Series capacitor banks to maintain Victorian values

Australia's first transmission voltage series capacitor banks (SCB) are installed in the Dederang to South Morang 330 kV No 1 and No 2 lines, which form part of the interconnection between New South Wales and Victoria. The SCBs are located at one end of the lines, at South Morang Terminal Station (EHV substation), on the outskirts of the Melbourne metropolitan area.

Why series capacitor banks?

The Victorian Power System has a maximum demand of some 7500 MWe, and is connected to the New South Wales Power System (maximum demand 10 000 MWe) via a relatively weak 330 kV interconnection of 1500 MWe nominal capacity. On hot summer days when real and reactive power demands are at their peak, maximum import into Victoria over the interconnection previously had to be reduced to around 1100 MWe, to prevent voltage collapse in the main Victorian load centre following a critical contingency on the Victorian system. The effect of installing the series capacitor banks is to decrease the net series impedance of the interconnection, thereby restoring the full 1500 MWe import capability. The series compensation was implemented at a relatively small cost (total project capital cost of series compensation for the two lines was approximately A$ 13 million). System stability is also enhanced by the presence of the series banks.

GPUPowerNet owns the existing transmission assets in the state of Victoria. The Victoria Power Exchange (VPX) is responsible for planning and directing augmentation of the transmission network in the state. As a mechanism to ensure that transmission costs are kept to a minimum, VPX is required under its licence to run a contestable process to source all major transmission augmentations. GPUPowerNet had to bid for the project, which was awarded on the basis of lowest cost to VPX of obtaining the series compensation services over a 20-year period.

The South Morang series capacitor banks provide 50 per cent compensation of the line inductance, and are rated at 186 MVAr (3 phase per line).

• Rated current – 1300 A rms
• 10 minute rating – 1950 A rms
• Nominal system voltage – 330 kV rms
• Impulse withstand – 1300 kV peak
• Varistor rating – 48 (MJ/ph/line)
• Varistor protective level – 169 kV peak

How the SCB works

Under normal operation, the bypass disconnector and bypass circuit breaker are open, the series disconnectors are closed, and the line current flows through the SCB. Under line fault conditions, the voltage across the capacitors increases as fault current flows through the bank. The series bank protective equipment consists of varistors (metal oxide non linear resistors) and triggered spark gaps which act to protect the capacitors against overvoltages. For an external fault, the varistor starts to conduct at the protective level, limiting the voltage across the capacitors. For an internal fault, (ie, a fault located between the circuit breakers at either end of the compensated line), the thermal rating of the varistor may be exceeded due to the higher fault current in the line. Under these circumstances, the triggered spark gap is fired bypassing and discharging the capacitor. The spark gap is deionized during the dead time of the reclose cycle, and when the line breakers reclose, the capacitor is still in circuit. The original requirement was for the bank to be bypassed and only reinserted after the line had reclosed successfully. However, this approach was abandoned to keep the installation simple and avoid the need for additional signalling. If, due to abnormal line breaker operation the gap conducts for too long, and its short-time rating is exceeded, the bypass circuit breaker is closed, thereby bypassing the capacitor.

The current limiting reactor limits the capacitor discharge current to an acceptable value when the spark gap is fired or the bypass circuit breaker is closed. The series disconnectors and associated earthing switches are used to take the series banks out of service to allow them to be worked on. The bypass disconnector is closed under these circumstances to keep the line in service. The bypass circuit breaker is used to insert and bypass the series bank.

Design and layout

It was a requirement that the series banks be located within the existing South Morang Terminal station grounds. The area available for the banks was constrained by transmission lines and line easements on three sides, and the station boundary on the fourth side. The banks also had to be located conveniently for connection to the existing 330 kV lines. This resulted in the asymmetrical arrangement shown. In addition, the banks were arranged so that maintenance can be carried out on them with the associated lines in service.

To facilitate cut-in of the Series Banks, and stablize the connection, the existing 330 kV line spans between the existing strain towers were diverted onto a series of concrete poles.

System studies

The varistors are non-linear devices. To optimize their rating, a computer study was carried out based on the specified worst case external fault for which the Series Bank must not be bypassed. This is usually based on an unsuccessful reclose for the maximum fault current. The maximum swing current for an unsuccessful reclosure on the other 330 kV line also has to be taken into account. In the case of the South Morang series capacitor banks, the cost of the varistors is about 25 per cent of the cost of the bank equipment.

