INTRODUCTION AND IMPLEMENTATION OF A NEW SWITCHGEAR TYPE CALLED RING MAIN UNIT WITH CIRCUIT BREAKER (RMU CB) FOR 22 KV AND 11 KV DISTRIBUTION SYSTEM IN TNB
[H Ahmad Rosli, Aset Management Department, Distribution Division TNB, Malaysia, +6019-6582910, [emailprotected]]
[MH Mohamad, Aset Management Department, Distribution Division TNB, Malaysia, +6019-3004320, [emailprotected]]
ABSTRACT
The pursuit to reduce System Average Interruption Duration Index (SAIDI) is ever continuous as it has direct impact to utility’s revenue and customer satisfaction. In Distribution Division of TNB, the approach to reduce SAIDI is holistic involving implementation of new technologies, effective maintenance practices and network automation.
A new type of switchgear combining circuit breaker with ring main unit (RMU) was introduced as part of new technology intervention to reduce SAIDI. Named as RMU CB, this switchgear provides circuit breaker function within a SF6 gas-insulated RMU. The tripping control is achieved by using self-powered relay. In one of configurations, HV energy meter is also included in a feeder for border metering purpose.
The advantages of RMU CB are manifold. The circuit breaker improves fault-sectionalizing capability especially for long and tee-off feeders and hence reducing SAIDI. The use of self-powered relay eliminates the need for LV supply and DC system which makes RMU CB suitable for non-building or outdoor substations hence cheaper and faster plant-up. RMU CB with meter provides facility to measure the import and export kWh units at substations located at bordering states or areas for accurate technical losses calculation.
This paper discusses experiences and challenges in introducing RMU CB in TNB system from user requirement identification, technical specification development, success stories and lessons learned from field trial implementation, and mass implementation roll-out.
KEYWORDS SAIDI, switchgear, circuit breaker, RMU
INTRODUCTION
The pursuit to reduce System Average Interruption Duration Index (SAIDI) is ever continuous as it has direct impact to utility’s revenue and customer satisfaction. In Distribution Division of TNB, the approach to reduce SAIDI is holistic involving implementation of new technologies, effective maintenance practices and network automation.
New technology implementation involves intervention through the use of new equipment or methodology to achieve the desired outcome. The process starts with the approval of concept paper which contains technical and commercial feasibility study of the new technology, followed by field trial installation of the equipment. The field trial unit is then assessed in terms of functionality, compatibility with the system and robustness to the installation surroundings. When the assessment proves that the new technology is suitable for mass implementation, technical specification of the equipment will then be developed based on TNB requirements as well as feedback from manufacturers. The technical specification shall then be complied by suppliers during the procurement process to ensure the right equipment are being utilized in the system.
For SAIDI reduction, many new technologies have been introduced in the distribution system in accordance to the TNB Distribution Technical Roadmap. In 2011, the concept of using a new type of switchgear combining circuit breaker with ring main unit (RMU) was introduced. This switchgear is termed as RMU CB.
MOTIVATION FOR THE USE OF ALTERNATIVE TO VACUUM CIRCUIT BREAKER PANELS
The need for RMU CB was identified for 11 kV and 22 kV system as an alternative to using vacuum circuit breaker (VCB) panels to achieve automated tripping and fault-sectionalizing capability. The TNB 11kV and 22 kV system consists of feeders which are energized from and are outgoing of the Transmission Main Intake Substation (Pencawang Masuk Utama-PMU) or Primary Distribution Substation (Pencawang Pembahagian Utama-PPU). A feeder is typically connected to two different sources in a ring configuration with normally off point (NOP) appropriately located in the ring network which breaks the feeder into two segments, each to be energized by a different source. Some feeders are spur feeders with single energizing source, and are typically tee-offs from the ring networks.
The 11 kV and 22 kV feeder feeds in and out of distribution substations (Pencawang Elektrik-PE) along the ring network. These substations use RMU extensively which functionally suit the system configuration and the protection practice of TNB. RMU is a high voltage switchgear which breaks on load but does not break the fault current that flows through it (load break switch). Depending on the loading, every feeder typically consists of about 10 PE substations from source to NOP. However, some feeders, especially in the sub-urban and rural areas with low loads, can be long and have for example up to 20 PE substations from source to NOP. Any fault in the system along this feeder will invariably trip the circuit breaker placed upstream of the fault location. If the only circuit breaker is at the outgoing of the PMU or PPU, then the whole feeder will be interrupted due to the tripping, affecting many customers and incurring high SAIDI especially for long feeders. This is even more critical for feeders with frequent breakdowns (more than 3 tripping per year) termed as worst performing feeders (WPF).
