Michael Faraday IET International Summit: MFIIS-2015, September 12 – 13, 2015, Kolkata, India (Paper ID: 176)
A PROPOSED METHOD OF LOAD SCHEDULING AND GENERATION CONTROL USING GSM AND PLCC TECHNOLOGY Subhra J. Sarkar*, Palash K. Kundu** *Batanagar Institute of Engg., Management & Sc., India,
[email protected] ** Jadabpur University, Kolkata, India,
[email protected] Keywords-PLCC, GSM, LDC, SCADA, Scheduling, Forecasting
Abstract With the various advances in load forecasting techniques, now a day it is possible to perform load scheduling effectively. Modern day wireless communication techniques are replacing the older wire based systems. In this paper it is proposed to implement an automated system employing GSM & PLCC for the smooth operation of power generation & control. Load dispatch centre (LDC) will communicate with the individual power generation units & send scheduling based information through GSM techniques. The generation plants, in turn will actuate the scheduled data for their generation control using power line communication based SCADA system.
I.
availability of state power plant & forward it to RLDC. RLDC then make the final changes in scheduling (if required)&circulate it to all SLDCs¢ral generating stations. SLDCs then forward the scheduling chart to the respective state power plant units for their power generation control. In this paper, it is proposed to implement a wireless communication scheme (GSM) for bidirectional communication between the units to LDCs & between LDCs. After obtaining the generator scheduling from LDCs, generator scheduling isdone by employing PLCC based SCADA systems in the respective units.
II.
Introduction
Load scheduling & dispatch are interrelated processes which are crucial for the smooth operation of any power system. Since electricity cannot be, so it is necessary that the energy generated must meet the load condition. Scheduling is the process of generation output variation from the anticipated load. The future prediction of the amount of load over various time periods i.e. load forecasting plays a critical role in load scheduling. But in real time scenario, it may happen that the demand requirement & load forecasted are not exactly same. So, some refining of scheduling process must be required which is categorized as dispatch processes [4]. In India, power is generated by power plants are maintained either by states or central. Central power plants generally fed a certain percentage of generated power to the neighboring states whereas state power plants generally fed the state itself. The entire power network in India is subdivided in five regions namely Northern zone, Western zone, Southern zone, Eastern zone & North-eastern zone each having few states. This load scheduling & dispatch is done by a series of Load Dispatch Centre (LDC) operating at state level (SLDC), regional level (RLDC) & national level (NLDC) [2]. The scheduling process is started after central power plants convey its availability of their units to RLDC which then conveys the information to SLDC. SLDC prepares the scheduling information accordingly after knowing the
Load Forecasting, Scheduling and Dispatch
As power system load varies greatly with time, it become extremely important to predict the load demand in advance in order to have a successful operational strategy. Load forecasting can be classified according to the duration of planning as Short Term Load Forecasting, STLF (up to 1 day forecasting), Medium Term Load Forecasting, MTLF (1 day to 1 year forecasting) & Long Term Load Forecasting, LTLF (1 to 10 year forecasting). Load forecasting error plays a significant role in power system operation. Operation economy & power system control is sensitive towards forecasting errors. Various load forecasting techniques like multiple regressions, adaptive load forecasting, fuzzy logic, neural network, ARMAX models based on genetic algorithm, iterative reweighted least-squares etc. are developed which gives good results [5]. Consider the daily demand graph for a particular system is as shown in Fig. 1 according to which the power plants must be scheduled so as to operate it at optimum cost. The system demand does not fall below a certain level termed as base load. During the peak hours, demand will be at its maximum & hence installed capacity of the system must be greater than the peak load. At other time instances, the load
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Michael Faraday IET International Summit: MFIIS-2015, September 12 – 13, 2015, Kolkata, India (Paper ID: 176) demand will lie between the two extremities& thus active power generation is controlled in a scheduled way [1]. Normally, load demand does not vary extensively during small time duration (say 10 or 15 minutes). In Indian Electricity Grid Code (I.E.G.C.), a day is sub-divided in 96 parts (each of 15 minutes) & thus scheduling information to each generating unit is provided for all these 96 periods [7]. LOAD DEMAND
Peak load
Controlled generation
Base load TIME Fig.1. A typical daily demand graph
For economic generator scheduling, unit commitment & online economic dispatch is essential. The selection of unit for supplying the anticipated system load over the required time period at minimum cost as well as to provide a specified margin of the operating reserve is called unit commitment. Whereas online economic dispatch is the load distribution among the generating units connected to the system such that the total fuel cost minimizes [3, 6].In India, Power System Operation Corporation Ltd (POSOCO), a wholly owned subsidiary of the Power Grid Corporation of India Limited (a Government Company) formed in March 2010 to handle the power management functions & operate 5 RLDCs and a NLDC [8]. Real time operations for grid control &electricity dispatch within a state through secure and economic operation of the state grid followingthegrid standards &state grid code is done by SLDC. The matching power demand with system integrity, reliability & security of generation and transmission facilities, system frequency regulation, optimum resource utilization & quick restoration of the system after severe disturbances are among the main objectives of LDCs. NLDC
NR LDC
SUB LDC
DCC
WR LDC
SR LDC
SLDC
ER LDC
Central generating stations
Sub-station / State generating stations
NER LDC
III.
