The main CSC equipment consists of two voltage-sourced converters, one shunt transformer and two series transformers. The CSC interacts with the transmission system through injection of either current, through the shunt transformer, and/or voltage components, through the series transformers.

The injected currents and voltages are fully controlled in magnitude and phase angle. The result is great flexibility in providing unprecedented dynamic shunt and/or series compensations in a way that maximises the power transfer capability of the New York transmission network.

The successful development of the CSC is the result of long-term collaborative research involving EPRI, Siemens, and energy companies, including NYPA, Tennessee Valley Authority (TVA), American Electric Power (AEP), and Western Area Power Administration (WAPA).

The NY transmission bottleneck

Most of New York state’s generating capacity is located in the western and northern regions. Many of the state’s major transmission lines, taking power to the eastern section, home to New York City and Albany, converge in the centre of the state, causing a bottleneck in the flow of power. Depending on system conditions, between 2200 and 2800 MW flows across this transmission interface, which runs from Utica to Albany.

This bottleneck is a strategic interface for both NYPA and the New York Independent System Operator (NYISO) – the non-profit corporation mandated with administering the state’s wholesale energy markets and operating New York’s high voltage electric transmission system.

The CSC helps relieve this bottleneck by increasing power transfer capability, enhancing voltage control, providing additional damping in response to system contingencies, and providing operational flexibility during system outages.

CSC structure and functions

The two voltage-sourced converters of the CSC can be connected to the 345 kV Marcy bus and two transmission lines through coupling transformers. Converter DC buses can either be operated independently (DC switch open) or in parallel (DC switch closed) to allow an exchange of real power between the converters through the DC bus. A number of disconnect switches and circuit switchers enable engineers to connect each converter to the system in various configurations.

For shunt compensation, both converters are connected to the secondary windings of the shunt coupling transformer. For series compensation, converter 1 connects to the series coupling transformer on one line, and converter 2 to the second series coupling transformer on the other line. Depending on control needs and objectives, manipulation of the disconnect switches, circuit switchers, and circuit breakers provides four primary control modes:

• STATCOM (Static Synchronous Compensator) provides shunt operation of the associated converter. The reactive output current of the converter is regulated to maintain the desired AC bus voltage. The real-power component of converter output-current is controlled indirectly to maintain the necessary level of DC bus voltage.

• SSSC (Static Synchronous Series Compensator) allows stand-alone series operation of the associated converter. The converter output voltage is controlled in quadrature with the prevailing line current and at the desired magnitude.

• UPFC (Unified Power Flow Controller) controls converter output voltage and influences the line’s real- and reactive-power flow with no restrictions on the phase of the injected voltage.

• IPFC (Interline Power Flow Controller) enables simultaneous control of power flow on two transmission lines so that the operators can automatically route power away from heavily loaded lines to underutilised lines. In this mode, the output voltage of the converter is controlled to influence the line power subject to the restriction that real power exchanged with the line via one converter must be balanced by the power exchanged by the other converter.

CSC hardware and control

The two voltage-sourced converters are each rated at 100 MVA. The converters use gate turn off thyristors (GTOs) rated at 4500 V blocking capability and 4000 A turn-off capability. There are 600 GTOs in the CSC.

A modular design is used for the converters, with each converter consisting of 12 three-level poles connected to a DC bus. The stacks of converter poles are about 5m high.

The control system for the two voltage-source converters and the auxiliary systems of the Marcy CSC installation is comprised of a central control suite and distributed controls for individual subsystems. The CSC is operated by a control system housed in five cabinets in the control room along with two additional cabinets associated with protective relaying functions.

The various distributed subsystems of the CSC are co-ordinated through the central controls. The controls are connected by serial communications to every part of the CSC.

This ranges from the local electronics at each converter pole, to relay logic and PLC controls on subsystems such as the transformer units, gating power supply, and conventional switchgear.

The central controls are responsible for the real-time algorithms that direct the two voltage-source converters in the exchange of real and reactive power based on operator input and system conditions. Both local and remote operator interfaces are provided to configure, start, and stop the equipment as well as change operating points and provide status and diagnostic information. Extensive sequencing and protection logic is required in the co-ordination of the two converters and their auxiliary subsystems.

The algorithms that govern the instantaneous operation of both converters are performed in real-time, using multiple digital signal processors. During run-time, the controls continually monitor the operation of all subsystems, collecting and analysing status information. The controls are responsible for all start-up and shutdown sequences and for organising and annunciating alarm conditions. A hierarchical arrangement of graphical display screens gives the operator access to all system set-points and parameters, and provides extensive diagnostic information right down to the individual GTO modules.

The next generation

The NYPA CSC installation represents the latest generation of power grid control equipment and displays the value of long-term collaborative research in creating cost-effective, customer-friendly solutions for today’s energy companies. The CSC enables both energy companies and regional independent system operators to overcome trouble spots and respond to the challenges of emerging open markets.

EPRI has identified numerous bottlenecks in the US grid that could be alleviated with FACTS equipment such as the CSC in the near future, cost-effectively increasing power flow capability by hundreds of megawatts.

The benefits arising from the NYPA CSC installation can be summarised as follows:

• An increase in the power transfer capability limit by over 200 MW without the need for new transmission lines.

• Unprecedented levels of power system control capability, which helps both the New York Power Authority and the New York Independent System Operator (NYISO) respond to the demands of open markets.