Because of trapped charges on the capacitor if the bank is not bypassed under all internal fault conditions, the transient recovery voltage (TRV) duty for the line circuit breakers can be more severe. The TRV is the transient voltage across the circuit breaker contacts when opening, and if in excess of the design values, can result in the circuit breaker failing to interrupt. Accordingly, an EMTP study was carried out to determine the values of TRV for different fault conditions. While the rates of rise of recovery voltage were not excessive, some of the TRV peak voltages were well above the rating associated with circuit breakers for a nominal 330 kV system. The highest TRV peaks were for a fault at the remote end (Dederang) of the line, for which the series capacitor would not normally bypass. GPUPowerNet was fortunate in that the line circuit breakers at South Morang were of the airblast type. While these breakers were quite old, they incorporate opening resistors which effectively reduces the TRVs to values within the rating of these breakers. The circuit breakers at the Dederang end had recently been replaced with modern SF6 breakers. It was fortunate that while the breakers had been purchased for a 330 kV system, the interruptors were designed for 400 kV, so there was some additional margin available.

Concerns were also raised about the impact of trapped charges on the capacitor during a reclose cycle on the switching surges that the terminal station equipment (particularly the line circuit breakers) would have to deal with. The switching surge is the line to ground transient overvoltage occurring on closing of the line. An analysis indicated that the probability of too high a switching overvoltage occurring was low.

Despite this, the following additional low cost measures were allowed for to provide a higher level of confidence that high TRV and switching levels would be fully dealt with:

• A signal from the line protection is sent to the spark gap to fire for all internal line faults. This results in the capacitor being discharged for all but the lowest level line faults. In the design selected, the line fault current needs to be high enough for the varistor to be conducting for the spark gap to fire. The added advantage of this measure is that the thermal duty on the varistors is reduced so that there is less chance that the varistor will be overheated for severe faults in quick succession.
• Line surge arrestors were installed at the 330 kV line entries to the Dederang and South Morang terminal stations. This ensures that the switching surge level be kept to acceptable levels for all conditions, and help reduce the TRV duty for the line circuit breakers when interrupting fault currents. The other benefit of the line surge arrestors is that they protect an open line circuit breaker from flashover across its contacts for a lightning strike to the line. This is possible during a reclose cycle at a time of severe lightning activity.

The presence of series capacitor banks leads to the possibility of subsynchronous resonance (SSR) and potential damage to the turbine-generator shaft systems. The closest machines electrically in the Victorian system are hydro generators. The SSR was analysed to see if it would be a potential problem for these hydro machines, or for the electrically more remote thermal machines. The analysis did not indicate any potential SSR problems, and system tests carried out during commissioning of the banks confirmed that SSR modes remained well damped following installation of the series banks.

Series capacitor bank design

The capacitors, varistors, spark gaps, damping reactors and platform cabinet are at line potential, and hence are mounted on large steel platforms supported on 330 kV post insulators. Fibre optic cables are used to transmit information from the platform mounted equipment to the ground controls. Fuseless capacitors were chosen for higher reliability and reduced cost. Provisions were made to install external fuses later if there were problems in service with the fuseless design.

In the chosen series bank design, information about the currents in the different parts of the platform mounted equipment is obtained using low voltage toroidal CTs on the platform. The CT signals are fed to a platform cabinet that contains electronic equipment which processes the information. This cabinet contains the fibre optic interface which converts CT analogue currents into digital light signals to be transmitted down the fibreoptic cables to the ground control. A CT-fed power supply is used to supply the platform electronics. The platform cabinet also contains a measuring circuit to determine the energy being absorbed by the varistors. If the selected thresholds are exceeded, a signal is sent to trigger the spark gap.

The ground controls contain PLCs which carry out the protection and control function. Information is displayed on a computer monitor. The information available includes alarms, status of the disconnectors and bypass circuit breakers, trending, sequence of events, varistor temperature, capacitor unbalance and line currents.

Control panels

All control and protection equipment is fully duplicated, including the CTs and the fibre optic cables. Because it is critical that the spark gap is always fired when required to do so, the firing system is also fully duplicated. The spark gap conducts within 0.3ms of being fired. Transient event recorders were also installed to capture fault voltage and current waveforms.

Duplicate 415V AC station supplies were derived from the existing supplies at the station. Duplicate 125V DC baterries and battery chargers were also provided. All cables used in the installation were screened, with screened twisted pair cables used to wire to the CTs on the platforms to limit cross-talk between the signals.