Global System Of Mobile (GSM) Communication
Global System for Mobile (GSM) Communications, actually known as Groupe Spécial Mobile(created by European Conference of Postal and Telecommunications Administrations, CEPT in 1982) is the standard developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second-generation (2G) digital cellular networks used by mobile phones [13, 15].GSM networks may operate in different carrier frequency ranges for different places but the common frequency bands are 900MHz & 1800MHz.When these bands were already allocated, the used frequency bands are 850MHz & 1900MHz [10, 13].A GSM network consists of Mobile Station (MS), Subscriber Identity Module (SIM), Base Transceiver Station (BTS), Base Station Controller (BSC), Transcoding Rate & Adaptation Unit (TRAU), Mobile Services Switching Centre (MSC), Home Location Register (HLR), Visitor Location Register (VLR) & Equipment Identity Register (EIR) which in together form Public Land Mobile Network (PLMN)[11, 12]. MS is the handheld or portable units of various types within a PLMN. In GSM, there is a difference between subscriber identity & mobile equipment identity. SIM is the database on user side which determines directory number & subscriber billing details. It communicates directly with VLR & indirectly with HLR. BTS takes care of radio related tasks & provides connection between the network and MS through air. On the other hand BSC takes care of all the central function & Base Station Subsystem (BSS) control. BTSs spread over a given area are connected with BSC through some interface termed as Abis-interface. For any mobile network, effectiveness with which it uses available frequency resources is very important. This effectiveness implies the no. of calls which can be made by using certain bandwidth. That can be improved by data compression, at least over airinterface. In GSM system, both MS & TRAU (which is a part of BSS) are responsible for this data compression. MSC is similar to a telephone exchange which performs the routing of incoming & outgoing calls. A large no. of BCSs is connected to it using A-interface. HLR is a large database to store the data of hundreds of thousands of subscribers. Each PLMN must contain at least one HLR. VLR is used to avoid overloading of data enquiries about the subscriber in HLR. It contains the data of only those subscribers roaming in the area for the VLR is responsible. After the subscriber leaves the area under VLR, HLR send a request for the removal the subscriber data. The service is provided over the entire range covered by the BTSs related with MSCs for which VLR is assigned for. As the mobile equipment may be stolen & can be reused with any other valid SIM, some unique identifier is required to prevent any such misuse. Now a day, almost all equipment has their unique International Mobile Equipment Identity (IMEI) no. EIR is the additional database in PLMN
Fig.2. LDC hierarchy in India
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Michael Faraday IET International Summit: MFIIS-2015, September 12 – 13, 2015, Kolkata, India (Paper ID: 176) where the IMEI no. of the stolen equipment is stored, so that they can be tracked [11, 12].
development of new communication schemes for PLCC, it can be used to provide high speed internet in-house network without any Ethernet cable [16]. Home automation & AMR system employing this power line communication is getting increasingly popular. In an in-house PLCC based network, comprising of various electrical equipment connected with one-another & to the system, individual equipment will transmit or receive the data as required by the system. The whole communication process is done by means of power lines through a suitable gateway placed near meter & PLCC modems at equipment placed near individual equipment. A typical in-house networking is as shown below. But obviously, data transmission employing through PLCC are not secured as LANs & thus some modification is necessary for having secured connection [17].
Fig.3. Generalized structure of PLMN
From LV network
Cellular mobile communication is based on the available frequency rangepartition & then reusing it. Consider the allocated spectrum is partitioned in p no. of nonoverlapping channel sets then applied in a regular repeated to a hexagonal cell grid. This p is dependent on environment of propagation, distribution of mobile equipment & cost of the system. On the other hand, cell size usually depends upon local traffic distribution & demand. Hexagonal shape is preferredas it approximated as a circle (the constant signal level contour from an omnidirectional antenna placed at the center) but forming a grid without any gap or overlap. This portioning of the spectrum reduces the interference between different BTSs [10, 11]. With the advancement in GSM technology, SMS is widely used for communication purposes. This SMS can be used to control electrical device at a distance place effectively [14]. IV.
