SCADA

In this modern world electricity plays a vital role. As electrical needs are increasing in our day-to-day life, the quality and continuity of power becomes most important.

Substations undoubtedly are the backbones of power transmission. So a good network between these substations will provide undisturbed power supply of good quality.

My Article deals with study of SCADA. It includes the study of components and applications of SCADA.

SCADA stands for Supervisory Control And Data Acquisition System. Supervisory and controlling is done using load dispatcher and the data acquisition is made possible through FEP and RTU.

1. INTRODUCTION  

In developing countries like our India our energy needs are increasing rapidly and doubled for every seven years. It is difficult to achieve the increased demand through additional capacity generates to better capacity utilization. Nearly we are losing 5% of power due to technical problems in transmission and distribution system. Employing several techniques like SCADA, HVDC, and FACTS can reduce this. So scada plays a major role in achieving objective like better plant management, higher availability, improved load management, reduced losses, Energy management, Energy audit.

SCADA stands for supervisory control and data acquisition system. It refers to a central system that monitors and controls the complete site in real time mode from a remote location with acquisition of data for analysis and planning from one control location. Data acquisition begins at RTU level. Includes several data like meter readings status position of devices are communicated to the SCADA. Data is then compiled and formatted in such a way that a control room operating using the scada can make appropriate supervision decisions.

2. ARCHITECTURE OF SCADA

The primary function of scada is data acquisitions. In the power system SCADA, required data will be acquired from the remote station \power houses to the master computer center. For this one computer called Remote Terminal Unit (RTU) is used in the remote sub-station \power house. It scans following data from the yard at fast periodic intervals of milliseconds.

Analog: MW, MVAR, Ampere, KV, Frequency, etc.

Digital:  Status indication of breakers, isolators, etc.

One computer at the master station called FEP (Front End Processor) collects data from different Ruts installed at various sub- stations sequentially polling one by one. The communication media may be P&T, PLCC, OFC, and microwave. FEP or another computer called sever\ Workstation may process the data. The processed data is displayed on single line or shift engineer. This computer (Dispatcher’s Console) is also called man machine Interface (MMI) through which the operator can interface or view the real time data of any selected station by using simple commands. The FEP, Server, Workstation, Dispatcher’s console, Printers etc are connected in LAN with redundancy.  The single line diagram illustrating the principle of operation of SCADA is enclosed below:

At Remote Location: 

         (a)Transducers (Analog Inputs):

                          In conventional systems the sub-station/power house metering of the analog parameters such as voltage, current, MW, MVAR, frequency etc are achieved making use of instrument transformers. The secondary of voltage transformers are rated for 110 volts and current transformers of 5A or 1A are commonly used to take above inputs and give out puts with proportionate (4-20 mA).

Transducers are named depending on the parameters for which they are used. Some of them are:

Frequency transducers

Power transducers (active/reactive)

Current transducers

Voltage transducers

Power factor transducers

Transducers are also classified as self- powered or auxiliary powered depending upon the usage of supply required for the functioning of the units. Self-powered transducers do not need separate voltage for normal functioning. They use pt secondary voltage given for it’s functioning the normal output will be (0-5 mA), (0-5v) etc. in respect to auxiliary powered transducers, they need extra input voltage say 220 v ac or 48 v dc etc for functioning. The outputs will be 4 to 20 mA where 4 mA corresponds to live zero value.

(b) Contact multiplier relays (digital input):

Contact multiplier relays (CMR) are used for providing potential free contacts from the used contacts to acquire the status of circuit breakers etc. these are used where it is difficult to get spare auxiliary contacts of devices from field. 

(c) Interposing relays: 

Interposing relays are used for carrying the commands from the yard to the master station to the remote equipments like CBs, Isolators and OLTS.separate relays are used for closing and opening of circuit breakers or Isolators.

    

(d) Remote Terminal Unit (RTU):

Remote Terminal Unit is nothing but a computer user for the remote station to scan all the analog and digital data from the yard RTU also accepts and faith fully executes the commands issued by the master station.  RTU consist of following cards along with microprocessor. A \D converted and signal conditioning units. A simplified block diagram is shown below:

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Article By:
K Praveen Kumar
Asst.Professor
EEE Department
Sphoorthy Engineering College

Sphoorthy Engineering College

Power Quality Improvement

Electrical energy, one of the most important forms of energy that has become a primary need to be fulfilled as almost all walks of life need electricity to accomplish various works. Electrical energy generated at the power generation stations has to be transmitted and distributed for utilization and in the process has to meet up with many a challenges. Through proper right of way the energy is supplied to the consumers at the utility centers. The electrical power is made use for a wide range of loads at the consumer end. And the course of load demands varies widely posing problems in the abridgement of the quality of power delivered to the consumers.