To reduce SAIDI due to this, circuit breaker function needs to be strategically placed along the feeder to provide automated tripping and fault-sectionalizing capability. Previously, this could only be achieved by installing VCB panels in PE substations. When VCB panels need to be installed, substation building is required to house the VCB with the required Low Voltage (LV) AC supply and DC system. The building of TNB substation costs about RM 100, 000 and takes about 8 months to be commissioned. Furthermore, some existing PE substations where the RMU needs to be upgraded to VCB panels, are outdoor substations, especially in the sub-urban and rural areas. As such, substation building needs to be built first before the VCB panels can be installed.
INTRODUCTION OF RMU CB
The introduction of RMU CB offers several advantages and is a suitable alternative to the use of VCB panels in certain conditions. RMU CB is a SF6 gas-insulated RMU which has circuit breaker function within the same block unit. The circuit breaker tripping is controlled by self-powered relay [1], which is a type of relay that is energised by the CT installed on the cable bushing to measure the secondary current of the feeder. It is sometimes referred to as CT-powered relay.
RMU CB is different from the conventional or existing RMU types used in TNB which have circuit breaker function as well. This is because the circuit breaker in these RMU is designed to control the transformer feeder as it is usually of the rotating arc type and has short time withstand current for the earth switch of 2.1 kA, 1 s only. In order to be suitably designed for ring feeders, the circuit breaker usually uses vacuum interrupter and the earth switch rating must be similar to the main switch rating of 20 kA, 3s.
Figure 1. Circuit diagram of conventional RMU with
circuit breaker function at transformer feeder (Left). An example of RMU in
TNB with circuit breaker for transformer protection
(Right)
The use of self-powered relay eliminates the need for LV AC supply and DC system to power up the relay for operation. As such, if the RMU CB is specified to have the necessary ingress protection (IP) for outdoor application, there is no requirement for substation building to be built to install RMU CB. This offers great advantage in terms of faster and cheaper construction compared to when installing VCB panels to achieve similar functionality [2], [3].
TECHNICAL SPECIFICATION OF RMU CB
The technical requirements of the RMU CB used by TNB are based on the technical specifications of existing RMU and VCB panels. These specifications refer to the governing IEC standards which are mainly the IEC 62271 series [4]. They can be described as follow:
a) Rating
The RMU CB shall have two rated voltages, 12 kV and 24 kV.
For 12 kV, the rated insulation level shall be as follow:
• Lightning impulse withstand voltage o 75 kV (across earth and between poles) o 85 kV (across isolating distances)
• Power frequency withstand voltage: o 28 kV (across earth and between poles) o 32 kV (across isolating distances)
Meanwhile for 24 kV, the rated insulation level shall be as follow:
• Lightning impulse withstand voltage o 125 kV (across earth and between poles) o 145 kV (across isolating distances)
• Power frequency withstand voltage: o 60 kV (across earth and between poles) o 60 kV (across isolating distances)
The load break switch, circuit breaker and earthing switch (except for earthing switch of transformer feeders) shall have the following rating:
• Continuous normal current: 630 A • Short time withstand current: 20 kA rms, 3 s • Short circuit making current: 50 kA peak
Meanwhile, for transformer feeder, the earthing switch shall have the following minimum rating:
• Continuous normal current: 630 A • Short time withstand current: 2.1 kA rms, 1 s • Short circuit making current: 5.4 kA peak
Rated Operating Sequence of the circuit breaker is O-t-CO-t'-CO where t = 3 min and t' = 3 min as per IEC 62271-100. This excludes the RMU CB from auto-reclosing or SCADA–ready function as the RMU CB is meant to be installed mainly in sub urban and rural feeders where these functions are not required. To ensure the RMU CB can be installed outdoor, the ingress protection must be rated at minimum IP 54 [5].