As the name suggests, it is a communication method where data can be transmitted over power line [19]. This is achieved by superimposing a high frequency low energy signal containing the information over conventional 50Hz electrical signal at sending end. This high frequency signal is then separated from power signal to decode the information being transmitted [3, 20]. Earlier PLCC was a good choice voice transmission & equipment protection. But in the recent times, it is getting more popular in Automatic Meter Reading (AMR), Home automation & networking, Network load control etc. Since it eliminates the need of additional wire, the system cost reduces as every building has a properly installed electricity connection [16, 18]. But a power line characteristic varies significantly at communication frequency resulting higher attenuation of signal. Generally a power line contains significant amount of noise produced by various devices (primarily electronics based non-linear devices) connected with line thereby degrading its performance [21]. With the
PLCC Modem
Equipment: 1
PLCC Modem
Equipment: 2
PLCC Modem
Equipment: n
Meter
Gateway
PLCC Access network
Fig.4. A typical in-house network
V.
Power Line Carrier Communication (PLCC) Based Inhouse Data Transfer
Power line network
Scada & Its Implementation using PLCC
Supervisory Control & Data Acquisition (SCADA) is the system of data acquisition for controlling & status monitoring using suitable communication channel. It can be used in monitoring & control in various systems like power generation process, refining process, water treatment & distribution process, electricity transmission and distribution etc. It may comprise of the Remote Terminal Units (RTUs) / Programmable Logic Controller (PLCs), Supervisory system, Human-Machine Interface (HMI), Historian, Data acquisition server & Data communication medium blocks [22 23]. Sensors are connected in the system by using RTUs/PLCs & covert the sensor output to digital data. They may transmit digital data to supervisory system or may receive digital command from supervisory system. Though PLC does not have telemetry option, they are more popular due to its economy, versatility, flexibility, and configurability. Supervisory system acquires the process data & sends commands. HMI is the interaction apparatus or device between the process of data acquisition & control & human being. Historian is the software service which contains various data, events & alarms in a database which can be used by
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Michael Faraday IET International Summit: MFIIS-2015, September 12 – 13, 2015, Kolkata, India (Paper ID: 176) HMI, if requested for. Data acquisition server is the server through which Historian can access data from field devices (RTUs / PLCs) maintaining the necessary protocols through any suitable communication medium. Communication medium between supervisory system & RTUs / PLCs might be wired or wireless.In a basic SCADA system, the control centre comprises of SCADA server, communication routers, HMI, engineering workstations & data historian connected through LAN. This control centre collects & processes all the data from field devices to display the information in HMI. It may also take any control action in case of event detection & have the provision of centralized alarming, analysis & reporting. The data communication between the control centre &field devices may be any type of serial communication with suitable communication protocol [24]. As the data rate is SCADA is not so high, it is also possible to use narrowband power line communication having lesser electromagnetic interference [25].
be scheduled at a new value, information is conveyed to the respective unit by voice call. In this method it is proposed to implement GSM based scheduling related data transfer between power plant & LDC. LDC will communicate with corresponding units to obtain their next day availability & prepare the scheduling chart by using suitable load forecasting techniques. As clear from the previous discussion, this chart will contain the forecasted power generation for 96 equal parts of 15 minutes each. Depending upon the value corresponds to zth part (0 < z 96), instruction is conveyed to the PLC responsible for alternator generation control through PLCC based SCADA to take necessary action. In case of any mismatch of load demand & scheduled load or breakdown of any unit (which can be determined by monitoring the system frequency), LDC sends the necessary instruction to the plants through GSM to control its generation accordingly. The simplified arrangement of such scheme is as shown below whose supposed to make the whole system automated.
Fig.5. Basic SCADA structure
VI.
Fig.6. Proposed load scheduling scheme employing GSM & PLCC
Proposed Method of GSM Based Load Scheduling & PLCC Based Scada for Alternator Generation Control
VII.
Now a days PLCC is already in practice for the data transfer between LDC & plants / sub-stations. LDC monitors various data like active & reactive power, voltage, frequency etc. at plants / sub-station level using PLCC. But the periodicity of monitoring is limited to the rate of data transfer via power line. Scheduling chart is generally sent to the corresponding plants one day advance, depending on which scheduling is done & power generation is controlled manually. In case of any emergency, when generation needs to
Discussion & Conclusions
From the above explanation, it is clear that the proposed system will make the operation of load scheduling & alternator control based on scheduling chart automatic. If the forecasting technique is accurate, then the system will automatically vary the generation based on scheduling data to meet the load at that particular time block. In case of any mismatch in load generation & system load, the system frequency falls which inform LDC to take necessary steps & reschedule the plant generation of the generating units based
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Michael Faraday IET International Summit: MFIIS-2015, September 12 – 13, 2015, Kolkata, India (Paper ID: 176) on its availability. In the modern power system, the time required for rescheduling the generation is about 1 hour (4 time block). This time might be reduced by employing this autonomous system. But obviously, this system has its own disadvantages. The system cannot work if there might have some GSM network problem. In that case, conventional method may be used or presently installed PLCC may be employedas an alternative. Beside that in India total energy generation is lesser total energy demand during most of the time block. So, generation variation is required for only a few instances & over a very small range.
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