Power Quality refers to the term used to describe electric power that drives an electrical load and the load's ability to function properly. Without proper power quality, the loads may not function properly and may even lead to the irrevocable damage to the load. Hence it is of vitality that power quality is maintained properly. To be precise, power quality actually refers to the quality of voltage and current delivered at the load end than power.

The quality of electrical power may be described as a set of values of parameters, such as:
• Continuity of service
• Variation in voltage magnitude
• Transient voltages and currents
• Harmonic content in the waveforms for AC power

Variation in voltage magnitude:
Ideally, AC voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards or system specifications. And under practical conditions, these may deviate in the following ways:

• Variations in the peak or RMS voltage.
• When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, the event is called a "swell".
• A "dip" also called as a "sag" is the situationwhere the RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute.
• Random or repetitive variations in the RMS voltage between 90 and 110% of nominal value can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level.
• Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges".
• "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1 minute. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light).
• "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute.
• Variations in the frequency.
• Variations in the wave shape – usually described as harmonics.
• Nonzero low-frequency impedance (when a load draws more power, the voltage drops).
• Nonzero high-frequency impedance (when a load demands a large amount of current, then stops demanding it suddenly, there will be a dip or spike in the voltage due to the inductances in the power supply line).
• Electrical line noise.

Causes of Voltage variations:

• Voltage Sags 
Sags are most often caused by fuse or breaker operation, motor starting, or capacitor switching. Voltage sags typically are non-repetitive, or repeat only a few times due to recloses operation.
• Voltage Swell
 A voltage swell takes place when the voltage is 110% or more above normal. The most common cause is heavy electrical equipment being turned off.
• Spikes
High-voltage spikes occur when there is a sudden voltage peak of up to 6,000 volts. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well.
• Switching Transients 
Switching transients take place when there is an extremely rapid voltage peak of up to 20,000 volts with duration of 10 microseconds to 100 microseconds. Switching transients take place in such a short duration that they often do not show up on normal electrical test equipment. They are commonly caused by machinery starting and stopping, arcing faults and static discharge.
• Frequency Variation 
A frequency variation involves a change in frequency from the normally stable utility frequency of 50Hz.This may be caused by erratic operation of emergency generators or unstable frequency power sources
• Electrical Line Noise 
 Electrical line noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) and causes unwanted effects in the circuits of computer systems.
• Brownouts 
 A brownout is a steady lower voltage state. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light). It comes from the noticeable to significant dimming of regular incandescent lights, during system faults or overloading etc., when insufficient power is available to achieve full brightness in (usually) domestic lighting. This term is in common usage has no formal definition but is commonly used to describe a reduction in system voltage by the utility or system operator to decrease demand or to increase system operating margins.

 An example of a brownout is what happens during  peak electrical demand in the summer, when utilities can't always meet the requirements and must lower the voltage to limit maximum power. When this happens, systems can experience glitches, data loss and equipment failure

• Blackouts 
A power failure or blackout is a zero-voltage condition that lasts for more than two cycles. It may be caused by tripping a circuit breaker, power distribution failure or utility power failure. A blackout can cause data loss or corruption and equipment damage.

A considerable solutions for these power quality abridgement problems have been proposed which make use of various Flexible AC Transmission System (FACTS)  devices. Enhancement of power quality with DSTATCOM is discussed below.

The enhancement of voltage sags, harmonic distortion and low power factor using Distribution Static Compensator (D-STATCOM) with LCL Passive Filter in distribution system can be achieved. power quality problems in transmission and distribution systems. The D-STATCOM is one of the most effective devices. A PWM-based control scheme can be implemented to control the electronic valves in the D-STATCOM. The D-STATCOM has additional capability to sustain reactive current at low voltage, and can be developed as a voltage and frequency support by replacing capacitors with batteries as energy storage.