b) Configuration
The RMU CB is of block type, and not modular. This means the switches are all assembled and connected inside the SF6 tank from factory according to specified switch combination or configuration. The configuration specified is to
suit the network configuration at TNB 11 kV and 22 kV PE substations. In total, 7 combinations of switches are specified. The nomenclature for the RMU CB is made up of the term ‘RMU CB’ followed by the first numeral which describes the number of load break switch, second numeral which describes the number of transformer switch and the third numeral which describes the number of circuit breaker. The configuration required by TNB is as follows:
• RMU CB 111: 1 load break switch + 1 transformer switch + 1 circuit breaker • RMU CB 121: 1 load break switch + 2 transformer switch + 1 circuit breaker • RMU CB 112: 1 load break switch + 1 transformer switch + 2 circuit breaker • RMU CB 102: 1 load break switch + 2 circuit breaker ring feeder • RMU CB 122: 1 load break switch + 2 transformer switch + 2 circuit breaker
Additionally, RMU CB 101 configuration is also introduced for loop in loop out installation at outgoing feeder of existing RMU that needs the circuit breaker function to be placed:
• RMU CB 101: 1 load break switch + 1 circuit breaker HV energy meter is also made available for this configuration and is differentiated in the nomenclature with notation M:
• RMU CB 101M: 1 load break switch + 1 circuit breaker with energy meter The purpose of having the energy meter is to record the kWh units imported or exported between two different TNB subzones which is one of the inputs required to determine the technical losses incurred within the subzone network. c) General arrangement and construction
All live parts shall be insulated within the SF6 tank except for the cable connection and fuse chamber. The SF6 tank shall be made of stainless steel and totally welded. The RMU CB shall be equipped with gas-type pressure gauge to indicate the SF6 content level within the tank. The transformer switch for transformer protection can employ either switch-fuse or circuit breaker. This circuit breaker tripping shall only be controlled by time lag fuse. For switch-fuse, HV DIN type fuse link of length 292 mm for the 12 kV and 442 mm for the 24 kV rated voltage is used in series with the load break switch. The load break switch, earthing switch and disconnector inside the tank are fitted with operating mechanism that are accessible from the external of the tank at the front part of the switchgear unit. Each load break switch shall be fitted with a direct manually operated mechanism having three positions, ON, OFF, and EARTH. The cable termination are located within cable boxes with minimum height of 650 mm and sufficient working clearance for TNB approved terminations. All cable boxes shall be fitted with cable clamps suitable to accommodate cables used in TNB system to support the cable weights and prevent damage to the bushing. The RMU CB shall have copper earth bar of 120 mm2 cross section bolted to the main frame and located so as to provide convenient facilities for earthing cable sheaths. The earth bar shall extend continuously along entire RMU CB body so as to ensure continuity for earth fault current to flow. d) Circuit breaker
The circuit breaker can either be vacuum type of SF6 type. It shall not be of the rotating arc type. Rotating arc circuit breakers interrupt low-level fault currents through the extinction of a free burning arc between the load current contacts but utilise an electromagnetic field for spinning the arc, transferred into a coil, for interrupting high-level fault currents [6]. Rotating arc type is not suitable for RMU CB because it generally has lower rated number of minimum switching operation at rated short circuit current compared to vacuum or SF6 circuit breakers.
The number of minimum switching operation at rated short circuit current (i.e. 20 kA) specified is 30. The tripping of circuit breakers shall be affected by means of adequate impulsive output energy generated internally by the self-powered relay. The trip coil must be of low energy type working with direct impulse output energy 24 volts DC, 0.1 J from self-powered relay.
