Iacdrive|Automation Solution

Difference between DCS and RTU

DCS distributed control system: you can control the system within a certain given facility from different locations, either control room or other places, and you should keep in mind this facility could be a in several locations but yet, hard-wired interconnected. while
RTU (remote Terminal Unit): you can control the system remotely through internet or a secure satellite connection which in not recommended for sensitive operations/process but it is ok for stand alone and not crucial systems. and more.

DCS as part of SAS (Substation Automation System) is based on local control of relays, meters and switchgear and automation as per required logic and programs that could be hardwired for serial protocols (like DNP 3.0) or through fiber optic when UCA 2.0 or IEC 61850 protocols are used.
For RTU, it’s just interface between substations’ I/O signals and dispatching center (SCADA) through communication links and specific protocols (such as IEC 101,104, Indactic, DNP 3.0, etc.). In other words, RTU has no controlling role by itself, but DCS as part of SAS has all programmed control logic within substations and without even connecting to dispatching center.

For Electrical Network Distribution, a System is required for controlling, Load dispatch as well as monitoring. Therefore Distribution Management System (DMS) or DCS to be adopted as an integrated System. They are simply like SCADA. Composed from Hardware, software, interfacing means & communication media / protocol as indicated above.
RTU (remote Terminal Unit) include Processor and all the required interfacing facilities as well as I/O(s) Modules.

The brief description of such system may be as follows:
The Substation prescribed Signals (MV switchgears, Transformer, Substation Auxiliary Equipment, etc.) to be hardwired to a marshaling box to Interface Cubicle where RTU located, RTU to be patched to the interface plate. Via the selected media “say FOC” the signals will be transferred to the DMC/DCS Control Centre. Accordingly, the real time status of the NW can be monitored and controlling can be achieved from remote.

The aforesaid Signals to be listed and sorted as per the required application to facilitate system configuration, integration and programming (unique address, function, type, is it required for control, monitor or both, which is digital & which is analogue, etc.).

How to select the right cable?

Before you select kind of cable for your consumer, you need to calculate expected operating current of cable which depends from rated power of your consumer. After that, before you select kind of cable for your consumer, you need to check size of cable which needs to satisfy next conditions:

1. you need to check cable if it satisfied limits in normal conditions without consequences in aspect of warming (normal work),

2. you need to check cable if he satisfied limits in abnormal conditions without consequences in aspect of warming (short circuit).

1. when you want to check cable if he satisfied limits in normal conditions, you need to choose installation place (trench, concrete channel etc.), you need to know heat resistance of land, you need to know appropriate temperature of land and you need to calculate number of cables in installation place.

Icalculate=number of cables*k1*k2*k3*k4*Irated cable>Irated (consumer)
k1 depends from installation place,
k2 depends from heat resistance of land,
k3 depends from appropriate temperature of land,
k4 depends from number of cables

2. when you want to check cable if he satisfied limits in abnormal conditions, you need to calculate expected current of short circuit and heat impulse in the place of installation.

If your cable satisfied these requirements, then you made the right choice.

What is the best laptop for field work?

Dell D630 – it is the best laptop for field use I have used. And for some applications standard RS232 port is a must. We have Freja 300 test set which totally refuses to communicate with PC via widely available cheap USB-to-serial adapters. The only usable adapter I have found is semi-industrial type, costing about 50 Euro. Not that a price is so much concern, but it is not very convenient to deal with additional boxes, power supply units for them, etc. when commissioning at field.

But I do not expect you will have problems connecting Omicron via converters. We have been used CPC256 via various USB-RS232 converters without serious problems.
For communication with relay protections from Siemens and AREVA never had problems too. Cannot remember how it was with older ABB relays (last case we used them was 4 years ago), but newer ABB series are all with Ethernet communications.

So my advice will be – by special laptop for field work, not mix it with that for everyday office use. Load it with the minimal necessary software – MS Word, Excel, Adobe Reader, Omicron’s Test Universe and software for relays which will test.
For all these needs most older type laptops (4-5 years older) would be sufficient and you can buy for 200-300 Euro solid business class laptop. And also very important: look for non-glossy displays only!

Power Transformer power losses

Power losses of ferromagnetic core depend from voltage and frequency. In case where is no-load secondary winding, power transformer has a power losses in primary winding (active and reactive power losses) which are very small, due to low current of primary winding (less than 1% of rated current) and power losses of ferromagnetic core (active and reactive power losses) which are the highest in case of rated voltage between ends of primary winding…

Of course, we can give voltage between the ends of primary winding of power transformer (voltage who is higher from rated voltage), but we need include some limits before that:

1. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), we need to set down frequency, otherwise ferromagnetic core of power transformer will come in area of saturation, where are losses to high, which has a consequence warming of ferromagnetic core of power transformer and finally, has a consequence own damage,

2. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), also intensity of magnetic field and magnetic induction will rise until “knee point voltage”: after that point, we can’t anymore increase magnetic induction, because ferromagnetic core is in area of saturation…

In that case, current of primary winding of power transformer is just limited by impedance of primary winding… By other side, in aspect of magnetising current, active component of this current is limited by resistance of ferromagnetic core, while is reactive component of this current limited by reactance of ferromagnetic core.