As the series bank control building is located some distance from the main control room for the Terminal Station, a fibre optic communication link was established between the two buildings. This enables all the information available at the ground controls to be read in the main control room, and provides a means of transmitting protection back-up trip signals. For failure of the bypass circuit breakers to close, the line circuit breakers are opened. In addition, to allow for remote control and monitoring from the state control centre, RTUs have been provided in the series bank controls with communication between South Morang and the control centres via existing communication facilities.

Line protection

The introduction of series compensation on existing lines results in the need to verify the performance of the protection schemes in the adjacent system, and usually results in the need to change the existing protection for the compensated lines. This stems from the change in the effective line impedance and the impact of the transients introduced by the Series banks and their protection during line faults. Specialist protection engineers carried out the necessary protection studies.

Segregated line protection schemes included distance relays with permissive overreaching intertripping using existing power line carrier links and digital current differential schemes operating on the single available digital radio link. Studies showed that these schemes will ensure adequate clearance times for all primary electrical faults.

Project organization

The main contractor for the project was GE Transmission Systems, which provided the series capacitor equipment, system studies, site supervision and testing of the series bank, and advice on the whole project. The disconnectors were purchased directly from Alstom T&D. Communications equipment, CVTs and line protection relays were purchased from several manufacturers.

Switchyard and line design, contract and project management was carried out by GPUPowerNet's internal engineering resources. Construction and testing was carried out by GPUPowerNet's field personnel. Civil design and construction was carried out by a local contractor.

The project duration was very short, being only 14 months from the contract award by VPX to full commercial service in December 1998. Liquidated damages were set relatively high to reflect the need for service of the plant prior to the peak summer load periods, which meant that delays could not be tolerated. On the other hand, the contract provides for rebates to be paid to VPX if the banks are taken out of service for repair and maintenance after the handover date. There was thus a need both to complete on time, and to ensure that the equipment was highly reliable at the handover date. The way this was managed was by commissioning a few weeks ahead of the actual handover date, so that any teething problems could be resolved prior to formal handover.

A fully detailed project scope document and project schedule was developed in the early stages. Monthly project meetings were required with VPX, and internal design and site meetings were held at frequent intervals. Project scheduling and cost control was carried out using GPUPowerNet's standard computer-based reporting systems.

Construction challenges

The key challenges here were the amount of space needed on the site for offloading - the equipment from GE arrived in 21 forty foot containers - and assembling on the ground, the lifting of the large steel platforms for the banks onto their insulators, and restrictions on crane movements due to the presence of adjacent transmission lines. There was a substantial amount of manual work needed on site for assembly of the steel platforms and the platform-mounted equipment.

The project programme required that most of the construction work took place during Melbourne's normally wet winter and spring. No extensions of time were allowed in the contract for inclement weather.

All equipment was fully production tested in the manufacturer's works prior to delivery to site. The protection and alarm settings, and the functionality of the Series Bank Controls, were checked using a simulator. No design testing was required, as equipment had previously tested up to the level for our application.

Site tests consisted of checks on wiring and the fibreoptic cabling, CT polarity, and AC and DC supplies. The operation of all devices and interlocks was proven from the devices to both the local and remote controls. Functional checks on the Series Bank protection and alarms were carried out by primary injection. Capacitor Bank balance was checked, and the triggered spark gaps were test fired. As part of the commissioning, the platforms and platform equipment were subjected to system voltage prior to inserting the Series Banks into the lines. The Series Banks were then inserted into the line with one end open, to prove that the platform cabinets - which are powered by CT's monitoring line current - were operational. Line opening and closure was carried out with the Banks inserted, and switching of an adjacent 500 kV line and transformer was carried out to assess the impact of the Series banks on the system. The definitive test to prove on site that the Series Bank protective equipment is working correctly is a Staged Line Fault Test. However, because of the inherent risks associated with placing a fault on the line, this test was not carried out.

As far as line protection is concerned, results of the studies were verified using the protection relays selected for the application in dynamic models of the system. Prior to commissioning on site, apart from the normal testing of the protection, end to end testing synchronised by GPS clocks was carried out.

Extensive training

As GPUPowerNet did not have any other Series Banks in its system, extensive training was provided to familiarise all necessary personnel with the special features of the equipment. During the installation phase, GE provided a specialist supervisor and test engineers, and GPUPowerNet personnel had the opportunity to become familiar with all aspects of the equipment. As part of the contract, GE also provided a 4-day formal training course which was addressed to engineers and field personnel. Particular emphasis was placed on GPUPowerNet gaining experience of the screen-based controls and monitoring system. Details on safe access procedures to the series bank equipment were covered with the field operators.