Bid Document No. CE/DSPC/LTLMS/T- 19/05-06
MAHARASHTRA STATE ELECTRICITY DISTRIBUTION COMPANY LTD
BID DOCUMENT FOR
LOW TENSION LOAD MANAGEMENT SYSTEM (LTLMS) IN MAHARASHTRA STATE
VOLUME II
TECHNICAL SPECIFICATION
EXECUTING AGENCY
CHIEF ENGINEER,
DISTRIBUTION SPECIAL PROJECT CELL
MAHARASHTRA STATE ELECTRICITY DISTRIBUTION COMPANY LTD.
MUMBAI
“Low Tension Load Management System” with Thyristorised Switching System.
1.0 MSEDCL: Technical Specifications
These specifications are for 27 kVAr (3×9 kVAr) thyristorised capacitor switched units to be connected on the secondary side (415 V) of 63kVA distribution transformer and 36 kVAr (4×9 kVAr) system for 100kVA distribution transformer. The unit should have the provision to add one more 9kVAr capacitor unit along with its necessary control and auxiliaries in both the cases at a later date. The system provided should be 3-phase 3-wire and recommended for no neutral connection requirement. The sites would be in various parts of Maharashtra.
Distribution systems are 3-phase 4-wire system and typically unbalanced. The compensating unit will be shunt connected with delta connected capacitor banks. Each bank should be switchable through thyristors. The control algorithm should be implemented on a digital platform preferably using DSPs. The processor has to monitor total active and reactive power, overall power factor, line currents and voltages etc. The processor should be able to meet the computational demands of numerical protection required for the compensator, the harmonics in the voltage and current signal seen in the compensator. The specifications of such units are detailed in this document.
The equipment offered shall be totally enclosed, metal clad and completely vermin – dust proof and suitable for continuous satisfactory operation in tropical area of Maharashtra. It should at least IP-55 complaint. The equipment offered shall be suitable for continuous satisfactory operation in tropical area of Maharashtra for following prevailing worse case climatic conditions.
A Normal Atmospheric Condition: Dry for eight months and humid for four months.
B Temperature: Maximum 50° C, Average 35° C
C Relative Humidity: 98% maximum
D Rainfall: 2000 mm concentrated in 3 months
E Basic Wind Speed: 60 m/sec.
2.0 Standards:
2.1 The Capacitor units shall comply with IEEE STD 18-2002 and IEEE STD 1036-1992 (or equivalent IS/IEC standards like IS: 13340/1998, IEC: 60831) and other components including MCBs, Thyristors etc., shall comply with the latest versions of relevant Indian Standards.
2.2 The Capacitor units shall be approved by NABL agency like CPRI / ERDA tested or at internationally accredited testing Laboratory like KEMA or UL.
2.3 The measuring and controlling instrumentation equipments should be tested for electrical measurement accuracy class of 1.0 from NABL approved laboratory like CPRI / ERDA/ETDC etc for measurement standards.
2.4 Applicable standard IS-13410, 13411 etc. for enclosure’s cover in case of Sheet Molding Compound (SMC) use as per clause 9.0 (Sub clause 9.4).
3.0 Basic Scheme:
The basic scheme is illustrated for convenience in Fig 1.
4.0 Capacitors:
The capacitors are proposed to be connected on the LT side of distribution transformer. The nominal voltage is 3Φ, 415V, 50 Hz system. The system frequency may vary from 48-51 Hz.
4.1 Capacitor Units (Manufacture and Construction)
· The capacitor shall be either dry design heavy duty metalized polypropylene self healing category or oil-immersed non-PCB type and all polypropylene film capacitors. The metallization shall be done preferably in-house to ensure good quality of metallization.
· The capacitor units shall be three phase units.
· The dielectric material shall be low loss less than 0.2 watts per kVAr.
· The capacitors shall be covered for FIVE YEARS of GUARANTEE against manufacturing, design or workmanship defects.
4.2 Capacitor Ratings and Service Conditions
The capacitors used should be consistent with the standards mentioned in section 2.0. A few important aspects are emphasized below.
4.2.1 Capacitance Tolerance
The capacitance of the unit shall not vary more than -0 % to +10 % of the nominal value based on rated kVAr, voltage and frequency measured at 25° C.
4.2.2 Maximum Continuous Operating Voltage, Current and kVAr
Capacitors are intended to operate at or below their rated voltage. Capacitors should be capable of continuous operation under contingency system and bank conditions provided that none of the following limitations are exceeded including harmonic contents.