There is a finite amount of energy or power that can be handled by various ferromagnetic materials used for core material. Current increases greatly with relatively small voltage increases when you are over the knee of the magnetization curve characterized by the hysteresis loop. Nickel/steel mix materials saturate at lower flux densities than silicon steel materials. 50ni/50fe materials saturate at about 12kG; 80Ni/20Fe as low as 6kG. Vanadium Permendur material saturates at levels as high as 22kGauss- Nano-crystallines- 12.5kG (type), Ferrites -typically over 4kG at room, decreasing as temperature rises. What causes saturation?: Exceeding material limits.

Overcurrent protection of generators

Overcurrent protection uses as back-up protection for protection generators from faults between two windings of stator (two phases of stator). Setting of overcurrent protection depends from two settings: current setting of relay protection and time setting of relay protection.

Current setting of relay protection represents minimal value of current under which relay protection will send signal to breaker to act and this value is higher from value of rated current in generator (higher from maximum allowed value of current in generator).

Time setting of relay protection represents time after that relay need to send signal to breaker to break fault. Of course, when we talk about time setting of relay protection, we need to have on mind time delay. Time delay represents time during other protections need to act before overcurrent protection acts in case where is overcurrent back-up protection for protection of generator.

Then there is voltage restrained time overcurrent protection (ANSI 51V) which is commonly applied on generators. The pickup setting of these relays reduces (becomes more sensitive) when the applied voltage reduces. It is supposed to aid in sensing faults that are electrically close to the generator terminals as there is insufficient fault impedance to maintain the voltage at the generator. It is especially useful in tripping out faults that have persisted long enough for the generator fault decrement curve to get to the portion where the synchronous reactance is the characteristic impedance. When this happens the fault current will be at the same levels as normal load currents and increased sensitivity is needed.

Transformer Saturation

Bushing insulation testing

In bushing insulation test there are three major current elements which any of those could affect the test result. These current elements are Capacitive current

Motor connection

Many years ago I had an experience of 4nos 37kW fin-fan motors wrongly connected at site to a star. After running for almost 1 year, the operators reported these motors were very warm and felt unusual. We removed one of them to the workshop and opened for inspection. All windings were OK but the rotor lamination surface had turned to light blue colour which showed a sign of abnormal heating.

I asked different experts in the industries for advices. From the advices, we suspected the motor could be designed for a delta connection even though the nameplate indicated a Star connection for 415V. We contacted the motor manufacturer by quoting the motor serial no. The manufacturer confirmed that the motors were designed for delta connection at 415V. The manufacturer apologized for the error in nameplate and gave us a free spare motor.

One clear sign that could lead us to believe that the motor was in a wrong star connection instead of delta was, for a 2 or 4-pole motor the no load running current should be more or less around 30% of FLC. When we tested run the motor in the workshop, the no load current was less than 15%xFLC.

After the rectification of all the 4 motors to delta connection, we had no complaint anymore. It was a good lesson out of this solved problem.

Torque ripple information from low resolution speed signal

Q:
I am trying to develop a controller for switched reluctance motor which minimizes torque ripple. My design is acquiring torque ripple information from speed signal. In simulation a high pass filter for speed gives me good ripple information. But in experiments I am using a 500 PPR optical absolute encoder to get the position and then calculate the speed using microcontroller (dspace) capture module. But the filtered speed signal does not provide much ripple information. Can you suggest any method to extract ripple information from low resolution speed signal.

A:
1. In simulation, do you consider motor inertia? Inertia filters out torque ripple’s impact on speed, resulting in a smooth speed signal. 2. Generally speaking, a low resolution position sensor produces speed signal of more noise, especially at low speed. I would expect more noise out of your high pass filter.

An encoder generally does not specify an accuracy for the A to A! channel or B to B! channel or it is so broad a spec that it is useless. If you have the ability to trigger a clock on A and B to determine the period between A and B channels the difference between successive reads will give you a good indication of your ripple.

In some cases of motor – encoder installations the mechanical alignment of the encoder to the exact center of motor shaft can cause misalignment noise to occur in the resolved speed signal. In theory the ripple signal could provide useful information however in practice there are too many other influences. Even the shaftless encoder mounting has some of these difficulties.

Add filters to frequency inverter to eliminate harmful

The high frequency edges of switched waveforms can cause capacitively coupled currents to flow from windings to frame, returning through the bearings, and these can accelerate corrosion in the bearings, causing early failure. Small filters on the motor leads allowing these currents to return locally to ground will avoid this.

The best way, though, is to use filters which can eliminate sharp transitions and leave only (like +/-10% ripple) fundamental frequency (motor’s RPM at given point) of the motor drives. However if somebody can handle 40 – 50kHz of the switching frequency the filter’s size shrinks dramatically and it is not too expensive anymore. Again, the problem is in ability to handle 100 (or so) kWs and 50kHz together.