The circuit breaker shall be connected in series with 3-position disconnector. Interlocking facility must be in place between the disconnector and the circuit breaker to ensure safe operation. e) Self-powered relay The relay is to be designed to be a part of the RMU CB. The relay shall be installed at the low voltage compartment of the circuit breaker. The relay shall be self-powered and to be energized from the main current transformers. Additionally, the relay shall have provision to be energised by external auxiliary AC supply voltage. The relay shall be provided with a long life lithium battery (minimum service life of 10 years) for maintaining the clock function and to allow setting and read out without external power supply. The relay shall be of numerical type which include but not limited to features of event log, fault current value and trip information. The relay shall provide impulse output energy of 24 volts DC, 0.1 J for direct tripping of the circuit breaker. The relay shall have three phase non-directional overcurrent and non-directional earth fault protection with selectable IDMT/definite time for low-set and high-set stage. Figure 2. Examples of self-powered relays: Woodward W1P1 (Left) and C&S CSDPR-V2 (Right) f) Current transformers (CT) There are two types of CT used in the RMU CB, which are protection CT and metering CT (applicable for RMU CB 101M). The CT shall be installed/mounted on bushing in the cable compartment with fixed mounting bracket. The electrical clearance and voltage stress management must be in place to avoid any partial discharge in the CT and bushing vicinity. In RMU CB 101 M, the metering current transformers shall be mounted on the bushing of the incoming LBS feeder while the protection current transformers shall be mounted on the outgoing circuit breaker bushing. The ratings of the CT used is as follows: Application Class Ratio Output
a) Energy Meter 0.2 300/5 7.5 VA
b) Overcurrent/Earth Fault Relays 5P10 300/1 7.5 VA
Determination on the rating also takes into account physical size of the CT to ensure that it can be accommodated and mounted on the cable bushings where there is space limitation. Cable-mounted CT is not allowed to eliminate the need for the CT to be handled during installation by cable jointers. Furthermore, to mount cable-mounted CT, the standard type of termination used in TNB will need to be modified to ensure the CT is placed in zero potential portion of the termination where the cable copper screen is still present.
Figure 3: Bushing-mounted CT (Left), cable-mounted CT (Right)
g) Safety features
Safety to operators is primary concern for all utilities. Safety features specified in the RMU CB are similar to the features in existing RMU. Capacitive voltage indication facilities are provided for all cable terminations at the front of the modules to enable operators to verify if the circuit if energised or not. The voltage indicator also has facilities to conduct secondary live phasing between feeders using phase comparators.
Inadvertent operation of the switches from ON, direct to EARTH or vice-versa shall be prevented by a mechanical interlocking arrangement. A mechanical interlocking facility shall be provided to prevent the following operations:
a) Access to the cable test bushing when the switch is in any other than EARTH ON position. b) The opening of the cable test bushing compartment when the switch is in any other than the EARTH ON
position. c) The movement of the switch to the ON position when the cable test bushing compartment is open
It shall also not be possible to operate the load break switch from ON to OFF or from EARTH to OFF for a minimum period of three seconds subsequent to achievement of the ON or EARTH position respectively. This is usually fulfilled by using anti-reflex handle, termed as the 3s handle. This feature is important to prevent possibility of the load break switch breaking fault current due to reflex when an operator accidentally close on fault which will cause failure to the load break switch.
Figure 4. Example of an anti-reflex handle
Padlocking facility shall be provided. It shall be possible to lock the operating mechanism in any of the three positions when the contacts have fully homed and also to independently lock off the ON and EARTH ON positions. The positions ON, OFF and EARTH ON of the switch shall be clearly indicated by position indicators such that the direction of movement of the operating handle(s) from one position to another is readily apparent.
For RMU CB with transformer switch, the fuse compartment shall also have interlocking to disallow access to it with the switch in other than EARTH position and the fuse earthed on the downstream side. Conversely it shall not be possible to switch to ON when access to the fuse or fuse carrier is still possible
h) Type test
The type tests are the tests conducted by manufacturers to prove the rating, performance and endurance of the design. For RMU CB, there are 12 type tests for primary part which must be complied before the product can be used by TNB. The type tests shall be conducted in laboratories with ISO/IEC 17025 accreditation for the tests to be conducted [7]. The type tests required are as follow:
a) Dielectric tests b) Temperature rise test c) Measurement of the resistance of main circuit d) Short-time and peak withstand current test e) Verification of making and breaking capacities for all switching devices and circuit breaker f) Electrical endurance tests (minimum E1) g) Cable charging current breaking tests h) Tightness test i) Arcing due to internal fault test for the SF6 tank:
• Classification IAC: AFL • Internal arc: 20 kA, 1s
j) Mechanical endurance test (minimum M1) k) Pressure withstand test for gas-filled equipment with pressure relief devices l) Verification of the degree of protection of enclosure (IP Test)
SUCCESS STORIES OF IMPLEMENTATION
Since introduction in June 2015, 80 units of RMU CB 22 kV and 11 kV from 3 manufacturers have been installed in the TNB system.