· 110 % of the rated rms voltage
· 120 % of the peak voltage
· 180% of rated rms current
· 135 % of the rated kVAr
4.2.3 Insulation Classes
The BIL of the capacitor should be 25 kV.
4.2.4 Overvoltage and Overcurrent withstand Capabilities
Capacitors shall be capable of withstanding with full life expectancy switching transients having crest voltages up to 2-times the peak of the capacitor rated voltage, and other transient overvoltages and overcurrents normally associated with the operation of shunt capacitors.
4.2.5 Internal Discharge Devices
Capacitor shall be equipped with an internal discharge device that will reduce the residual voltages to 50 V or less within 1minute after the capacitor is disconnected from the peak of rated voltage. In order to avoid human contact with charged capacitor, door interlock has to be provided.
4.2.6 Radio Influence Voltage
RIV generated by capacitor shall not exceed 250 micro volts.
4.3 Testing
Compliance certificate from CPRI/ERDA on design tests and productions shall be provided. The tests as per IEEE STD 18-2002 or equivalent IEC/IS standards.
5.0 Capacitor Bank Protection;
Delta connected units which are popular at low voltages are configured with a single series group of capacitors rated at line to line voltages. With only one series group of units, no overvoltage occurs across the remaining capacitor units from the isolation of the faulted capacitor unit. Therefore, unbalance detection may not be required for protection, but may be used to detect outage of the units with in the bank. In this arrangement, the individual capacitor fuses shall be capable of interrupting the system short circuit phase-to-phase fault current. This design may necessitate current limiting fuses. The capacitor bank may be subjected to overvoltages resulting from abnormal system operating conditions. If the system voltage exceeds the capacitor voltage with capacitor bank online, the bank should be removed with minimum time delay. The capacitors may be exposed to overvoltages as a result of combined fundamental and harmonic content. The manufacturer should be asked to furnish peak voltage stress levels as a function of time and temperature. Also, protection against loss of bus voltage should be considered. In the event single phasing the capacitor unit should be disconnected. Lightning and switching transient overvoltages on capacitors as well as thyristors should be controlled by using standard overvoltage protection equipment, such as surge arresters. A capacitor bank generally absorbs overvoltages because it acts temporarily as a short circuit for step voltage changes. For additional information one may refer IEEE guide for protection of shunt capacitor banks (IEEE Std C37.99-2000).
6.0 Thyristor Switched Block for 9 kVAr:
6.1 Thyristor Switch Rating
The continuous current rating of the thyristors used to switch the line connected delta capacitor bank (9 kVAr) should be a minimum of 90 A rms and they are of standard make like EUPEC/IRF/Mistubushi/Semicron/Hind Rectifier. This number is obtained after taking into account the factor of safety in design and de-rating factor for climatic conditions and life expectancy of the equipment. The blocking voltage of the thyristor switch during off condition should be a minimum of 1800V. Every thyristor switch block should be capable of handling a minimum dv/dt of 1000Volts/ms. This is to avoid spurious turn on while powering up as well as due to supply transients. These devices should be protected using switching aid networks taking into account initial capacitor voltages and capacitor inrush current. Thyristors used for every 9 kVAr bank should have independent heat sink. Operating conditions such as no forced cooling, dusty weather condition etc should be taken into account while determining the size of the heat sink. It should be noted that the unit will be disconnected from the system if the heat sink temperature rises beyond 75° C. Manufacturers will have to provide a design document which outlines these issues and should be willing to carryout necessary design modifications based upon the feedback of relevant experts.
6.2 Differential Voltage Switching:
The thyristors should be turned on when the voltage across the device is less than 6 Volts. Trigerring circuit should be immune to EMI and it should not maloperate due to multiple zero crossings of the input voltage waveform if any.
6.3 Response Time:
The time delay between turn off and subsequent turn on should in between 2-4 seconds. While turning on the unit, the inrush current should be minimum. For this purpose, incoming phase voltage and voltage across the capacitor have to be sensed. Under healthy conditions, turn on to turn off delay should be of the same order as above.
6.4 Protection Against Supply Transients and Current Spikes:
The thyristor switching units should be provided with adequate protections against any external transients that can cause current and/or voltage spikes. In case of a power failure and subsequent restoration, the capacitor unit should not be energized for at least 3-min after transformer energization. Adequate arrangement of forced commutation to prevent the thyristors from exceeding its I2t rating be present in every thyristor switched block.
