Increase of power transmission capacity by means of reactive compensation equipment

Foreword

In an electrical power system featured by long transmission lines and poorly meshed grids, voltage regulation is a source of concern. Due to thisissue could occur in the Chilean Central Interconnected System (SIC), specifically in the northern area where 600 miles transmission lines can be found, it will be the focus of the analysis herein performed.

The triple circuit Maitencillo – Cardones, with 80 miles of 220kV transmission lines and 780 MVA of rated power, represents the interconnection of two clearly segregated blocks in the SIC northern area: the southern subsystem (with Maitencillo substation as boundary) behaves as an active node, with high voltage and active power control capability, but low short-circuit power (2400MVA); on the other hand, the northern subsystem (Cardones to Diego de Almagro substations, 94 miles of simple circuit) is mainly featured by great blocks of mining demand, passive nodes, no active power or voltage control capability, and high-cost power generation available.

The following image depicts a geographic representation of the SIC northern area, and a single-line diagram which shows power plants, transmission lines lengths, equipment, etc.

It is expected a power transference in thesouth-north direction, with much lower operative limits than conductors thermal ampacity though, due to the low short-circuit level at the transmitter side and a poor voltage regulation at the receiver side. Even considering load shedding schemes in case of fault events(to improve voltage control), operative limits established for the triple circuit resulted in 345MW, requiring thermal generation unit to be dispatched (high cost). Higher levels of power transference are extremely risky, withhigh levels of dU/dP sensitivity and likely undamped power oscillations as limiting factors.

In order to improve the power transference, without increasing operating cost, it is mandatory to perform exhaustive electrical studies, both static and dynamic, that allow the specification of reactive compensation equipment.

Actualsystem performance: Voltage stability

In order to increase the power transmission capacity, it is necessary to fit the following requirements:

• Verification of operating criteria established by national regulatory entities.
• Avoid lowering of current operative conditions.

Basing on these requirements,it is first developed a detailed analysis regarding the actual limiting operative conditions that will allow verifyingoperative standards fulfillment and, at the same time, defining other performance indices.

The main features of the operation scenario are:

• High demand
• 4 units for Guacolda plant in service, with reactive power margin for voltage control.
• The absence of power generation from Cardones to north.
• SVC’s Maitencillo and Pan de Azúcar in service withreactive power margin for voltage control.
• Power transference through triple circuit Maitencillo – Cardones of 345MW.

A prior analysis considers the evaluation of dU/dP y dQ/dP sensitivity levels that power system presents in those conditions, with the aim of determining voltage variations and the increase of reactive power induced by small load growth. These values will allow estimating the system behavior against small perturbations in the grid.

The following picture shows PV characteristic curves for this base case, due to the increase of loads in 110kV voltage level located in the northern side. It can be observed the reduced distance between the operation and collapse points.

Evidently, the power system demands in order to face load variations located at the end of the grid are extremely high, resulting in transference levels of 86% (345MW over 401MW) for N grid condition, and 90% (315MW over 351MW) for N-1 grid condition. Both levels found by static simulations are above the recommended limits for the operation of the power system (70% for N grid conditions and 80% for N-1 grid).Once border conditions are statically stated, dynamic simulations are performed in order to determine the power system transient stability after being applied a severe fault event (design fault).

The most critical fault is a 2-phase short-circuit in the simple circuit of the Maitencillo – Cardones 220kV transmission line, at Maitencillo end. It can be seen in the following picture that the power system is barely capable of recovering the voltage above 0.7pu,10ms after clearingthe fault.

Basing on this analysis, not only is possible to validate the SIC operation with a transference of 345MW through the Maitencillo – Cardones link, but also sensitivity values wereestablishedthat will have, at least, to be sustainedfor the analysis expected to define an increase in power transference.

Power transmission increase capability

The Maitencillo – Cardones 220kV link is composed by a 54.37W circuit (C1), with a rated power of 197MVA, and a 55.26W double circuit (C2 and C3), each one with a rated power of 290MVA.Therefore, circuit 1 represents the bottleneck of the link due to its minimum capacity and impedance. Specifically, transmitted power through circuits 2 and 3 will be, at most, the 98.4% of the transmitted power through circuit 1 because of the impedance relationship between the simple and double circuits.

The limiting power transference for N-1 grid condition, regarding conductors capacity, is 418MW, which states that in order to increase the transmission level through the link because of a circuit outage, an ALSS (Automatic Load Shedding Scheme) would berequired so that no unacceptable overloads could be induced. Basing on this, and considering ALSS out of service, the study cases take into account a maximum power transference of  420MW.

