducing of maintenance expenditures. It becomes possible due to integration of microprocessor-based devices in control and monitoring systems of the entire electric power system. Protection functions require the availability of signal exchange between multiple relay protection and automation devices. Such exchange is implemented by application of wiring between inputs and outputs in various microprocessor-based devices. The advance in solving of this problem is expressed in replacement of an expensive wiring system by a signal exchange system arranged between microprocessor-based devices via computer networks. However, advantages of the integration become evident insufficiently today, when different communication protocols are available, because additional hardware, such as protocol conversion units, software (various interface programs) are required for real-time data acquisition, which in turn increases costs associated with development and training of the personnel.

The International standard IEC 61850 was developed for solving of the problem concerning the creation of a multifunctional platform that allows arranging of interaction between protection and control microprocessor-based devices at the computer network level. This standard defines the architecture for unified communication protocols, including those using real-time signals for synchronization of control responses and protections in electric power systems and ship systems.

IEC 61850 provides recommendations for a peer-to-peer communication and a Client-Server communication, procedure for testing of networks equipped with microprocessor-based devices, design of distributed computer networks used for solving of engineering problems.

The peer-to-peer communication in the integrated control and protection system is based on transferring of messages about an object-oriented event via protocols named Generic Object Oriented Substation Event (GOOSE). This term is interpreted as a system of «horizontal» data exchange between devices based on asynchronous reporting about the state of digital outputs in microprocessor-based devices to be transferred to other peer-to-peer devices. Taking into account the importance of messages transferred over GOOSE protocol, the reliability of a fast peer-to-peer communication is to be not lower than that, which is available under the conditions, when the existing «wire» technologies are used, i.e. the total communication time between devices must not exceed 4 ms. To reach the high reliability level, messages are to be continuously repeated for the period defined for existence of the monitored state. The peer-to-peer communication is to be used for transferring of blocking signals and circuit breaker failure protection signals instead of a wire communication to provide safety in the Main Switchboard.

Status information on circuit breakers and switching units is available for microprocessor-based devices via an input/output unit (IOU). Conditioning of measuring channels for measured values digitizers in the relay protection and automation, control and monitoring channels is to be arranged in accordance with IEC 61850-9-1 by means of a Merging Unit (MU) that receives input measured current/voltage values, computed values and discrete signals, from which time synchronized fetches for measured values are formed and transmitted to multiple microprocessor-based devices over the computer network. This term is defined as a «bus data processing» for many cases. Quantity of digital data processing stages in the distributed relay protection and automation system increases due to IOU, interface modules MU, МП, the control unit for the circuit breaker and the switching device in the computer network, but the total actuation time is to be equal to or less than in standard microprocessor-based devices. It should be noted, that the problem with wiring of connectors is solved in case of using of a process bus.

Requirements established for the communication equipment in the electric power system and interacting engineering systems are stated in IEC 61850-5 in which requirements to functioning are specified as well, including the «transfer time» (the sum of all time delays for data conversion and transmission), that is verified during acceptance tests of the distributed relay protection and automation system and the distributed monitoring. IEC 61850-6 establishes the general engineering principle and the configuration philosophy for such network.

In accordance with the requirements stated in IEC 61850, networks become an important part of the protection and automation system and data transfer network graph is required to be tested in accordance with the IEEE802.1q:2003 standard (Virtual bridged local area networks), configuration speed is to be tested in accordance with IEEE802.1w:1998 (Rapid Spanning Tree Protocol), availability of all microprocessor-based devices is to be verified. If GOOSE is used for protection and monitoring functions related to protection, time between events (analog data selection) and response time of microprocessor-based devoices is to be measured. Requirements established in IEC 61850 radically change the approach to testing of protections.

In relation to expansion of application fields for electric power rated for 6 (10) kV for electric power systems of marine facilities, including unified ships’ electric power systems with electric propulsion, Russian shipbuilders are facing a task connected with continuous monitoring of networks and units’ electrical equipment, delivery of corrective controlling actions and registration of the actual state, operability and the remaining life for heavy duty units when navigated.

Key conclusions are as follows:
• design of ship electric power systems, development of protection systems and monitoring facilities for them is to be carried out on a system level for transferring to the central part of the automated control system for hardware components and the ship in whole sufficient, but total information about all important events and responses of lower level control and regulating systems. Data on protection actuation in case of critical and alarm situations are not sufficient;
• not only primary processing of measured parameters and default tripping in an emergency but interaction with units regulating and diagnostic systems for prevention of critical events are to be set as tasks for relay protection and automation systems within the structure of electric power systems;
• integration of gathered and combined information from different subsystems is to be processed by monitoring facilities for its transfer to the central part of the automated control system for hardware components and the ship when values are reaching hazard limits, that makes it possible to provide timely generation of control actions with regard to the required equipment complex.

References
1. IEC 61850 - Communication Networks and Systems in Substations.
2. Magazine «Novosti electotekhniky» №No. (48) 2008. Intellect for protection of electric systems. Materials of the International Relay Specialists Congress. Valentin Alexandrovich Sushko PhD in Technical Sciences, associate professor in the Chuvash State university, Cheboksary.

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# 3(41), 2012
Ocean and shelf exploration