Figure 5. RMU CB 102 11 kV (Top left), RMU CB 112 11 kV (Top right), RMU CB 101 (Left)
Number of successful tripping recorded since September 2015 uo till 30th May 2016 is 12. For example, for the first 3 months of installation in PE Seberang Guai, Pahang, the RMU CB 111 11 kV tripped 3 times to isolate faults. In this case, the saving on SAIDI is apparent. Without the RMU CB installed and functioning at this substation, those tripping would have tripped the outgoing circuit breaker (CB1) in PMU Kerayong hence interrupting supply to 8 additional substations, including ones supplying to important premises such as the District Office and library.
Figure 6. Schematic diagram of PE Seberang Guai where RMU CB is installed
Relay operation Relay data Fault
L1, L2, L3 L1 – 432A L2 – 429A) L3 – 429A E – 137.4A
Termination fault
L2, L3 L1 – 468A L2 – 474A) L3 – 462A E – 78.3A
Termination fault
Earth fault L1 – 357A L2 – 84A L3 – 66A
E – 297.9 A
Joint fault
LESSONS LEARNED
Newly introduced technology and product are usually faced by issues at the initial stage of implementation. The RMU CB is no exception. While TNB is benefiting from adopting this new technology through the successful tripping, it also faces issues which are mainly on product quality and certain limitation on the self-powered relay.
The product quality issues mostly revolve around the circuit breaker mechanism of a particular RMU CB brand manufactured by a local vendor which results in either maltripping or the circuit breaker cannot be switched on after charging. Post mortem done on failed units revealed that the root cause is out of tolerance in measurement for some critical components, and not subjecting the units to full routine test aft the factory. Rectification is being carried out by the vendor on units in TNB warehouses, units not yet delivered as well as units already in operation at sites, all monitored by the Quality Assurance Unit of TNB. The local vendor has also engaged switchgear design consultant to improve on their design and quality.
Another lesson learned is the inherent limitation of the self-powered relay technology where currently most of the self-powered relays demonstrate delay time (called as boot time) of about 100 ms before the relay sends tripping signal. This inherent boot time varies from one relay brand to another. For some applications at site, the boot time led to maltripping as the operating time margin with either upstream or downstream relay was compromised. The relay manufacturer has reduced the boot time from 300 ms to 80 ms through firmware upgrade and is conducting relay replacement for all installed units. For future, maximum allowable boot time shall be included in the technical specification. This allowable boot time will be taken into calculation when doing protection grading at site.
CONCLUSION
TNB has embarked on using RMU CB as an initiative to reduce SAIDI and technical losses. Based on successful tripping cases reported and reduction in SAIDI achieved, this technology adoption is proven effective and hence it will be continued. It is targeted for 1000 units of RMU CB to be installed within the next 2 years. The performance, endurance and quality of the RMU CB are continuously monitored and any improvements required will be channeled to manufacturers for their actions. In future, subject to feasibility study, RMU CB can be motorised and SCADA-ready for wider application to offer cheaper circuit breaker option to existing VCB panel with further advantage of being fixed type (non-withdrawal).
REFERENCE
[1] Self-Powered Feeder Protection 605 series REJ603 Product Guide, ABB, 2012
[2] Substation Design Manual, Asset Management Department of TNB Distribution Division, December 2012
[3] Pekeliling Pengurus Besar Kanan (Pengurusan Aset) A12/2014 - Pengenalan Ring Main Unit with Circuit Breaker (RMU CB) untuk Sistem 22 kV dan 11 kV Sebagai Inisiatif Pengurangan SAIDI dan Technical Losses, Asset Management Department of TNB Distribution Division, April 2014
[4] TNB Technical Specification -12 kV SF6 Gas Insulated Ring Main Unit With Circuit Breaker (RMU CB), TNB Distribution Division, September 2012
[5]I EC 60529:1989+A1:1999+A2:2013 Degrees of protection provided by enclosures (IP Code), International Electrotechnical Commission, 2013
[6] Arc transfer in a rotary arc circuit breaker Article in Generation, Transmission and Distribution [see also IEE Proceedings-Generation, Transmission and Distribution], IEE Proceedings C 135(6):511 – 517, December 1988
[7] Dasar dan Prosedur Jaminan Kualiti Produk TNB, Quality Assurance Governing Council, Second Edition, 2011