6.5 Capacitor Over Current Protection:
Apart from the spike protection, every thyristor block should be protected if capacitor continuous current remains above 130% for more than 1 minute. This block should not have the auto restart facility.
6.6 Thyristor Damage Monitoring:
If thyristor gets damaged (short or open), the thyristor block should be able to recognize the same and give a digital fault feedback to the ACU to log this as faulty condition. In case of a thyristor failure, the corresponding compensator should be taken out of service. An alarm indication should be made available on the unit and GSM/GPRS.
6.7 Over-Temperature of Heat-Sink:
The heat sink on which the thyristors are mounted should be monitored continuously for over temperature. If the temperature increases above the specified limit fault that particular bank should be disconnected from the system. The recommended trip level is at 75°C. This fault should have an auto-restart function once the temperature comes down below 65°C or 15 minutes to avoid frequent hunting on resumption of operation.
6.8 Operational Conditions:
· Mains voltage: Line to Line: 300V to 456V .
· Min. temperature: 0°C, Max. Temperature: 50°C.
7.0 Controls:
7.1 All controls shall be mounted on enclosure door for easy inspection and service.
7.2 All the control wiring in either Incomer/Control block or kVAr block should be of adequate rating minimum of 1.5 sq.mm Copper insulated cable with relevant IS standards. The cables used should have temperature-withstanding strength of 85°C.
7.3 Automatic Control Unit: ACU (Load Manager + Auto PF Controller + Data Logger):
The functions of “Load Manager”, “Auto PF controller” and “Data Logger” should be as a part of electronic controlling unit.
a) The display in the front should be either backlit LCD,16 characters × 4 lines or suitable seven segment LED display. This display should be clearly visible even in dark.
b) Various display parameters and parameters settings to be entered in the unit / units should be through front keypad, which should be in front of ACU and should be user friendly. The keypad can be either membrane type or flush type electro-mechanical switches. It should be enclosed and accessible to only authorized personnel. The denominations on the keypad should be clearly printed on the switches.
c) The unit should be powered up through a 415V ac mains supply and should be capable of giving the full functionality from 70% to 115% of the mains Voltage range for continuous operation. None of the control unit / units should require neutral connection either for its auxiliary supply or for measurement. This is because the entire scheme is without neutral connection.
d) The unit should be in an enclosed cabinet (metallic or plastic) such that it should be seen as a free standing instrument/s. All the control and supply terminals should be snap on type terminals. ( No necessity to screw/un-screw the connections for connection / removal). The housing should be such as no internal electronics should be accessible for human touch from any side of the unit / units.
e) The unit should be capable of functioning and measuring with specified accuracy for ambient temperature from 0°C to + 50°C and independent of internal inside temperature of the panel. Temperature inside the cubicle should not exceed 55°C. This figure is arrived keeping in mind that capacitor life reduces with rise in temperature. If the temperature increases above this limit, the unit should be shut down and LED indication on the panel, along with event trigger through GSM/GPRS should be provided. The make of unit manufacturer and the certificate from NABL approved laboratory like CPRI / ERDA / ETDC etc for the said unit/s for its temperature functionality should be furnished.
7.4 ACU: Function of Load Manager
a) Should be capable of measuring the electrical parameters through three phases, three wire system for A.C. Voltage and through 5Amp Current Transformer (CT). Three CTs for line current measurement would be placed in the secondary of the distribution transformer as shown in Fig: 1. The use of single CT is not advisable as the distribution system is inherently unbalanced in nature. Therefore, other line currents cannot be inferred from one CT alone. Additional CTs in the line end of the compensator should be placed to achieve adequate control and protection requirements of the compensator. Manufacturers would have to provide a design document which outlines these issues and should be willing to carryout necessary design modifications based upon the feedback of relevant experts.
b) Should be able to measure and user friendely display the following parameters.
· RMS values of line voltages, line currents (I_r, I_y, I_b), capacitor current (Ic_r, Ic_y, Ic_b), overall PF (measured and displayed up-to two decimal digit), total kW, kVAr and kVA of the system, C-kVAr (kVAr compensated by capacitors), mains frequency (measurement and display up-to second decimal digit), system kWh, kVAh and kVArh (energy parameters cumulative value up-to minimum seven digits).