Study case 1: Synchronous compensator in Taltal power plant

The first alternative, and with base on previous technical-economic analysis, is to evaluate the installation of a synchronous compensator within the Taltal power plant at15kV voltage level, which woulduse a power transformer already installed for one of the generation units.

Additionally, taking into account maximum transference expected by the triple circuit Maitencillo – Cardones link, increase of demand, and mining projects planned in the area, an operation scenario is designed in order to reach a power transference of 420MW at the time the synchronous compensator would be available for the operation.

With the same fault event considered for the base case, it is observed that the power system considerably improves its staticbehavior: voltage levels are close to rated values with enough reactive power margin for regulation, and sensitivity operation levels which have been considerably reduced, implying more stable conditions. Nevertheless, synchronous compensator connection point and characteristics make this solution impractical from the transient stability point of view.

The following pictures show the voltage waveforms in 220kV Diego de Almagro busbar, and the synchronous compensator rotor angle after a short-circuit in Maitencillo – Cardones link. As it can be seen, both variables exceed operative limits imposed by regulations.

These instability phenomena are due to the synchronous machine inertia to be installed in the power plant (backswing effect): once the fault has been cleared, voltage tends to recover and the machine rotor angle reaches ahigh amplitude of negative values regarding the SIC inertia frame of reference. Therefore, the electric power absorbed by the machine increases the power transference from Maitencillo to thenorth and induces a voltage drop.

Although a higher inertia value could improve the voltage recovery, it would negatively affect angular variation. Thus, since both magnitudes are exceeding the operative limits, this solution is impractical for the purpose of the study.

Study case 2: SVC in Diego de Almagro substation

Once the necessity of a synchronous compensatorin Taltal power plant had been discarded because of a backswing effect caused by the machine inertia, it is studied the installation of an SVC (Static Var Compensator) in a strategic electrical point: Diego de Almagro substation. For this purpose, it is selected an equipment of +110/-69MVAr to be installed at220kV voltage level with a step-up power transformer.

In order to compare the system response with the solution proposed in the study case 1, the simulation performed considers the same short-circuit event application. The following pictures show 500ms of the simulation where it can be seen the recovery voltage and the reactive power supplied by the SVC (measured on the HV power transformer side).

It is noted that the voltage recovery is rather different than the obtained with the synchronous machine. However, 10 ms after clearing the fault, the voltage level does not reach the minimum operative limit established of 0.7pu.

Concluding, the proposed solution does not fit the voltage recovery requirements because of theSVS time response, since the TCR remains at its minimum value until the voltage level becomes acceptable. The SVS behaves, until the fault clearing in 1.154s,  as a capacitive bank. For this reason, this technology is not suitable to face this fault.

Study case 3: SATCOM in Diego de Almagro substation

Judging by the solutions previously proposed, it is considered the installation of a STATCOMin Diego de Almagro substation with the same characteristics than the one located in Cerro Navia. In order to compare the system response with the solutions proposed in study cases 1 and 2, the simulation performed considers the same short-circuit event application.

The following pictures show the main specifications of the equipment considered for the analysis, and the simulation results for the fault event under analysis. It can be clearly seen that with reactive power supplied during and after fault application, the recovery voltage behavior results acceptable without exceeding operative limits. The higher transient overload acceptable level, the faster the recovery voltage over 0.7pu after clearing thefault.

Finally, with base on these preliminary results, the proposed solution of installing a STATCOM would represent the best option in order to increase power transference through Cardones – Maitencillo link without violating power system requirements.

Conclusions

With base on the feasibility studies performed in order to install reactive compensation equipments, it was finally selected a ±100 MVAr STATCOM (two ±50 MVAr modules) with a capacitors bank (MSC) of 533.3µF to be installed in Diego de Almagro substation, which represents in total a +140 MVAr/-100 MVAr total reactive power supply/absorption capability.

Capacitors bank does not serve as a filter, which allows its connection in case the power system requiring for a voltage regulation purpose. Additionally, the fast control capability of the SVC Plus allows Diego de Almagro’s reactor to be out of service, which also improves the voltage regulation. The combined solution of two SVC Plus modules and an MSC brings major flexibility and security to the electrical power system.

Definitive studies have been performed with detailed models of the equipment, particularly delivered by themanufacturer, basing on specific scenarios according to expected connection dates, and considering maximum voltage regulation criteria. Power transference limits have beenfinally established in 420MW, confirmed by numerous fault events (include the SVC Plus itself).

Finally, the increase of power transmission capacity through Cardones – Maitencillo link was possible by means of installing a STATCOM, initially studied by simulations and eventually implemented arriving at a real power transference of 420MW in the triple circuit under study.