DISTRIBUTION STATIC COMPENSATOR (D-STATCOM)

A D-STATCOM is a shunt compensating FACTS device. It consists of a two-level VSC, a dc energy storage device, controller and a coupling transformer connected in shunt to the distribution network. Figure below shows the schematic diagram of D-STATCOM.

Voltage Source Converter (VSC) 

A voltage-source converter is a power electronic device that connected in shunt or parallel to the system. It can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. The VSC used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. It also converts the DC voltage across storage devices into a set of three phase AC output voltages.

Controller

Proportional-integral controller (PI Controller) is a feedback controller which drives the system to be controlled with a weighted sum of the error signal (difference between the output and desired set point) and the integral of that value. In this case, PI controller will process the error signal to zero. The load r.m.s voltage is brought back to the reference voltage by comparing the reference voltage with the r.m.s voltages that had been measured at the load point. It also is used to control the flow of reactive power from the DC capacitor storage circuit. PWM generator is the device that generates the Sinusoidal PWM waveform or signal. To operate PWM generator, the angle is summed with the phase angle of the balance supply voltages equally at 120 degrees. Therefore, it can produce the desired synchronizing signal that required. PWM generator also received the error signal angle from PI controller. The modulated signal is compared against a triangle signal in order to generate the switching signals for VSC valves.

Energy Storage Circuit

DC source is connected in parallel with the DC capacitor. It carries the input ripple current of the converter and it is the main reactive energy storage element. This DC capacitor could be charged by a battery source or could be recharged by the converter itself.

LCL Passive Filter

LCL passive filter reduces the harmonics within the limits and thus the damage caused due to total harmonic distortion can be reduced.

The DSTATCOM is connected to the  distribution system and thus the power quality can be improved. 

There are also some other methods of improving power quality. Some of the solutions .There are five basic categories of solutions to some of the power quality problems, each having different capabilities, strengths and weaknesses. 

1. Surge Suppressors

2. Voltage regulators.

3. Power conditioners

4. Uninterruptible power supplies.

5. Generators.

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Article By:

Minnie Florence V

Asst. Professor

EEE Department

Sphoorthy Engineering College

Sphoorthy Engineering College

Transients in Circuits

1. How Transients will occur in the circuit?

(a) Energy storage elements present in the circuit.
(b) Switching operations(opening the switch, closing the switch, moving the switch from one position to other)
(c) Sudden change in circuit configuration(connecting the source suddenly, disconnecting the source suddenly, short circuit, open circuit)

2. What are Transients?

Voltages, current during transient period are called Transients.

3. Fundamental concepts in Transient analysis:

(a) Transient state: The state during which the circuit voltages, currents are not constant, changes with time is called Transient state.
(b) Steady state:    A network is said to be in steady state if it consists of constant sources and branch currents, node voltages are not changing with time. Circuits with currents ad voltages having a constant amplitude and constant frequency sinusoidal functions are considered to be in steady state,this means the amplitude and frequency of a sinusoidal wave never changes in steady state circuit.
(c) Transient Response:  The current and votage during transient period are called Transients response of the circuit. This will vary with the time and becomes zero after some time
(d) Steady state Response:  The current and voltage during Steady state period are called Steady state response of the circuit. This will not vary with the time.
(e) Natural Response or free response: The response of the circuit without source or when no external source applied, the response of the circuit due to energy stored in its energy storage elements.
(f) Forced response:  The response of the system when an external source is applied is called as forced response.
(g) Initial conditions or boundary conditions:
1. We assume that at reference time t=0, network state is changed by switching operation and let switch operates in zero time. The network conditions at this instant are called Initial conditions in network.
2. t(0-): It is the instant at which the condition of the network is not yet changed ,but it is about to be changed .(Just before the switching operation)
t(0+): It is the instant at which the condition of the network is just changed. (Immediately after the switching operation).
i(0-): current in the circuit just before the switching operation
i(0+): current in the circuit immediately after the switching operation
v(0-): voltage in the circuit just before the switching operation
v(0+): voltage in the circuit immediately after the switching operation.
3. Initial conditions in the network depend on the past history prior to t=0-
4. Initial conditions are used to obtain the natural response of the circuit.
5. Initial conditions of basic circuit components

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Article By:
K Praveen Kumar
Asst. Professor
EEE Department
Sphoorthy Engineering College

Sphoorthy Engineering College